ON MUCOR MUCEDO, MUCOR FLAVUS AND RELATED SPECIES

M. A. A. SCHIPPER
Centraalbureau voor Schimmelcultures, Baarn

 
Summary

Results of a study on Mucor mucedo, M. piriformis, M. saturninus, M. aligarensis, M. plasmaticus, M. psychrophilus, M. strictus, M. minutus and M. flavus are compiled. Descriptions and original line drawings of cultures grown under standard conditions are given. Additionally the variability, as influenced by environmental conditions such as temperature, light and medium, and the outcome of mating experiments are mentioned. Furthermore a key to the accepted species and a list of doubtful taxa are provided.

 
Introduction

In a previous paper (Schipper, 1973) variability in Mucor hiemalis and related species was discussed as part of a taxonomic treatment of the genus Mucor. In continuation of that study the present paper summarizes the results of an investigation of the large species of the genus that show optimal development at temperatures below 20C, viz. Mucor mucedo, M. piriformis, M. saturninus, M. aligarensis, M. plasmaticus, M. psychrophilus, M. strictus, M. minutus and M. flavus.
Many species have been described in the genus Mucor. For reasons of convenience the species were grouped into sections. Zycha (1935) placed the larger forms of Mucor, with sporangia over 100 m in diameter, in the sections Mucedo and Flavus. Section Flavus was characterized by sympodially branched tall sporangiophores while the tall sporangiophores in section Mucedo were never sympodially branched.
Hesseltine (1954) slightly modified Zychas system of grouping. Apart from the sections Mucedo and Flavus, he distinguished a section Macromucor to include the largest forms, with sporangia over 500 m in diameter and sporangiophores usually up to 10 cm in height and 50-100 m in diameter. His section Mucedo included species that produce colonies with tall, erect sporangiophores usually unbranched above, and a short fertile region of smaller sporangiophores with smaller sporangia near the surface of the substrate, the short sporangiophores being repeatedly branched. Section Flavus had simple sporangiophores or branched usually only [p. 2] once or twice; colonies without an abundant region of fruiting near the substrate. Zycha et al. (1969) used sections Mucedo and Flavus in the sense of Zycha (1935) again. However, in the key to sections, section Mucedo was characterized by tall unbranched sporangiophores with large sporangia and next to these by short sporangiophores with small sporangia.
It is rather confusing that Pidoplichko & Milko (1971) introduced a section Griseo-ochraceus (syn. Mucedo Zycha sensu Hesseltine), while their section Mucedo included species with sporangiophores not differentiated into distinct regions. Thus Mucor mucedo was treated as a species with undifferentiated sporangiophores, and placed in their section Mucedo together with species that Zycha and Zycha et al. classified in the section Flavus.
Zycha (1935) and Zycha et al. (1969) mentioned the influence of environmental conditions on height and colour of the colony in members of the section Mucedo; they made no further use of this knowledge. And yet, in Mucor mucedo and M. piriformis light may stimulate the production of tall sporangiophores to the extent that the colony consists of tall sporangiophores only. The same authors placed Mucor alboater in the section Mucedo and M. piriformis in the section Flavus; the species however proved to be identical. Otherwise, though a second fertile region is rare in Mucor flavus, it does occur. Furthermore they placed Mucor flavus in the section Flavus, M. peacockensis and M. attenuatus in the section Mucedo; all three however represent a single variable species.
A reason to reject the recent classifications has been the fact that a species may be found in two different sections, since the variability within the species was not taken into account.
In a previous paper (Schipper, 1973) it was put forward that in grouping Mucor species into sections or otherwise, special attention should be given to zygosporic stages. After some years of study of the genus the conviction took shape that Mucor species that belong together from a general morphological point of view, produce similar zygospores. It seemed to open perspectives for the realization of a natural grouping of Mucor species: an indication of the relationship will be revealed whilst studying zygospores only.
In order to check the validity of this conception, extensive mating experiments were performed between strains of similar general morphology. As a matter of fact, mating experiments were even more successfull than expected: rather often species with similar zygospores were not only related, but were found to be able to produce zygospores with each other as well, thus indicating synonymy according to the proposition that mating partners that can produce normal zygospores are considered one species. The mating experiments revealed once more that Mucor species may be surprisingly variable.
The species discussed in the present paper are not considered a defined group of Mucor, but they are closely related to M. mucedo or represent forms intermediate between M. mucedo and M. flavus. They have the following characteristics in common: [p. 3] Sporangiophores up to 20 m in diam. at the least, bear sporangia beyond 100 m in max. diam. The sporangiophores may be produced at distinct levels: tall and either single or branched, short and usually repeatedly branched sympodially; furthermore very short and slender sporangiophores bearing very small sporangia may occur. Typically the young sporangiophores are erect. In young colonies of species that produce unbranched tall sporangiophores, these sporangiophores show numerous droplets adhering to the walls, except at the tips. Sporangial diameter and structure of their walls, size and shape of the columellae and of the sporangiospores usually vary in accordance with the level at which they are borne, but these characters may also be rather variable in each level. The larger sporangia are over 100 m in max. diam. with echinulate walls, mature sporangia are powdery in aspect; larger columellae mostly obovoid to pyriform in shape, rarely ovoidal, with or without reddish to yellowish-brownish contents; sporangiospores smooth, with cloudy (Fig. 2) to granular contents.
The zygosporic stage is known in Mucor mucedo, M. piriformis, M. saturninus, M. plasmaticus and in M. flavus. In each of these species the zygospores result from compatible matings at low temperature, are black, roughened in outline and vary from 180-250 m in max. diam. In Mucor mucedo, M. piriformis, M. saturninus and M. plasmaticus the zygospores are produced in the under portion of the colony, while in M. flavus they are formed in the upper portion.

 
Material and methods

The cultures used in this study are monosporangial isolates from Mucor strains in the CBS collection, in addition to strains sent for identification or received from colleagues, and own isolates.
Routinely descriptions of the species were prepared from cultures on beerwort agar at 20C, in crystallizing dishes to ensure an undisturbed development of the sporangiophores and sporangia. Exceptions were made in two cases: Mucor psychrophilus was described at 15C, since it developes poorly at 20C and Mucor strictus at 10C and 20C in view of temperature-dependent variability.
Since synthetic Mucor agar (SMA) proved to be a rather unsatisfactory medium for the cultures under study, beerwort agar was chosen as a standard medium for descriptions.
The influence of light was determined by growing simultaneous cultures in uncovered dishes and in dishes wrapped in black paper, in a light incubator (Heraeus, type KKB 600 L; light: 4 X TL 20W/34, 12 h on/12 h off.) When it seemed useful to check the influence of the medium, species were grown on other media as well. (cf. Schipper, 1973).
Size of the sporangia was established with the aid of a stereomicroscope [p. 4] in undisturbed cultures and afterwards checked in slides, if possible, and so was the branching type of the sporangiophores. Sizes of sporangia, columellae and zygospores are given up to the largest of a continuous series of sizes; sporangiospore-sizes are noted as smallest and largest of a continuous series of sizes. Rare extremes, not connected with the series by intermediate measurement, are given between brackets.
Capitalized colour names are after Ridgway (1912). Terms used to describe the shape of the columellae are given in Fig. 1.

 

Fig.1. Descriptive terms adjusted to Ainsworth (1971). a. globose; b. applanate; c. ellipsoidal; d. cylindrical; e. ovoid; f. obovoid; g. pyriform; h. conical. (Figures after Pidoplichko &. Milko, 1971).

The methods used in intraspecific matings were those described in a previous paper (Schipper, 1973). The choice of partners in interspecific mating experiments of each species was guided primarily by morphological similarities and secondarily to establish reactions towards species outside the relationship. Also contrasts were made with strains of remote relationship but usually strong sexual activity, in attempts to determine the mating reaction type.
Blakeslees standard tester strain D = CBS 850.71, M. lamprosporus (-), Blakeslees = CBS 851.71, M. lusitanicus (-), Blakeslees IV(-) = CBS 852.71, M. lusitanicus (+).
As both slow linear growth and heavy sporulation might have an unfavourable effect on mating results, observations on influence of external conditions on growth and sporulation are noted. [p. 5]

 
Key to the accepted species

1a.

Colonies about 1 mm in height

M. aligarensis

1b.

Colonies higher

2

     

2a.

Sporangiospores spherical or nearly so when grown at 20C, ellipsoidal when grown at 10C

M. strictus

2b.

Shape of sporangiospores independent of growth temperature

3

     

3a

Sporangiospores spherical or nearly so, 4-5 m in diam.

M. minutus

3b

Sporangiospores larger, usually ellipsoidal

4

     

4a

Columellae ovoid to conical

5

4b

Columellae obovoid, ellipsoidal or pyriform

6

     

5a

Mature sporangia dorsiventrally flattened, up to 400 m (and beyond) in diam.; sporangiospores 25 m and upwards in average length

M. plasmaticus

5b

Mature sporangia slightly dorsiventrally flattened, up to 200 m in diam.; sporangiospores usually 7.5-10 m in lengt; psychrophilic: poor sporulation at 20C; slow surfacial growth at all temperatures

M. psychrophilus

 

 

Fig. 2. Mucor mucedo, CBS 640.67. a. base of a tall sporangiophore; b. short sporangiophore; c. columellae; d. sporangiospores from sporangia on tall sporangiophores, with cloudy granular contents (a different strains); e. sporangiospores from sporangia on short sporangiophores. [p.6]

 

Fig. 3. Mucor piriformis, CBS 169.25. a. base of a tall sporangiophore b. short sporangiophore; c. germinating sporangium; d. columellae; e. sporangiospores from sporangia on tall sporangiophores; f. sporangiospores from sporangia on short sporangiophores.

 

Fig. 4, a-f. Mucor saturninus, CBS 974.68. a. branching fashion of the tall sporangiophores; b. branching fashion of the short sporangiophores; c. branching fashions schematic; d. columellae; c. sporangiospores; f. zygospores; g-i. Mucor aligarensis, CBS 993.70. g. sporangiophore; h. columella; i. sporangiospores. [p.7]

 

6a

Tall sporangiophores usually unbranched above; in colonies grown in darkness a second fertile region of short, repeatedly branched sporangiophores common; columellae obovoid, ellipsoidal or pyriform; sporangiospores averaging over 7 x 4 m

7

6b

Tall sporangiophores branched sympodially; with or without a second fertile region; columellae typically obovoid

8

     

7a

Rapid growth on cherry agar or acid beerwort agar; sporangiospores usually ellipsoidal, up to 10 m in length

M. piriformis

7b

Restricted growth on cherry agar or acid beerwort agar; sporangiospores typically cylindrical-ellipsoidal, over 10 m in max. length

M. mucedo

     

8a.

Colonies grown in darkness Pale Smoke Gray to Smoke Gray; a second fertile region of short sporangiophores common

M. saturninus

8b

Colonies grown in darkness mostly Olive Buff; a second fertile region of short sporangiophores rare, if present it produces larger sporangiospores than found in the sporangia on the tall sporangiophores

M. flavus

 

 

Fig. 5, a-c. Mucor plasmaticus, CBS 402.68. a. base of a sporangiophore; b. columella; c. sporangiospores. e-f. Mucor psychrophilus, CBS 288.71. d. branched sporangiophore; e. columellae; f. sporangiospores. [p. 8]

 

Fig. 6, a-d. Mucor strictus, CBS 576.66. a. branched sporangiophore (20C); b. columellae; c. sporangiospores (20C); d. sporangiospores (10C); e-f. Mucor minutus, CBS 586.67. e. branching fashions of the tall sporangiophores; f. short sporangiophore; g. columellae; h. sporangiospores.

 

 
Descriptions

  
1. Mucor mucedo L. ex Fr. - Fig. 2

Mucor mucedo L. Spec. Plant. 2: 1655; ex Fr. Syst. mycol. 3: 320. 1832.
Mucor griseo-ochraceus Naumov Mater. Mikol. Fitopat. Ross. 1 : 9. 1915.
Mucor murorum Naumov Mater. Mikol. Fitopat. Ross. 1: 11. 1915.
Mucor coprophilus Povah Bull. Torrey bot. Club 44: 297. 1917.
Mucor mucedo L. ex Fr. f. dependens Burgeff Z. Bot. 12: 3. 1920 Mucor dependens (Burgeff) Burgeff Bot. Abb. 4: 17. 1924.
Mucor murorum Naumov var. macrosporus, var. robustus, var. rufescens, var. coloratus Naumov Opredelitel Mukorovykh (Mucorales), Ed. 2, pp. 50, 51. 1935.
Mucor murorum Naumov var. bitabulatus Pidoplichko & Milko Atlas mucoral'nykh gribov, p. 80. 1971. [p. 9]


 


Fig. 7. Mucor flavus. a-b. CBS 234.35. a. branching fashion of the sporangiophores; b. columella, with sporangiospores sticking to the wall; c-i. sporangiospores. c. CBS 234.35; d. CBS 235.35; C. CBS 230.35; f. CBS 127.70; g. CBS 126.70; h. CBS 210.71; i. CBS 715.73; j-n. characteristics of CBS 197.71. j. columellae on tall sporangiophores; k. sporangiospores from sporangia on tall sporangiophores; l short sporangiophore; m. columella on short sporangiophore; n. sporangiospores from sporangia on short sporangiophores.

The following description is of CBS 640.67 on beerwort agar at 20C, in darkness.
Colony up to 25 mm in height, Drab-Gray, aromatic, composed of a sparse layer of tall, usually simple sporangiophores and a well developed layer of short, repeatedly branched sporangiophores; tall sporangiophores up to 40 m in diam. usually unbranched or with few secondary branches, though large branches infrequently occur, at first with numerous adhering droplets, bearing sporangia up to 250 m in diam., brownish grey at maturity, with deliquescent spiny walls; columellae obovoid to ellipsoidal, [p. 10] up to 160 x 125 m, with or without reddish-brownish contents; the sporangia that are supported by the short sporangiophores are slightly darker in colour, have persistent spiny walls and are variable in size; the columellae found in the under-region are cylindrical ellipsoidal or, rarely, campanulate, up to 85 x 53 m, with reddish-brownish contents; sporangiospores cylindrical-ellipsoidal, broadly ellipsoidal, 10.5-13.5 x 5.7-7.5 m, or subspherical, 8-9 m in diam., thick-walled, with more or less cloudy granular contents; zygospores [CBS 640.67 (-) x CBS 144.24 (+), on beerwort agar at 15C] globose to compressed between suspensors, up to 250 x 200 m, with warty protuberances up to 15 m in length; suspensors equal or nearly so, swollen and then constricted at the attachment to the zygospore or without a swelling, yellowish to brownish.
Influence of temperature: at 30C no growth, at 25-5C growth and sporulation. At 15C and below recurved short sporangiophores occur, the columellae are narrow cylindrical-ellipsoidal and the sporangiospores tend to be larger.
Influence of light: cultures grown in daylight at 20C produced hardly any short sporangiophores or none, but tall unbranched sporangiophores only. Simultaneous cultures in darkness produced a dense layer of short sporangiophores intermingled with rather few tall sporangiophores. Light-cultures were yellowish-brownish in colour; yellowish spots occurred on the tall sporangiophores; only few sporangiophores were covered with droplets.
Influence of medium: on cherry agar and on acid beerwort agar (pH 4) linear growth slow; on SMA at 20C in darkness, colony Light Grayish Olive, tall unbranched sporangiophores rare.

Material examined

CBS 109.16 (), received from K. Saito.
CBS 144.24 (+), CBS 145.24 (), received from H. Burgeff. According to the CBS files Burgeff received a (+) and a () strain from the CBS originating from Saito, and returned these to the CBS in 1924. So most probably CBS 109.16 and CBS 145.24 are of one stock.
CBS 228.29 (+), type strain of Mucor murorum.
CBS 524.66 (+), received as M. griseo-ochraceus, BKM F-1085, fr. water, Kiev district, USSR.
CBS 640.67 (), neotype, isolated from nasal discharge of cow.
CBS 525.68 (), received as M. coprophilus, BKM F-1245, from fieldmouse dung, Armenian SSR.
CBS 983.68 (+), CBS 984.68 (+), isolated from rabbit droppings, and other isolates from dog-, sheep-, rabbit-droppings collected in the Netherlands, and from pig- and horse-dung, collected in Germany.

Discussion

The strains CBS 109.16, 144.24 and 145.24 have a strong sexual ability, but in asexual aspect they are poor. Short, repeatedly branched sporangiophores [p. 11] were never observed in these strains, tall unbranched sporangiophores were produced in rather small numbers. Attempts to improve the sporangial stage, by varying the external conditions (temperature, light, medium, size culture vessel) remained without success. The concentration of the medium may affect the shape of the sporangiospores: on 2% beerwort agar they were mainly ellipsoidal and only few were spherical; on 8% beerwort agar the sporangiospores were practically spherical at first, then broadly ellipsoidal or spherical. The strains showed a more intense pigmentation than the recent isolates. Since Burgeff's paper (1924) these strains have been the test organisms in all experiments connected with the study of sexual substances in Mucor mucedo matings.

Variability. - The tall sporangiophores of other isolates than above may be over 50 mm in height, either simple or ramifying with short branches, rarely also with tall branches. The sporangia vary from 175 to 250 m in maximum diameter. The colour of the immature sporangia is whitish or yellowish, mature sporangia are light brownish grey or dark grey; infrequently germinating sporangia occur.
Special attention was given to the shape and the size of the sporangiospores. Individual large sporangia, which looked mature and which just had opened under the microscope, released sporangiospores of one shape only, either spherical or broadly ellipsoidal or cylindrical-ellipsoidal with or without the addition of a few spherical spores.
Spherical sporangiospores originate from sporangia with dry walls, that were usually isolated from tall sporangiophores. Still moist sporangia usually contain cylindrical-ellipsoidal spores, sticking together.

On beerwort agar at 20C, the sizes of the ellipsoidal spores were:
in CBS 109.16: 9.5-11(-15) x 5.4-6(-7) m
in CBS 228.29: 11-13 x 6-7 m
in CBS 542.66: 16.5-19 x 7.5-9 m
in CBS 640.67: 10.5-13.5 x 5.7-7.5 m
in CBS 525.68: 9.5-19.5 x 5.5-9.5 m
in CBS 984.68: 11-13.5 x 6-8 m

At lower temperatures the sporangiospores tend to be larger in each strain, e.g.
in CBS 228.29 at 15C sporangiospores of 21.5 x 11 m were observed,
in CBS 542.66 at 5C the spores were up to 21 x 10 m,
in CBS 640.67 at 5C sporangiospores of 17 x 10 m were not rare,
in CBS 984.68 at 5C the average size was 15 x 9 m, maximum size 19 x 11 m.

Synonymy. - Since no type material of Mucor mucedo is known to exist, the strain CBS 640.67 was chosen as neotype.
The synonyms of M. mucedo listed by Fischer (1892), Lendner (1908) and others will not be discussed. The older diagnoses are too brief for definite identification and no material is known to exist. As for the other [p. 12] species and varieties that have been reduced to synonymy, with the exception of the type strain of Mucor murorum no authentic material of those was available either. However, the demonstrated variability of Mucor mucedo encompasses the slight differences of features indicated by their diagnosis.
M. griseo-ochraceus was described as a species with spherical to subspherical sporangiospores, 7-10 m in diam. and, from the same sporangium, cylindrical ellipsoidal spores, 8-14 x 5.5-8.7 m. Naumov (1915) described the sporangiospores of M. murorum as 12-18 x 6-9.6 m, mainly 9.6-16.5 x 5.5-8.5 m. In 1935 the sizes were given as (8.3-)12-15 (-22) x 5.5-11 m. Furthermore 4 varieties were distinguished:

var. macrosporus: sporangiospores 8-19 (26) x 5-9 (12) m,
var. robustus: sporangiophores unbranched,
var. rufescens: colony salmon; sporangiophores branched,
var. coloratus: colony yellowish; columellae dark grey; sporangiospores 8-14 x 5-7 m.
Pidoplichko and Milko (1971) distinguished Mucor murorum (syn. M. murorum var. macrosporus), M. murorum var. robustus and Mucor murorum var. bitabulatus (syn. M. murorum sensu Naumov, 1935).
Burgeff (1920, 1924) attempted without success to extend the number of (+) and () strains of M. mucedo for his studies, with fresh isolates from horse dung in Germany; none of his isolates, though strongly resembling M. mucedo in morphology, showed sexual response in contrasts with the Saito-strains or with each other. He named these sexual inactive strains M. mucedo dependens' (1920) or M. dependens (1924).

Intraspecific matings. The strains CBS 109.16 () and CBS 144.24 (+) were chosen as tester pair. Matings of the strains studied with the tester pair resulted in the production of zygospores. Though the yield was not as good as the zygospore production in matings of the tester pair, the results were satisfactory. The best results were obtained in matings on beerwort agar + peptone at 15C, the partners inoculated at a distance of about 10 mm from each other. The partners had been grown at 15C.

Interspecific matings. All tested strains of Mucor mucedo responded to the presence of either the (+) or the () mating tester of M. piriformis in the production of progametangia; the conjugation was not completed.
Likewise, incomplete conjugations were formed in contrasts of
CBS 144.24, M. mucedo (+) and CBS 137.40, M. saturninus ();
CBS 109.16, M. mucedo () and CBS 974.68, M. saturninus (+);
CBS 109.16 or CBS 525.68, M. mucedo () and CBS 571.70, M. plasmaticus (abundant progametangia, but peculiar in shape);
CBS 109.16, M. mucedo () and CBS 576.66, M. strictus (progametangia, but peculiar in shape).
No response was observed in contrasts of the tester pair of M. mucedo with M. aligarensis, M. psychrophilus and M. minutus. [p. 13

  
2. Mucor piriformis Fischer - Fig. 3

Mucor piriformis Fischer -Phycomycetes. In Rabenhorst's Kryptogamenflora, Die Pilze, IV: 191. 1892 (not sensu Pidoplichko & Milko - Atlas mucoral' nykh. gribov, p. 75. 1971).
Mucor wosnessenskii Schostakowitsch - Ber. dt. bot. Ges. 16: 92. 1898.
Mucor albo-ater Naumov - Mater. Mikol. Fitopat. Ross. 1: 8. 1915.
Mucor albo-ater Naumov var. sphaerosporus Naumov - Opredelitel' Mukorovykh (Mucorales), Ed. 2, p. 49. 1935.

The following description is of CBS 169.25 on beerwort agar at 20C in darkness.
Colony up to 45 mm in height, whitish to Pale Olive-Gray, aromatic; sporangiophores of two kinds, tall and short, up to 40 m in diam. (rarely wider); tall sporangiophores covered with droplets at the basal part, unbranched at first, then branched with short secondary branches; short sporangiophores branched sympodially; sporangia blackish, up to 350 m in diam. (mostly smaller), with deliquescent spiny walls; columellae variable in shape, the larger columellae obovoid, the smaller columellae cylindrical-ellipsoidal, pyriform or subglobose, up to 170 x 150 (190 x 175) m, with or without brownish contents; sporangiospores ellipsoidal, 7-9.5 x 4-7 m (rarely larger), a few subspherical to spherical sporangiospores may occur, originating from the lower sporangia and increasing in number with the age of the colony; zygospores [CBS 169.25 (-) x CBS 225.29 (+), on cherry agar at 15C] slightly compressed between suspensors, up to 210 x 90 (240 x 210) m, roughened in outline, black, formed just above the substrate; suspensors equal or slightly unequal, tinged yellowish to brownish.
Influence of temperature: at 30C no growth, at 25C slow growth, poor sporulation, atypical, at 20-5C growth and sporulation, optimal development at 15-10C, sporangia (and columellae) larger in average size than at 20C.
Influence of light: cultures grown in daylight produced a denser layer of tall sporangiophores than cultures grown in the dark. Colonies with tall sporangiophores only, bearing large sporangia with broadly pyriform columellae and oblong-ellipsoidal sporangiospores were reported in a previous paper (Schipper, 1969). At the time this effect was ascribed to the influence of the rice medium. Most probably an even more important factor is light of a particular wave-length.
Influence of medium: simultaneous cultures of CBS 169.25 on beerwort agar, SMA, cherry agar, prune agar, PDA and V8-agar, did not essentially influence the development of the strain. Colonies on SMA at 20C in darkness were Pale Pinkish Buff-Pinkish Buff, sterile mycelium present, sporangia on the tall sporangiophores brownish-greyish.

Material examined
CBS 169.25 (-), neotype, received from C. Wehmer, isolated from decaying pear.[p. 14]
CBS 175.27 (-), received as M. wosnessenskii from Ling Young
CBS 225.29 (+), received as M. albo-ater from N. A. Naumov.
CBS 327.68 (-), isolated from gooseberry.
CBS 328.63 (-), CBS 329.68 (-), isolated by W. Gams from soil of a wheatfield, Kiel, Germany.
CBS 400.68 (-), isolated from soil near Gandasee, Montafon, Austria.
CBS 527.68, received as M. wosnessenskii, BKM F-964, isolated from water of Danube.
CBS 528.68 (--), received as M. wosnessenskii, BKM F-1325, isolated from anthill by A. A. Milko,
and other isolates from soil or decaying fruit.

Discussion

Variability. - The strains vary in: height of the colony (up to 70 mm), diameter of the sporangiophores (up to 70 m), and the ratio tall/short sporangiophores. Short sporangiophores may be absent or formed in patches only. In general the sporangia are white when very young, but pallid yellowish immature sporangia may occur. The shape of the columellae may tend towards cylindrical-ellipsoidal or towards piriform-ellipsoidal. The main shape of the sporangiospores in some strains is ellipsoidal. In others they are ellipsoidal, broadly ellipsoidal and spherical. In aging cultures sprouting of both sporangia and columellae was observed.

Synonymy. - Fisher (1892) gave a clear description of the species. Unfortunately no original material is known to exist. No type material of M. wosnessenskii has been preserved either; Schostakowitsch's (1898) description probably is of a strongly developed strain of M. piriformis (sporangiophores up to 100-120 mm in height and 50-100 m in diam., sporangia averaging 500 m in diam., the size of the sporangiospores is given as 8.6 x 5 m).
An authentic strain of M. albo-ater (CBS 225.29) was available for study. It rarely forms globose to broadly ellipsoidal sporangiospores in large amounts. The variability of M. albo-ater appears from the varying descriptions given by Naumov. First description (Naumov, 1915b): Colony white, 3(-8-9) cm in height; sporangiophores 20-90 m in diam., covered with droplets, rarely branched; sporangia 300-400 m, white at first then black, diffluent spiny walls; columellae cylindrical, piriform, 100-200 x 60-165 (275 x 250) m, greyish; sporangiospores variable in shape, now globose, ovoid, ellipsoidal, now cylindrical with obtuse ends, 5.5-12 m in diam., 6-15 x 4-10 m'. In the Key to the Mucorineae' Naumov (1915a) states that the sporangiophores in M. albo-ater are 2-4 cm in height, 20-50 m in diam.; the columellae may be yellowish or greyish; the sporangiospores measure 2,5 m in diam., 5 x 4, 6 x 4,8 x 6, 10 x 7, 11 x 5.5 m. And only in the 2nd edition of the Key' (1935/1939) is a layer of short, branched sporangiophores mentioned. The scent is described as: like wine or candied fruit. A new variety was added: if the sporangiospores are globose, 4.5-7-14 m diam., it is M. albo-ater var. sphaerosporus (Naumov, 1935).
A careful study was made of CBS 378.66 -- BKM F-1003, one of the [p. 15] two strains from which Pidoplichko & Milko (1971) prepared their description of M. piriformis Fischer. Differences from Fischers description (1892) are its yellowish tinged, repeatedly branched sporangiophores without droplets adhering to the walls, the sandy sporangia up to 125 m and the sizes of the sporangiospores. The strain is re-identified as M. flavus.

Relationships. Mucor mucedo and M. piriformis are closely related. Simultaneous cultures of several strains of both species under varying environmental conditions showed the following differences. In M. mucedo usually more yellowish-brownish pigment is produced than in M. piriformis and the colony-colour is brighter. Generally the sporangia produced in the upper-region of the colony are brownish-greyish (sand colour) in M. mucedo, but of a darker greyish tinge in M. piriformis. The columellae are ellipsoidal in both species, though narrow cylindrical-ellipsoidal columellae, rather common in M. mucedo, are rare in M. piriformis. In M. mucedo the sporangiospores, typically oblong, are up to 14 um in length or over; in M. piriformis the sporangiospores are usually ellipsoidal, up to 10 m in length. Whereas on beerwort agar (pH 7-8) at 20C growth of both species was fairly equal, on cherry agar (pH 4) radial growth of each showed a marked difference: M. piriformis strains grew well, but M. mucedo strains were slow in linear growth. To check whether this effect of cherry agar was due to its acidity, both species were grown on acid beerwort agar pH 4) in 90 mm petri dishes at 20C. After 3 days colonies of M. piriformis filled the dishes, strains of M. mucedo had produced colonies of 50-70 mm in diam.

Intraspecific matings. - For mating testers were chosen CBS 169.25 (-) and CBS 225.29 (+). Abundant zygospores were obtained on cherry agar at 15 C, though occasionally matings between at other times prolific partners might fail completely or resulted in few incomplete conjugations or abortion of abundant well developed progametangia. In the most successful matings a continuous line of zygospores was produced, but in others the line was interrupted, being most distinct in the distal ends, restricted to the distal parts or restricted to one end of the line of contact only.
The distance between (H+) and (-) inoculations (1-7 cm) did not markedly influence the results, neither did the humidity of the agar-surface (newly poured dishes stored dishes) or the age of the stock cultures (1-9 weeks). Cultures were stored at 15C as a routine; 20C-cultures have been used too, but as the first mating results were the same as those obtained with the 15C-cultures, the 20C-cultures were omitted in further tests. Replacing the usual petri dishes by 15 cm deep vessels did not alter the mating results.
Abundant zygospores can be found in matings of compatible strains of the M. piriformis, M. albo-ater and M. wosnessenskii habitus.

Interspecific matings. - As a routine, M. piriformis contrasts were performed [p. 16] on cherry agar. This medium proved to be unfavourable in view of linear growth in M. mucedo, M. saturninus, M. aligarensis, M. plasmaticus, M. psychrophilus, M. strictus, M. minuta and also in M. flavus. Several strains of M. piriformis reacted in contrasts with the mating partners of M. mucedo, though the conjugations were far from complete. Mucor mucedo partners produced abundant zygospores on beerwort agar at 15C, but none on cherry agar; M. piriformis partners were able to produce a few zygospores on beerwort agar at 15C but matings on cherry agar are by far more prolific. Since both species are closely related in view of general morphology, various media were tried to check whether the inability to conjugate completely was due to different demands in respect of a favourable medium for matings. The tester pairs of both M. mucedo and M. piriformis each produced a fair yield of zygospores on cherry agar + horse-dung, on a mixture of beerwort agar + cherry agar and on a mixture of cherry agar + peptone-glucose-saccharose agar oatmeal agar. On Ypss agar and on oatmeal agar + horse-dung zygospores were obtained in matings of M. mucedo, but the M. piriformis partners failed to react. On all media tested, contrasts of (+) and () strains of M. mucedo and M. piriformis with each other resulted in the production of rather unusually shaped (misshaped) progametangia. From these results it is concluded that the conjugation is blocked by internal conditions, not by external conditions.
Apart from the response in M. mucedo x M. piriformis contrasts, M. piriformis was never observed to react sexually to other species mentioned in this study.

  
3. Mucor saturninus Hagem Fig. 4 a-f

Mucor saturninus Hagem - Annls. mycol. 8: 265. 1910.

The following description is of CBS 974.68 on beerwort agar at 20C in darkness.
Colony up to 45 mm in height, Pale Smoke Gray to Smoke Gray; sporangiophores up to 20 m in diam., infrequently up to 35 m, tall and short, branched sympodially, in older cultures short monopodial branches occur; sporangia on the tall sporangiophores yellowish when young, then brownish grey, with encrusted deliquescent walls, up to 175 m in diam., on the short sporangiophores yellow when young, then blackish brown, with persistent, slightly spiny walls, up to 150 m in diam.; sporangia of about 25 m in diam. with persistent spiny walls occur on the short monopodial branches; columellae on the tall sporangiophores obovoid or ellipsoidal up to 90 x 70 m (rarely larger), on the short sporangiophores cylindrical-ellipsoidal, up to 85 x 63 m; sporangiospores ellipsoidal 4.5-8 x 3-5.4 m, mostly 7 x 4 m, with rather thick walls; zygospores [CBS 974.68 (+) x CBS 137.40 () on beerwort agar at 15C] formed in the [p. 17] aerial mycelium just above the agar surface, globose to slightly compressed between suspensors, up to 180 x 170 m, with protuberances up to 11 m in length, black; suspensors equal or unequal, up to 55 m in diam., with or without reddish to yellowish contents; zygophores slightly unequal, up to 17 m in diam., with encrusted walls.
Influence of temperature: at 30C no growth, at 25-5C growth and sporulation, growth slow at 5C. At lower temperatures short sporangiophores are more numerous than tall sporangiophores.
Influence of light: cultures grown in light develop a denser layer of tall sporangiophores than simultaneous cultures in darkness.
Influence of medium: on SMA at 20C, in darkness, essentially as above.

Material examined
CBS 137.40 (), received from K. Zobl.
CBS 521.64 (-), isolated from pigeon-dung.
CBS 636.65 (+), isolated from oak leaf litter.
CBS 974.68 (+), neotype, received from J. H. van Emden, Wageningen, isolated from soil,
and isolates from dung, leaf litter, toad-stools, soil and a hens leg.

Discussion

No original material is known to exist anymore; CBS 974.68 is proposed as the neotype strain. Hagem (1910) named the species after the lead-grey colour of the tall sporangiophores in his cultures. In the strains listed above, a colouring that justified the name was never found.
The examined strains varied in height and colour of the colony, the latter connected with the degree of development of the upper- and lower-region. The colour of the young sporangia varied from pallid yellow to bright yellow. Some of the strains formed sterile aerial mycelium, especially at lower temperatures.

Intraspecific matings. - 16 strains, similar in general morphology, were mated with the testerpair CBS 974.68 (+), CBS 137.40 (-), and with each other. Not all strains formed zygospores when paired with a tester; in matings with other strains of the species, the conjugation was complete.
Examples:
CBS 636.65 (-) x CBS 137.40 (-) ----------4 progametangia only
CBS 636.65 (+) x CBS 521.64 (-)----------4 zygospores
CBS 974.68 (+) x CBS 521.64 (-)----------4 zygospores
CBS 974.68 (+) x CBS 137.40 (-)----------4 zygospores

Similar mating-results were obtained in other species of the genus Mucor, e.g. in M. hiemalis (Schipper, 1973).

Interspecific matings. - Incomplete conjugations of the tester pair of [p. 18] M. saturninus were observed in contrasts with the appropriate partner of the M. mucedo pair, and also in CBS 137.40, M. saturninus () and CBS 571.70, M. plasmaticus (a line of contacts, a few progametangia distinguishable, but as a whole a very poor reaction), as well as in CBS 137.40, M. saturninus () and CBS 205.68, M. griseocyanus (+).
No response was observed in contrasts with M. piriformis, M. aligarensis, M. psychrophilus, M. strictus, M. minuta, nor with M. circinelloides and M. mousanensis.

  
4. Mucor aligarensis B.S. & B.R. Mehrotra Fig. 4 g-i

Mucor aligarensis B.S. & B. R. Mehrotra - Sydowia 23: 183. 1969.
Mucor petrinsularis Naumov var. ovalisporus G. Smith - Trans. Br. mycol Soc. 40: 482. 1957 = Mucor ovalisporus (G. Smith) Pidoplichko & Milko - Atlas mucoral'nykh gribov, p. 61. 1971.

The description is of CBS 993.70 on beerwort agar at 20C in darkness.
Colony up to 1 mm in height, Pale Smoke Gray; sporangiophores up to 25 (40) m in diam., infrequently with a constriction just below the sporangium, often preceded by a widening of the stalk, branched sympodially or in an irregular fashion; sporangia white at first, then a pale brownish grey (sand colour), up to 125 m, rarely up to 175 m in diam., with persistent, spiny, transparent walls; columellae ellipsoidal to obovoid, the smallest ones subspherical to applanate, up to 85 x 74 m, with large collars; sporangiospores variable in size and shape, but mainly ellipsoidal, 6.5-12 x 4.5-6.5 (14 x 9) m, average 8 x 6 m, with cloudy contents. Minuscule sporangiophores may be present interspersed with the taller sporangiophores, bearing sporangia 20 m or more in diam.
Influence of temperature: at 25C growth and sporulation, but abnormal; at 20-5C good growth and sporulation, though rather slow at the lower temperatures. At the lower temperatures the sporangiospores tend to be slightly larger.
Influence of light: daylight and black light did not notably alter the morphology.
Influence of medium: growth and sporulation were more profuse on beerwort agar + 10% saccharose than on beerwort agar, on SMA the colony was whitish (abundant sterile mycelium), on cherry agar development was rather poor.

Material examined
CBS 244.58, type strain of M. petrinsularis var. ovalisporus.
CBS 993.70 = MX-18 = NRRL 3099, type strain of M. aligarensis.[p. 19]

Discussion

Variability. - Strain CBS 244.58 showed a poorer development under the above conditions: columellae subspherical, sporangiospores broadly ellipsoidal to subspherical, 6.5-11 x 5.5-6.5 m.

Synonymy. - Smith (1957) described the species as an oval-spored variety of M. petrinsularis Naumov. However, the type strain is unlike M. petrinsularis (= M. fuscus Bainier) in colony height, branching of the sporangiophores, shape of the columellae and shape of the smooth sporangiospores.

Relationships. - M. aligarensis formed only very short sporangiophores under all conditions studied. The colony resembles the lower zone of short, repeatedly branched sporangiophores in M. mucedo, M. piriformis and M. saturninus, bearing in mind that in M. aligarensis the sporangia are always light coloured.

Mating experiments. - No sexual response was evoked in contrasts of the 2 strains of M. aligarensis, either with each other or with M. mucedo, M. piriformis, M. saturninus or M. minuta.

 
5. Mucor plasmaticus van Tieghem - Fig. 5 a-c

Mucor plasmaticus van Tieghem - Annls Sci. nat. z: 33. 1875.
Mucor mucilagineus Bref. - Botanische Untersuchungen ber Schimmelpilze IV, p. 58. 1881.
Mucor irkutensis Schostakowitsch - Bet. dt. bot. Ges. is: 472. 1897.

The following description is of CBS 402.68 on beerwort agar at 20C in darkness.
Colony up to 70 (80) mm in height, Light Buff, reverse yellowish, slightly aromatic; sporangiophores arising separately, up to 75 m in diam., covered with droplets but for the distal part, glassy hyaline in reflected light, brownish with crystals on the walls in microscopic view, typically unbranched; sporangia whitish to yellowish, dorsiventrally flattened, up to 450 x 350 m, slowly deliquescing walls with short spines (a few persistent sporangia as small as 110 x 120 m may be seen in slides); columellae ovoid to conical, up to 290 x 190 m, with or without brownish or reddish-brownish contents, with collar; sporangiospores rather uniform in shape, oblong ellipsoidal, infrequently slightly reniform, with more or less granular ('cloudy') contents, yellowish, 17.5-24.5 x 9.5-12.5 m, mixed with an interstitial substance; substrate hyphae narrower than the sporangiophores, lumpy.
Influence of temperature: at 30C no growth; at 25-5C growth and sporulation. At 15C sporangiospores were found up to 30 x 14 m. Though at 5C growth is extremely slow, the resulting culture is well [p. 20] developed: sporangia pale violet-greyish, over 500 m in diam., columellae up to 350 x 245 sporangiospores 21-32 x 13-15 m, rarely larger (47 x 17 m).
Influence of light: daylight did not notably alter the morphology.
Influence of medium: on SMA at 20C in darkness: colony irregularly zonate, restricted; substrate hyphae deep down into the medium, sporangiophores up to 50 mm in height; sporangia up to 550 m in diam., sporangiospores up to 33.5 x 17 m in diam.

Material examined

CBS 122.23, received as M. mucilagineus from H. Burgeff.
CBS 177.46, recevied from C. G. Dobbs, isolated from rabbit droppings, England.
CBS 275.49, isolated by G. A. de Vries from mouse droppings.

CBS 402.68, isolated by R. A. Samson from horse dung.
CBS 571.70 (+), received from N. Naganishi, isolated from horse dung, Japan (see: Naganishi et al., 1969).
CBS 402.73 = BKM F-852.
CBS 403.73 = BKM P-1801.

Discussion

Variability. Grown at 20C on beerwort agar, the strains vary in height of the colony (20-80 mm); with the exception of CBS 402.68, the sporangiophores show a few yellowish spots on the wall in reflected light; the colour of the sporangia may tend either to yellowish or to brownish-greyish, and the production of sporangiospores of the larger size is variable: in some strains only a few occur, in others they are more abundant.
External conditions may influence the size of the sporangiospores considerably: on beerwort agar at 20 C the sporangiospores in most strains average 25 x 14 m, while spores of 45 x 20 m are rare, but on horse-dung agar at 20C they average 35 x 15 m. At lower temperatures the sporangiospores are always somewhat larger than at the higher temperatures.
In young undisturbed colonies, the sporangiophores are unbranched, in aging cultures secondary branching may occur. Unfavourable conditions and mechanical injury seem to stimulate the branching of the sporangiophores.

Synonymy. No type material is known to exist of M. plasmaticus, M. mucilagineus and M. irkutensis. The first descriptions of M. plasmaticus (van Tieghem, 1875) and of M. mucilagineus (Brefeld, 1881) are rather brief. There are no indications to make it clear that the species concerned are different. The characters of M. irkutensis (Schostakowitsch, 1897) are in agreement with those of M. plasmaticus and M. mucilagineus, as far as data are available. Since Brefeld (1881) stressed the point that the sporangia of M. mucilagineus are not yellow at any stage of their development, it has [p. 21] to be remembered that Brefeld compared it with M. mucedo, of which the young sporangia may show a very bright yellow colouring. It was observed that when grown on beerwort agar at 20C the mature sporangia of some of the strains mentioned above are greyish to yellowish-greyish, while in the others the sporangia are pallid-yellowish to yellowish, but cultured at lower temperatures every strain produced greyish sporangia. Brefeld (1881) did not indicate temperature conditions.
Naumov (1935, 1939) introduced the variety microsporus with sporangiospores 14-18 x 9.6-14 m. Since no material of this variety is available, its position is uncertain.

Relationships. - M. plasmaticus differs from all other Mucor species mainly in having distinctly flattened sporangia and by taller sporangiophores, larger sporangia and larger sporangiospores. On account of these features M. plasmaticus shows some resemblance to the genus Phycomyces Kunze ex Fr. However, the zygosporic stage, reported by Pidoplichko & Milko (1971) is of the usual Mucor type.

Intraspecific matings. Contrasts of the available strains of M. plasmaticus with each other on beerwort agar, on beerwort agar + peptone and on horse-dung agar, at 15C, were without result. According to Pidoplichko & Milko (1971) the zygospores are globose to slightly compressed, 100-180 (220) m, black, with spirally arranged warty protuberances, and are produced near the substrate in a small band between mating-partners; zygophores rather parallel, orginating from substrate hyphae. The mating partners are no longer available.

Interspecific matings. - Contrasts of the available M. plasmaticus strains with the tester pair of M. mucedo were without result but for the strain CBS 571.70. The latter also responded to CBS 137.40, M. saturninus (-). With CBS 283.35, Phycomyces blakesleeanus Burgeff and CBS 305.58, Phycomyces nitens Kunze a weak reaction was observed. No response was observed in contrasts with M. piriformis and M. psychrophilus or with M. aromaticus and M. oblongiellipticus.

 
6.
Mucor psychrophilus Milko - Fig. 5 d-f

Mucor psychrophilus Milko in Pidoplichko & Milko - Atlas mucoral'nykh gribov, p.73. 1971.

The description is of CBS 288.71 on beerwort agar at 15C in darkness.
Colony up to 25 mm in height, white to Pale Gull Gray, rapidly attaining full height but slow in linear growth, collapsing tall sporangiophores giving rise to sporangial colonies; tall sporangiophores arise singly, while short sporangiophores, sterile aerial mycelium and sprouting sporangia and hyphae compose a flat rather dense cottony layer; tall sporangiophores up to 35(-50) m in diam., unbranched or weakly branched sympodially in [p. 22] the distal part and/or branched monopodially, infrequently slightly recurved when young, with a few droplets occurring on the surface; sporangia watery white when young, changing to watery blue, then brownish grey, applanate, up to 200 x 180 m, with slightly echinulate walls, a few droplets may top the sporangia; columellae ovoid, sagging to slightly panduriform, up to 140 x 120 m, with brownish contents, collars rather small; sporangiospores ellipsoidal, 7.5-10 x 6.5-7.5 m, thick-walled, with greyish cloudy' contents; short sporangiophores bear sporangia that are whitish at first, then blackish, up to 200 m in diam., dorsiventrally flattened, with columellae narrow ellipsoidal, bluish to mouse-colour, provided with large collars, sporangiospores being spherical, 8-10 m in diam.
Influence of temperature: at 20C poor growth and sporulation, irregularly shaped sporangiospores; at 15-5C growth and sporulation.
Influence of light: colonies grown on beerwort agar at 20 and 15C in light were more restricted and lower than simultaneous cultures in darkness; the sporangiospores in light-cultures were subglobose, in dark-cultures ellipsoidal.
Influence of medium: colonies on SMA were more restricted, the (usual) sporangiospores mixed with unusual large, subglobose spores' up to 20 m in diam.

Material examined
CBS 288.71 = BKM F-1441, type strain.

Discussion

Since cultures at 20C were unsatisfactory, the description is based on a culture at 15C, in darkness, instead.
M. psychrophilus resembles M. mucedo in its typically unbranched, tall sporangiophores, in the colour of the sporangia, and especially in its thick-walled sporangiospores with greyish cloudy' contents. Sporangia in M. psychrophilus however are slightly applanate, while in M. mucedo they are globose, at least at maturity, and the columellae in the upper portion of the colony are ovoidal.

Interspecific matings. M. psychrophilus never showed a visible response in interspecific contrasts (M. mucedo, M. piriformis, M. saturninus, M. plasmaticus, M. minutus or with M. aromaticus, M. flavus, M. oblongiellipticus, M. oblongisporus, M. sinensis). However, not much value was set on those contrasts since in M. psychrophilus linear growth was extremely slow in all conditions used. Its colony was rapidly overgrown by the contrasting species. [p. 23]

  
7. Mucor strictus Hagem - Fig. 6 a-d

Mucor strictus Hagem - Untersuchungen ber norwegische Mucorineen 1: 18. 1908 (not sensu Zycha Pilze II. Mucorineae, Kryptogamenflora der Mark Brandenburg VIa, p. 80. 1935).
Mucor kanivcevii Pavlenko & Milko - Nov. Sist. niz. Rast. p. 101. 1965.

Since the influence of temperature on the morphology of this species is very distinct, descriptions are given of CBS 576.66 on beerwort agar at 20C and at 10C in darkness.
At 20C: colony up to 6 mm in height, Pale Smoke Gray to Smoke Gray; sporangiophores up to 33 (42) m in diam., rather abruptly tapering towards the sporangia, repeatedly branched sympodially, with incrusted walls; some minuscule sporangiophores may be present, interspersed with the typical sporangiophores; sporangia hyaline at first then tinged pale yellowish changing to brownish and finally blackish or dark brownish grey, up to 150(-175) m in diam., with incrusted walls; columellae subglobose, slightly applanate to broadly ellipsoidal, obovoid to slightly pyriform, up to 70 x 65 m, with a smaller collar, with or without brownish contents; sporangiospores subspherical to broadly ellipsoidal, 3.5-7 m in diam., a few cylindrical-ellipsoidal, 7 x 3.5 m.
At 10C: colony up to 17 mm in height, Deep Mouse Gray; sporangiophores up to 30 m in diam., constricted next to the sporangia, unbranched, infrequently branched either monopodially or sympodially, with incrusted walls; some minuscule sporangiophores may be present; sporangia brownish-dark greyish to blackish, up to 220 m in diam., with incrusted walls; columellae obovoid to slightly pyriform, up to 105 x 80 m, with a small collar, with or without brownish contents; sporangiospores mainly cylindrical-ellipsoidal, 5.5-8 x 3-4 m, a few spherical to ellipsoidal. In aging cultures sprouting sporangia are not rare.
Influence of temperature: at 25C hardly any growth or none; 20-0C growth and sporulation, fastest at 20C, slowest at 0C, height of the sporangiophores increasing and branching decreasing with falling temperature; at 20C and 15C sporangiospores mainly spherical, at 10, 5 and 0C sporangiospores mainly cylindrical; spherical sporangiospores produced at 20C have rather smooth walls, spherical sporangiospores in cultures grown at lower temperatures have more often verruculose walls. The sporangiospores of each sporangium are rather uniform in shape and size.
Influence of light: insignificant or none.
Influence of medium: colonies on bread, at 10C, produce taller sporangiophores, larger sporangia (up to 325 m, and almost black in colour) with larger columellae (up to 155 x 122 m), than simultaneous colonies on beerwort agar. On SMA: at 20C colony up to 13 mm in height, whitish (much sterile mycelium), sporangiophores simple or weakly branched, retarded sporulation; at 10C colony up to 15 mm, white.
Influence of diameter of culture-vessel: colonies grown on beerwort agar [p. 24] at 0C in 40 mm dishes produced both cylindrical-ellipsoidal and spherical to ellipsoidal sporangiospores, while simultaneous colonies in 90 mm dishes produced cylindrical-ellipsoidal sporangiospores only.

Material examined

CBS 100.66 = BKM F 789, type culture of M. kanivcevii.
CBS 575.66, received from W. Gams as B 247, isolated from soil, Gtt, Pitztal, Austria.
CBS 576.66 (+), neotype, isolated from soil near Gandasee, Montafon, Austria.
CBS 368.71 A, B, and C, isolated from marshy soil near Frederiksseter, Norway.

Discussion

Variability. The strains differ at 20C on beerwort agar in colony colour (Pale Smoke Gray-Smoke Gray to Smoke Gray with a touch of Drab Gray), shape of the larger columellae (ellipsoidal, obovoid-ellipsoidal to slightly pyriform) and size of the columellae (up to125 x100 m); at 0C the size of the columellae may be up to 160 x 120 m.

Synonymy. Zycha (1935) indicated a neotype' of Mucor strictus Hagem. Schipper (1967) did not accept the identity of Zycha's neotype with M. strictus Hagem, and indicated another neotype, CBS 576.66.

Relationships. Low-temperature cultures of M. strictus produce tall unbranched sporangiophores, similar to those found in M. mucedo, M. piriformis, M. plasmaticus and M. psychrophilus. Sporangia, columellae and sporangiospores too are comparable. The most striking difference is the influence of temperature on branching and height of the sporangiophores, and on the shape of the sporangiospores.

Intraspecific matings. Zygospores were not obtained.

Interspecific matings. All available strains of M. strictus were contrasted with the tester pairs of M. mucedo, M. piriformis, M. saturninus and of M. flavus. A slight response of the strain CBS 576.66 to CBS 109.16, M. mucedo () was the only positive result.

  
8. Mucor minutus (Baijal & B.S. Mehrotra) Schipper, comb. nov. Fig. 6 e-h

Mucor griseo-ochraceus Naumov var. minutus Baijal & Mehrotra Sydowia 19: 206. 1965 (basionym) = Mucor saturninus Hagem var. minutus (Baijal & Mehrotra) Milko in Pidoplichko & Milko Atlas mucoral'nykh gribov p. 70. 1971.

The description is of CBS 586.67 on beerwort agar at 20C in darkness.
Colony rather loose, up to 19 mm in height, Smoke Gray, in a fully [p. 25] developed culture tall and short sporangiophores are present, while minuscule sporangiophores occur as well; tall sporangiophores up to 20 (30) m in diam., with slightly roughened, striate walls, at first unbranched then branched sympodially (mainly in the upper part of the main axis) and monopodially; sporangia at first white then greyish-brown, up to 175 m in diam., rarely larger, with slightly echinulate, slowly diffluent walls; columellae obovoid-ellipsoidal, infrequently pyriform, the outline bluish in water mounts, up to 110 x 85 (135 x 97) m; sporangiospores sub-spherical or broadly ellipsoidal, mostly 4-5.5 m (rarely up to 7 m) in diam. or 5.5 x 4.7 m; in the short fertile region sporangiophores are of the same diameter and wall structure as above, but repeatedly branched in a sympodial fashion; sporangia pale sand colour, mat glittering, dry, mostly 125 m in diam., with slightly echinulate, slowly diffluent walls; columellae cylindrical-ellipsoidal, the smaller columellae conical to applanate; sporangiospores subspherical, mostly 4(-5) m in diam. The slender, very short minuscule' sporangiophores branch repeatedly in a sympodial fashion.
Influence of temperature: at 37C no growth; at 30C restricted growth, incomplete maturation of sporangia; at 25-10C good growth and sporulation. At 25C the branching of the tall sporangiophores is mainly monopodial, only few sympodial branches are formed. Development slows down with decrease of temperature; at 5C development is extremely slow, the columellae are conical to applanate and some of the sporangiospores irregular in shape.
Influence of light: in cultures grown continuously in light (beerwort agar, 20C) columellae of truly pyriform shape occur. Furthermore some sporangiophores are swollen just below the septa, and infrequently columellae have a few small projections. The latter characteristics were mentioned by Baijal & Mehrotra (1965), are absent in cultures grown under other conditions. Cultures grown intermittently in light (12 hr/12 hr) are taller than simultaneous cultures grown in darkness.
Influence of medium: on cherry agar linear growth is slow, but the sporangiophores reach their usual height in a comparatively short time. At first the culture has the appearance of a pin-cushion, then the sporangiophores collapse, thus initiating marginal colonies (cf. M. flavus). On 1% beerwort agar a rather dense lower zone is formed, and a loose upper zone. On SMA the colony is rather poor, whitish, with a wavy margin; substrate hyphae growing deep into the medium.

Material examined

CBS 586.67, type strain.

Discussion

The pattern of branching of the tall sporangiophores is unique amongst the known larger Mucor species. Though the sympodial branching equals [p. 26] the branching system of M. flavus, the appearance of monopodial branches at the same time as sympodial branches is unusual. In M. mucedo (= M. griseo-ochraceus), M. saturninus and also in M. flavus, monopodial branches may occur, but only as secondary branching in aging cultures.
The pale sand colour of the sporangia in the lower zone of the colony differs from the sporangium colour found in M. mucedo and M. saturninus. The shape of the columellae and the presence of sporangiospores sticking to the columella-wall is like in M. flavus.

For above reasons the strain CBS 586.67 should be treated as a separate species M. minutus, inapt as the name may be for a species with the above measurements.

Interspecific matings. -- No sexual response was observed in contrasts of M. minutus with M. mucedo, M. saturninus, M. aligarensis, M. psychrophilus and M. flavus, neither with M. mousanensis, M. racemosus, M. ucrainicus, M. zonatus. Slow development of the strain compared with the growth rate of the partners in those contrasts (except M. aligarensis) may have checked reactions.

  
9. Mucor flavus Bainier - Fig. 7

Mucor flavus Bainier - Bull. Soc. mycol. Fr. 19: 157. 1903.
Mucor sciurinus Naumov - Mater. Micol. Fitopat. Ross. 1: 7. 1915.
Mucor petropolitanus Naumov - Mater. Micol. Fitopat. Ross. 1: 12. 1915.
Mucor petropolitanus Naumov var. macrosporus Naumov - Opredelitel' Mukorovykh (Mucorales), Ed. 2, p. 54. 1935.
Mucor attenuatus Linnemann - Beitrag zu einer Flora der Mucorineae Marburgs, p. 192. 1936 = Mucor sciurinus Naumov var. attenuatus (Linnemann) Pidoplichko & Milko - Atlas mucoral'nykh gribov, p. 77. 1971.
Mucor peacockensis Mehrotra & Nand - Sydowia 20: 71. 1966.
Mucor mephitis Ellis & Hesseltine - Mycologia 61: 869. 1969.
Mucor meridionalis Milko & Kormilizina in Pidoplichko & Milko -Atlas mucoral'nykh gribov, p. 71. 1971.
Mucor piriformis Fisher sensu Pidoplichko & Milko - Atlas mucoral'nykh gribov, p. 75. 1971.

The following description is of CBS 234.35 on beerwort agar at 20C in darkness.
Colony up to 43 mm in height, Olive-Buff; sporangiophores arising from clearly visible superficial substrate hyphae, light yellowish, up to 35 m in diam. with a wider base and a constriction next to the sporangium, smooth or slightly roughened (striate), unbranched at first then branched sympodially in the upper part of the main axis, in older cultures a few slender monopodial branches may occur and infrequent slender, repeatedly branched, very short sporangiophores may arise from the substrate; sporangia whitish when young changing to watery blue ('soap and water' colour) to watery yellow-brown while moist in appearance, in aging cultures turning to yellowish sand colour, mat glittering, dry, up to 175 (-200) m [p. 27] in diam., with echinulate deliquescent walls; columellae obovoid-ellipsoidal to slightly pyriform, up to 100 x 80 (130-110) m, with or without light brownish contents and small collars, while often sporangiospores are found sticking to the wall in water mounts (older columellae that lose the turgor may sag to a pyriform shape); sporangiospores ellipsoidal, occasionally slightly flattened on one side, 7-12 x 4-6.5 m, rarely larger, with granular (cloudy) contents; zygospores [CBS 234.35 () x 235.35 (+), on PDA at 15C] borne chiefly in the upper portion of the aerial mycelium, often in a ladderlike fashion, spherical, compressed between suspensors, up to 180 x 165 m, roughened by short projections, glossy black; suspensors mostly slightly unequal, up to 50 (60) m in diam., with slightly incrusted walls, with or without brownish contents; zygophores up to 18 m in diameter, tinged brownish, and with slightly incrusted walls.
Influence of temperature: at 25C no growth, at 20-5C good growth and sporulation, optimal at 15C, slow at 5C.
Influence of light: none.
Influence of medium: on SMA at 20C, in darkness, colony up to 20 mm in height, Ivory Yellow with patches of Colonial Buff, sterile mycelium covering the surface of the medium. On cherry agar the colour of the colony is a deeper yellow (in comparison with cultures on beerwort agar). On this medium linear growth is slow to very slow and the colonies are usually irregularly zonate. The sporangiophores reach full height in a comparatively short time. First these cultures have the appearance of pin-cushions, then the sporangiophores collapse, thus initiating marginal colonies.

Material examined

CBS 173.27 (), received as M. murorum from Ling Young.
CBS 230.35, type strain of M. attenuatus.
CBS 234.35 (), neotype, CBS 235.35 (+), both received from G. Linnemann.
CBS 378.66 () = BKM F 1003, received as M. piriformis from A.A. Milko.
CBS 672.66 (), isolated from soil, Vergaldnerjoch, Austria.

CBS 673.66 (+), CBS 674.66 (+), isolated from mouse-droppings.
CBS 675.66 (+), isolated from soil, Bettlerkopf, Austria.
CBS 664.67 (+), received from V. Hintikka, at first identified as M. oblongisporus (Hintikka, 1971).
CBS 665.67 (), isolated from soil, Ladizkopf (c. 1930 m) Karwendel, Austria.
CBS 988.68, CBS 989.68, CBS 990.68, received from W. Gams as C 268, C 371, C 837, isolations from soil wheat-field, N. Germany.
CBS 991.68, received from G. Hennebert as 885-1, M. piriformis, isolated from bones of Talpa europaea, Han-sur-Lesse, Belgium.
CBS 126.70 () = NRRL 2477, type strain of M. mephitis.
CBS 127.70 (+) = NRRL 2597 received as M. mephitis.
CBS 197.71 () = BKM F 1396, type strain of M. meridionalis.
CBS 210.71 () = IMI 136 624, type strain of M. peacockensis.
CBS 678.73 () = BKM F 1371, CBS 679.73 () = BKM F 1372, labeled M. piriformis.
CBS 680.73 (+.), host of Syncephalis intermedia in CBS 150.69 = BKM F 1383.
CBS 681.73 (), isolated from soil of wheat-field, Kiel-Kitzeberg, Germany, by W. Gams, as C 789. [p. 28]
CBS 715.73 (-), isolated from nursery soil.
CBS 846.73, isolated from soil, Chile, by J. Grinbergs.
CBS 893.73 = BKM F 1326, neotype strain of M. sciurinus,
and other isolates from soil in Austria, Germany and the Netherlands.

Discussion

Variability. Depending on strain properties and on environmental conditions, the height of the colony varies from 12 to 60 mm. Mostly the colonies are Light Buff in colour, but may be tinged either yellowish or brownish. Some strains have a faint smell, others give out a strong scent, strain CBS 234.35 is practically odourless. The scent varies from a rather sweet fruity odour to a nasty smell (M. mephitis was named after the skunk because of the odour they have in common). The medium may influence the nature of the odour e.g. agreeable on beerwort agar but rather nasty on rice. Sporangiophores may be slightly wider than in CBS 234.35, up to 50 m in diam.
Though the number of branches varies in simultaneous cultures, all strains studied have in common that the initation of the branching of the sporangiophores starts only after about one week at 20C on beerwort agar. The growth of these branches is slow at this temperature, and often no more than one or two fertile branches are formed at the time the aging sporangiophores collapse; at 15C branching is stronger. Rather characteristic is the turn-over type of branching (see fig. 7a), occurring more or less regularly in all cultures studied.
Unlike the dwarf sporangiophores that occur in all aging cultures of M. flavus, the short sporangiophores in CBS 197.71 and CBS 378.66 bear full-grown sporangia. The number of those sporangiophores is inconstant, varying from an incomplete second layer to small patches in the colony. Mostly the short sporangiophores are born from substrate hyphae, infrequently they occur as short branches on the basal part of tall sporangiophores. Branching is rare in the short sporangiophores. The immature sporangia show a deeper purplish tinge than those on the tall sporangiophores. One-sporangium cultures, originating from each type, produced tall as well as short sporangiophores.
Size of sporangia and columellae may vary slightly from those of the strain CBS 234.35.
The most obvious variability in respect of morphological features is found in the size of the sporangiospores. The following sizes were noted after culturing on beerwort agar at 20C.

 

+

-

mating reaction unknown

       

CBS 665.67

 

7-9.5 x 4-5.5 (rarely larger)

 

CBS 235.35

8-10 x 5.5-7

   

CBS 234.35

 

6.7-12 x 4-6.5

 

[p. 29]

     

CBS 664.67

11-12 x 5.5-7

   

CBS 672.66

 

9-12.5 x 5.5-7

 

CBS 680.73

9.6-13.2 x 6-7.4

   

CBS 678.73

 

8-15 x 4.5-6.5

 

CBS 173.27

 

8-15 x 4.5-7.5

 

CBS 126.70

 

8-20 X 5.5-10
(mostly 13.5-15 x 8)

 

CBS 127.70

7-19 x 4.5-7.5
(mostly 7-14 x 4.5-7.5)

   

CBS 210.71

 

11-9 x 5.5-8.5

 

CBS 715.73

 

9.5-19 x 5.5-8

 

CBS 230.35

   

7-9 x 5.5

CBS 893.73

   

7-9.5 x 4.5-6.5

The strains CBS 197.71 and 378.66 differ from the other strains studied in their ability to produce sporangiospores of two types. In CBS 197.71, sporangiospores formed on tall sporangiophores are 7-9.5 x 4.5-5.5 m in size, sporangiospores produced in sporangia on short sporangiophores are 10-20 x 5-8 m. CBS 378.66 produced similar sporangiospores. The studied strains differ in temperature requirements: CBS 197.71 and 210.71 grow and sporulate well at 25C. The other strains either do not grow at this temperature or show abnormalities in their morphology. Strains originating from cool areas like CBS 664.67 (forest litter in winter, Finland) and some isolates from alpine soils at c. 2000 m altitude in Austria develop poorly even at 20C.
Variability of Mucor flavus can be summarized as follows. Common characters of the strains studied are: yellowish-greyish colonies; sporangiophores up to 35 m in diam. with a constriction next to the sporangium, unbranched at first then branched sympodially in the upper part of the main-axis; sporangia whitish when young changing to watery blue or watery brown, and moist in appearance then turning to yellowish sand colour, mat glittering, dry, up to 175 m in diam., with echinulate deliquescent walls; columellae obovoid-ellipsoidal to slightly pyriform; sporangiospores ellipsoidal, contents granular (cloudy); zygospores borne in the upper portion of the aerial mycelium, globose, compressed between suspensors, roughened by short projections, glossy black.
Variation is found especially in: odour of the colony, speed of the formation of side branches on the sporangiophores, occurrence of full grown sporangia on short sporangiophores and maximum size of the sporangiospores.

Synonymy. - No original material of M. flavus is known to be preserved. The description given by Bainier (1903) is incomplete in that the branching fashion of the sporangiophores is described as infrequently branched irregularly and that the sizes of sporangia and columellae are not given. However, the height of the colony, colour of the sporangia, shape and size of the sporangiospores and the zygosporic stage strongly indicate a [p. 30] species as represented by CBS 234.35. This strain, exhaustively studied by Linnemann (1936), is therefore proposed as neotype. Both Hagem (1908) and Linnemann (1936) report the isolation of strains separable into two different colony types according to height and colour of the colony and shape of the sporangiospores. A linkage of mating reaction type and colony type, as surmised by Linnemann (1936), does not exist. Different features are found in various combinations.

Mucor sciurinus Naumov (1915) was insufficiently described and no type material is known to exist. Pidoplichko & Milko (1971) indicated BKM F 1326 (= CBS 893.73) as neotype. This strain differs from Naumovs description in the production of smaller sporangiospores and larger columellae. However, in view of the variability of the species these deviations are not significant. CBS 230.35, type culture of M. attenuatus is almost identical. Both mentioned strains differ from CBS 234.35, proposed neotype of M. flavus, in the production of slightly shorter, more profusely branched sporangiophores and colonies Pale Olive Buff in colour, instead of Olive Buff, and furthermore the sporangiospores are rather small.
Type strains were studied of M. peacockensis, M. mephitis and M. meridionalis; synonymy was concluded. CBS 378.66, described by Pidoplichko & Milko (1971) as M. piriformis, and CBS 197.71 deviate from the type in the production of large sporangiospores in sporangia in the under-portion of the colony, and smaller ones in sporangia on tall sporangiophores.
No authentic material is known to exist of M. petropolitanus and its variety macrosporus. Since the descriptions given by Naumov (1915b, 1935) fit the concept of M. flavus as given above, both taxa are enlisted into synonymy.
Though in Naumovs description of M. oblongisporus (1915b) some of the features are reminiscent of M. flavus, the strain CBS 569.70, indicated as neotype by Naganishi et al. (1969) differs from M. flavus e.g. by usually unbranched sporangiophores and by transitorily recurved young sporangiophores.

Intraspecific matings. Since Linnemann (1936) was unable to determine the mating reaction types of her strains of M. flavus, she called the mating partners a and b. Unfortunately, these partners were received at the CBS as + and , with an annotation that the mating signs were arbitrary. They were listed as CBS 234.35 (+) and CBS 235.35 (-). After a response of CBS 234.35, M. flavus with Blakeslees tester strain IV (-1-) (CBS 852.71) the mating reaction types proved to be:

CBS 234.35 ()
CBS 235.35 (+)

Matings of CBS 234.35 and CBS 235.35 on PDA at 15C yielded a fair amount of zygospores; on beerwort agar and on cherry agar no zygospores were obtained. However, CBS 126.70 x 127.70 produced zygospores both on PDA and on beerwort agar. [p. 31]
The usual condition in matings is darkness, matings of above strains in light were not successfull.
In M. flavus matings 30 ml agar was used instead of the usual 15 ml. Several matings that had failed on 15 ml layers produced (few) zygospores on 30 ml. PDA. No further investigation into the subject was done. A survey of all contrasts performed between strains that showed sexual responses is given in table 1.

Table 1. Contrasts of Mucor flavus strains on potato-dextrose agar at 15C.

-\+

235.35

673.66

674.66

675.66

664.67

127.70

680.73

173.27

p

o

o

o

z

z

p

234.35

z

o

p

p

p

z

p+z

672.66

o

o

o

o

o

7

z

378.66

o

o

o

o

o

z

o

665.67

p

p

p

o

o

z

p+z

126.70

p

z

p

p

p+z

z.

z.

197.71

z

o

o

o

o

z

z

210.71

p

z

z

p?

p+z

z

z

673.73

z

z

z

o

p

z

z.

679.73

z

z

z

o

p

z

z

681.73

p+z

o

o

o

p?

z

p

715.73

p

x

o

o

o

p+z

p+z

z= zygospores
p= progametangia (incomplete conjugation only)
p+z= most conjugations incomplete, few complete
o= no reaction observed

The yields of the matings vary from abundant to few zygospores. It seems noteworthy that among the prolific matings are:
CBS 127.70, M. mephitis (+) x CBS 234.35, M. flavus ()
CBS 127.70, M. mephitis (+) x CBS 210.71, M. peacockensis ()
CBS 127.70, M. mephitis (+) x CBS 197.71, M. meridionalis ()
CBS 235.35, M. flavus (+) x CBS 197.71, M. meridionalis ()

Though a variety of contrasts, under various conditions, were studied, no mating reactions were observed in strains CBS 230.35, type of M. attenuatus, CBS 893.73, neotype of M. sciurinus, CBS 988.68, CBS 989.68, CBS 990.68 and CBS 991.68.
Though all non-responding strains are of similar morphology, a distinct connection between morphology and (negative) mating reaction was not established. Relatively small sporangiospores and profuse branching are [p. 32] not a fixed combination of features. Strains that do have both features have been found to produce zygospores in matings with strains with large sporangiospores and/or weak branching of the sporangiophores.

Interspecific matings. Imperfect hybridization was observed in contrasts of M. flavus with M. aromaticus, M. inaequisporus, M. luteus var. indicus, M. mucedo, M. saturninus, Blakeslees tester strains IV and D, and with NRRL 1909, M. subtilissimus, a strain that has shown response to a great number of (+) strains of various Mucor species.

 
Doubtful species

Mucor microsporus Naumov Tablitsy delya opredeleniya predstaviteleni Mucoraceae, p. 27. 1915 (not Mucor microsporus Namyslowski Bull. int. Acad. Sci. Lett. Gracovie, Ser. B. sci. nat. p. 517. 1910) = Mucor humilis Naumov Mater. Mikol Fitopat. Ross. 1: 10. 1915 (name change) -Mucor sciurinus Naumov var. humilis (Naumov) Pidoplichko & Milko Atlas mucoral'nykh gribov, p. 76. 1971.
No type material is known to exist. Though most of the features described by Naumov (1915a, b) might indicate a Mucor species that is very close to M. flavus, none of the strains available formed uniform sporangiospores 6-7 x 4 m in size, as were reported for Naumovs strain.

Mucor ingricus Naumov Mater. Mikol. Fitopat. Ross. 1: 6. 1915.
No type material is known to exist. The characteristics mentioned in the description, unbranched sporangiophores, large sporangia and uniform cylindrical-ellipsoidal sporangiospores 7.5 x 3.6-5 m in size, might indicate relationship to M. strictus; however spherical to subspherical columellae are unlike that species.

M. albidus Naumov Opredelitel Mukorovykh (Mucorales), Ed. 2, p. 52. 1935.
No type material is known to exist, and neither is a strain available that fits the description: two fertile regions, heavily branched sporangiophores, non-hygroscopic whitish-grey sporangia mostly 40-160 m in diam., columellae elliptical to pyriform, sporangiospores ellipsoidal, (5.5-)9.6-15 m.

 
Acknowledgements

The author wishes to express her gratitude to Dr. J. A. von Arx, Dr. G. S. de Hoog, and Dr. R. A. Samson for valuable criticism during the preparation of the manuscript. She is indebted to Mr. D. Yarrow for correcting the [p. 33] English text. Many thanks are due to Miss L. van Dijk for technical assistance, to Mrs. N. Brouwer and Miss E. Timmer for typing the manuscript and to Miss J. Sytema for inking the drawings.

 
References

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