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Remarks (internal):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 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 µm 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.
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 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 µm in diam. and, from the same sporangium, cylindrical ellipsoidal spores, 8-14 x 5.5-8.7 µm. Naumov (1915) described the sporangiospores of M. murorum as 12-18 x 6-9.6 µm, mainly 9.6-16.5 x 5.5-8.5 µm. In 1935 the sizes were given as (8.3-)12-15 (-22) x 5.5-11 µm. Furthermore 4 varieties were distinguished:
var. macrosporus: sporangiospores 8-19 (26) x 5-9 (12) µm,
var. robustus: sporangiophores unbranched,
var. rufescens: colony salmon; sporangiophores branched,
var. coloratus: colony yellowish; columellae dark grey; sporangiospores 8-14 X 5-7 µm.
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).
Description type:Non-original description 
Description: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.
The following description is of CBS 640.67 on beerwort agar at 20°C, 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 µm 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 µm in diam., brownish grey at maturity, with deliquescent spiny walls; columellae obovoid to ellipsoidal, up to 160 x 125 µm, 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 µm, with reddish-brownish contents; sporangiospores cylindrical-ellipsoidal, broadly ellipsoidal, 10.5-13.5 x 5.7-7.5 µm, or subspherical, 8-9 µm in diam., thick-walled, with more or less 'cloudy' granular contents; zygospores [CBS 640.67 (-) x CBS 144.24 (+), on beerwort agar at 15°C] globose to compressed between suspensors, up to 250 x 200 µm, with warty protuberances up to 15 µm 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 30°C no growth, at 25-5°C growth and sporulation. At 15°C 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 20°C 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 20°C in darkness, colony Light Grayish Olive, tall unbranched sporangiophores rare.
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