MUCOR - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION

            A. CLASSIFICATION:

Division – Mycota

    Sub-division – Eumycotina

         Class – Zygomycetes

               Order – Mucorales

                      Family – Mucoraceae

                            Genus – Mucor

            The genus Mucor comprises about 42 species is a saprophytic fungus which is cosmopolitan in distribution. The species of Mucor are found living on moist soil, dung and decaying plant and animal matter.

            Mucor mucedo, is a common mould fungus growing sporophytically on various decayed and rotten organic matter, moist leather, horse and cattle dung, etc. The species also spoils human food like jam, jellies, bread, cheese, etc. During rainy season, this fungus appears as white or grayish cottony mould on the substratum.

            B. STRUCTURE OF THE VEGETATIVE BODY:- The vegetative body is stout and well-developed mycelium which consists of a much branched filamentous coenocytic hyphae that spread in all direction over the substratum. Some of the hyphae penetrate deep into the substratum and serves as fixative and absorptive hyphae. Though, the hyphae are coenocytic, septa may appear later at the bases of the reproductive organ and occasionally on the older hyphae.

            Cell wall of Mucor contains abundant chitosan. Besides, glucosamine, galactose, proteins, lipids, calcium, etc., are also present. The hyphae contains granular cytoplasm, large number if vacuoles, irregularly shaped numerous nuclei, droplets of oil and glycogen as reserve food.

                  C. REPRODUCTION:- Mucor reproduces by two methods or reproduction – asexual and sexual reproduction

            1. Asexual Reproduction:- Asexual reproduction takes place by the production of sporangiospore and chlamydospore formation.

            Sporangiospore formation – In this process, some of the unbranched hyphae, slightly broader then other grow up vertically in the air and are known as sporangiophores. The tips of sporangiophores swell up to form a spherical sac-like structure called sporangia.

            Through the sporangiophore, numerous nuclei along with cytoplasm flow into the sporangium and divide. The protoplasm collects densely in the peripheral region of the sporangium rather than the central region. Next, several small vacuoles forming a dome-shaped foamy area appear within the protoplasm towards the base of the sporangial wall and is known as columellaplasm. The columellaplasm develops into columella. The denser portion of protoplasm with numerous nuclei constitutes the sporeplasm. The sporeplasm undergoes progressive cleavage, as a result of which develops into numerous non-flagellate sporangiospores.

            Mature sporangiospores are thin walled, smooth, ovate and hyaline or coloured in mass. At maturity, the sporangiospores are liberated by bursting of the sporangial wall and are disseminated by means of insects or wind. On reaching a suitable substratum, the sporangiospore germinates by producing one or more germ tube and ramify forming a new mycelium.

            Chlamydospore formation – These spores are formed when some of the hyphae break up by transverse wall into thick-walled in chains. Later these cells round off and represent chlamydospores. Chlamydospore germinates to produce a new mycelium.

            Oidia formation – When species of Mucor is allowed to grow in anaerobic culture medium especially in presence of CO₂, a transverse breakage of hyphae takes place which results in the formation of oidia.

           2. Sexual Reproduction:- Species of Mucor may be either homothallic or heterothallic. The sexual reproduction in Mucor takes place by gametangial copulation method. The copulating gametangia are isogametangia, i.e., morphologically similar gametes. In case of heterothallic species, the two copulating isogametangia arise from two physiologically different and compatible hyphae of (+) and (–) strains. Numerous nuclei and cytoplasm flow into the tip of these hyphae which produces somewhat short and swollen lateral branch celled progametangia.

            Two progametangia of opposite strains, i.e., (+) strain and (–) strain come in contact with each other through their apex. The progametangia press together at their apex – as a result the hyphae bearing progametangia are pushed apart. Now the tip of each progametangium is cut off by a transverse wall dividing the progametangium into a distal gametangium and basal suspensor cell. In the meantime, the common wall at the point of contact between the two gametangia break down and the nuclei of (+) and the (–) strains fuse to form diploid nuclei. As a result, a multinucleate zygospore is formed. The diploid nuclei of zygospore undergo meiotic division and hence, the zygospore contains haploid nuclei. The zygospore is set free by disintegration of the parent mycelia.

            After a periof of rest over a month, zygospore germinates by producing a long germ tube called promycelium. The tip of promycelium develops a typical sporangium called zygosporangium or germ sporangium. Inside the zygosporangium, one kind of spores, either (+) or (–) strain spores are produced.  Each of the spores after liberation from the zygosporangium germinates by producing a germ tube to give rise to new mycelium.    

            Sometimes gemetangia fails to fuse and then a single gametangium may directly develop into a thick walled structure called azygospore or parthenospore. Azygospore directly germinates into new mycelium in due course.

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