Zygomycetes Mucorales

In most members of the Mucorales, numerous spores are contained in globose sporangia borne at the tips of aerial sporangiophores. Within the sporangium the spores may surround a central core or columella, although in some species the columella is greatly reduced.

Some species possess fewspored sporangia, termed sporangiola, which are often dispersed as a unit, and in some groups the spores are arranged as a single row inside a cylindrical sac termed a merosporangium. Yet other Mucorales reproduce by means of unicellular propagules which are sometimes termed conidia. It is believed that ‘conidia’ may have evolved several times within different groups of Mucorales from forms with monosporous sporangiola.

A distinction between sporangiospores and conidia is that germinating sporangiospores lay down a new wall, continuous with the germ tube, within their original spore wall, whilst within germinating conidia there is no new wall layer formed.

Habitat:

The Mucorales are mostly saprotrophic and are abundant in soil, on dung and on other organic matter in contact with the soil. They may play an important role in the early colonization of substrata.
Sometimes, however, they can behave as weak pathogens of soft plant tissues, e.g. Rhizopus stolonifer can cause a rot of sweet potatoes or fruits such as apples, tomatoes and strawberries. Such infections may cause spoilage of food. Some species are parasitic on other fungi, a common example being Spinellus fusiger which forms a tuft of sporangiophores on the caps of moribund fruit bodies of Mycena spp.

Others cause diseases of animals including man, especially patients suffering from diabetes, leukaemia and cancer. Lesions may be localized in the brain, lungs or other organs, or may be disseminated, e.g. at various points in the vascular system. Species of Rhizopus and Mucor are reported from human lesions, and these genera together with species of Absidia may also infect domestic animals.

A number of species have been used in the production of oriental foods such as sufu, tempeh and ragi  and some are used as starters in the saccharification of starchy materials before fermentation to alcohol.

In modern biotechnology, many mucoralean fungi are employed in biotransformation processes.

Growth and asexual reproduction:

Mycelium

The mycelium is coarse, coenocytic and richly branched, the branches tapering to fine points. Later, septa may appear. Thick-walled mycelial segments (chlamydospores) may be cut off by such septa.  In anaerobic liquid culture, especially in the presence of CO2, several species of Mucor (e.g. M. rouxii) grow in a yeast-like instead of a filamentous form but revert to filamentous growth in the renewed presence of O2.

Cell walls

The cell walls of Mucorales are chemically complex. Chitin microfibrils are present. Other compounds such as poly-D-glucuronides, polyphosphates, proteins, lipids, purines, pyrimidines, magnesium and calcium have also been detected.

Comparison of the structure and composition of yeast-like and filamentous cells of Mucor rouxii shows that the yeast-like cells have much thicker walls. They also have a mannose content about five times as great as that of filamentous cell walls. The synthesis of chitin microfibrils takes place within chitosomes which have been described from sporangiophores of Phycomyces.

Asexual reproduction

is by aplanospores (sporangiospores) contained in globose or pearshaped sporangia, which are borne singly at the tip of a sporangiophore or on branched sporangiophores. In Absidia the sporangia are arranged in whorls on aerial branches, and in many species of Rhizopus the sporangiophores arise in groups from a clump of rhizoids.  Sporangiophores are often phototropic. 

 

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