Zygomycota

The Zygomycota is a relatively small phylum, comprising less than a thousand species. Its members are typically found in soil, or on decaying organic matter, including animal droppings. Some members of the group are of great importance in the formation of a mutualistic association with plant roots known as a mycorrhiza. This will be discussed in more detail in Chapter 15.

Members of the Zygomycota are characterised by the formation of a dormant form, the zygospore, which is resistant to unfavourable environmental conditions. Hyphae are coenocytic, with numerous haploid nuclei, but few dividing walls or septa.

Familiar examples of this group are Mucor and the black bread mould Rhizopus. The life cycle of Rhizopus is shown in Figure 8.2. Hyphae spread rapidly over the surface of the substrate (bread, fruit etc.) and penetrate it, absorbing soluble nutrients such as sugars. Upright hyphae develop, carrying at their tip sporangia, full of black haploid spores. The spores give the characteristic colour to the mould; they are the asexual reproductive structures, and are released when the thin wall of the sporangium ruptures. When conditions are favourable, Rhizopus reproduces in this way; each spore, upon finding a suitable substratum for growth, is capable of germinating and initiating a new mycelium.

Sexual reproduction occurs when environmental conditions are unfavourable. Most species of Rhizopus are heterothallic; that is, there exist two distinct mating strains known as + and —. Sexual reproduction is only possible between a member of the + -strain and a member of the — -strain. Although reproductively distinct,

A sporangium is a structure inside which spores develop. It is held aloft on an aerial hy-pha called a sporango-phore.

A dicaryon is a structure formed by two cells whose contents, but not nuclei, have fused.

Adult mycelium (n)

Adult mycelium (n)

Mucor Gametangia

strain

Gametangia

Figure 8.2 Zygomycota: the life cycle of Rhizopus. Both sexual and asexual cycles involve the production of sporangiospores. In sexual reproduction, hyphae from different mating strains fuse to form a diploid zygospore, via a short-lived dicaryotic intermediate. Germination of the zygospore gives rise to an aerial sporangium; this contains many haploid sporangiospores, which give rise to another vegetative mycelium strain

Zygospore (2n)

Gametangia

Figure 8.2 Zygomycota: the life cycle of Rhizopus. Both sexual and asexual cycles involve the production of sporangiospores. In sexual reproduction, hyphae from different mating strains fuse to form a diploid zygospore, via a short-lived dicaryotic intermediate. Germination of the zygospore gives rise to an aerial sporangium; this contains many haploid sporangiospores, which give rise to another vegetative mycelium these two types are morphologically identical; because of this it is not appropriate to refer to them as 'male' and 'female'. When hyphae of opposite mating types come into contact, a cross-wall develops a short distance behind each tip, and the regions thus isolated swell to produce gametangia (Figure 8.2). These fuse to form a single large multinucleate cell. Note that at this stage, the nuclei from each parent have paired up but not fused, forming a dicaryon. Dicaryon formation is found in all fungal phyla apart from the chytrids, and may be regarded as an intermediate stage between the haploid and diploid conditions (Figure 8.3). The proportion of the life cycle it occupies varies considerably. A thick protective covering develops around the dicaryon in Rhizopus, forming the zygospore, which can survive extremes of draught and temperature and may remain dormant for months. When conditions are favourable again, the nuclei from each strain fuse in pairs, to give a fully diploid zygote. Just before germination, meiosis occurs, then an aerial sporangiophore emerges, terminating in a sporangium. Production and dispersal of haploid spores then occur as in the asexual life cycle and a new mycelium forms when a spore germinates.

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Figure 8.3 Dicaryon formation. The dicaryon (n + n) represents an intermediate stage between the haploid (n) and diploid (2n) states. The participating cells have undergone plasmogamy (fusion of cytoplasm) but not caryogamy (fusion of nuclei). Each dicaryotic cell contains one nucleus from each parent cell. Fusion of nuclei leads to the formation of a true diploid

In Rhizopus, therefore, we have a life cycle in which the haploid form predominates (the zygospore is the only diploid stage), and we have sexual reproduction without the involvement of motile gametes.

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