Ascomycota

The Ascomycota are characterised by the production of haploid ascospores through the meiosis of a diploid nucleus in a small sac called an ascus. For this reason they are sometimes called the sac fungi or cap fungi. Many of the fungi that cause serious plant diseases such as Dutch elm disease and powdery mildew belong to this group. They include some 30 000 species, among them yeasts, food spoilage moulds, brown fruit rotting fungi and truffles. Note that the latter, often regarded as the most prized type of mushrooms by gourmets, are assigned to a completely different group to the true mushrooms, which belong to the Basidiomycota. Around half of ascomycote species exist in associations with algae to form lichens; these will be discussed more fully in Chapter 15. Most ascomycetes produce mycelia that superficially resemble those of zygomycetes, but differ in that they have distinct, albeit perforated cross walls (septa) separating each cell.

Asexual reproduction in most ascomycetes involves the production of airborne spores called conidia. These are carried on the ends of specialised hyphae called conidiophores, where they may be pinched off as chains or clusters (Figure 8.4). Note that the conidia are not contained within sporangia; they may be naked or protected by a flask-like structure called the pycnidium. Asexual reproduction by conidia formation is a means of rapid propagation for the fungus in favourable conditions. The characteristic green,

Reproduction Ascomycota

Figure 8.4 Asexual reproduction in the Ascomycota. Chains of conidia develop at the end of specialised hyphae called conidiophores

A lichen is formed by the symbiotic association of a fungus (usually an as-comycete) and an alga or cyanophyte.

Figure 8.4 Asexual reproduction in the Ascomycota. Chains of conidia develop at the end of specialised hyphae called conidiophores

Figure 8.5 Yeast cells in various stages of budding. A protuberance or bud develops on the parent yeast; the nucleus undergoes division and one copy passes into the bud. Eventually the bud is walled off and separated to form a new cell

Plasmogamy is the fusion of the cytoplasmic content of two cells. Caryogamy is the fusion of nuclei from two different cells.

pink or brown colour of many moulds is due to the pigmentation of the conidia, which are produced in huge numbers and dispersed by air currents. The conidia germinate to form another mycelium (haploid).

In the case of the unicellular yeasts, asexual reproduction occurs as the result of budding, a pinching off of a protuberance from the cell, which eventually grows to full size (Figure 8.5).

Sexual reproduction in some ascomycetes involves separate + and — mating strains similar to those seen in zygomycetes, whilst in other cases an individual will be self-fertile, and thus able to mate with itself.When there are separate strains, the hyphae involved in reproduction are termed the ascogonium (—strain) and antheridium (+ -strain). In either case, two hyphae grow together and there is a fusion of their cytoplasm (Figure 8.6). Within this fused structure, rather like the zygospore of Rhizopus, nuclei pair, but do not fuse; thus the resulting structure is a dicaryon. Following cytoplasmic fusion (plasmogamy), branching hyphae develop. These hyphae are septate, i.e. partitioned off into separate cells, but each cell is dicaryotic, having a nucleus from each parental type.

As we have seen, a key characteristic of ascomycetes is the production of sexual spores in sac-like structures called asci. These develop in distinct macroscopic fruiting bodies called ascocarps, which arise from the aggregation of dicaryotic hyphae (Figure 8.6). At the tip of each dicaryotic hypha, pairs of nuclei fuse to give a diploid zygote; this is followed by one meiotic and one mitotic division, giving rise to eight haploid ascospores. An ascocarp may contain thousands of asci, each with eight ascospores. Tetrad analysis of ascospores has proved a valuable technique in genetic mapping.

When the ascus is mature, it splits open at its tip and the ascospores are released. They are dispersed, often over long distances, by air currents. If a mature ascocarp is disturbed, it may release smoke-like puffs containing thousands of ascospores. The germinating ascospore forms a new mycelium. Frequently there are many rounds of asexual reproduction between successive rounds of sexual reproduction by ascospore production.

Life Cycle Ascomycota

Figure 8.6 Sexual reproduction in the Ascomycota. Nuclei in the cells at the tips of the ascogenous hyphae fuse to give a diploid zygote. Meiotic and mitotic divisions result in the formation of eight haploid ascospores inside a tubular ascus. On germination, each ascospore is capable of giving rise to a new mycelium. From Black, JG: Microbiology: Principles and Explorations, 4th edn, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Figure 8.6 Sexual reproduction in the Ascomycota. Nuclei in the cells at the tips of the ascogenous hyphae fuse to give a diploid zygote. Meiotic and mitotic divisions result in the formation of eight haploid ascospores inside a tubular ascus. On germination, each ascospore is capable of giving rise to a new mycelium. From Black, JG: Microbiology: Principles and Explorations, 4th edn, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

As with the zygomycetes, the diploid stage in ascomycetes is very brief. The life cycles of the two groups differ, however, in the greater role played by the dicaryotic form in ascomycetes.

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