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Fungi

Ascomycota

Problems

Problems

The Ascomycota are morphologically diverse. The group includes organisms from unicellular yeasts to complex cup fungi. There are 2000 identified genera and 30,000 species of Ascomycota. The unifying characteristic among these diverse groups is the presence of a reproductive structure known as the ascus, though in some cases it has a reduced role in the life cycle. As a group, they are economically important. Many are tree diseases, such as dutch elm disease and apple blights. The yeasts are used to produce alcoholic beverages and breads. The mold Penacillium is used to produce the anit-biotic penicillin.

Structure

Like basidiomycota, most ascomycota sprout from spores into hapliod mycelia. These mycelia can produce two types of reproductive structures. First, they can produce conidiophores for asexual reproduction. Conidiaphores may simply branch off from the mycelia or they may be formed in fruiting bodies. Secondly, ascomycota produce structures for sexual reproduction called gametangia. These structures are either male or female. The male gametangia may be anything from a detached cell (called a spermatium) to a differentiated region called an antheridium. The female structure is always a differentiated region known as the ascogonium. Many Ascomycota form a fruiting body, or ascoma, similar to that of the Basidiomycota, but with an important difference. The ascomycota fruiting body is composed mainly of entangled monokaryotic hyphae from the male and female mycelia rather than of dikaryotic hyphae formed from the joining of hyphae from the two mycelia, as in the basidiomycota. The only dikaryotic structures in the fruiting body are those produced by the gametangia after plasmogamy.

Figure %: a)Some varieties of Ascomycota. b) Structure of the Ascomycota Fruiting Body

The exceptions to the above discussion of structure are the unicellular ascomycota or yeasts. These organisms are non-motile single cells with chitinous cell walls that earn them classification as fungi. Though they mainly reproduce by budding and fission, yeasts also engage in sexual reproduction that results in the production of an ascus, placing them in the Ascomycota. Most varieties of yeast do not form multicellular filiments like the mycelia and hyphae of other fungi, though they do live in massive groupings called colonies.

Reproduction

Like all fungi, Ascomycota can undergo both asexual and sexual reproduction.

Asexual Reproduction

Asexual reproduction among the different groups of fungi are very similar. Like Basidiomycota, Ascomycota reproduce asexually through budding or the formation of conidia.

Sexual Reproduction

Figure %: Sexual Reproduction in the Ascomycota

Sexual reproduction in the Ascomycota differs from that in the Basidiomycota and Zygomycota because Ascomycota have male and female gametangia in their haploid stage. These structures, discussed in Heading , form on the mycelia. Plasmogamy, or the transfer of cytoplasm and nuclei, takes place when a part of the ascogonium, the trichogyne, fuses with the antheridium. This produces a binucleate, dikaryotic condition in the ascogonium. This phase is prolonged and a series of dikaryotic cells called an ascogonius hypha is produced. At the tip of this hypha, karyogamy or nuclear fussion takes place, resulting in the formation of a diploid ascus. Within this structure, the diploid nucleus undergoes meiosis, producing four haploid nuclei. These nuclei then undergo mitosis to form eight haploid ascospores. Notice that this is twice as many spores as produced in the basidium.

Yeast Reproduction

Figure %: Life Cycle of Yeast

The different kinds of yeast most commonly reproduce by budding and fission, both forms of asexual reproduction. Budding occurs when a small portion of the cytoplasm of the parent cell becomes separated into small daughter cell. Fission involves an equal division of the cytoplasm into two daughter cells.

Yeast can also reproduce sexually, and usually do so under starvation conditions. Most yeast have two mating types. In the most commonly studied species, Saccharomyces cerevisiae, these are designated a and alpha. When yeast of opposite mating types meet, the cells fuse (plasmogamy), followed by the fusion of their nuclei (karyogamy). This diploid cell can produce more diploid cells by budding. Eventually, a diploid cell will become and ascus and enter meiosis. This produces four haploid nuclei that are then surrounded by thick protective coats and become spores. These spores are released and become new haploid cells.

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