Phylum Ascomycota: Sac fungi produce sexual spores in saclike asci

•Mycologists have described over 60,000 species of ascomycetes, or sac fungi.

•They range in size and complexity from unicellular yeasts to elaborate cup fungi and morels.

•Ascomycetes live in a variety of marine, freshwater, and terrestrial habitats.

•Some are devastating plant pathogens.

•Many are important saprobes, particularly of plant material.

•About half the ascomycete species live with algae in mutualistic associations called lichens.

•Some ascomycetes form mycorrhizae with plants or live between mesophyll cells in leaves where they may help protect the plant tissue from insects by releasing toxins.

•The defining feature of the Ascomycota is the production of sexual spores in saclike asci.

•In many species, the spore-forming asci are collected into macroscopic fruiting bodies, the ascocarp.

•Examples of ascocarps include the edible parts of truffles and morels.

•Ascomycetes reproduce asexually by producing enormous numbers of asexual spores, which are usually dispersed by the wind.

•These naked spores, or conidia, develop in long chains or clusters at the tips of specialized hyphae called conidiophores.

•Ascomycetes are characterized by an extensive heterokaryotic stage during the formation of ascocarps.

(1) The sexual phase of the ascomycete lifestyle begins when haploid mycelia of opposite mating types become intertwined and form an antheridium and ascogonium.

(2) Plasmogamy occurs via a cytoplasmic bridge and haploid nuclei migrate from the antheridium to the ascogonium, creating a heterokaryon.

(3) The ascogonium produces dikaryotic hyphae that develop into an ascocarp.

(4) The tips of the ascocarp hyphae are partitioned into asci.

(5) Karyogamy occurs within these asci and the diploid nuclei divide by meiosis, (6) yielding four haploid nuclei.

(7) Each haploid nuclei divides once by mitosis to produce eight nuclei, often in a row, and cell walls develop around each nucleus to form ascospores.

(8) When mature, all the ascospores in an ascus are dispersed at once, often leading to a chain reaction of release, from other asci.

(9) Germinating ascospores give rise to new haploid mycelia.

(10) Asexual reproduction occurs via conidia.