ascomycota

Pylum Ascomycota

(sac fungi)

30,000 species of non -lichen forming, plus another 15-20,000 species occuring as lichens chacrteristics: - no flagellated cells at any stage - assimilative stage usally a haploid and monokaryyotic mycelium composed of septate hyphae - formation of a special dikaryotic cell (the young ascus) within which karyogamy and meiosis occur, leading to formation of ascospores within the mature ascus - sexual life cycles all haplontic involing gametangial contact or spermatization leading to a relatively brief dikaryophase within the fruiting bodies - usally posess some type of organized fruiting body(the asccoma) which bears the asci and ascospores9some are very simple ex. yeasts) - mostly terrestrial decomposers or symbionts of the parastic or mutualistic types

The Ascomycota, or sac fungi, is monophyletic and accounts for approximately 75% of all described fungi. It includes most of the fungi that combine with algae to form lichens, and the majority of fungi that lack morphological evidence of sexual reproduction. Among the Ascomycota are some famous fungi: Saccharomyces cerevisiae, the yeast of commerce and foundation of the baking and brewing industries (not to mention molecular developmental biology), Penicillium chrysogenum, producer of penicillin, Morchella esculentum, the edible morel, and Neurospora crassa, the "one-gene-one-enzyme" organism. There are also some infamous Ascomycota, a few of the worst being: Aspergillus flavus, producer of aflatoxin, the fungal contaminant of nuts and stored grain that is both a toxin and the most potent known natural carcinogen, Candida albicans, cause of thrush, diaper rash and vaginitis, and Cryphonectria parasitica, responsible for the demise of 4 billion chestnut trees in the eastern USA (Alexopoulos et al., 1996). Asexual Ascomycota, such as Penicillium or Candida species, used to be classified separately in the Deuteromycota because sexual characters were necessary for Ascomycota classification. However, the comparison of nucleic acid sequence, as well as nonsexual phenotypic characters, have permitted the integration of asexual fungi into the Ascomycota Breeding and Courtship

Ascomycota are either single-celled (yeasts) or filamentous (hyphal) or both (dimorphic). Yeasts grow by budding or fission and hyphae grow apically and branch laterally. Most yeasts and filamentous Ascomycota are haploid, but some species, Saccharomyces cerevisiae for example, can also be diploid. Mitospores may simply reproduce the parent, or may also act as gametes to fertilize a compatible partner. Some Ascomycota must outbreed (heterothallic), others can also self, and some can only self (homothallic) (Alexopoulos et al. 1996).

Genetic regulation of sex expression and mating is well-understood in some model Ascomycota such as yeast, where there are two sexes and mating is coordinated by oligopeptide pheromones (Marsh, 1991; Glass and Lorimer, 1991). In hyphal species, cytoplasmic fusion may not be immediately followed by nuclear fusion, leading to a short dikaryotic phase. The dikaryotic hyphae may be protected and nourished by differentiated haploid hyphae which form a fruiting body (the ascoma; plural ascomata). Ascomata may be closed (cleistothecium), open by a narrow orifice (perithecium), or broadly open like a cup (apothecium). Ascospores are released from the ascoma and germinate to form a new haploid mycelium.

Sexual Ascomycota all have asci. Comparison of nuclear small subunit ribosomal RNA gene sequence demonstrates a monophyletic Ascomycota, although support for the basal branch is not strong (Berbee and Taylor, 1993; Bruns et al., 1992). Early diverging Ascomycota have been grouped into the Archaeascomycetes, although support for the monophyly of this group is not strong (Nishida and Sugiyama, 1994). The placement of Neolecta among the Archaeascomycetes is surprising because of the presence of an ascoma, a feature not found in other Archaeascomycetes or in any Hemiascomycetes (Landvik et al. 1992). However, there is no reason that the Hemiascomycetes could not have lost ascomata as hyphal growth became suppressed in favor of yeasts. The Hemiascomycetes form a well-supported monophyletic taxon, as do the Euascomycetes (Gargas et al., 1995). Asexual fungi sharing morphological or molecular characters of sexual Ascomycota are classified in the Ascomycota; examples include Candida albicans (Hemiascomycetes) and Pencillium chrysogenum (Euascomycetes).

By comparing nucleic acid sequences, the timing of Ascomycota evolution has been estimated (Berbee and Taylor, 1993). The Archaeascomycetes, Hemiascomycetes and Euascomycetes all became established in the coal age, a bit more than 300 million years ago. Fossils of these early Ascomycota are not going to be easy to recognize, because they probably lacked ascoma and their spores were not distinctive. Fungal-like fossils claimed to be older than 1.0 to 1.2 billion years are probably artifactual. The earliest ascomycete fossil ascomata and spores are controversial because their age of deposition significantly predates molecular estimates of their time of origin. The fruiting bodies may be zygomycetous, and the spores may have washed into older sediments, or the molecular estimates may be erroneous.

Reproductive stategy

aquatic ecology

terrestial ecology

predators and defense

community ecology

other useful links... http://www.perspective.com/nature/fungi/index.html

refereces