Ascomycetes, or sac fungi, make up the largest group of fungi, having 50,000 species including yeasts, licens, and truffles. These fungi are about 75% of all known species. While a very important group of fungi (we couldn't make bread without it), ascomycota is also responsible for many plant diseases such as Dutch elm disease. The common name sac fungi is given because of the sacs in which Ascomycetes form their spores.
The ascomycetes are a very diverse group, but they have one unifying characteristic in the ascus. In the ascus, nuclear fusion and meiosis take place for reproduction. Usually, one cell undergoes mitosis and then meiosis to produce eight new haploid cells, eventually becoming ascospores. When released, these ascrospores are very resistant to environmental hardships, but can eventually develop into a new haploid fungus, the next step in a the fungus lifecycle (see the fungi main page). Younger species like hyphal members of phylum Ascomycota develop 8-spored asci, while yeasts, much simpler organisms, develop 4 spores.
Ascomycetes can be single celled or divided into cellular divisions, in hypha form. The wall around the eukaryotic cells of these organisms is made of chitin and beta glucans, but in different amounts. They are heterotrophic and can be found all over the world.
There are three main groups of ascomycetes. The most primitive group, which seems to have diverged very early in evolution, contains the Archaeasomycetes. The other two groups, the Hemiascomycetes and Euascomycetes, both are less primitive and diverged later, an idea supported by DNA comparison. The fossils of all three of these groups have been found as far back as 300 million years ago.
1. View through dissecting microscope.