The ascomycetes, or sac fungi, form a very large group and include the blue-green, pink and brown molds (responsible for food spoilage), powdery mildews (plant parasites), ergot on rye, Dutch elm disease, yeast (important in fermentation and baking), morels, truffles, cup fungi, and most of the fungi that participate in lichen symbioses. By some counts, there are as many as 56,000 species in this diverse group. Some, like the yeast, are unicellular. The plant bodies of most are filamentous. Growth at the tip of a filament produces new cells, with wall in-growths behind the newly formed cells (i.e., septate filaments). A central pore occurs in the newly formed cross-wall, similar to pit connections between cells in red algae.

Sexual reproduction in the sac fungi involves the production of an ascus, or sac-like cell. Haploid spores in an ascus are produced by meiosis and are called ascospores. Each ascus usually contains eight ascospores. Asci may be formed direct, as in the yeast (Fig.1), or in specialized fruiting bodies called ascocarps (e.g., on the upper surface of a cup Fig.2, Fig.3). Sexual reproduction in the ascomycetes can occur by various mechanisms. In yeasts, two haploid cells unite to initiate ascus development. In others, haploid filaments produce female sex organs called ascogonia and male ones called antheridia, each similar in structure to the sex organs produced in red algae. In yet other species, differentiated sex organs are lacking. Compatible nuclei are brought together by union of haploid filaments. With or without the presence of sex organs on filaments, compatible nuclei in fused cells fail to unite immediately. Instead, they continue to divide synchronously, resulting in a dikaryotic rather than diploid condition. Union of nuclei in young asci is the culmination of sexuality in these species. In the filamentous sac fungi, the dikaryotic condition usually initiates the formation of the fruiting ascocarp.

Lichens are grouped here and not with algae even though both groups are involved in a lichen symbiosis. Lichens are named after the fungal component because the fungal partner in the lichen is never found free-living. The algae are. Look for more information about the algal component of lichens under Trebouxia in the green algae and under Nostoc in the blue-green bacteria.

If you followed this description closely, you may have noted some similarity between sac fungi and red algae in morhological and sexual characteristics. Many mycologists suggest that this group evolved from the red algae by loss of photosynthetic pigments. The fossil record is consistent with this hypothesis, with the oldest known ascomycetes found in 438 million year old Silurian rocks, close to or slightly later than the first red algae fossils so far found. Others suggest that the fungi evolved from flagellated protozoa, and therefore have a closer relationship to animals than to plants. More data on the evolution of the group is needed.