Ascomycota, Sac Fungi
Species in this phylum reproduce sexually by forming a spore-filled structure called an ascus, which means literally "a sac." The hyphae of the sac fungi are divided by septa with pores, that is, they have perforated walls between adjacent cells. They reproduce asexually by producing spores, called conidia, which are born on specialized erect hyphae, called conidiophores. The sac fungi are typically prolific producers of conidia.
The sac fungi also have a sexual reproduction phase of their life cycles. In the first step of this process, compatible hyphae fuse together by one of several different methods. Second, the nuclei from the different hyphae move together into one cell to form a dikaryon, a cell with two haploid nuclei. Third, several cell divisions occur, resulting in several cells with two different haploid nuclei per cell. Fourth, nuclear fusion of the two haploid nuclei occurs in one of these cells, the ascus mother cell. Fifth, the ascus mother cell develops into an ascus. Then, meiosis occurs in the diploid cells and, depending on the species, four or eight haploid ascospores form inside the ascus. In some species, such as the fleshy and edible morels, a large number of asci are massed together to form an ascocarp.
This large phylum of fungi includes many species which are beneficial to humans. For example, the yeasts are a major group of ascomycetes. Different yeasts in the genus Saccharomyces are employed by bakers, brewers, and vintners to make their bread, beer, or wine. Truffles are subterranean ascomycetes which grow in association with tree roots. Traditionally, pigs have been used to sniff out these underground fungi, so that French chefs could use truffles to complement their finest cuisine.
Some other ascomycetes are significant plant pathogens. For example, Endothia parasitica is an ascomycete which causes chestnut blight, a disease which virtually extirpated the American chestnut as a mature forest tree. Ceratocystis ulmi is a pathogenic ascomycete which causes Dutch elm disease, a scourge of American elm trees. Claviceps purpurea, the ergot fungus, infects agricultural grains, and when ingested can cause intense hallucinations or death due to the presence of LSD (D-Lysergic acid diethylamide).
Another well known ascomycete is Neurospora crassa, the red bread mold. The ordered manner in which the eight spores of this fungus align during sexual reproduction allows geneticists to construct a map of the genes on its chromosomes. Earlier in this century, biologists used Neurospora as a model organism to investigate some of the basic principles of genetics and heredity. More recently, biologists have shown that the mycelium of this species can produce spores at approximately 24 hour intervals, a circadian rhythm, in a constant environment. Many biologists are currently using Neurospora crassa as a model organism for investigation of circadian rhythms, which occur in a wide diversity of organisms, including humans.