Major Groups of Fungi

The five major groups of fungi are the chytrids, the zygomycetes, the glomeromycetes, the ascomycetes, and the basidiomycetes.

Chytrids

Chytrids are traditionally classified into Phylum Chytridiomycota; however, this group is most likely paraphyletic and reclassification is currently in progress. Chytrids are fungi with motile, flagellated spores. This is an ancestral character that other fungal groups have lost. Most chytrids are unicellular, but some species are multicellular with coenocytic hyphae. There are about 1,000 described species of chytrids. Many are saprobes, while others are parasites.

Zygomycetes

Zygomycetes are fungi that use zygospores to reproduce sexually. There are over a thousand species, and are classified in Phylum Zygomycota. However, this is a paraphyletic group that is currently being revised. Zygomycetes generally have coenocytic hyphae. They reproduce asexually by means of spores produced by mitosis. Different mating types can also reproduce sexually. During sexual reproduction, a hypha from each mating type forms a gametangium, which is separated from the rest of the hypha by a cell wall. The two gametangia fuse into a single structure, the zygospore. The zygospore is dikaryotic, containing haploid nuclei from each mating strain until karyogamy, when the nuclei fuse and then undergo meiosis to produce spores.

Glomeromycetes

There are about 230 species of glomeromycetes. They form arbuscular mycorrhizae with up to 90% of land plants. Glomeromycetes usually have coenocytic hyphae and reproduce asexually. We will not examine them in lab, but you should be aware of their critical importance in terrestrial ecosystems.

Ascomycetes

The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans.

Asexual reproduction is frequent and involves the production of conidia that release haploid spores. Sexual reproduction takes place in saclike structures called asci. These result from fusion of hyphal structures to produce dikaryotic hyphae, which then produce asci at their tips. Within the asci, karyogamy takes place, resulting in a zygote. The zygote undergoes meiosis. In some species, the haploid cells that result from meiosis divide by mitosis to form eight ascospores.

Basidiomycetes

The fungi in the Phylum Basidiomycota are easily recognizable by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cell of a hypha. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom. Spores are generally produced through sexual reproduction, rather than asexual reproduction. The club-shaped basidium carries spores called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dikaryotic stage of the basidiomyces lifecyle and and it is the dominant stage. Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.