Diversity of the Fungi
The following is our usual diversity table, which somewhat overemphasizes the basal Fungi. Recent work suggests that fungal diversity may be undersampled even at the highest taxonomic levels. Specifically, a taxonomic survey of alpine fungal communities which flourished under snow cover suggests that there may be 1-2 high-level fungal taxa between Basidiomycota and Ascomycota. Schadt et al. (2003).
The chytridomycotes, or "chytrids," are usually aquatic, either marine or freshwater. Presumably this is the original domain of the chytrids, and of all Fungi, but chytrids are also found in terrestrial communities almost as soon as there were terrestrial communities to be found in. So, for example, several different groups of chytrids are known from the Early Devonian Rhynie Chert. The implication is that they had begun to radiate even before the Devonian. They are a remarkably diverse lot, as one might expect from a basal radiation of the Fungi, and there is some possibility that the Chytridomycota may be paraphyletic, i.e. that all Fungi are descended from chytrids.
The chytrids are mostly single-celled forms, traditionally classified as protists. In fact, some sources still classify them with the Chromista, even though, so far as we can tell, chytrids have no light-sensitive pigments at all. What chytrids do have is a single- celled zoospore with an anterior flagellum, which is distinctly odd for a fungus. In fact, chytrids are the only large taxon of Fungi which produces a zoospore of any kind.
However, there's no real doubt about their position any more. For example Borneman & Hartin (2000) showed that rDNA primers from all of four basic fungal phyla (Trichomycota was not included) permit amplification of rDNA in the other fungal groups, including Chytridomycota, but don't amplify anything else. This strongly suggests that that rDNA from all four groups was very similar and that all are closely related.
That same conclusion can be reached for any number of other reasons. Chytrids have an absorptive mode of nutrition, like other Fungi. Chytrids have cell walls composed of chitin. Chytrids form hyphae. They share, with the other Fungi, key enzymes and metabolic pathways that are not found in other fungus-like protozoans (slime molds and water molds), in addition to oddities of molecular structure. Alkemar & Nygard (2003). The chytrids are surely the most basal Fungi, but Fungi they are.
Zygomycotes, like chytrids, are known from the Rhynie Chert, although, in this case, the identification is more tentative. What are actually seen are fungal hyphae which appear to pierce other cells, a characteristic of many zygomycotes. Definitive zygomycotes are found in Carboniferous exposures. A more familiar present-day example of a zygomycote is Rhizopus, the black bread mold.
The Zygomycota are named for their characteristic teleomorph, which is referred to as a zygosporangium. The images at the glossary entry for gametangium are of Phycomyces and Rhizopus, both zygomycotes. They illustrate how the zygosporangium is formed from the head-on meeting of two hyphae whose ends have specialized as gametangia. The contents of the gametangia are mixed in the zygosporangium, which develops between them. The haploid nuclei from the gametangia then fuse. The zygosporangium develops a hard, thick chitin shell, which is frequently ornamented and may bear spines or other appendages. The remains of the gametangia protrude from the sides and are referred to as suspensors. The zygosporangium also serves as a resting phase, which will develop when conditions are favorable.
Zygomycotes also reproduce asexually. The haploid spores develop in a bulbous mitosporangium at the tips of specialized vertical hyphae referred to as sporangiophores.
The basidiomycotes are the rusts, smuts, gilled mushrooms, puffballs, stinkhorns, and club, shelf or coral fungi. They are one of the two major divisions of Fungi, the other being the Ascomycota. Definitive Basidiomycote fossils are known from the Late Devonian, although there has been a recent report of a possible Early Devonian lichen incorporating a probable basidiomycote fungus.
The Basidiomycota is such a large and diverse group, that the living members have little in common. The basidiomycote life cycle has a four unique properties which are probably synapomorphies, but which are no longer shared by all members of the group:
(1) The taxon is named for the basidium where sexual spores are produced.
(2) The life cycle generally includes a persistent dikaryon, frequntly large (e.g., a mushroom) in which each cell in the thallus contains two haploid nuclei, typically as the resulting of a mating event
The Ascomycota are the largest and most diverse group of Fungi. They include the yeasts, most of the fungal elements of lichen, and such famous Fungi as Saccharomyces, Aspergillus, Candida and Neurospora, as well as morels, truffles and similar delicacies. The current understanding is that supposed pre-Devonian (even Proterozoic!) lichens are probably artifacts, making the earliest known ascomycote of Carboniferous age.
The Ascomycota are united by the presence of asci (see glossary entry). Like Basidiomycota, ascomycotes remain indefinitely in the dikaryon state, with the fungal filaments (hyphae) partitioned into cells each containing two haploid nuclei -- one from each parent. Also as in basidiomycotes, nuclear fusion (karyogamy) occurs only in connection with the formation of sexual spores. At that time the newly diploid nucleus undergoes one (sometimes more) round of mitosis, followed by meiosis, to yield eight (or a multiple of eight) haploid nuclei. The nuclei are then partitioned by internal membranes into individual ascospores. The Ascomycota also share with Basidiomycota the use of conidia for the development of asexual spores.
Another unique character (but not present in all ascomycotes) is the presence of Woronin bodies on each side of the septa separating the hyphal segments. The septae are pierced by pores which allow most cytoplasmic constituents (but not nuclei) to travel freely between hyphae. However, if an adjoining hypha is ruptured, the Woronin bodies block the pore to prevent loss of cytoplasm into the ruptured compartment.
(3) Clamp connections (explained at the glossary entry) are unique to Basidiomycota and are used to maintain the dikaryon state during hyphal division.
(4) Many basidiomycotes can launch spores into the air in a process referred to as ballistospory.
The basidiospores bear a single haploid nucleus. They germinate into hyphae with a single nucleus in each compartment, a monokaryon. A mating event results from end-to-end fusion of hyphae, as in Zygomycota, or fusion of a hypha with an oidium, a specialized mating spore. Then the resulting dikaryon divides through clamp connections so that the dikaryon state is maintained. Many basidiomycotes remain in the dikaryon state indefinitely. Under appropriate conditions, the dikaryon will produce fruiting bodies. Some of these hyphae produce basidia, such as the cells lining the "gills" under the cap of gilled mushrooms. Ultimately, the two haploid nuclei in each basidium fuse (karyogamy) to form a diploid nucleus. This then undergoes meiosis to produce four haploid nuclei whch migrate into the basidiospores and are dispersed into the environment.