Introduction to the Chytridiomycota

First among fungus

The oldest fossil fungi so far known are chytrid-like forms from the Vendian of northern Russia. Older fossils of Precambrian "fungi" are now usually considered to be empty sheaths of filamentous cyanobacteria, or else are not distinct enough to be placed in any taxon with certainty. Amazingly fine chytrid fossils are known from the Devonian Rhynie Chert, where they occur alongside representatives of other major fungal groups. Some Devonian forms are parasitic on rhyniophytes and closely resemble the modern genus Allomyces (Blastocladiales), both in form and in reproductive features of the life cycle. Other Rhynie chytrids resemble members of the Spizellomycetales, suggesting that chytrids had diversified by this early date.

Chytrids are not merely "first" because of the age of their fossils, however. Studies of the evolutionary relationships between chytrids and other fungi indicate that they are the sister group to the remaining fungal groups, or that they may be a paraphyletic basal assemblage. This means that chytrids may give us a good picture of what the ancestors of fungi were like.

So what do chytrids tell us about the origin of fungi? First of all, chytrids are predominantly aquatic, and not terrestrial. This means that fungi probably got their start in the water, as did plants and vertebrates.

Secondly, chytrids have flagellated gametes -- their reproductive cells have a flagellum that allows them to swim. No other fungi have flagella, which suggests that the other fungi lost this trait at some point in their evolutionary history. This is also consistent with what we know about the closest relatives to the fungi, which also have flagellae.

Finally, like other fungi, chytrids have chitin strengthening their cell walls, and one subgroup (Hyphochytrids) have cellulose as well, a trait unique among living fungi. The presence of chitin is thus an important defining feature of the fungi.

There is considerable variation in the morphology and ecology of chytrids. Some are freshwater, some marine; some are parasites on plants and dipterans, while others live on decaying plants and insect parts. Some are unicellular, some coenocytic, and still others produce a mycelium much like other fungi. Few have any noticeable impact on humans, with the exception of a few that parasitize algae, cause potato wart (Synchytrium endoboticum), and those used in experimental research (e.g. Allomyces).