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PHYLUM CHYTRIDIOMYCOTA

INTRODUCTION TO THE CHYTRIDIOMYCOTA

Chytridiomycota (ka-tri-de-o-mi-KO-ta) is made of two Greek roots that mean little cooking pot (chytridion -χυτριδιον); and fungus (mykes -μύκης).  The reference is to the cooking pot appearance of the sporangia of holocarpic taxa.

Chytrids occur mainly in aquatic or moist habitats where they live as parasites or saprobes.   Thus, they superficially resemble the water molds to which they were thought to have been affiliated.  Chytrids seem to have kept a suite of primitive characters in the fungal line.  Most importantly, they are the only members of the fungi in which motility has been retained.   In overall growth habit, chytrids can be holocarpic or eucarpic.  By and large, the chytrids are divided into two groups, each with holocarpic and eucarpic taxa.  

As a group, the Chytridiomycetes (Figures A&B) is primarily haploid.  That is, in its life history [see Polyphagus Life History], the zygote undergoes meiosis and all other stages are haploid.  Still, the holocarpic Polyphagus varies in form from a swimming cell to an amoeba to a thallus, to a sporangium, and a spore. 

The Blastocladiomycetes does exhibit a complete alternation of generation between a haploid gametophyte (Figure C) and a diploid sporophyte (Figure D), the phase in which meiosis occurs [see Allomyces Life History].  Allomyces, a eucarpic taxon, forms cottony mycelia in both phases such that they would be indistinguishable without seeing gametangia or sporangia on the hyphae.  The gametophyte of Allomyces (Figure C) has tandem paired gametangia that terminate the hyphae such that the terminal gametangium (usually smaller of the two) is the "male", that is, it produces the smaller of the two motile anisogametes.  The larger proximal gametangia release larger motile anisogametes.  The sporophyte of Allomyces (see Figure D) produces terminal single sporangia within which meiosis occurs and zoospores are formed.  The released zoospores germinate and develop into the gametophyte.  Spizellomyces is a holocarpic genus with alternation of generation (Figure E).

Chytrids infect many different insects.  In fact, some aquatic insects have been implicated as vectors for chytridiosis of salmon and other salmonids, which is leading to a dramatic global loss of populations of these taxa (Nichols et al. 2001).  A particular chytrid, Batrachochytridium dendrobatidis, has been implicated in the global decline and extinctions of amphibians, particularly frogs in the Americas and Australia (Lips et al. 2005).  First reported in the African Clawed Frog (Xenopus), one of the most widely transported taxa in the world, Batrachochytridium may have been carried almost as if by a vector to regions of susceptible frogs.  Laboratory studies suggest that the chytrid is susceptible to warm temperatures, which may help to explain why the maximum impact of chytridiomycosis is among frog species in cooler climates (higher latitudes or mountainous areas of the tropics).

SYNOPTIC DESCRIPTION OF THE CHYTRIDIOMYCOTA

SYSTEMATICS OF THE CHYTRIDIOMYCOTA

In general, the following taxonomy follows that of Barr (1990) and Margulis and Schwartz (1988, Pr-26 and 1998, Pr-29). In particular, Barr (1990) has divided the chytrids into 4 orders according to zoospore ultrastructure, but he does not group them into 2 classes as I have. Other details of the taxonomy come from Sleigh et al. (1984), Alexopoulos and Mims (1979), and Alexopoulos et al. (1996).

The barren basal body near the active one in the zoospores of the chytrids indicates that they must have evolved from a biflagellate ancestor.  Barr (1990) believes that the flagellar rootlets are similar to astral rays, structures which radiate from centrioles in many organisms during interphase, so the chytrids may be derived from the earliest flagellated eukaryotic groups. Thus, in Barr's view, the differences in ultrastructural details between the orders may simply reflect a very long phylogenetic history.  However, Bruns et al. (1992) in their examination of 18S rRNA nucleotide sequences, confirmed that the chytrids and the Zygomycota were basal groups within the clade of the Kingdom Fungi and seemed to close the book on the question of the chytrids.  More recently, further molecular evidence (see Tudge 2000; Patterson 1999; and Baldauf 2003 for a synopsis) has confirmed their position near the root of the fungi [see also Lang, The Fungal Mitochondrial Genome Project], which is part of a larger clade called the opisthokonts, a group that includes the choanoflagellates and the metazoans (Patterson 1999).  Margulis and Schwartz (1998) still maintain that the fungi are a kingdom of conjugating taxa, and, therefore, continue to exclude the chytrids.

More recent molecular evidence has called into question the monophyly of the Chytrids as defined here (Lutzoni et al. 2004).  The Blastocladiales cluster with some of the orders in the Zygomycota into a group that likely will be defined as a new phylum.  As of now, however, all of the Chytrids are contained within one phylum.

HIERARCHICAL CLASSIFICATION TO THE CHYTRIDIOMYCOTA

This page is maintained by Jack R. Holt & Carlos A. Iudica.  Last revised 03/12/2008.