INTRODUCTION TO THE CHYTRIDIOMYCOTA

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 among the only members of the fungi in which motility has been retained.

In overall growth habit, chytrids tend to be holocarpic (text with tooltip) Entire thallus functions as sporangium. (Figures 1 and 2), but some are eucarpic (text with tooltip) Thallus consists of rhizoids and sporangium. (Figure 3). As a group, the chytrids are primarily haploid, characterized by the life cycle of Polyphagus (see Figure 4). That is, the zygote undergoes meiosis (Figure 4 M-N) and all other stages are haploid. Still, the holocarpic Polyphagus varies in form from a zoospore (Figure 4 A) to an amoeba (Figure 4 B-E) to a thallus (Figure 4 G-L), and to a sporangium (Figure 4 F and N-P). The released zoospores germinate and develop into the gametophyte (Figure 4 B-L). Spizellomyces is a holocarpic genus with alternation of generation (Figure 3).

FIGURE 1. Synchytrium, a holocarpic chytrid that infects potatoes. Image from http://io.uwinnipeg.ca/~simmons/2152web/2152/fungi1a.htm	FIGURE 2. Spizellomyces, a holocarpic chytrid is growing in broth and shows rhizoids and a sporangium. Image from http://www.bsu.edu/classes/ruch/msa/barr/4-15.jpg	FIGURE 3. Monoblepharis, a eucarpic chytrid showing a developing sporangium. Image from http://www.botany.uga.edu/zoosporicfungi/monfoamy.htm

FIGURE 4. Life Cycle of Polyphagus. A. Zoospore with a posteriorly-directed flagellum B. Zoospore in a cyst C-D. Germinating spore E-F. Development of a sporangium G. Large (female) thallus H. Smaller (male) thallus with a copulation tube connecting G & H I-J. Plasmogamy (fusion of cytoplasm) K-L. Dikaryotic “zygote” M. Fusion of nuclei N-O. Meiosis followed by development of sporangium P. Release of zoospores Image taken from Figure 31-2 of Bold et al. (1987)

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).

SYSTEMATICS OF THE CHYTRIDIOMYCOTA

In general, the following taxonomy is a reduction of that of Barr (1990) and Margulis and Schwartz (1988, Pr-26 and 1998, Pr-29). In particular, Barr (1990) divided the chytrids into 4 orders according to zoospore ultrastructure. One of those orders, the Blastocladiales, has been raised to phylum status and removed from the Chytridiomycota (James et al. 2006a and 2006b, Hibbett et al. 2007).

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. Further molecular evidence (see Tudge 2000; Patterson 1999; and Baldauf 2003 for a synopsis) 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 conjugating fungi (Figure 5) as shown by James et al. (2006a and 2006b), Hibbett et al. (2007), and Porter et al. (2011). These taxa, the core Chytrids (Hibbett et al. 2007), make up the formal taxon of the Chytridiomycota, which should be understood to be the Chytridiomycota of Barr (1990), Alexopoulos and Mims (1979), and Alexopoulos et al. (1996) minus the current phyla Blastocladiomycota and Neocallimastigomycota.