Department of Biological Sciences, Box 870344, The University of Alabama Tuscaloosa, AL 35487, United States of America

E-mail: *[email protected] [email protected]

Chytridiomycota characteristically reproduce with posteriorly uniflagellate zoospores and are known as zoosporic fungi. Electron microscopic studies and molecular analyses of gene sequences have transformed our understanding of relationships within the Chytridiomycota. From syntheses of these data, systematics of zoosporic fungi is being rewritten, including the separation of Blastocladiomycota, Neocallimastigomycota and Monoblepharidomycota from the Chytridiomycota as new phyla. This chapter traces the recent history of zoosporic fungal taxonomy and summarizes our current classification of Chytridiomycota. Since the 1970s, the concept of distinct “zoospore types” based on a constellation of characters allowed zoospore ultrastructure to defi ne orders in the Chytridiomycota. During the 1990s, phylogenetic analyses of gene sequences provided greater resolution needed to refi ne chytrid relationships, and by the 2000s classification decisions, even at the species level, applied molecular-based monophyly. Unexpectedly, molecular analyses of zoosporic fungi revealed the order Chytridiales as polyphyletic and composed of multiple distinct lineages. Revisionary systematic studies have now characterized several of these lineages as new orders for taxa previously classifi ed in the Chytridiales. How lineages of zoosporic fungi evolved is controversial, but molecular-based

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hypotheses of phylogeny suggest that holocarpic parasitic zoosporic fungi are most basal in the radiation of fungal evolution and hyphal forms are more primitive than simple eucarpic thallus forms.