Ultrastructural and molecular delineation of the Chytridiaceae (Chytridiales)

The Chytridiomycota is in need of taxonomic revision, especially the largest order, the Chytridiales. We analyzed 25 isolates in, or allied to, the Chytridium clade of this order. Isolates were selected based on one or more of the following criteria: (i) having a large subunit molecular sequence similar to that of the type of the genus Chytriomyces, (ii) having specific zoospore morphology, and (iii) currently classified as a species in the genus Chytriomyces. We examined ultrastructural characters and partial sequences of large subunit and small subunit rDNA and generated a phylogenetic hypothesis using maximum parsimony and Bayesian analyses. The sequence analyses strongly supported the Chytridiaceae, Phlyctochytrium, and Chytriomyces angularis clades, and each clade had a specific zoospore type. Developmental morphology of the thallus did not mirror the DNA-based phylogeny. Based on the results of phylogenetic analyses of sequences and ultrastructural characters, we emend the Chytridiaceae by including exogenous and polycentric development and define the family on the basis of a single zoospore type. Species identified as being in the genus Chytriomyces occur in several separate, well-supported clades along with species currently classified in seven other genera (Asterophlyctis, Entophlyctis, Obelidium, Physocladia, Podochytrium, Rhizoclosmatium, and Siphonaria), indicating that Chytriomyces as currently defined is polyphyletic.