Molecular phylogenetics of the Chytridiomycota supports the utility of ultrastructural data in chytrid systematics

The chytrids (Chytridiomycota) are morphologically simple aquatic fungi that are unified by their possession of zoospores that typically have a single, posteriorly directed flagellum. This study addresses the systematics of the chytrids by generating a phylogeny of ribosomal DNA sequences coding for the small subunit gene of 54 chytrids, with emphasis on sampling the largest order, the Chytridiales. Selected chytrid sequences were also compared with sequences from Zygomycota, Ascomycota, and Basidiomycota to derive an overall fungal phylogeny. These analyses show that the Chytridiomycota is probably not a monophyletic group; the Blastocladiales cluster with the Zygomycota. Analyses did not resolve relationships among chytrid orders, or among clades within the Chytridiales, which suggests that the divergence times of these groups may be ancient. Four clades were well supported within the Chytridiales, and each of these clades was coincident with a group previously identified by possession of a common subtype of zoospore ultrastructure. In contrast, the analyses revealed homoplasy in several developmental and zoosporangial characters.