Pseudorhizidium is a new genus with distinct zoospore ultrastructure in the order Chytridiales.

A chytrid isolate (JEL 221) we identified as the rarely reported species, Rhizidium endosporangiatum Karling, was cultured axenically for the first time. The purposes of this study are to characterize the developmental morphology of isolate JEL 221 and to elucidate its zoospore ultrastructural features. Thallus development and morphology of isolate JEL 221 are characteristic of R. endosporangiatum as it was originally described. However, thallus morphology of R. endosporangiatum is not entirely typical of the genus Rhizidium, especially that of the type R. mycophilum. The presence of an endosporangium, a layer of material encapsulating the edges of the protoplast protruding through multiple discharge pores, makes this a distinctive species. Consistent with its published molecular-based phylogenetic placement, we found that isolate JEL 221 shared ultrastructural features with the two major zoospore types described for the Chytridiales but had distinct zoospore architecture. A new genus, Pseudorhizidium, is erected for this chytrid based on its thallus morphology, molecular phylogenetic placement and unique zoospore ultrastructure. This new genus does not fit into either of the described families (Chytridiaceae or Chytriomycetaceae) in the Chytridiales because of its unique zoospore ultrastructure, especially the two-layered nature of the electron-opaque plug in the base of the flagellum.