RESUMEN
While surveying chytrid diversity in lakes and streams, we found on cellulosic bait a chytrid that had both monocentric and polycentric thallus forms. We brought this chytrid into axenic culture from three sites in eastern North America, studied its thallus development and zoospore ultrastructure, and compared its 28S rDNA sequence with those of other members of the Chytridiomycota. Thallus morphology matched that described for the rare chytrid, Cladochytrium polystomum Zopf. Sporangia were spherical and produced numerous long discharge tubes. After discharge, zoospores remained in spherical clusters at the tips of the inoperculate openings of discharge tubes. After 10-30 min zoospores either swam away or encysted in place. Zoospore ultrastructural features included a cell coat, flagellar plug, and paracrystalline inclusion, features typical of members of the Chytridiales. However, the flagellar apparatus structure and organellar organization differed from that of zoospores previously described. Based on its molecular phylogeny and its zoospore ultrastructural features, we classify C. polystomum as a member of the Chytridiaceae in the Chytridiales. Because its thallus development and its ribosomal DNA sequences diverged decidedly from those of Cladochytrium tenue Nowak, the type species of Cladochytrium, we erected Zopfochytrium as a new genus for this chytrid.