ABSTRACT: Chytridium has long been an accepted and inclusive genus of Chytridiomycota, underpinning the nomenclature of the Chytridiaceae and Chytridiales. However, Chytridium is a morphologically variable and unweildy taxonomic assemblage. Karling divided the genus into a nonapophysate group, Chytridium sensu stricto, and an apophysate group, his new genus Diplochytrididum. We analyze and compare the major morphological variations in Chytridium sensu stricto and Diplochytridium. The criteria by which Karling recognized Diplochytridium (the presence of an apophysis and a sometimes alternating, endo-exogenous mode of thallus develpment) do not both occur in all species included within Diplochytridium. Furthermore, some members of Chytridium sensu stricto exhibit, to a degree, one or the other of these traits. Endo-exogenous development is poorly known except in a few taxa. We conclude that the segregation of genera from Chytridium sensu lato based on the presence or absence of apophysis alone is of questionable validity. In addition to the morphological inconsistencies, Diplochytridium is nomenclaturally suspect because there is no citation of a type and because Diplochytridium could be interpreted as an obligate synonym of Chytridium.

KEYWORDS: Chytridiomycota, development, morphology, nomenclature

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