The question of the separation of Diplochytridium from Chytridium sensu lato

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 unwieldy taxonomic assemblage. Karling divided the genus into a nonapophysate group, Chytridium sensu stricto, and an apophysate group, his new genus Diplochytridium. 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 development) 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 segregation of genera from Chytridium sensu lato based on 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 a synonym of Chytridium.