Hyphochytriomycota

Abstract

The Hyphochytriomycota are a small group of zoospore-producing organisms that, although they live like fungi, i.e., they are nonphotosynthetic organisms that usually possess cell walls during their growth phases and acquire nutrients by absorption, are more closely allied with algal groups. The diagnostic feature of the group is the presence of a single, mastigoneme-bearing flagellum that most often is at the anterior end of the zoospore as it swims. The Hyphochytriomycota have been classified with the Protista (Whittaker 1969) and Protoctists (Margulis et al. 1990), but more recent molecular data place them in a clade with the Oomycetes and heterokont algae. This clade has been called Kingdom Cromista (Cavalier-Smith 1993, and Chap. 1, this Vol., and Kingdom Stramenopila (Alexopoulos et al. 1996); others simply refer to the group as stramenopiles (Leipe et al. 1994). Molecular studies (Van der Auwera et al. 1995) of ribosomal RNA sequences of Hyphochytrium catenoides and inverted repeat sequences of mitochondrial DNA in H. catenoides (Mc Nabb et al. 1988) indicate that the Hyphochytriomycota are close relatives of the Oomycota (see Dick, Chap. 2, this Vol.). Our own molecular studies (S. Lee and M.S. Fuller, unpubl.) of the group support inclusion of the Hyphochytriomycota in a clade with the Oomycota and heterokont algae, but suggest that the Hyphochytriomycota are more closely related to some groups of the heterokont algae than to the Oomycota.