Hyphochytrium

Abstract: This is an account of the molecular systematics and phylogeny of the osmotrophic stramenopile lineages traditionally studied by mycologists, which include the Labyrinthulomycota, Hyphochytriomycota, and Oomycota. All three groups produce zoospores with a mastigonate anterior flagellum, and all fall within Kingdom Straminipila, which is part of the larger stramenopile/alveolate/rhizaria superkingdom. The labyrinthulids and thraustochytrids are part of the same monophyletic clade as the bicoecids, protermonads, and opalinids, whereas the hyphochytrids and oomycetes are part of a sister clade, together with all the ochrophyte algae and some bacteriotrophic flagellates. Both are believed to have evolved from a common mixotrophic flagellate ancestor. The Labyrinthulomycota contains two order-level clades, the Thraustochytridiales (encompassing a half dozen or so thraustochytrid families and Diplophrys like protists) and the Labyrinthulales (containing the Labyrinthulaceae and Aplanochytridiaceae). The Hyphochytriomycota (based on sequence data for Hyphochytrium and Rhizochytrium) fall into a well-supported monophyletic clade that sits between the Ochrophyta and the Oomycota, although recent unpublished evidence suggests that the Anisolpidiaceae should be excluded. The Oomycota are the largest and most complex stramenopile clade. Four early-diverging order-level clades (Eurychasmales, Haptoglossales, Olpidiopsidales s.lat., and Haliphthorales) are recognized but have not been assigned to classes. Nearly all early-diverging genera are marine organisms that are parasites of algae, acheleminthes, or crustaceans. The later-diverging oomycetes are mostly found in freshwater and terrestrial ecosystems and fall into two class-level clades: the Saprolegniomycetes and Peronosporomycetes. Both have been split into three orders (Atkinsiellales, Leptomitales s. lat., and Saprolegniales for the former and Rhipidiales, Albuginales, and Peronosporales s. lat. for the latter). Morphological characteristics associated with zoosporogenesis and sexual reproduction are discussed in relation to our revised phylogenetic framework. Finally, a possible evolutionary timeline based on both molecular and fossil evidence is discussed.