Hyphochytriomycota and Oomycota

Abstract

The anteriorally uniflagellate Hyphochytriomycota and biflagellate Oomycota are in the Kingdom Straminipila (commonly referred to as stramenopiles) which are part of the SAR superkingdom. Both appear to be basal to the large assemblage of golden-brown algae, the Ochrophyta. Both feature osmotrophic nutrition and have traditionally been considered as zoosporic “fungi,” but are unrelated to organisms in the monophyletic kingdom Mycota. The Hyphochytriomycota is a small group encompassing around half a dozen genera, which have simple nonmycelial, holocarpic thalli, traditionally encompassing three families: the endobiotic Anisolpidiaceae, the polycentric Hyphochytriaceae, and the monocentric Rhizidiomycetaceae. Recently the former have been shown to be placed among the early diverging Oomycota, leaving just the latter two families in the monophyletic Hyphochytriomycota clade. Hyphochytriomycota are widespread in occurrence, and most are saprotrophs or parasites, infecting the resting spores of Oomycota and Glomeromycota. In contrast, the Oomycota are a large and diverse assemblage, consisting of two major (class level) clades, the Saprolegniomycetes and Peronosporomycetes, and several early diverging classes most of which are simple holocarpic organisms that lack mycelial organisation. Many of these early-diverging clades are as yet poorly resolved because of sparse taxon sampling. The early-diverging orders include the Eurychasmales and Olpidiopsidales, both of which are marine seaweed parasites, the nematode infecting Haptoglossales and crustacean infecting Haliphthorales. The Saprolegniomycetes mostly have fungal-like mycelial thalli and include the orders Atkinsiellales s.lat., Leptomitales, and Saprolegniales, which are mostly saprophytes or parasites of invertebrates and, occasionally, vertebrates such as fish and amphibians. A few species in the Saprolegniales are root infecting parasites of plants. The Peronosporomycetes are the second major fungal-like class, and include the largely saprotrophic Rhipidiales, the facultively parasitic Pythiales s.lat., which can infect both animals and plants and the predominantly plant pathogenic Albuginales and Peronosporales sensu lato. Indeed, the Oomycota are significant parasites of both animals and plants, impacting both natural ecosystems and causing significant economic losses in both aquacultural and agricultural systems. The molecular systematics of the Oomycota is still in a state of flux, and in this account a relatively conservative approach has been taken. It is apparent that most of the early-diverging genera are almost exclusively marine and that the Peronosporales represents the main terrestrial and plant pathogenic lineage. Most early-diverging genera lack the oogamous sexual reproduction that characterizes this group and suggests that the oogenesis evolved around the time of emmergence from the sea to the land and freshwater ecosystems. It is also clear that obligate biotrophy in the white blister rusts (Albuginales) and downy mildews (Peronosporales s.str.) has evolved independently.