Incipient loss of flagella in the genus <i>Geolegnia</i>: the emergence of a new clade within <i>Leptolegnia</i>?

The genus <i>Geolegnia</i> represents a poorly documented group of saprolegnialean oomycetes isolated from soils as free-living organisms. Although it is morphologically similar to the facultative parasitic genus <i>Leptolegnia</i>, <i>Geolegnia</i> presents the uncommon property of having lost a flagellate stage in its lifecycle. Based on ITS and large subunit (LSU) rRNA sequence data, we show <i>Geolegnia</i> to be basal to <i>Leptolegnia</i>, and also introduce <i>Geolegnia helicoides</i> sp. nov. Using sequence data of <i>Leptolegnia</i> available in GenBank, supplemented by data derived from culture collections, we show that <i>Geolegnia</i> is nested within <i>Leptolegnia</i>, a genus characterised by its “conventional” biflagellate life cycle. The emergence of <i>Geolegnia</i> is therefore seen as a recent event, and we suggest here an evolutionary context where this loss might have been advantageous. Based on this study, <i>Leptolegnia</i> remains paraphyletic, awaiting the redefinition of genera in this complex.