Adding a missing piece to the puzzle of oomycete phylogeny: the placement of Rhipidium interruptum (Rhipidiaceae)

Abstract

Oomycetes are a group of fungus-like organisms, which phylogenetically comprise early diverging lineages that are mostly holocarpic, and two crown classes, the Peronosporomycetes and Saprolegniomycetes, including many well-investigated pathogens of plants and animals. However, there is a poorly studied group, the Rhipidiales, which placement amongst the crown oomycetes is ambiguous. It accommodates several taxa with a sophisticated vegetative and reproductive cycle, as well as structural organisation, that is arguably the most complex in the oomycete lineage. Despite the remarkable morphological complexity and their notable perseverance in the face of faster-growing saprotrophic oomycetes and fungi, the knowledge on Rhipidiales is limited to date, as the most complex members are not easily cultured, even by targeted approaches. This also leads to inadequate sequence data for the order, which was sourced from only the two least complex out of seven introduced genera, i.e. Sapromyces and Salispina. In the present study, ex-situ baiting was done using various fruit substrates, and naturally-shed twigs or fruits acquired from water bodies were examined. As a result of these efforts, the species Rhipidium interruptum was obtained and gross cultivation was accomplished using poplar (Populus nigra) twigs as substrate, which allowed further documentation of both asexual and sexual reproduction. This enabled phylogenetic and detailed morphological study, as well as an epitypification of the species. Phylogenetic analyses based on cox2 and nrLSU sequences revealed Rhipidium as the sister genus of Sapromyces. The morphological studies done support a conspecificity of R. interruptum and R. continuum, which might in turn be conspecific with R. americanum. Though several further studies will be required to fit the scattered missing pieces of knowledge on Rhipidiales together revealing a more complete picture of oomycete evolution, we hope that the current study can serve as a cornerstone for future investigations in the group.