Adding a missing piece to the puzzle of oomycete phylogeny: the placement of ().

Oomycetes are a group of fungus-like organisms, which phylogenetically comprise early diverging lineages that are mostly holocarpic, and two crown classes, the and , including many well-investigated pathogens of plants and animals. However, there is a poorly studied group, the , 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 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, and . In the present study, 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 was obtained and gross cultivation was accomplished using poplar () 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 2 and nrLSU sequences revealed as the sister genus of . The morphological studies done support a conspecificity of and , which might in turn be conspecific with . Though several further studies will be required to fit the scattered missing pieces of knowledge on 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. Tsai I, Thines M (2023). Adding a missing piece to the puzzle of oomycete phylogeny: the placement of (). : 95-108. doi: 10.3114/fuse.2023.11.08.