Rediscovery of Araiospora Thaxter: new insights into a neglected chapter of oomycete evolution

Abstract The genus Araiospora , belonging to the family Rhipidiaceae of the Oomycota , had received some interest from the mycological community for a few decades since the late-nineteenth century. Members of the genus were occasionally found forming tufts on submerged twigs or fruits in water bodies of diverse kind. The four described species of Araiospora were distinguished from other rhipidiacean genera, by exhibiting two different forms of zoosporangia, depending on environmental conditions. Unfortunately, similar to the related genus Rhipidium , Araiospora was neglected over the past decades in mycological studies, due to the lack of molecular data and failures in establishing long-lasting cultures. In the current study aiming to close this knowledge gap, thalli of Araiospora were searched for and ultimately encountered on submerged twigs, and several individuals were isolated, documented using molecular phylogenetics and light microscopy, and were eventually identified as Araiospora spinosa . Phylogenetic reconstruction was done on the basis of concatenated sequence data of nuclear LSU rDNA and mitochondrial COX 2 regions, revealing that Araiospora formed the sister lineage to Rhipidium and Sapromyces , confirming its position within the Rhipidiaceae , while Salispina was inferred to group together with the aforementioned genera, reinforcing the monophyly of the Rhipidiales . To settle the taxonomy of Araiospora spinosa , an epitype specimen was fixed for this species, and the morphology of its asexual characters was depicted using differential interference contrast microscopy. Thus, this study provides a cornerstone for the investigation of Rhipidiaceae , connecting the knowledge of the late-nineteenth century with present-day techniques.