Taxonomy and phylogeny of Aphanomycopsis bacillariacearum, a holocarpic oomycete parasitoid of the freshwater diatom genus Pinnularia

Abstract

Investigations into simple holocarpic oomycetes are challenging, because of the obligate biotrophic nature of many lineages and the periodic presence in their hosts. Thus, despite recent efforts, still, the majority of species described remains to be investigated for their phylogenetic relationships. One of these species is Aphanomycopsis bacillariacearum, the type species of the genus Aphanomycopsis. Species of Aphanomycopsis are endobiotic holocarpic parasites of diverse hosts (e.g., diatoms, desmids, dinoflagellates). All species classified in this genus were assigned to it based on the presence of branching hyphae and the formation of two generations of zoospores, of which the first one is not motile. Originally, Aphanomycopsis with its type species, A. bacillariacearum, had been classified in the Saprolegniaceae. However, the genus has undergone multiple taxonomic reassignments (to Ectrogellaceae, Lagenidiaceae, and Leptolegniellaceae) in the past. To settle the taxonomy and investigate the phylogenetic placement of Aphanomycopsis, efforts were undertaken to isolate A. bacillariacearum from its original host, Pinnularia viridis and infer its phylogenetic placement based on nrSSU (18S) sequences. By targeted isolation, the diatom parasitoid was rediscovered from Heiðarvatn lake, Höskuldsstaðir, Iceland. Phylogenetic reconstruction shows that A. bacillariacearum from Pinnularia viridis is embedded within the Saprolegniales, and largely unrelated to both diatom-infecting oomycetes in the Leptomitales (Ectrogella, Lagenisma) and those placed within the early-diverging lineages (Miracula, Diatomophthora) of the Oomycota.