Diatomophthoraceae – a new family of olpidiopsis-like diatom parasitoids largely unrelated to Ectrogella

Abstract

The oomycete genus Ectrogella currently comprises a rather heterogeneous group of obligate endoparasitoids, mostly of diatoms and algae. Despite their widespread occurrence, little is known regarding the phylogenetic affinities of these bizarre organisms. Traditionally, the genus was included within the Saprolegniales, based on zoospore diplanetism and a saprolegnia/achlya-like zoospore discharge. The genus has undergone multiple re-definitions in the past, and has often been used largely indiscriminately for oomycetes forming sausage-like thalli in diatoms. While the phylogenetic affinity of the polyphyletic genus Olpidiopsis has recently been partially resolved, taxonomic placement of the genus Ectrogella remained unresolved, as no sequence data were available for species of this genus. In this study, we report the phylogenetic placement of Ectrogella bacillariacearum infecting the freshwater diatom Nitzschia sigmoidea. The phylogenetic reconstruction shows that Ectrogella bacillariacearum is grouped among the early diverging lineages of the Saprolegniomycetes with high support, and is unrelated to the monophyletic diatom-infecting olpidiopsis-like species. As these species are neither related to Ectrogella, nor to the early diverging lineages of Olpidiopsis s. str. and Miracula, they are placed in a new genus, Diatomophthora, in the present study.

Phylogeny and cultivation of the holocarpic oomycete Diatomophthora perforans comb. nov., an endoparasitoid of marine diatoms

“…Even though monophyly of the parasites of red algae was not resolved in Badis et al 2019, a monophyletic grouping was observed in Buaya et al (2019c) and Buaya and Thines (2020). Interestingly, the genus Ectrogella with its type species E. bacillariacearum, which was previously speculated to belong to the early-diverging lineages, was found to be embedded in the basal Saprolegniomycetes (Buaya and Thines 2020) and epitypified with a specimen exhibiting the typical thallus and exit-pore morphology described for the species by Zopf (1884). Consequently, the olpidiopsis-like (i.e.…”

“…Miracula helgolandica was inferred to Section Editor: Marc Stadler be the earliest-diverging oomycete lineage together with the recently described freshwater parasitoid M. moenusica, while D. drebesii formed a monophyletic group together with D. gillii and uncultured environmental sequences (Buaya and Thines 2019;Buaya et al 2017;2019a). Only recently, phylogenetic affinities of the genus Olpidiopsis and Ectrogella were resolved and their type species epitypified, leading to the taxonomic revisions of these two important taxa of early-diverging oomycetes (Buaya et al 2019c; Buaya and Thines 2020). The genus Olpidiopsis with its type species O. saprolegniae was found to be unrelated to the parasites of red algae and diatoms parasitoids (Buaya et al 2019c).…”

“…Consequently, the olpidiopsis-like (i.e. those holocarpic oomycetes with simple thalli and zoospores that start swarming within the mature thallus) diatom parasitoids were assigned to a new genus, Diatomophthora, while the parasites of red algae were re-classified to the order Pontismales (Buaya et al 2019c;Buaya and Thines 2020), following the classification first suggested by Dick (2001).…”

“…The parasitoid usually has multiple broadly conical discharge tubes, a characteristic similar to several Ectrogella species (Petersen 1905;Sparrow 1960). However, its phylogenetic placement in Ectrogella remains uncertain in the light of a recent study that demonstrated that the type species of Ectrogella is a member of an early diverging group of the Saprolegniomycetes (Buaya and Thines 2020). Buaya and Thines (2020) have shown that E. bacillariacearum, which has a clearly defined diplanetism belongs to the Saprolegniomycetes that are characterised by this.…”

“…However, its phylogenetic placement in Ectrogella remains uncertain in the light of a recent study that demonstrated that the type species of Ectrogella is a member of an early diverging group of the Saprolegniomycetes (Buaya and Thines 2020). Buaya and Thines (2020) have shown that E. bacillariacearum, which has a clearly defined diplanetism belongs to the Saprolegniomycetes that are characterised by this. In contrast, two parasitoids without clear-cut diplanetism belonged to the early-diverging oomycetes and were, thus, classified in the new genus Diatomophthora (Buaya and Thines 2020).…”