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

• Published in: Mycological progress Vol. 19; no. 5; pp. 441 - 454
• Main Authors: Buaya, Anthony T., Ploch, Sebastian, Kraberg, Alexandra, Thines, Marco
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-05-2020; Springer Nature B.V
• Subjects: Affinity; Bacillariophyceae; Biomedical and Life Sciences; Cross-infection; Cryptic species; Cultivation; Cultural characteristics; Ecology; Ectrogella; Endoparasites; Fjords; Freshwater environments; Genetic distance; Host range; Life cycles; Life Sciences; Microbiology; New genera; Original Article; Parasites; Parasitoids; Phylogenetics; Phylogeny; Placement; Plant Sciences; Pure culture; Species; Species classification; Swarming; Taxonomic revision; Taxonomy; Zoospores; Oomycetes; Phylogeny
• ISSN: 1617-416X; 1861-8952
• DOI: 10.1007/s11557-020-01569-5

Oomycetes infecting diatoms are biotrophic parasitoids and live in both marine and freshwater environments. They are ubiquitous, but the taxonomic affinity of many species remains unclear and the majority of them have not been studied for their molecular phylogeny. Only recently, the phylogenetic and taxonomic placement of some diatom-infecting, early-diverging oomycetes was resolved, including the genera
Ectrogella
,
Miracula
,
Olpidiopsis
, and
Pontisma
. A group of holocarpic diatom parasitoids with zoospores swarming within the sporangium before release were found to be unrelated to the known genera with diatom-infecting species, and were re-classified to a new genus,
Diatomophthora
. However, about a dozen species of holocarpic diatom parasitoids with unclear affinity remained unsequenced, which includes a commonly occurring species so far identified as
Ectrogella perforans.
However, this assignment to
Ectrogella
is doubtful, as the species was not reported to feature a clear-cut diplanetism, a hallmark of
Ectrogella
s. str. and the whole class Saprolegniomycetes. It was the aim of the current study to clarify the phylogenetic affinities of the species and if the rather broad host range reported is correct or a reflection of cryptic species. By targeted screening, the parasitoid was rediscovered from Helgoland Roads, North Sea and Oslo Fjord, Southern Norway and investigated for its phylogenetic placement using small ribosomal subunit (18S) sequences. Stages of its life cycle on different marine diatoms were described and its phylogenetic placement in the genus
Diatomophthora
revealed. A stable host-parasite axenic culture from single spore strains of the parasitoid were established on several strains of
Pleurosigma intermedium
and
Coscinodiscus concinnus
. These have been continuously cultivated along with their hosts for more than 2 years, and cultural characteristics are reported. Cross-infection trials revealed the transferability of the strains between hosts under laboratory conditions, despite some genetic distance between the pathogen strains. Thus, we hypothesise that
D. perforans
might be in the process of active radiation to new host species.