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

Abstract

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 reclassified 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.

Setting scientific names at all taxonomic ranks in italics facilitates their quick recognition in scientific papers

“…With increasingly well-resolved phylogenetic hypotheses, it has become possible to define synapomorphies for higher level groups and to redefine many names at the family level and above, e.g. in oomycetes (Beakes and Thines 2017 ; Buaya et al 2020 ). Thus, consensus views on higher-level designations are starting to become feasible (e.g.…”

Microeukaryotic Communities Associated With the Seagrass Zostera marina Are Spatially Structured

“…), being closely related to Pontisma lagenidioides (Buaya et al. 2020), a pathogen of green filamentous alga Chaetomorpha media (Raghukumar and Chandramohan 1988) and red algae such as Ceramium rubrum (Buaya et al. 2019).…”

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

“…Samples preserved in 70% ethanol were deposited in the herbarium collection of the Senckenberg Museum of Natural History (Herbarium Senckenbergianum, FR), Cryptogams Section, Frankfurt am Main, accession number FR-0046138. Morphological characterization of the parasite was done as described earlier (Buaya et al 2020) using a compound light microscope (Imager2, Carl Zeiss Göttingen, Germany) with DIC, and photographs were taken using a Zeiss Axiocam MRc5 (Carl Zeiss, Göttingen, Germany). In addition, cellulose presence in the thallus wall of the parasitoids was tested using zinc iodine chloride (Carl Roth GmbH, Germany) solution.…”

“…To obtain high-quality nrSSU sequences of the oomycete infecting P. viridis, PCR amplicons were also cloned. Initially, the PCR amplicons were purified using magnetic bead-based nucleic acid purification (AMPure XP, Beckman Coulter Inc., USA) as previously described (Buaya et al 2020). Subsequently, PCR amplicons were diluted by a factor of ten, and the mixture was cloned into competent Escherichia coli using a StrataClone TA cloning kit (Agilent Technologies, Santa Clara, United States) following instructions of the manufacturer.…”

“…Many of the oomycete parasitoids of freshwater diatoms were documented in studies from more than half a century ago (Karling 1942;Sparrow 1960) and were classified into various genera, mainly Aphanomycopsis (Scherffel 1925), Ectrogella (Zopf 1884), Lagenidium (Zopf 1878), and Olpidiopsis (Cornu 1872). Interest in these endobiotic parasitoids has resurged recently, with studies focusing on systematics (Buaya et al 2017;Buaya and Thines 2019;Buaya and Thines 2020a;Buaya et al 2020), axenic dual culture of host and parasite (Buaya et al 2019b;Buaya et al 2020), and environmental sequencing (Garvetto et al 2018;Hassett et al 2019). Despite these Section Editor: Marc Stadler advances, the taxonomic affinities of most oomycete diatom parasitoids remain unresolved because of lacking sequence data, including also the type species of Aphanomycopsis, Aphanomycopsis bacillariacearum (Buaya and Thines 2020c).…”