Morphology, phylogeny, and host range of the novel early-diverging oomycete Sirolpidium dinoletiferum sp. nov. parasitizing marine dinoflagellates.

Oomycetes are fungus-like heterotrophic organisms with a broad environmental distribution, including marine, freshwater, and terrestrial habitats. They function as saprotrophs that use the remains of other organisms or as parasites of a variety of eukaryotes, including protists, diatoms, dinoflagellates, macroalgae, plants, fungi, animals, and even other oomycetes. Among the protist hosts, the taxonomy, morphology, and phylogenetic positions of the oomycete parasitoids of diatoms have been well studied; however, this information concerning the oomycete parasitoids of dinoflagellates is poorly understood. During intensive sampling along the east and west coasts of Korea in May and October 2019, a new species of oomycetes was discovered and two strains of the new parasitoid were successfully established in cultures. The new oomycete parasitoid penetrated the dinoflagellate host cell and developed to form a sporangium, which was very similar to the perkinsozoan parasitoids that infect marine dinoflagellates. The most distinctive morphological feature of the new parasitoid was a central large vacuole forming several long discharge tubes. The molecular phylogenetic tree inferred based on the small subunit (SSU) ribosomal DNA (rDNA) revealed that the new parasitoid forms a distinct branch unrelated to other described species belonging to early-diverging oomycetes. It clustered with species belonging to the genus Sirolpidium with strong support values in the cytochrome c oxidase subunit 2 (cox2) tree. Cross-infection experiments showed that infections by the new parasitoid occurred in only six genera belonging to dinoflagellates among the protists tested in this study. Based on the morphological and molecular data obtained in this study, we propose to introduce a new species, Sirolpidium dinoletiferum sp. nov., for this novel parasitoid, conservatively within the genus Sirolpidium.