Dinopallor Comventus n. gen., n. sp., a Novel Parasitoid of Marine Dinoflagellates Representing a Sister Lineage to Seagrass Parasites and Expanding the Host Range of Phytomyxea (Rhizaria, Endomyxa).

Abstract

Phytomyxea (SAR: Rhizaria: Endomyxa) is a group of obligate biotrophic parasitic protists comprised of two orders: Plasmodiophorida, found in terrestrial or freshwater environments, and Phagomyxida, found in marine environments. While Plasmodiophorida has been extensively studied due to its economic importance as plant pathogens, Phagomyxida remains poorly investigated despite its ecological significance in marine ecosystems. During intensive sampling along the Korean coast from April to December 2023, novel parasitoids infecting dinoflagellates were discovered in seawater collected at 10 coastal stations. A total of 23 isolates were successfully established in culture, and the morphology of infected host cells resembled that of known Phagomyxa infections. The newly identified parasitoid exhibits a life cycle that includes zoospore penetration, multinucleate plasmodium development, and formation of a sporangiosorus composed of numerous zoosporangia. Each zoosporangium produces three biflagellate zoospores, and no resting spores were observed. A key morphological feature distinguishing this parasitoid from Phagomyxa species is the presence of a sporangiosorus wall enclosing the zoosporangia. Phylogenetic analysis based on small subunit (SSU) ribosomal DNA (rDNA) revealed that this parasitoid forms a distinct clade with Marinomyxa and the environmental sequence TAGIRI-5, suggesting a disparity between its morphological similarity to Phagomyxa and its molecular phylogenetic position. The SSU rRNA gene sequence of the new parasitoid showed 99.87% identity to the TAGIRI-5 sequence obtained from an anoxic sediment in Kagoshima Bay, Japan. Cross-infection experiments demonstrated that infections occurred only in five dinoflagellate genera among the taxa tested. Based on morphological and molecular data obtained in this study, we propose a new genus and species, Dinopallor comventus n. gen., n. sp., for this newly discovered parasitoid.