New Perkinsea Parasitoids of Dinoflagellates Distantly Related to Parviluciferaceae Members

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.

Global perspective of environmental distribution and diversity of Perkinsea (Alveolata) explored by a meta-analysis of eDNA surveys

Perkinsea constitutes a lineage within the Alveolata eukaryotic superphylum, mainly composed of parasitic organisms. Some described species represent significant ecological and economic threats due to their invasive ability and pathogenicity, which can lead to mortality events. However, the genetic diversity of these described species is just the tip of the iceberg. Environmental surveys targeting this lineage are still scarce and mainly limited to the Northern Hemisphere. Here, we aim to conduct an in depth exploration of the Perkinsea group, uncovering the diversity across a variety of environments, including those beyond freshwater and marine ecosystems. We seek to identify and describe putative novel organisms based on their genetic signatures. In this study, we conducted an extensive analysis of a metabarcoding dataset, focusing on the V4 region of the 18S rRNA gene (the EukBank dataset), to investigate the diversity, distribution and environmental preferences of the Perkinsea. Our results reveal a remarkable diversity within the Perkinsea, with 1568 Amplicon Sequence Variants (ASVs) identified across thousands of environmental samples. Surprisingly, we showed a substantial diversity of Perkinsea within soil samples (269 ASVs), challenging the previous assumption that this group is confined to marine and freshwater environments. In addition, we revealed that a notable proportion of Perkinsea ASVs (428 ASVs) could correspond to putative new organisms, encompassing the well-established taxonomic group Perkinsidae. Finally, our study shed light on previously unveiled taxonomic groups, including the Xcellidae, and revealed their environmental distribution. These findings demonstrate that Perkinsea exhibits far greater diversity than previously detected and surprisingly extends beyond marine and freshwater environments. The meta-analysis conducted in this study has unveiled the existence of previously unknown clusters within the Perkinsea lineage, solely identified based on their genetic signatures. Considering the ecological and economic importance of described Perkinsea species, these results suggest that Perkinsea may play a significant, yet previously unrecognized, role across a wide range of environments, spanning from soil environments to the abyssal zone of the open ocean with important implications for ecosystem functioning.

Comparative biological traits of perkinsozoan parasitoids infecting marine dinoflagellates

The number of perkinsozoan parasitoid species known to infect dinoflagellates has increased to 11 over the last two decades. However, most of the current knowledge about the autecology of perkinsozoan parasitoids of dinoflagellates has derived from studies of one or two species, thereby making it difficult to directly compare their biological traits at the same time and even their potentials as biological control agents if they are to be exploited to mitigate harmful dinoflagellate blooms in the field. This study investigated total generation time, the number of zoospores produced per sporangium, zoospore size, swimming speed, parasite prevalence, zoospore survival and success rate, and host range and susceptibility for five perkinsozoan parasitoids. Four of the species (Dinovorax pyriformis, Tuberlatum coatsi, Parvilucifera infectans, and P. multicavata) were from the family Parviluciferaceae and one (Pararosarium dinoexitiosum) was from the family Pararosariidae, with dinoflagellate Alexandrium pacificum employed as a common host. Distinct differences in the biological traits of the five perkinsozoan parasitoid species were found, suggesting that the fitness of these parasitoids for the common host species differs. These results thus offer useful background information for the understanding of the impacts of parasitoids on the natural host population and for the design of numerical modeling including the host-parasitoid systems and biocontrol experiments in the field.

Development of a novel electroporation method for the oyster parasite Perkinsus marinus

Gene manipulation techniques are fundamental to molecular biology and are continuously being improved. However, gene transfection methods are not established for many unicellular eukaryotes (protists), thereby hindering molecular biological investigations. The oyster parasite Perkinisus marinus is one of the few protists with established gene transfection and drug selection. Nevertheless, the present protocols are tedious, requiring a specific electroporator and pulse conditions which limits the accessibility of this technique across different research groups. Here, we present alternative buffer and electroporation conditions that make the protocol less restrictive. We revealed the pulse condition that enables the introduction of plasmids into P. marinus cell using Ingenio electroporation buffer and NEPA21 electroporator. We found that number of cells and plasmid concentration were critical parameters for the electroporation system. We also constructed a simpler expression plasmid that is removed needless regions for gene expression in the parasite. Our findings resolved the equipment restriction in electroporation of P. marinus and would be a good reference for electroporation in other protists, in particular other Perkinsozoa parasites and core dinoflagellates.

A diversified and segregated mRNA spliced-leader system in the parasitic Perkinsozoa

Spliced-leader trans-splicing (SLTS) has been described in distantly related eukaryotes and acts to mark mRNAs with a short 5′ exon, giving different mRNAs identical 5′ sequence-signatures. The function of these systems is obscure. Perkinsozoa encompasses a diversity of parasitic protists that infect bivalves, toxic-tide dinoflagellates, fish and frog tadpoles. Here, we report considerable sequence variation in the SLTS-system across the Perkinsozoa and find that multiple variant SLTS-systems are encoded in parallel in the ecologically important Perkinsozoa parasite Parvilucifera sinerae. These results demonstrate that the transcriptome of P. sinerae is segregated based on the addition of different spliced-leader (SL) exons. This segregation marks different gene categories, suggesting that SL-segregation relates to functional differentiation of the transcriptome. By contrast, both sets of gene categories are present in the single SL-transcript type sampled from Maranthos, implying that the SL-segregation of the Parvilucifera transcriptome is a recent evolutionary innovation. Furthermore, we show that the SLTS-system marks a subsection of the transcriptome with increased mRNA abundance and includes genes that encode the spliceosome system necessary for SLTS-function. Collectively, these data provide a picture of how the SLTS-systems can vary within a major evolutionary group and identify how additional transcriptional-complexity can be achieved through SL-segregation.

Emerging Parasitic Protists: The Case of Perkinsea

The last century has witnessed an increasing rate of new disease emergence across the world leading to permanent loss of biodiversity. Perkinsea is a microeukaryotic parasitic phylum composed of four main lineages of parasitic protists with broad host ranges. Some of them represent major ecological and economical threats because of their geographically invasive ability and pathogenicity (leading to mortality events). In marine environments, three lineages are currently described, the Parviluciferaceae, the Perkinsidae, and the Xcellidae, infecting, respectively, dinoflagellates, mollusks, and fish. In contrast, only one lineage is officially described in freshwater environments: the severe Perkinsea infectious agent infecting frog tadpoles. The advent of high-throughput sequencing methods, mainly based on 18S rRNA assays, showed that Perkinsea is far more diverse than the previously four described lineages especially in freshwater environments. Indeed, some lineages could be parasites of green microalgae, but a formal nature of the interaction needs to be explored. Hence, to date, most of the newly described aquatic clusters are only defined by their environmental sequences and are still not (yet) associated with any host. The unveiling of this microbial black box presents a multitude of research challenges to understand their ecological roles and ultimately to prevent their most negative impacts. This review summarizes the biological and ecological traits of Perkinsea-their diversity, life cycle, host preferences, pathogenicity, and highlights their diversity and ubiquity in association with a wide range of hosts.

A Novel Parasitoid of Marine Dinoflagellates, Pararosarium dinoexitiosum gen. et sp. nov. (Perkinsozoa, Alveolata), Showing Characteristic Beaded Sporocytes

The phylum Perkinsozoa is known as an exclusively parasitic group within alveolates and is widely distributed in various aquatic environments from marine to freshwater environments. Nonetheless, their morphology, life cycle, the identity of the host, and physiological characteristics remain still poorly understood. During intensive sampling along the west coast of Korea in October and November 2017, a new parasitoid, which shares several characteristics with the extant families Perkinsidae and Parviluciferaceae, was discovered and three strains of the new parasitoid were successfully established in cultures. Cross-infection experiments showed that among the examined planktonic groups, only dinoflagellates were susceptible to the new parasitoid, with infections observed in species belonging to eight genera. Even though the new parasitoid shared many morphological and developmental characteristics with other Perkinsozoan parasites, it differed from them by its densely packed trophocyte structure without a large vacuole or hyaline material during the growth stage. These characteristics are common among Parviluciferaceae members. Furthermore, through palintomic extracellular sporogenesis, it produced characteristic interconnected sporocytes resembling a string of beads. Phylogenetic analyses based on the small subunit and large subunit ribosomal DNA sequences revealed that the new parasitoid was distantly related to the family Parviluciferaceae and was more closely related to the families Perkinsidae and Xcellidae. Morphological, ultrastructural, and molecular data on the new parasitoid raised the need to erect a new family, i.e., Pararosariidae, within the phylum Perkinsozoa with Pararosarium dinoexitiosum gen. et sp. nov. as the type species. The isolation and establishment in culture of the new parasitoid outside the family Parviluciferaceae in the present study would contribute to the better understanding of the diversity of Perkinsozoan parasites and provide useful material for comparisons to other parasite species in the further study.