Phylogeny, Evidence for a Cryptic Plastid, and Distribution of Chytriodinium Parasites (Dinophyceae) Infecting Copepods

8 pages, 3 figures, supporting information https://doi.org/10.1111/jeu.12701Spores of the dinoflagellate Chytriodinium are known to infest copepod eggs causing their lethality. Despite the potential to control the population of such an ecologically important host, knowledge about Chytriodinium parasites is limited: we know little about phylogeny, parasitism, abundance, or geographical distribution. We carried out genome sequence surveys on four manually isolated sporocytes from the same sporangium, which seemed to be attached to a copepod nauplius, to analyze the phylogenetic position of Chytriodinium based on SSU and concatenated SSU/LSU rRNA gene sequences, and also characterize two genes related to the plastidial heme pathway, hemL and hemY. The results suggest the presence of a cryptic plastid in Chytriodinium and a photosynthetic ancestral state of the parasitic Chytriodinium/Dissodinium clade. Finally, by mapping Tara Oceans V9 SSU amplicon data to the recovered SSU rRNA gene sequences from the sporocytes, we show that globally, Chytriodinium parasites are most abundant within the pico/nano‐ and mesoplankton of the surface ocean and almost absent within microplankton, a distribution indicating that they generally exist either as free‐living spores or host‐associated sporangiaThis work was supported by a grant from the Gordon and Betty Moore Foundation (GBMF3307) to P.J.K., T.A.R., A.Z.W., and A.E.S., and from NSERC (RGPIN‐2014–03994) to P.J.K. Ship time was supported by a grant from the David and Lucile Packard Foundation through MBARI and GBMF3788 to A.Z.W. J.dC. was supported by a Marie Curie International Outgoing Fellowship grant (FP7‐PEOPLE‐2012‐IOF-331450 CAARL), and N.O., M.K., and J.dC. were supported by a grant from the Tula Foundation to the Centre for Microbial Biodiversity and Evolution at UBCPeer Reviewe