Plastid genes in a non-photosynthetic dinoflagellate

Dinoflagellates are a diverse group of protists, comprising photosynthetic and heterotrophic free-living species, as well as parasitic ones. About half of them are photosynthetic with peridinin-containing plastids being the most common. It is uncertain whether non-photosynthetic dinoflagellates are primitively so, or have lost photosynthesis. Studies of heterotrophic species from this lineage may increase our understanding of plastid evolution. We analyzed an EST project of the early-diverging heterotrophic dinoflagellate Crypthecodinium cohnii looking for evidence of past endosymbiosis. A large number of putative genes of cyanobacterial or algal origin were identified using BLAST, and later screened by metabolic function. Phylogenetic analyses suggest that several proteins could have been acquired from a photosynthetic endosymbiont, arguing for an earlier plastid acquisition in dinoflagellates. In addition, intact N-terminal plastid-targeting peptides were detected, indicating that C. cohnii may contain a reduced plastid and that some of these proteins are imported into this organelle. A number of metabolic pathways, such as heme and isoprenoid biosynthesis, seem to take place in the plastid. Overall, these data indicate that C. cohnii is derived from a photosynthetic ancestor and provide a model for loss of photosynthesis in dinoflagellates and their relatives. This represents the first extensive genomic analysis of a heterotrophic dinoflagellate.