In the photosynthetic dinoflagellate Dinophysis, pigment composition and ultrastructural characters place the origin of the chloroplast with the cryptophytes. Presented here is the first molecular data from the plastid of Dinophysis. We cloned and sequenced the coding regions of psbA and small subunit (SSU) rDNA from four populations of D. acuminata and from D. acuta. These data were used to test two hypotheses about Dinophysis plastid evolution: 1) The plastid is of cryptophyte origin, and 2) The plastid is a permanent replacement. The psbA coding region was identical among the populations of D. acuminata and between this species and D. acuta. On the other hand, the SSU rDNA showed polymorphisms, both between species and among populations. Phylogenetic analysis shows that both psbA and SSU rDNA sequences firmly place the Dinophysis spp. plastid within the cryptophytes, confirming its origin through plastid replacement. The polymorphic SSU rDNA sequences group together and are sister to a cryptophyte ultraplankter (OCS20). The SSU rDNA sequence heterogeneity between species (about 1%) is comparable to the heterogeneity among different isolates of D. acuminata, suggesting these are population-level differences and not indicative of different plastid captures by Dinophysis. Interestingly, a second class of cloned coding regions was also isolated from each population. These psbA and SSU rDNA sequences were evolutionarily more divergent and specifically related to florideophyte red algae. Given the mixotrophic habit of Dinophysis, the possibility arises that this second class may have come from red algal preys in the food vacuoles of our single-cell isolates. These results highlight the importance of sampling multiple genes and populations in determining the complex evolutionary history of the Dinophysis plastid.

Key words: dinoflagellate, Dinophysis, endosymbiosis, evolution, plastid