Evolutionary relationship between dinoflagellates bearing obligate diatom endosymbionts: insight into tertiary endosymbiosis

The marine dinoflagellates Peridinium balticum and Peridinium foliaceum are known for bearing diatom endosymbionts instead of peridinin-containing pla stids, While evidence clearly indicates that their endosymbionts are closel y related, the relationship between the host dinoflagellate cells is not se ttled. To examine the relationship of the two dinoflagellates, the DNA sequ ences of nuclear small-subunit rRNA genes (SSU rDNA) from Peridinium baltic um, Peridinium foliaceum and one other peridinin-containing species, Peridi nium bipes, were amplified, cloned and sequenced. While phylogenetic analys es under simple models of nucleotide substitution weakly support the monoph yly of Peridinium balticum and Peridinium foliaceum, analyses under more so phisticated models significantly increased the statistical support for this relationship. Combining these results with the similarity between the two endosymbionts, it is concluded that (i) the two hosts have the closest sist er relationship among dinoflagellates tested, (ii) the hypothesis that the diatom endosymbiosis occurred prior to the separation of the host cells is most likely to explain their evolutionary histories, and (iii) phylogenetic inferences under complex nucleotide evolution models seem to be able to co mpensate significant rate Variation in the two SSU rDNA.