Dinoflagellate nuclear SSU rRNA phylogeny suggests multiple plastid lossesand replacements

Dinoflagellates are a trophically diverse group of protists with photosynth etic and nonphotosynthetic members that appears to incorporate and lose end osymbionts relatively easily. To trace the gain and loss of plastids in din oflagellates, we have sequenced the nuclear small subunit rRNA gene of 28 p hotosynthetic and four non-photosynthetic species, and produced phylogeneti c trees with a total of 81 dinoflagellate sequences. Patterns of plastid ga in. loss, and replacement were plotted onto this phylogeny. With the except ion of the apparently early-diverging Syndiniales and Noctilucales, all non -photosynthetic dinoflagellates are very likely to have had photosynthetic ancestors with peridinin-containing plastids. The same is true for all dino flagellates with plastids other than the peridinin-containing plastid: thei r ancestors have replaced one type of plastid for another, in some cases mo st likely through a nonphotosynthetic intermediate. Eight independent insta nces of plastid loss and three of replacement can be inferred from existing data, but as more non-photosynthetic lineages are characterized these numb ers will surely grow.