New home for tiny symbionts: dinophytes determined as Zooxanthella are Peridiniales and distantly related to Symbiodinium.

Endosymbiotic dinophytes are diverse and are found in a large variety of aquatic partners. They are colloquially coined zooxanthellae, and knowledge about those dinophytes with a coral as partner (i.e., Symbiodinium) is extensive. However, Zooxanthella nutricula has been specifically described based on material isolated from radiolarians, and its phylogenetic position within the dinophyte tree is unclear at present. We isolated genomic DNA and sequenced the ribosomal RNA genes from an endosymbiotic dinophyte found in a radiolarian. In phylogenetic analyses, the endosymbiont was distantly related to Symbiodinium and the Suessiales, but clustered together with members of the Peridiniales. Specifically, it was the sister lineage of a small group, whose members host a diatom as endosymbiont (i.e., the dinotoms). Endosymbiosis is thus of multiple origin within the dinophytes, and more research is necessary to work out ecological and morphological character traits that are congruent to the DNA trees.