Dinotoms Illuminate Early Pathways to the Stable Acquisition of Photosynthetic Endosymbionts

It is a widely accepted hypothesis that plastids and mitochondria arose by endosymbiosis; however, many of the evolutionary processes of organellar acquisition and development are still unknown. While many organism models represent points in a possible transition from facultative photosynthetic endosymbionts to highly reduced plastid organelles, only ‘dinotoms’ can be applied to answer the question: How do free-living microalgae transform into stable plastid-bearing endosymbionts? Many of the proposed models for plastidogenesis are either still in a kleptoplastidic stage requiring ongoing endosymbiont gain or have already transitioned to fixed endosymbionts with dramatic reduction of the symbiont nuclei and other organelles. Dinotom dinoflagellates, on the other hand, provide a missing link. Their plastids are derived from diatoms at various stages of association and permanence within dinoflagellate hosts. Some are at the kleptoplastid stage, where ingested diatoms are maintained only for months; some are at a stage in which multiple diatoms are stably maintained; and some are in a permanent partnership with only a single diatom endosymbiont. Dinotoms maintain all core diatom organelles regardless of their evolutionary stage, except for the plasma membrane and the frustule. In this chapter, we summarise the current knowledge of the dinotom system with respect to cell biology and molecular biology and discuss the applicability of dinotoms as model organisms for endosymbiosis studies.