Positive Selection of a Starch Synthesis Gene and Phenotypic Differentiation of Starch Accumulation in Symbiotic and Free-Living Coral Symbiont Dinoflagellate Species

Symbiosis is a basis for species diversification through interactions between organisms. In tropical and subtropical oceans, dinoflagellate symbionts belonging to the family Symbiodiniaceae, including the genus Symbiodinium, support the flourishment of cnidarian hosts, including corals, and thereby the ecology of oligotrophic oceans through their photosynthate carbon transfers. Although the genus Symbiodinium includes both free-living and symbiotic species, the detailed genetic background of their lifestyle differences remains unclear. In this study, we identified candidate genes involved in the evolutionary acquisition or maintenance of symbiosis in Symbiodinium spp. by detecting genes that have undergone positive selection during symbiotic and free-living lifestyle diversification. Using multiple Symbiodinium genomes to detect positive selection, 35 genes were identified, including a gene encoding soluble starch synthase SSY1 and genes related to metabolite secretion, which may be preferred for symbiotic lifestyles. In particular, our in silico analyses revealed that the SSY1 gene family has undergone extensive gene duplications in an ancestral dinoflagellate, and that the mutations detected as positive selection have occurred in the intrinsically disordered region of one of the homologs. Consistent with molecular evolution, the phenotypes of intracellular starch synthesis/accumulation were distinct between the symbiotic and free-living species of Symbiodinium when cultured under different pH and nitrogen conditions. These results provide molecular and phenotypic insights into symbiotic Symbiodinium-coral relationships.