Evolution of the soritids-Symbiodinium symbiosis

Coral reef ecosystems worldwide gather a myriad of invertebrates, including sponges, jellyfishes, anemones, corals, and mollusks that are hosts to a diverse group of dinoflagellates of the genus Symbiodinium. Among protists, Symbiodinium endosymbionts have been reported in ciliates and large soritid foraminifera. Recent molecular phylogenetic studies on the symbionts of soritids have revealed an extraordinary diversity of Symbiodinium lineages, most of which are specifically associated with this relatively small group of foraminifera. Additional ecological and evolutionary studies have shown that (1) the specificity between soritids and Symbiodinium is greater than previously thought and can also be found at a lower taxonomic level within Soritinae, (2) the diversity of soritid-specific Symbiodinium spp. is much greater in the Indo-Pacific than in Western Atlantic, hence correlates positively with the distribution of soritid diversity, (3) soritid symbionts did not present faster evolutionary rates compared to the Metazoan symbionts, suggesting that other factors such as the predominantly vertical transmission of symbionts and/or biogeographic isolation may be responsible for the host-symbiont specificity and diversity observed in Soritinae. Additionally, a relaxed molecular clock method was applied on 105 Symbiodinium rDNA sequences to estimate when the various clades diversified from each other. Our results suggested that the genus originated in early Eocene, and that the majority of extant lineages diversified since mid-Miocene, about 15 million years ago. Here we review the history of our research on soritid-Symbiodinium relationship and discuss its future perspectives.