Events

Of the >1600 species of stony corals (Scleractinia), 55% form a nutritional endosymbiosis with photosynthetic dinoflagellates. This 'photosymbiosis' is key to the success of corals forming shallow tropical reefs, yet we still know little of its evolutionary history. Sampling the posterior set of a comprehensive species-level supertree and fitting hidden-rates models allowing among-lineage variation in the rate of trait evolution, we reconstruct the history of photosymbiosis within Scleractinia and characterize the evolutionary stability of the interaction. The ancestral states of Complexa and Robusta were confidently reconstructed as non-photosymbiotic. Photosymbiosis has been independently gained at least seven times in Robusta and at least five times in Complexa. Almost all photosymbiotic species are found in large clades and are evolutionarily stable for the trait, that is, they show no evidence of loss. Nevertheless, several smaller clades across Scleractinia evince high rates of gains and losses of photosymbiosis. This pattern of variation in the rate of acquisition and reversal suggests that there exist multiple strategies for coexistence between corals and their photosymbionts, ranging from epochal-scale dynamism to irreversibility. This study identifies model taxa for comparative genomic approaches to elucidate the proximate mechanisms governing these broad evolutionary patterns.