Whole-genome duplication in an algal symbiont serendipitously confers thermal tolerance to corals
The algal endosymbiont Durusdinium trenchii enhances the resilience of coral reefs under thermal stress1,2. As an endosymbiont, D. trenchii is generally expected to have a reduced genome compared to its free-living relatives, due in part to the lack of selective pressure for maintaining redundant gene functions in a stable intracellular environment within the host3. However, D. trenchii can live freely or in endosymbiosis, and the analysis of genetic markers4 suggests that this species has undergone whole-genome duplication (WGD). Here we present genome assemblies for two D. trenchii isolates, confirm WGD in these taxa, and examine how selection has shaped the duplicated genome regions. We assess how the competing free-living versus endosymbiotic lifestyles of D. trenchii have contributed to the retention and divergence of duplicated genes, and how these processes have enhanced thermotolerance of corals hosting these symbionts. We find that lifestyle is the driver of post-WGD evolution in D. trenchii, with the free-living phase being most important, followed by endosymbiosis. Adaptations to both lifestyles collectively result in increased cellular fitness for D. trenchii, which provides enhanced thermal stress protection to the host coral. Beyond corals, this polyploid alga is a valuable model for understanding how genome-wide selective forces act to balance the often, divergent constraints imposed by competing lifestyles.