Whole-genome duplication in an algal symbiont bolsters coral heat tolerance

Abstract

The algal endosymbiont <i>Durusdinium trenchii</i> enhances the resilience of coral reefs under thermal stress. <i>D. trenchii</i> can live freely or in endosymbiosis, and the analysis of genetic markers suggests that this species has undergone whole-genome duplication (WGD). However, the evolutionary mechanisms that underpin the thermotolerance of this species are largely unknown. Here, we present genome assemblies for two <i>D. trenchii</i> isolates, confirm WGD in these taxa, and examine how selection has shaped the duplicated genome regions using gene expression data. We assess how the free-living versus endosymbiotic lifestyles have contributed to the retention and divergence of duplicated genes, and how these processes have enhanced the thermotolerance of <i>D. trenchii</i>. Our combined results suggest that lifestyle is the driver of post-WGD evolution in <i>D. trenchii</i>, with the free-living phase being the most important, followed by endosymbiosis. Adaptations to both lifestyles likely enabled <i>D. trenchii</i> to provide enhanced thermal stress protection to the host coral.