Abstract
Coral reefs are fundamentally sustained by symbioses involving dinoflagellate algae in the Family Symbiodiniaceae. The coral symbiont Durusdinium trenchii is notable for enhancing the resilience of coral holobionts under thermal stress. Believed to have experienced whole-genome duplication (WGD), D. trenchii offers a valuable model system to understand how selection acts on the genome of a facultative symbiont after WGD. We present genome assemblies for two isolates of D. trenchii and confirm WGD in these taxa, providing the first example of this phenomenon in a single-celled eukaryotic symbiont. We assess how the facultative lifestyle has contributed to the retention and divergence of duplicated genes, and how these results intersect with the observed thermotolerance of corals hosting D. trenchii symbionts. Our findings reveal that the free-living lifestyle is the main driver of post-WGD evolution, however, they also implicate symbiosis in this process, with both lifestyles increasing algal fitness. Our results demonstrate that WGD, driven by selection in the free-living phase, has converted D. trenchii into a coral symbiont that serendipitously provides increased thermal stress protection to the host coral.
Competing Interest Statement
The authors have declared no competing interest.
