Genetic diversity of the symbiont as a driver of the holobiont differentiation in the temperate sea-anemone Anemonia viridis?

As many Cnidarians, the sea anemone Anemonia viridis lives in an intimate symbiotic trophic relationship with microorganisms, in particular dinoflagellates of the genus Symbiodinium. The holobiont A.viridis-Symbiodinium can live in a wide geographical and habitat range, and previous studies showed that A.viridis  harbours a genetically diverse population of Symbiodinium sp. which could therefore play a major adaptive  role. For these reasons, the system A.viridis-Symbiodinium is a first-class model to study holobiont adaptation. Our work aims at clarifying the importance of the symbiont in the response to environmental variation and will highlight the possible genotype-environment associations (GEA) at the scale of the whole holobiont. To identify the factors that drive the holobiont differentiation, we characterized the distribution of in hospite Symbiodinium genetic diversity by sampling ecologically differentiated A.viridis populations from the English Channel and the Mediterranean Sea. We assessed this genetic diversity by combining locus specific genotyping (obtained by NGS and an ad-hoc analysis pipeline for in hospite diversity) for the symbiont and SNPs (RADsequencing) for the animal host. This allowed us to unravel a complex co-evolutionary dynamics of the two partners. Independant host genetic lineages can harbour similar symbiont populations, but within a host lineage, symbionts present a stronger association to the environment than their host.