Decrypting colours: From colour morphotypes to holobiont genetic diversity

As many Cnidarians, the sea anemone Anemonia viridis lives in an intimate symbiotic trophic relationship with microorganisms, in particular dinoflagellates of Temperate Clade A from the Symbiodiniaceae family. The holobiont A. viridis - Dinoflagellate can live in a wide geographical and habitat range and displays different colour morphs. Previous studies showed that A. viridis harbours a genetically diverse population of Dinoflagellates but also displays a differential bathymetric distribution of the morphs, which could be a signal of an adaptative process. For these reasons, the system A. viridis - Dinoflagellate is a first-class model to study holobiont adaptation. Our work aims to clarify the taxonomic nature of the colour morphs in A. viridis and the potential correlation to the symbiotic diversity. To identify the factors that drive holobiont differentiation, we characterized the distribution of in hospite Dinoflagellate genetic diversity by sampling 3 colour morphs of A.viridis in ecologically differentiated populations from the English Channel and the Mediterranean Sea. We assessed this genetic diversity by combining NGS genotyped microsatellite markers for the symbiont and RADseq markers for the animal host. This allowed us to unravel the co-evolutionary dynamics of the two partners along a geographical and/or environmental continuum. The barriers to gene flow were not found where expected, and the patterns of symbiont differentiation were more due to the host's genetic discontinuities than to local ecological conditions