Horizontal acquisition of Symbiodiniaceae in the Anemonia viridis genetic data

All metazoans are in fact holobionts, resulting from the association of several organisms, and organismal adaptation is then due to the composite response of this association to the environment. Deciphering the mechanisms of symbiont acquisition in a holobiont is therefore essential to understanding the extent of its adaptive capacities. In cnidarians, some species acquire their photosynthetic symbionts directly from their parents (vertical transmission) but may also acquire symbionts from the environment (horizontal acquisition) at the adult stage. The Mediterranean snakelocks sea anemone, Anemonia viridis (Forskål, 1775), passes down symbionts from one generation to the next by vertical transmission, but the capacity for such horizontal acquisition is still unexplored. To unravel the flexibility of the association between the different host lineages identified in A. viridis and its Symbiodiniaceae, we genotyped both the animal hosts and their symbiont communities in members of host clones in five different locations in the North Western Mediterranean Sea. The composition of within-host symbiont populations was more dependent on the geographical origin of the hosts than their membership to a given lineage or even to a given clone. Additionally, similarities in host symbiont communities were greater among genets (i.e. among different clones) than among ramets (i.e. among members of the same given clonal genotype). Taken together, our results demonstrate that A. viridis may form associations with a range of symbiotic dinoflagellates and suggest a capacity for horizontal acquisition. A mixed-mode transmission strategy in A. viridis, as we posit here, may help explain the large phenotypic plasticity that characterises this anemone.