Contrasting patterns of seascape genetics in Acropora cf. tenuis and their symbiotic algae.

Theory suggests that pairs of mutualist species will often co-disperse and share the same dispersal patterns, though the extent to which this happens remains unclear. Photosynthetic corals constitute an example of this dynamic, as they rely on algal symbionts to meet their energetic needs, yet many acquire their symbionts environmentally after their larval dispersal phase. Consequently, corals and their symbionts may exhibit similar or contrasting patterns of genetic variation across the seascape, with implications for their evolutionary and ecological processes. Here, we densely sample corals of the key reef-building taxon Acropora cf. tenuis and their algal symbionts across a reefscape in the central Philippines to examine genetic variation across space. Four distinct coral taxa show genetic evidence of long distance dispersal, including weak or absent isolation by distance signals and parent-offspring pairs at widely spaced sites. These coral taxa all host a single group of algal symbionts from the genus Cladocopium, which shows landscape genetic structure independent from its coral hosts. In fact, Cladocopium genetics vary with both latitude and depth, potentially indicating genome-wide local adaptation at a finer spatial scale than that seen in their hosts. Genetic variation at markedly different spatial scales between host and symbiont may be beneficial for hosts if these differences enable them to acquire symbionts adapted to their settlement environments.