RT Journal Article SR Electronic T1 Contrasting population genetic structure in Acropora coral hosts and their algal symbionts across multiple spatial scales JF bioRxiv FD Cold Spring Harbor Laboratory SP 575183 DO 10.1101/575183 A1 SW Davies A1 DC Wham A1 MR Kanke A1 MV Matz YR 2019 UL http://biorxiv.org/content/early/2019/03/13/575183.abstract AB Many broadly-dispersing corals acquire their algal symbionts (Symbiodiniaceae) from their environment upon recruitment. This horizontal transmission strategy has the potential to promote coral fitness across diverse environments provided that corals can associate with diverse algae across their range and that these symbionts exhibit reduced dispersal potential. Here we quantified genetic structure of algal symbionts in symbiosis with two hosts (Acropora hyacinthus, Acropora digitifera) across two spatial scales (across islands, within islands) in Micronesia using microsatellites. We contrast these symbiont genetic structures to previously published coral host structures across the same spatial gradient. We find that both hosts associated with two genetically distinct Cladocopium lineages (C3, C40), confirming that Acropora coral hosts associate with diverse algae across this range. In addition, each Cladocopium lineage exhibited signatures of moderate host specialization. Across locations, algal populations were significantly more genetically structured compared to their hosts: they demonstrated higher FST among islands and often exhibited significant divergence among reefs on the same island, which was never observed in coral hosts. While Cladocopium C3, like their hosts, broadly followed an isolation-by-distance pattern across islands, there was one notable case of no genetic structure across more than 2,000 km (between Yap and Pohnpei), demonstrating that Cladocopium genetic structure can be more complex than their hosts. Overall, our results support the view that horizontal transmission – establishing novel symbioses with local symbionts - has the potential to facilitate local fitness for broadly dispersing coral species, and highlight the complexity of factors affecting the population biology of Cladocopium.