Genetic structure in a cnidarian symbiont is correlated with geographic location, environment, and host species

Corals and cnidarians form symbioses with dinoflagellates across a wide range of habitats from the tropics to temperate zones. Notably, these partnerships create the foundation of coral reef ecosystems and are at risk of breaking down due to climate change. This symbiosis couples the fitness of the partners, where adaptations in one species can benefit the holobiont. However, the scales over which each partner can match their current - and future - environment are largely unknown. We investigated population genetic patterns of temperate anemones (Anthopleura spp.) and their endosymbiont Breviolum 'muscatinei', across an extensive geographic range to identify the spatial scales over which local adaptation is possible. Similar to previously published results, two solitary host species exhibited isolation-by-distance across hundreds of kilometers. However, symbionts exhibited genetic structure across multiple spatial scales, from geographic location to depth in the intertidal zone, and host species, suggesting that symbiont populations are more likely than their hosts to adaptively mitigate the impact of increasing temperatures.