D. J. Thornhill, E. J. Howells, D. C. Wham, T. D. Steury, S. R. Santos

Symbiodinium is a diverse genus of unicellular dinoflagellate symbionts associating with various marine protists and invertebrates. Although the broad-scale diversity and phylogenetics of the Symbiodinium complex is well established, there have been surprisingly few data on fine-scale population structure and biogeography of these dinoflagellates. Yet population level processes contribute strongly to the biology of Symbiodinium, including how anthropogenic-driven global climate change impacts these symbionts and their host associations. Here, we present a synthesis of population-level characteristics for Symbiodinium, with an emphasis on how phylogenetic affinities, dynamics within and among host individuals, and a propensity towards clonality shape patterns on and across reefs. Major inferences include: 1.Symbiodinium populations within individual hosts are comprised mainly of cells belonging to a single or few genetic clones. 2.Symbiont populations exhibit a mixed mode of reproduction, wherein at least one sexual recombination event occurs in the genealogy between most genotypes, but clonal propagation predominates overall. 3.Mutualistic Symbiodinium do not perpetually persist outside of their hosts, instead undergoing turnover and replacement via the continuous shedding of viable clonal cells from host individuals. 4.Symbiont populations living in the same host, but on different reefs, are often genetically subdivided, suggesting low connectivity, adaptation to local conditions, or prolific asexual reproduction and low effective population sizes leading to disproportionate success within and amongst hosts. Overall, this synthesis forms a basis for future investigations of coral symbiosis ecology and evolution and delimitation of species boundaries in Symbiodinium as well as other eukaryotic microorganisms. This article is protected by copyright. All rights reserved.