title: Population genetics of reef coral endosymbionts (Symbiodinium, Dinophyceae).
abstract: Symbiodinium is a diverse genus of unicellular dinoflagellate symbionts associating with various marine protists and invertebrates. Although the broadscale 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 the following: (i) Symbiodinium populations within individual hosts are comprised mainly of cells belonging to a single or few genetic clones. (ii) 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. (iii) Mutualistic Symbiodinium do not perpetually persist outside their hosts, instead undergoing turnover and replacement via the continuous shedding of viable clonal cells from host individuals. (iv) 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 among hosts. Overall, this synthesis forms a basis for future investigations of coral symbiosis ecology and evolution as well as delimitation of species boundaries in Symbiodinium and other eukaryotic microorganisms.
authors.affiliation: Department of Biological Sciences and Molette Biology Laboratory for Environmental and Climate Change Studies, Auburn University, 101 Rouse Life Sciences Building, Auburn, AL, 36849, USA.
authors.forename: D J
authors.identifier: http://orcid.org/0000-0002-9540-6892
authors.initials: DJ
authors.lastname: Thornhill
authors.affiliation: Center for Genomics and Systems Biology, New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates.
authors.forename: E J
authors.initials: EJ
authors.lastname: Howells
authors.affiliation: Department of Biology, Pennsylvania State University, 208 Mueller Laboratory, University Park, PA, 16802, USA.
authors.forename: D C
authors.initials: DC
authors.lastname: Wham
authors.affiliation: School of Forestry and Wildlife Sciences, Auburn University, 3301 Forestry and Wildlife Building, Auburn, AL, 36849, USA.
authors.forename: T D
authors.initials: TD
authors.lastname: Steury
authors.affiliation: Department of Biological Sciences and Molette Biology Laboratory for Environmental and Climate Change Studies, Auburn University, 101 Rouse Life Sciences Building, Auburn, AL, 36849, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Santos
mesh_terms: D000818:Animals; D037421:Anthozoa; D057231:Climate Change; D057894:Coral Reefs; D004141:Dinoflagellida / Q000254:growth & development*; D005828:Genetics, Population; D010802:Phylogeny; D013559:Symbiosis
keywords: connectivity; coral reef; microsatellite; symbiosis; symbiotic dinoflagellate; zooxanthellae