Host Specificity and Regional Endemicity in Symbiotic Dinoflagellates (Symbiodinium, Dinophyta) Associated with Sea Anemones in the Genus Aiptasia

Abstract

Recent investigations of coral reef biology have focused on the global biogeography and host specificity of Symbiodinium, a diverse group of dinoflagellates that symbiotically associate with many marine organisms, including reef-building corals. Despite this, few studies have investigated the genetic structure of Symbiodinium at the population level. One suitable system to investigate Symbiodinium population genetics of a single host across a global range is the facultatively symbiotic anemone Aiptasia. In order to determined the specificity and population genetic structure of Symbiodinium communities associated with Aiptasia spp., 356 anemones were sampled from 18 locations throughout the world. Symbiodinium diversity was assessed using a variety of v molecular markers that measure diversity from the level of sub-generic clades to populations, including restriction fragment length polymorphisms (RFLPs) of 18S-rDNA, denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer 2 (ITS2) rDNA, flanking region sequences of two microsatellite loci, and allelic variation at six microsatellite loci specific for Symbiodinium Clade B. These data revealed that, with the exception of individuals from the Florida Keys, a single phylotype of Symbiodinium clade B (ITS2 “type” B1) associates with Aiptasia throughout the world. Furthermore, strong population structure was detected across local, regional, and global geographic scales, suggesting limited gene flow among most Symbiodinium populations. The high genetic structure of Symbiodinium populations and the association with one particular symbiont lineage across large geographic scales suggests strong regional endemism and the existence of specificity in Aiptasia-Symbiodinium symbioses. This work represents a contribution towards our understanding of the ecology and evolution of cnidarian-Symbiodinium endosymbioses.