Spatial and temporal genetic structure of Symbiodinium populations within a common reef‐building coral on the Great Barrier Reef

Abstract

The dinoflagellate photosymbiont Symbiodinium plays a fundamental role in defining the physiological tolerances of coral holobionts, but little is known about the dynamics of these endosymbiotic populations on coral reefs. Sparse data indicate that Symbiodinium populations show limited spatial connectivity; however, no studies have investigated temporal dynamics for in hospite Symbiodinium populations following significant mortality and recruitment events in coral populations. We investigated the combined influences of spatial isolation and disturbance on the population dynamics of the generalist Symbiodinium type C2 (ITS1 rDNA) hosted by the scleractinian coral Acropora millepora in the central Great Barrier Reef. Using eight microsatellite markers, we genotyped Symbiodinium in a total of 401 coral colonies, which were sampled from seven sites across a 12-year period including during flood plume-induced coral bleaching. Genetic differentiation of Symbiodinium was greatest within sites, explaining 70-86% of the total genetic variation. An additional 9-27% of variation was explained by significant differentiation of populations among sites separated by 0.4-13 km, which is consistent with low levels of dispersal via water movement and historical disturbance regimes. Sampling year accounted for 6-7% of total genetic variation and was related to significant coral mortality following severe bleaching in 1998 and a cyclone in 2006. Only 3% of the total genetic variation was related to coral bleaching status, reflecting generally small (8%) reductions in allelic diversity within bleached corals. This reduction probably reflected a loss of genotypes in hospite during bleaching, although no site-wide changes in genetic diversity were observed. Combined, our results indicate the importance of disturbance regimes acting together with limited oceanographic transport to determine the genetic composition of Symbiodinium types within reefs.

Corals in the hottest reefs in the world exhibit symbiont fidelity not flexibility

“…Genetic dissimilarity corresponded with oceanographic distance, where the composition of ITS2 sequences was distinct between the nearby sites of Saadiyat and Ras Ghanada (39 km apart) and most dissimilar from the isolated site of Delma (219–248 km away; Figure ). This fine‐scale spatial genetic structure is consistent with patterns observed with microsatellite markers for Cladocopium populations on the Great Barrier Reef (Howells, van Oppen, & Willis, ; Howells, Willis, Bay, & Oppen, ) and in the Caribbean (Thornhill et al, ). Thus, it is likely that Cladocopium populations in the Persian Gulf are also differentiated due to limited dispersal and recruitment among sites and predominant asexual reproduction within sites (Thornhill, Howells, Wham, Steury, & Santos, ).…”

Deep-Sequencing Method for Quantifying Background Abundances of Symbiodinium Types: Exploring the Rare Symbiodinium Biosphere in Reef-Building Corals

“…individual haplotypes) as well, as suggested by the numerous positively correlated haplotypes found for all symbiont types tested here. Thus far, only microsatellites have exhibited the ability to discern below the type level [84], however, specific microsatellites must be developed for most clades and types [52] and detection is limited to targeted loci, eliminating the possibility of finding novel diversity. The development of new loci for amplicon sequencing [85], possibly applied together with historically used markers such as ITS, will enable enhanced resolution to differentiate both clades and types.…”

Population genetics of reef coral endosymbionts (Symbiodinium, Dinophyceae)

“…; Howells et al . , ). Higher within‐host population diversity can also be detected with increased survey effort, either by genotyping biopsies from different areas, or by repeated temporal sampling, of the same host individual (Thornhill et al .…”