What drives phenotypic divergence among coral clonemates?

Abstract

Evolutionary rescue of populations depends on their ability to produce phenotypic variation that is heritable and adaptive. DNA mutations are the best understood mechanisms to create phenotypic variation, but other, less well-studied mechanisms exist. Marine benthic foundation species provide opportunities to study these mechanisms because many are dominated by isogenic stands produced through asexual reproduction. For example, Caribbean acroporid corals are long lived and reproduce asexually via breakage of branches. Fragmentation is often the dominant mode of local population maintenance. Thus, large genets with many ramets (colonies) are common. Here, we observed phenotypic variation in stress response within genets following the coral bleaching events in 2014-and 2015 caused by high water temperatures. This was not due to genetic variation in their symbiotic dinoflagellates (Symbiodinium fitti) because each genet of this coral species typically harbors a single strain of S. fitti. Characterization of the microbiome via 16S tag sequencing did not provide evidence for a central role of microbiome variation in determining bleaching response. Instead, epigenetic changes were significantly correlated with the hosts genetic background, the position of the sampled polyps within the colonies (e.g. branch versus base of colony), and differences in the colonies condition during the bleaching event. We conclude that microenvironmental differences in growing conditions led to long-term changes in the way the ramets methylated their genomes contributing to, but not fully explaining, the differential bleaching response. This research provides novel data to understanding intra-genet variability in stress phenotypes of sessile marine species.