Phenotypic and transcriptomic differences between colonies of staghorn coral inhabiting disparate microenvironments – implications for coral restoration
Lesneski, Kathryn C.
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In the Caribbean, Acropora cervicornis (staghorn coral) exemplifies the worldwide anthropogenic decline of reef-building corals. From the mid-Pleistocene through the mid-1900s, A. cervicornis was a dominant framework builder, providing complex habitat for reef organisms. Since the 1980s, populations of A. cervicornis have declined by as much as 98%. Despite the overall decline, scattered remnants persist, and some appear to be thriving. As in recent studies on other acroporids, if we can identify variation in traits related to resilience in the remaining A. cervicornis, and understand the genetic basis of such variation, we could better forecast the species’ future response to climate change, and inform ongoing restoration efforts. Here, I compare phenotypic and transcriptomic indicators of resilience in A. cervicornis from two nearby but environmentally-disparate habitats on Turneffe Atoll, Belize: Calabash Caye forereef and Blackbird Caye backreef. Blackbird exhibits significantly higher flow, light, average temperature, and temperature variation. Over four years, I conducted a longitudinal study of 122 tagged coral colonies. Corals from Blackbird and Calabash, which I confirmed to be genetically distinct based upon single nucleotide polymorphisms, exhibited pronounced differences in traits related to resilience including the proportion of healthy tissue, chlorophyll, growth, and wound-healing. By most measures, Blackbird corals displayed superior indicators of resilience. Through a two-year reciprocal transplant study involving 120 corals, I identified substantial environmental plasticity in these traits, e.g., Blackbird corals transplanted to Calabash exhibited higher chlorophyll levels and more rapid wound healing than when grown in Blackbird, exceeding the native Calabash corals. RNA sequencing and assembly of site-specific transcriptomes revealed greater diversity of transcripts and genes from photosynthetic symbionts at Blackbird but greater diversity of bacterial associates at Calabash. Single nucleotide polymorphism (SNP) analyses using RNAseq data determined that corals from the two sites were separate putative populations. Principal components analysis of gene expression in natives and transplants revealed a clear distinction based on site of origin, but also a clear effect of environment. Thousands of differentially expressed genes distinguished the sites, including many genes implicated in heat stress, oxidative stress and UV-light stress. This genetic and phenotypic diversity of remnant staghorn populations on Turneffe represents a potential basis for future re-expansion of this important framework builder through natural or assisted shifts toward resilient populations.