Transcriptomic plasticity of mesophotic corals among natural populations and transplants ofMontastraea cavernosain the Gulf of Mexico and Belize

While physiological responses to low-light environments have been studied among corals on mesophotic coral ecosystems worldwide (MCEs; 30-150 m), the mechanisms behind acclimatization and adaptation to depth are not well understood for most coral species. Transcriptomic approaches based on RNA sequencing are useful tools for quantifying gene expression plasticity, particularly in slow-growing species such as scleractinian corals, and for identifying potential functional differences among conspecifics. A tag-based RNA-Seq (Tag-Seq) pipeline was applied to quantify transcriptional variation in natural populations of the scleractinian coralMontastraea cavernosafrom mesophotic and shallower environments across five sites in Belize and the Gulf of Mexico: Carrie Bow Cay, West and East Flower Garden Banks, Pulley Ridge, and Dry Tortugas. Regional site location was a stronger driver of gene expression patterns than depth. However, mesophotic corals among all sites shared similar regulation of metabolic and cell growth functional pathways that may represent common physiological responses to environmental conditions at depth. Additionally, in a transplant experiment at West and East Flower Garden Banks, colonies transplanted from mesophotic to shallower habitats diverged from the control mesophotic group over time, indicating depth-regulated plasticity of gene expression. When the shallower depth zone experienced a bleaching event, bleaching severity did not differ significantly between transplants and shallow controls, but gene expression patterns indicated variable regulation of stress responses among depth treatments. Coupled observational and experimental studies of gene expression among mesophotic and shallowerM. cavernosaprovide insights into the ability of this depth-generalist coral species to persist under varying environmental conditions.