Presence–Absence Polymorphisms of Highly Expressed FP Sequences Contribute to Fluorescent Polymorphisms in Acropora digitifera

Despite many hypotheses regarding the roles of fluorescent proteins (FPs), their biological roles and the genetic basis of FP-mediated color polymorphisms in Acropora remain unclear. In this study, we determined the genetic mechanism underlying fluorescent polymorphisms in A. digitifera. Using a high-throughput sequencing approach, we found that FP gene sequences in FP multigene family exhibit presence–absence polymorphism among individuals. A few particular sequences in short-to-middle wavelength emission and middle-to-long wavelength emission clades were highly expressed in adults, and different sequences were highly expressed in larvae. These highly expressed sequences were absent in the genomes of individuals with low total FP gene expression. In adults, presence–absence differences of the highly expressed FP sequences were consistent with measurements of emission spectra of corals, suggesting that presence–absence polymorphisms of these FP sequences contributed to the fluorescent polymorphisms. The functions of recombinant FPs encoded by highly expressed sequences in adult and larval stages were different, suggesting that expression of FP sequences with different functions may depend on the life-stage of A. digitifera. Highly expressed FP sequences exhibited presence–absence polymorphisms in subpopulations of A. digitifera, suggesting that presence–absence status is maintained during the evolution of A. digitifera subpopulations. The difference in FPs between adults and larvae and the polymorphisms of highly expressed FP genes may provide key insight into the biological roles of FPs in corals.