The synaptophysin protein family consists of membrane proteins involved in vesicle-trafficking events, but the physiological function of several members is unknown. Notably, a member of the synaptophysins, pantophysin (Pan I), has been shown to be highly polymorphic among populations of Atlantic cod and we have also previously revealed that the stationary and migratory ecotypes of Atlantic cod display near fixed alleles at this locus (e.g. Fevolden et al 2012). The divergence between the ecotypes of Atlantic cod at this locus has been suggested to reflect local adaptation, e.g. to alternative regimes of light, during settling of the YOY and to vertical behavior of the adults. However, whether the adaptive divergence between the ecotypes at the Pan I locus is caused by “divergence hitch-hiking” of Pan I with other genes/synaptophysins (e.g. rhodopsin, known to be directly linked to perception of alternative wavelengths of light) or by directional selection against unfavorable Pan I alleles during settling and vertical migration also remains to be illuminated. Additionally, in order to understand the adaptive significance of these genes, we also need a better understanding of the evolution of the gene family and whether the protein phenotypes produced by the allelic variants differ in composition.
The study was led by Øivind Andersen, Nofima and together with several other collaborators we showed that the synaptophysin gene family may have expanded in vertebrates via four successive gene duplications. We did not identify any major structural differences in between allelic variants of the Pan I protein, suggesting that the adaptive importance of these allelic protein variants, by them self, may be of limited functional importance. The genomic distance between Pan I and rhodopsin was mapped to >2.5Mb, but we identified highly significant linkage disequilibrium between these two physins when genotyping the ecotypes of shallow and deep water juvenile settlers. This suggests that Pan I alleles may have been fixed via selection on other strongly linked genes. This was further supported by the similar predicted allelic variants of rhodopsin proteins.
Although we did not settle the long lasting puzzle of the Pan I divergence between ecotypes of Atlantic cod, this study propelled our understanding of adaptive loci (such as Pan I) forward and seeded important new directions for understanding of adaptive population divergence in marine species.