Adaptive and Neutral Polymorphisms of the Onne-DAB Gene from the Major Histocompatibility Complex (MHC) in Sockeye Salmon Populations on the Asian Range

Abstract

The variability of an MHC complex gene in sockeye salmon populations throughout the Asian range was studied to identify “footprints” indicative of pathogen-mediated selection and neutral demographic processes that have influenced these populations in both the recent and distant past. Genotype frequencies of a haplotype block consisting of two SNP loci (One_MHC2_109 and One_MHC2_190v2) in the Onne-DAB gene encoding the β-chain of the MHC class II molecule as well as allelic frequencies of 29 putative neutral SNPs have been traced in 27 sockeye salmon populations in the Asian Pacific coast. Differently directed clines of genetic diversity at the MHC2 loci were observed in sockeye salmon populations inhabiting the Sea of Okhotsk and Pacific coasts of the Kamchatka Peninsula. The formation of these clines can be attributed to a combination of historical processes associated with the colonization of the Asian range and the latitudinal gradient of abiotic and biotic factors influencing the variability of the Onne-DAB gene. In continental populations of sockeye salmon, balancing selection was not intense enough to conceal the impacts of demographic and historical processes associated with the fragmentation of the area in the late Pleistocene. In contrast, in island populations, balancing selection effectively maintained the diversity of the Onne-DAB gene despite a significant decrease in polymorphism observed in neutral regions of the genome.