Inferring Adaptive Nature of Major Histocompatibility Complex (MHC) Polymorphism from Single Nucleotide Substitutions in Asian Sockeye Salmon Populations: I. Different Forms of Selection Act in Sockeye Salmon Populations from the Ozernaya and Kamchatka Rivers

Abstract

Variability of the Onne-DAB gene, which encodes the β-chain of class II major histocompatibility complex molecule (MHCII), is studied using One_MHC2_109, One_MHC2_190v2, and One_MHC2_251v2 single nucleotide polymorphisms (SNP) in the two largest Asian-Pacific coast populations of sockeye salmon that reproduce in the Ozernaya River and Kamchatka River basins. Differences in the nature of inheritance and degree of polymorphism of One_MHC2_190v2 and One_MHC2_251v2 are observed in samples from both lake-river systems, whereas One_MHC2_109 was uninformative. Samples collected from the lake and river spawning grounds of Lake Kurilskoe are characterized by high estimates of intrapopulation genetic diversity, whereas no intersample differences are revealed in the frequencies of haplotypes and phenotypes of the joint MHC2 locus. This can be interpreted as the result of balancing selection in sequences encoding the MHCII peptide-binding region in this sockeye salmon population. This hypothesis is also confirmed by Ewens–Watterson neutrality tests. On the contrary, low genetic diversity estimates and significant heterogeneity of haplotype and phenotypic frequencies at the MHC2 locus are observed in the samples of early (spring) sockeye salmon from the Kamchatka River basin, apparently owing to the action of directional pathogen-induced selection in the Onne-DAB gene in some localities of the lake-river system.