Allelic variation of Mhc class II in atlantic salmon; a population genetic analysis

We have studied the degree of genetic variation at a variable Mhc class II β gene in four populations of Atlantic salmon Salmo salar by using RFLPs. The class II β gene encodes the part of the Mhc class II molecule that contains the antigen binding region and is therefore essential for disease resistance. There was extensive genetic variation in all four populations; the expected heterozygosity (H(Si)) varied between 0.50 and 0.81. Heterozygosity tended to be higher in broods surviving a syndrome causing high mortalities (60-95%) in Swedish salmon hatcheries. Populations that had experienced more incidences of genetic bottlenecks (years when fewer than 60 adults had been used for breeding) and had a lower average effective population size (N(e)), had a lower degree of heterozygosity. The four populations differed significantly in allele frequencies as measured by F(ST) = 0.13 and Nei's genetic distance (D = 0.09 - ∞). Pairwise F(ST) values varied between 0.01 and 0.23, all but one being highly significant, indicating a differentiation between the populations in Mhc class II β. This study shows that the four populations of Atlantic salmon have a high degree of polymorphism in the Mhc class II β gene. However, there was great variation between different hatcheries, both in heterozygosity levels and allele frequencies.