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 clas s II beta gene in four populations of Atlantic salmon Salmo salar by u sing RFLPs. The class II beta gene encodes the part of the Mhc class I I molecule that contains the antigen binding region and is therefore e ssential for disease resistance. There was extensive genetic variation in all four populations; the expected heterozygosity (H-si) varied be tween 0.50 and 0.81. Heterozygosity tended to be higher in broods surv iving a syndrome causing high mortalities (60-95%) in Swedish salmon h atcheries. Populations that had experienced more incidences of genetic bottlenecks (years when fewer than 60 adults had been used for breedi ng) and had a lower average effective population size (N-e), had a low er degree of heterozygosity. The four populations differed significant ly in allele frequencies as measured by F-ST = 0.13 and Nei's genetic distance (D = 0.09 - infinity). Pairwise F-ST values varied between 0. 01 and 0.23, all but one being highly significant, indicating a differ entiation between the populations in Mhc class II beta. This study sho ws that the four populations of Atlantic salmon have a high degree of polymorphism in the Mhc class II beta gene. However, there was great v ariation between different hatcheries, both in heterozygosity levels a nd allele frequencies.