MHC DIVERSITY IN PACIFIC SALMON - POPULATION-STRUCTURE AND TRANSSPECIES ALLELISM

Geographic variation at an Mhc class I A1 exon was surveyed in 14 popu lations of coho salmon (Oncorhynchus kisutch) and 15 populations of ch inook salmon (O. tshawytscha) inhabiting rivers of British Columbia, C anada. A total of 2,504 fish were sampled using denaturing gradient ge l electrophoresis (DGGE), which distinguished 17 alleles in coho salmo n and 20 alleles in chinook salmon. Heterozygosity at the AI locus was moderately high for both coho (0.7) and chinook (0.6) salmon, but seq uence divergence was low, with mean inter-and intraspecific nucleotide similarities of approximately 0.96. In a maximum parsimony tree, all of the observed alleles clustered into two trans-specific lineages. Wi thin each lineage, coho and chinook alleles tended to fall into specie s-specific subclusters. Much of the intraspecific allelic variation wi thin each lineage could be accounted for by nonsynonymous point mutati on, indicative of balancing selection. The F-ST values for both coho ( 0.11) and chinook (0.13) salmon indicated that much of the allelic div ersify was partitioned among populations. Neighbor-joining analyses of Al allelic frequencies among coho and chinook salmon populations show ed strong patterns of geographic differentiation similar to those base d on neutral genetic markers such as microsatellite loci. Both natural selection and the salmonid zoogeographic history of frequent populati on bottlenecks have shaped the patterns of diversity observed al this and other Mhc exons in Pacific salmonids.