D. Garrigan et Pn. Hedrick, Class I MHC polymorphism and evolution in endangered California Chinook and other Pacific salmon, IMMUNOGENET, 53(6), 2001, pp. 483-489
Twelve MHC class 1 exon 2 sequences were uncovered in a sample from the end
angered Sacramento River winter-ran Chinook salmon in the central valley of
California. Phylogenetic analysis of the 12 sequences indicates that the a
lleles descend from two of six major allelic lineages found among four Paci
fic salmon species. Nine of the 12 alleles belong to an allelic lineage tha
t began diversifying 8 million years ago, just prior to the estimated time
of Chinook speciation. The most recent common ancestor of all 12 winter-run
alleles is estimated to be 15 million years ago, approximately 5 million y
ears before the radiation of the Pacific salmon species. The average nonsyn
onymous distance among the peptide binding-region codons, of exon 2 for the
12 alleles is significantly higher than the average synonymous distance in
these codons. We estimate the symmetrical overdominant selection coefficie
nt against homozygotes for this exon to be 0.038. Thus, strong positive and
balancing selection has maintained functional diversity in the peptide-bin
ding region of the exon over millions of years and this variation has not y
et been substantially eliminated by increased genetic drift due to the rece
nt dramatic decline in abundance of this Chinook salmon population.