![]() ![]() ![]() New alleles may arise through point mutations, and selective pressures then act on the substitution rate of such point mutations causing higher frequencies of nonsynonymous than synonymous replacements at those amino acid residues responsible for peptide binding (the peptide-binding residues ) ( Garrigan and Hedrick 2001). Several molecular mechanisms are thought to be involved in producing high levels of polymorphism at MHC loci. 2001) and neutral forces such as genetic drift and gene flow may also have a role ( Landry and Bernatchez 2001). However, sexual selection ( Jordan and Bruford 1998 Landry et al. Pathogen-driven balancing selection, either through overdominance or negative frequency–dependent selection or both, is thought to be the main evolutionary force that promotes and maintains the extensive variability in these genes ( Edwards and Hedrick 1998 Jeffery and Bangham 2000 Hedrick 2002 Penn, Damjanovich, and Potts 2002). Recognition of the MHC–non-self-peptide complex by a specific T cell receptor initiates either a cellular or humoral immune response depending on the classes of MHC molecule and T cell involved. MHC genes encode cell-surface molecules that bind small self-peptides or peptides from non–self-proteins derived from infectious pathogens within the cell and present them on the cell surface to T cells. The genes of the major histocompatibility complex (MHC) are some of the most variable protein-coding loci detected in vertebrates and among the most suitable candidate genes currently available to study processes of adaptive evolution due to their well-established role within the immune system ( Hedrick 1994). MHC, recombination, positive selection, Atlantic salmon, Salmo salar Introduction Moreover, selection acting on peptide-binding residues seems to favor new recombinant alleles and is likely to be responsible for the rapid divergence between populations. Although there is a strong signal of point mutation that predates population divergence, recent recombination, acting in similar, but not identical, ways in both populations appears to be a significant force in creating new alleles. Contrary to our prediction, and in contrast to the situation for other genetic markers, the two populations have almost nonoverlapping sets of major histocompatibility class I alleles. We also examined the evidence for adaptive molecular divergence at this locus by analyzing the rate of amino acid replacement in relation to a neutral expectation. We compared major histocompatibility class I allelic diversity in two currently reproductively isolated Atlantic salmon ( Salmo salar) populations (Irish and Norwegian) with a common postglacial origin in order to test for among-population differences in allelic composition and patterns of recombination and point mutation. ![]()
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