Nucleotide polymorphism and natural selection at the pantophysin (Pan I) locus in the Atlantic cod, Gadus morhua (L.)

Molecular studies of nucleotide sequence variation have rarely attempted to test hypotheses related to geographically varying patterns of natural sele ction. The present study tested the role of spatially varying selection ill producing significant linkage disequilibrium and large differences in thr frequencies of two common alleles at the pantophysin (Pan I) locus among fi ve populations of the Atlantic cod, Gadus morhua. Nucleotide sequences of 1 24 Pan I alleles showed strung evidence for an unusual mix of balancing and directional selection but no evidence of stable geographically varying sel ection. The alleles were highly divergent at both the nucleotide level (dif fering on average by 19 mutations) and at amino acid level (each having exp erienced thr ec amino acid substitutions since diverging fr-om a common anc estral allele). All six amino acid substitutions occurred in a 56-residue i ntravesicular loop (IV1 domain) of the vesicle protein and each involved a radical change. An analysis of molecular variation revealed significant het erogencity in the frequencies Of recently derived mutations segregating wit hin both allelic classes, suggesting that two, selective sweeps may be pres ently occurring among populations. The dynamic nature of the Pan I polymorp hism in G. morhua and clear departure from equilibrium conditions invalidat e a simple model of spatially varying selection.