Nucleotide polymorphism at the alcohol dehydrogenase locus of pocket gophers, genus Geomys

Using the strictly neutral model as a null hypothesis, we tested fordeviations from expected levels of nucleotide polymorphism at the alcoholdehydrogenase locus (Adh-1) within and among four species of pocket gophers(Geomys bursarius major, G. knoxjonesi, G. texensis llanensis, and G.attwateri). The complete protein-encoding region was examined, and 10unique alleles, representing both electromorphic and cryptic alleles, wereused to test hypotheses (e.g., the neutral model) concerning themaintenance of genetic variation. Nineteen variable sites were identifiedamong the 10 alleles examined, including 9 segregating sites occurring insynonymous positions and 10 that were nonsynonymous. Several statisticalmethods, including those that test for within-species variation as well asthose that examine variation within and among species, failed to reject thenull hypothesis that variation (both within and between species of Geomys)at the Adh locus is consistent with the neutral theory. However, there wassignificant heterogeneity in the ratio of polymorphism to divergence acrossthe gene, with polymorphisms clustered in the first half of the codingregion and fixed differences clustered in the second half of the gene. Twoalternative hypotheses are discussed as possible explanations for thisheterogeneity: an old balanced polymorphism in the first half of the geneor a recent selective sweep in the second half of the gene.