NUCLEOTIDE POLYMORPHISM AT THE ALCOHOL-DEHYDROGENASE LOCUS OF POCKET GOPHERS, GENUS GEOMYS

Using the strictly neutral model as a null hypothesis, we tested for d eviations from expected levels of nucleotide polymorphism at the alcoh ol dehydrogenase locus (Adh-1) within and among four species of pocket gophers (Geomys bursarius major, G. knoxjonesi, G. texensis ilanensis , and G. attwateri). The complete protein-encoding region was examined , and 10 unique alleles, representing both electromorphic and cryptic alleles, were used to test hypotheses (e.g., the neutral model) concer ning the maintenance of genetic variation. Nineteen variable sites wer e identified among the 10 alleles examined, including 9 segregating si tes occurring in synonymous positions and 10 that were nonsynonymous. Several statistical methods, including those that test for within-spec ies variation as well as those that examine variation within and among species, failed to reject the null hypothesis that variation (both wi thin and between species of Geomys) at the Adh locus is consistent wit h the neutral theory. However, there was significant heterogeneity in the ratio of polymorphism to divergence across the gene, with polymorp hisms clustered in the first half of the coding region and fixed diffe rences clustered in the second half of the gene. Two alternative hypot heses are discussed as possible explanations for this heterogeneity: a n old balanced polymorphism in the first half of the gene or a recent selective sweep in the second half of the gene.