Genic Differentiation and Evolution in the Ground Squirrel Subgenus Ictidomys (Genus Spermophilus)

Abstract

The genetic structure of 34 natural populations of three species belonging to the ground squirrel subgenus Ictidomys (genus Spermophilus) was analyzed using starch-gel electrophoresis. Of the 28 loci examined, 14 were considered monomorphic, with the same allele fixed in all species. The mean proportion of polymorphic loci per population for the three species was 0.232, whereas the mean proportion of loci heterozygous per individual in mainland populations was 9.3 percent. Heterozygosity in a population of S. spilosoma from Padre Island (0.9 percent) was considerably lower than that in mainland populations of this species. The mean coefficients of genetic similarity (S) and genic identity (I) between the three species were 0.826 and 0.869, respectively. Spermophilus tridecemlineatus and mexicanas showed the highest genic identity, 0.965, and shared predominant alleles at 86 percent of their loci. Hybridization between these two species and other lines of evidence suggest that S. tridecemlineatus and S. mexicanus have evolved only to the level of semispecies. Divergence time estimated from biochemical data indicates the species diverged within the last 155,000 years.