Studies of a Contact Zone between Chromosomally Characterized Populations of Geomys bursarius

Abstract

A 5-square-km zone of hybridization occurs between two chromosomal races of Geomys bursarius, which are designated by Arabic letters B (2N = 70; FN = 70) and C (2N = 70, 71, or 72; FN = 72), in eastern New Mexico. Within and near this contact zone, race B is distributed to the southwest in aeolian sands, race C occupies more indurate soils to the north and east, and hybrids occur in an ecologically intermediate zone. Eighteen percent of the animals captured were apparent F1 hybrids (some of these may represent backcross or F2 individuals). A combination of behavioral interactions and soil-related ecological factors appear to limit hybridization to a narrow zone. The two cytotypes are separable on the basis of morphological characters, with chromosomal race B being singificantly smaller than race C. Apparent F1 hybrids are not morphologically intermediate, but more closely resemble race C. This situation may result from genetic interactions, hybrid reproduction, or a combination of these two factors. Chromosomal data are not capable of distinguishing backcross or F2 offspring. Chromosomal differences between the two cytotypes are less than those found between Thomomys bottae and T. umbrinus (Patton, 1973), yet the zones of hybridization are similar.