Aspects of a Contact Zone between Two Chromosomal Races of Peromyscus leucopus (Rodentia: Cricetidae)

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Abstract
G- and C-banded karyotypes of 269 specimens of Peromyscus leucopus were analyzed from along an east-west transect in central Oklahoma. A zone of contact between two previously described chromosomal races is documented, and is associated with an ecotone between the Great Plains Grassland and Eastern Deciduous Forest biomes. Parental cytotypes are allopatric, being separated by populations of hybrid individuals. The zone is further characterized by concordance of the three chromosomal clines and by asymmetrical distribution of marker chromosomes. Hybrid genotypes from the center of the zone occur in Hardy-Weinberg equilibrium and no linkage disequilibrium was detected. Field reproductive data demonstrate that hybrid mice are reproductively active, and that if negative heterosis is present, it does not appear to be severe. The data indicate that the zone is the result of secondary contact of allopatrically diverged populations, and is stable. Potential factors responsible for the structure and maintenance of the zone are discussed.