DIRECTION OF INTROGRESSIVE HYBRIDIZATION BETWEEN SPECIES OF NORTH-AMERICAN DEER (ODOCOILEUS) AS INFERRED FROM MITOCHONDRIAL-CYTOCHROME-B SEQUENCES

We used the polymerase chain reaction to amplify a 359-base-pair portion of the mitochondrial-cytochrome-b gene from 157 deer in the genus Odocoileus. The deer examined included Texas white-tailed deer (Odocoileus virginianus texanus) and Desert mule deer (O. hemionus crooki) from an area of sympatry in west Texas, allopatric populations of both species on either side of the area, and black-tailed deer from California (O. hemionus columbianus) and Alaska (O. hemionus sitkensis). One moose (Alces alces) was included as an outgroup. Direct determination of the DNA sequences revealed 53 variable nucleotide positions that define 11 distinct genotypes. All of the mule deer and a majority of the white-tailed deer within the area of sympatry share a common sequence genotype. Cladistic analysis identifies clades corresponding to populations of O. hemionus and O. virginianus west and east, respectively, of the area of sympatry. The shared sequence is a member of the western, O. hemionus clade, which implies that genetic introgression of mitochondrial DNA has been from mule deer into white-tailed deer. Genotypes found in black-tailed deer from Alaska and one population in northern California form a sister group that is quite divergent (7.5%-sequence difference) from other black-tailed and mule deer. Interspecific sequence divergences are in several cases smaller than intraspecific divergences. Mitochondrial-DNA sequences in O. hemionus appear to be paraphyletic. Possible explanations include sorting of genetic lineages between species, extensive introgression of mitochondrial DNA between subspecies of O. hemionus or between species, or a recent derivation of 0. virginianus from O. hemionus.