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 porti on of the mitochondrial-cytochrome-b gene from 157 deer in the genus O docoileus. The deer examined included Texas white-tailed deer (Odocoil eus virginianus texanus) and Desert mule deer (O. hemionus crooki) fro m an area of sympatry in west Texas, allopatric populations of both sp ecies on either side of the area, and black-tailed deer from Californi a (O. hemionus columbianus) and Alaska (O. hemionus sitkensis). One mo ose (Alces alces) was included as an outgroup. Direct determination of the DNA sequences revealed 53 variable nucleotide positions that defi ne 11 distinct genotypes. All of the mule deer and a majority of the w hite-tailed deer within the area of sympatry share a common sequence g enotype. Cladistic analysis identifies clades corresponding to populat ions 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 mitoch ondrial 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 seque nce divergences are in several cases smaller than intraspecific diverg ences. Mitochondrial-DNA sequences in O. hemionus appear to be paraphy letic. Possible explanations include sorting of genetic lineages betwe en species, extensive introgression of mitochondrial DNA between subsp ecies of O. hemionus or between species, or a recent derivation of 0. virginianus from O. hemionus.