A Contact Zone Between Karyotypically Characterized Taxa of Uroderma Bilobatum (Mammalia: Chiroptera)

Abstract

Baker, R. J., W. J. Bleier, and W. R. Atchley (Department of Biological Sciences and The Museum, Departments of Biological Sciences and Faculty of Statistics, Texas Tech University,,Lubbock, Texas 79409.) A contact zone between karyotypically characterized taxa of Uroderma bilobatum (Mammalia: Chiroptera) Syst. Zool. 24:133-142.-A total of 191 specimens of Peters' tent-making bat, Uroderma bilobatum, were collected from the zone where two chromosomal races, representing two subspecies, meet. Eighty-eight specimens had 2n = 38, four had 2n = 39, one had 2n = 40, one had 2n = 41, one had 2n = 42, 14 had 2n = 43, and 82 had 2n = 44. This chromosomal variation is best explained as resulting from between two cytotypes with the 2n = 38 and the 2n = 44 being the parental (pure) types, the 2n = 41 individual being of the F1 type and the 2n = 43, 42, 40, and 39 individuals representing backcross or F2 products. The two parental cytotypes were not found to be sympatric at any individual collecting station. The zone of is approximately 200 kilometers long on the Pacific versant of Honduras, eastern El Salvador and northwestern Nicaragua. Somne chromosomally intermediate individuals were reproductively active although the frequency of reproductive activity was not so great as in individuals with parental cytotypes. Measurements of the cranial and wing morphology of chromosomally intermediate individuals indicated that in general there was a correspondence between chromosome number and phenetic similarity to one or the other of the two parental stocks. Chromosomal and cranial and wing morphological data suggest that there is considerable gene flow between the two cytotypes and that chromosomal divergence has occurred in the absence of speciation. The magnitude of chromosomal differentiation between the two subspecies serves as a caveat to those cases where specific recognition is based on chromosomal distinctness of allopatric samples. [Karyotypes; Chiroptera; Uroderma.] Although there have been numerous studies on the divergence of subspecific populations of mammals, such studies have been concerned primarily with the description of size (cranial and external measurements) and pelage color variations that serve as a basis for trinominal application (for a critique see Lidicker, 1962). Such studies have documented that many mammalian species can be subdivided into morphologically distinct units, but they have failed to reveal the nature of the interactions at zones of contact between such units. However, when two subspecific units come into contact, it is the interaction between the units that determines their evolutionary future. Perhaps the greatest factor that inhibits studies of contact zones is that the degree of morphological divergence is insufficient to identify all members of the respective populations, plus F1 and backcross individuals. In the tent-making bat, Uroderma bilobatum, the subspecies U. b. davisi and U. b. convexum are easily identified by chromosomal features (Baker et al., 1972). The degree of chromosomal divergence is also adequate to identify presumed F1 and backcross individuals. The present study was conducted to determine the nature of the contact zone between the subspecies davisi and convexum in hopes that such a study would help elucidate the nature and significance of such zones in mammals. In the following discussion the term hybridization refers to successful reproduction between two unlike parental stocks (in this case chromosomally characterized) and is not meant to infer specific status (or the lack of it) to the taxa involved. Cytotype