CHROMOSOME FLOW BETWEEN CHROMOSOMALLY CHARACTERIZED TAXA OF A VOLANT MAMMAL, URODERMA BILOBATUM (CHIROPTERA: PHYLLOSTOMATIDAE)

Peters tent-making bat (Uroderma bilobatum) is the only known organism that is volant (with presumably relative high vagility and large deme size) and has a low reproductive rate, and yet possesses a tension zone (Key, 1974) between chromosomally characterized taxa. The zone of hybridization is itself unique in that it is the widest known (over 400 km) and because the cytotypes involved differ by three chromosomal rearrangements, each of which theoretically should result in moderate to severe meiotic malassortment (White, 1978a; Lande, 1979). An understanding of the nature of this contact zone will be a valuable resource in determining why tension zones exist and in documenting the role of chromosomal change in evolution. This report presents karyotypic data for 210 additional specimens (bringing the total sample size to 543) and discusses how these data relate to our understanding of chromosomal evolution. Some aspects of this zone have been described (Baker et al., 1972, 1975; Baker, 1979) based on 333 specimens. Populations of Uroderma bilobatum studied were distributed along the Pacific versant of Middle America from Mexico through Costa Rica. As U. bilobatum is found only in lower tropical forest, the central mountain ranges in Mexico, Guatemala, El Salvador, Honduras and the northern part of Nicaragua restrict the distribution of this species into two linear groups with no indication of interchange of individuals between the two (Davis, 1968) north of the Nicaraguan lowlands. On the Pacific versant, at the northwestern end of its distribution, U. bilobatum davisi has a karyotype with a diploid number (2n) of 44 and a fundamental number (FN) of 48. At the southeastern end of the study area, the entire Atlantic versant, and northwestern South America, the species has 2n = 38, FN = 44. Karyotypically intermediate individuals occur over a 400 km region, extending from central Guatemala to northwestern Nicaragua. Samples of U. bilobatum from this region include individuals with diploid numbers ranging from 38 to 44, with all intermediate diploid values being represented within the zone (Baker et al., 1972, 1975; Baker, 1979). The nature of the three rearrangements which distinguish the 2n = 38 and 2n = 44 parental types were described from Gand C-band studies (Baker, 1979) as (1) the fusion of a medium-sized acrocentric to a small biarmed element (or alternatively, the fissioning of a medium-sized biarmed element into a small biarmed element and an acrocentric); (2) a Robertsonian variation involving a medium-sized subtelocentric element and two acrocentric elements, one small and the other medium-sized; and (3) a telomere-centromere translocation of a small acrocentric to the largest acrocentric in the karyotype. Chromosomes involved are diagramatically represented in Figure 1. None of these rearrangements involves extensive C-band areas. The karyotype of U. bilobatum contains little C-band positive material with most heterochromatin being restricted to the centromeric regions (Baker, 1979). Gand C-band studies have been performed on 11 specimens (three with 2n = 44, two with 43, one with 42, one with 41, one with 40, one with 39, two with 38). The nature of the chromosomal rearrangements, as determined by the banding analyses, is such that the chro-