POPULATION CYTOGENETICS OF THE COMMON CHIMPANZEE PAN TROGLODYTES

Abstract

First, the literature on hominoid cytogenetics is reviewed and evaluated. It is suggested that there are significant deficiencies in the ways in which chromosomal data have been used with regard to primate evolution. The most robust chromosomal data support an orthodox phylogeny of the Hominoidea. Heterodox phylogenies which have been suggested on the basis of chromosomal data are not well supported. The present investigation considered the problem of intra-specific variation in karyotypes of the common chimpanzee. Blood cultures were cultivated on 25 chimpanzees, constituting the largest sample of chimpanzees in a single cytogenetic study. These were studied by G-banding, C-banding and nucleolar organizer (Ag-NOR) staining. No inversions, translocations, fissions or fusions were detected in this sample. However, several variations of the constitutive heterochromatin and nucleolar organizers were noted. One individual had a chromosome 22 which lacked the heterochromatic short arm and satellite entirely. The most common variants were those in which the amount of telomeric heterochromatin differed significantly between the two homologous chromosomes. One such variant for chromosome 19 was found in 8 individuals. Two of the common chimpanzees possessed a chromosome 23 with a large heterochromatic short arm, although this feature has been reported only for the pygmy chimpanzee. To compare the observable range of variation in the common chimpanzee with its sister group, fibroblast cultures were obtained on three pygmy chimpanzees. Some of the cytotaxonomic distinctions between the two chimpanzee species are called into question. Three main conclusions are drawn from this work. First, the most common kinds of variations are nucleolar organizer and C-band heteromorphisms, as with the chimpanzee's close relative Homo sapiens. Second, inversions and translocations, which seem to be very common among gibbons but not among macaques/baboons, are not common in chimpanzees. This is in accordance with the hypothesis that such structural chromosomal diversity is a property of the social structure of the species rather than a property of the clade to which the species belongs. Third, there is some overlap between Pan paniscus and Pan troglodytes for characters which have been thought to be cytotaxonomically distinctive of each species.