Localization of Repeated DNA Sequences in Xenopus Chromosomes

The karyotypes of two species of toads, Xenopus laevis and Xenopus mulleri, are almost indistinguishable (Tymowska and Kobel, 1972) and matings between individuals of the two species produce fertile hybrid animals. Yet, in spite of this evidence that X. laevis and X. mulleri are closely related, certain groups of repeated DNA sequences show marked evolutionary divergence between the two species (Brown and Sugimoto, this volume). The spacer DNA interspersed between the repeated genes coding for 18 and 28 S ribosomal RNA and also the spacer DNA interspersed between the genes coding for 5 S RNA are almost entirely species-specific. X. mulleri has a family of repeated AT-rich DNA sequences which is not found in the genome of X. laevis (Stern, 1972). An understanding of the spatial relationships between members of a family of repeated sequences might help to explain the mechanism by which the sequences evolve together within a species.