The nucleolus organizers of Plethodon and Aneides located by in situ nucleic acid hybridization with Xenopus 3H-ribosomal RNA.

The main clusters of ribosomal genes, or nucleolus organizers, have been located by in situ nucleic acid hybridization of Xenopus laevis 3H labelled ribosomal RNA to mitotic chromosomes in squash preparations of intestinal epithelium from 7 species of Plethodon and 3 species of Aneides. The species used were chosen on account of having well known karyotypes and genome sizes. The Plethodon species covered a range of genome size of 20--69.4 pg. The locations of those nucleolus organizers that could be detected by autoradiography after in situ hybridization varied from species to species, and in Aneides there were differences between two populations of the same species. On the other hand, some distantly related species of Plethodon, with widely different genome sizes, had nucleolus organizers at corresponding positions. The results are discussed in relation to ideas on karyotype stability, homosequentiality and chromosome repatterning.