Chromosome banding in amphibia. XI. Constitutive heterochromatin, nucleolus organizers, 18S + 28S and 5S ribosomal RNA genes in Ascaphidae, Pipidae, Discoglossidae and Pelobatidae.

The karyotypes of 14 species of Anura from 9 genera of the suborders Amphicoela, Aglossa, Opisthocoela and Anomocoela were analysed with various banding techniques and conventional cytogenetic methods. The 18S + 28S and 5S ribosomal RNA genes were localized by means of in situ hybridization. No Q-, R- and G-banding patterns in the euchromatic segments of the metaphase chromosomes could be demonstrated in any of the species; this does not seem to be caused by a higher degree of spiralization of the amphibian chromosomes, but by the special DNA organization in these organisms. In most karyotypes, constitutive heterochromatin is present at centromeres, telomeres and nucleolus organizer regions (NORs), but rarely in interstitial positions. The heterochromatic regions are either quinacrine positive and mithramycin negative or vice versa. All species examined possess only one homologous pair of NORs: these display the brightest mithramycin fluorescence in the karyotypes. Many specimens exhibited unequal labelling of the two NORs both after silver and mithramycin staining as well as after in situ hybridization with 3H-18S + 28S rRNA. In four species, between one and six chromosome pairs with homologous 5S rRNA sites could be identified. The 5S rRNA genes and the 18S + 28S rRNA genes are closely linked in two species. In the male meiosis of the Amphicoela and Opisthocoela, there are intersitial, subterminal and terminal chiasmata in the bivalents, whereas only terminal chiasmata are observed in the bivalents of the Aglossa and Anomocoela. No heteromorphic sex-specific chromosomes could be demonstrated in any of the species. The differential staining techniques revealed that the chromosomal structure in these four suborders is largely the same as in the highly evolved anuran suborders Procoela and Diplasiocoela.