Chromosome banding in Amphibia

Highly differentiated, heteromorphic ZZ♀/ZW♂ sex chromosomes were found in the karyotypes of the neotropical leptodactylid frogs <i>Eleutherodactylus euphronides</i> and <i>E. shrevei</i>. The W chromosomes are the largest heterochromatic, female-specific chromosomes so far discovered in the class Amphibia. The analyses of the banding patterns with AT- and GC base-pair specific fluorochromes show that the constitutive heterochromatin in the giant W chromosomes consists of various categories of repetitive DNA sequences. The W chromosomes of both species are similar in size, morphology and banding patterns, whereas their Z chromosomes exhibit conspicuous differences. In the cell nuclei of female animals, the W chromosomes form very prominent chromatin bodies (W chromatin). DNA flow cytometric measurements demonstrate clear differences in the DNA content of male and female erythrocytes caused by the giant W chromosome, and also shows that these <i>Eleutherodactylus</i> genomes are among the smallest of all amphibian genomes. The importance of the heteromorphic ZW sex chromosomes for the study of Z-linked genes, the similarities and differences of the two karyotypes, and the significance of the exceptionally small genomes are discussed.