Chromosome banding in Amphibia

The karyotype of the common bell-ring frog, <i>Buergeria buergeri, </i>is characterized by primitive ZW/ZZ (female/male) sex chromosomes. The only difference between the Z and W is the presence of a nucleolus organizer region (NOR) and its associated constitutive heterochromatin in the Z. This causes a sex-specific difference in the number of ribosomal RNA genes, with a male:female ratio of about 2:1. In the cell nuclei of the various tissues analyzed the NORs on both Z chromosomes are transcriptionally active. During meiosis in females, the ZW chromosomes form a characteristic lampbrush sex bivalent with a conspicuous absence of chiasmata along half of their long arms. No further morphological differences are present in these primitive sex chromosomes. The results indicate that there is no Z-chromosome inactivation mechanism operating in this amphibian. Experimental sex reversal of genetic (ZW) females to functional males was achieved by estrogen treatment of tadpoles. Mating of sex-reversed females with normal females yields the expected 25 % lethal nucleoliless WW embryos, which die at the tail-bud stage. The sex of the remaining viable embryos with one or two nucleoli determined after metamorphosis was in a female:male ratio of 1.57:1. These genetic experiments confirm that a primitive ZW/ZZ (female/male) type of chromosomal sex determination operates in this species.