A comparison of the karyotype, constitutive heterochromatin, and nucleolar organizer regions of the new tetraploid species Xenopus epitropicalis Fischberg and Picard with those of Xenopus tropicalis Gray (Anura, Pipidae).

A cytological analsis of the recently discovered tetraploid species Xenopus epitropicalis was carried out, using, in addition to the classical orcein method, silver staining and alkaline Giemsa banding techniques. The chromosome number of X. epitropicalis was found to be 40. The chromosomes can be grouped in to sets of four similar chromosomes (quartets), resembling the karyotype of X. tropicalis (2n = 20). However, C-band patterns revealed heterogeneity within the quartets, dividing each of them into two pairs of homologous chromosomes ("duets"). Moreover, there are differences in the position and distribution of constitutive heterochromatin between the karyotypes of X. epitropicalis and X. tropicalis. The secondary constrictions stained by silver and representing the nucleolar organizer regions (NOR's) appear in both species on chromosome pair 5. During meiosis, usually only bivalents appear in X. epitropicalis. The question of whether this species is of autopolyploid or allopolyploid origin cannot be answered with certainty; however, it seems to have a common ancestor with X. tropicalis.