A cytological analysis of the anuran species Xenopus tropicalis was performed. The diploid chromosome number is 2n = 20, whereas two other Xenopus species previously analyzed, laevis and muelleri, have 2n = 36. In X. tropicalis the morphology of the various chromosome groups differs from the other two species. The karyotype arranged according to size of chromosomes and centromere position falls into six morphological groups of chromosomes. Two distinct pairs bear secondary constrictions. Neither of these is similar to the constriction-bearing chromosomes of X. laevis (nucleolar organizer) or X. muelleri. The aspect of their association is also different. There is no evidence for sex-chromosome hetero-morphism. The karyotype analysis suggests that X. tropicalis is systematically quite separate from the two other species mentioned above.
A cytological analysis 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.
A cytogenetic analysis of two anuran species, Xenopus laevis laevis and Xenopus muelleri, was performed. The diploid number of chromosomes in both species was 2n = 36. The two karyotypes, arranged according to size of chromosome and centromere position, showed no pronounced differences. However, the chromosomes which had a secondary constriction were different. In X. laevis this constriction (nucleolar organizer) was found on one pair of acrocentric chromosomes only, the pair showing association in about 50% of the metaphases. In X. muelleri, on the other hand, two distinct pairs of submetacentric chromosomes contained secondary constrictions. Neither of these two chromosome pairs was morphologically similar to the constriction-bearing chromosomes of X. laevis. Association similar to that in X. laevis was found in one pair only, the one with a terminal constriction. In neither of the two species was there any evidence of sex-chromosome heteromorphism.
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