Chromosomal rearrangements as the source of variation in the number of chromosomes in Pseudis (Amphibia, Anura).

A cytogenetic study of Pseudis specimens from three localities in Rio Grande do Sul State, the southernmost Brazilian, was performed to identify karyotypic characteristics that could account for differences in vocalization pattern and body size. Individuals from around Tainhas were compared to those of São Jerĵnimo and Eldorado do Sul. Specimens from these latter two localities were identified as Pseudis minuta, while those from the former were classified as Pseudis sp. (aff minuta). The populations from São Jerĵnimo and Eldorado do Sul had 2n = 24 chromosomes, classified as metacentric. submetacentric and subtelocentric. The population from Tainhas had 2n = 28 chromosomes, with four pairs of telocentric chromosomes. Modelling of these 28 chromosomes and testing for fusion in the centromeric/telomeric regions yielded a karyotype of 2n = 24 chromosomes, similar to that of the other populations. The similarity was reinforced by the location of the NORs and heterochromatin. The Tainhas population showed an increase in heterochromatin, as seen by the presence of additional C-bands, especially in the telocentric chromosomes. These data suggest that the two karyotypes described in this work had a common ancestry. There is evidence that the differentiation of these karyotypes may have occurred by chromosome fission and heterochromatin addition. Based on the present karyotype (2n = 28) and on morphological and vocalization studies by other researchers, we conclude that the Tainhas population may represent a new species.