Karyological analyses of Pseudhymenochirus merlini and Hymenochirus boettgeri provide new insights into the chromosome evolution in the anuran family Pipidae

Karyological studies continue to clarify the evolutionary history of many taxa. We report here on the first chromosome survey of the frog Pseudhymenochirus merlini (Pipidae) in comparison with the related species Hymenochirus boettgeri.In addition to the 2n = 20 regular (A) chromosomes (eight metacentric and two subtelocentric pairs), H. boettgeri has an accessory B chromosome in about half of the scored karyotypes (2n = 21). P. merlini showed a karyotype of 2n = 20 chromosomes, differing from the set of A chromosomes of H. boettgeri by three inversions.Other differences are in the location of the nucleolus organizer regions (NORs), on the 10th chromosome pair in P. merlini instead of the 4th pair in H. boettgeri. Heterochromatin is less abundant in P. merlini, and localized in centromeric and telomeric regions of all chromosomes. The patterns in H. boettgeri resemble those of another pipid species, Silurana tropicalis, as to chromosome number (2n = 20), morphology, and G-banding patterns. We propose that these characteristics are symplesiomorphic in Pipidae, allowing us to present a scenario for the chromosome evolution in this family. Our results identify a third mechanism of increase in chromosome numbers among pipid frog species, the addition of B chromosomes in Hymenochirus, besides the known processes of polyploidization in Xenopus/Silurana, and centric fission in Pipa.