Karyological analyses of Pseudhymenochirus merlini and Hymenochirusboettgeri provide new insights into the chromosome evolution in theanuran family Pipidae

tKaryological studies continue to clarify the evolutionary history of many taxa. We report here on thefirst chromosome survey of the frog Pseudhymenochirus merlini (Pipidae) in comparison with the relatedspecies 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. merlinishowed a karyotype of 2n = 20 chromosomes, differing from the set of A chromosomes of H. boettgeri bythree inversions.Other differences are in the location of the nucleolus organizer regions (NORs), on the 10th chromosomepair in P. merlini instead of the 4th pair in H. boettgeri. Heterochromatin is less abundant in P. merlini, andlocalized in centromeric and telomeric regions of all chromosomes. The patterns in H. boettgeri resemblethose of another pipid species, Silurana tropicalis, as to chromosome number (2n = 20), morphology, andG-banding patterns. We propose that these characteristics are symplesiomorphic in Pipidae, allowing usto present a scenario for the chromosome evolution in this family. Our results identify a third mecha-nism of increase in chromosome numbers among pipid frog species, the addition of B chromosomes inHymenochirus, besides the known processes of polyploidization in Xenopus/Silurana, and centric fissionin Pipa.