Cytogenetic analysis and description of the sexual chromosome determination system ZZ/ZW of species of the fish genus Serrapinnus (Characidae, Cheirodontinae).

Four populations of Serrapinnus notomelas and one population of Serrapinnus sp.1, both belonging to the subfamily Cheirodontinae, were analyzed by Giemsa and silver nitrate impregnation techniques. We found 2n = 52 chromosomes for all populations, with interspecific differences in the karyotype formula; S. notomelas showed 16 m + 22 sm + 10 st + 4a, with fundamental number (FN) = 100 for males, and 16 m + 23 sm + 10 st + 3a, with FN = 101 for females. Serrapinnus sp.1 had 8m + 16 sm + 4 st + 24 a, with FN = 80 for males, and 8m + 15 sm + 4 st + 25 a, with FN = 79 for females. The difference in FN for the two sexes is due to a pair of heteromorphic chromosomes in the females of both species, which characterizes a ZZ/ZW-type mechanism of chromosome sexual determination. Interspecies differences were also found in nucleolus organizer regions (NORs). A simple NOR system was detected in three of four S. notomelas populations, while Serrapinnus sp.1 had two chromosome pairs with NOR. Although S. notomelas and Serrapinnus sp.1 have the same diploid number, differences in the karyotype structure indicate that these are different species. Apparently there was pericentric inversion during the karyotype evolution of these species.