Diversification of a ZZ/ZW sex chromosome system in Characidium fish (Crenuchidae, Characiformes).

Karyotype and other chromosomal markers of Characidium cf. gomesi were analyzed using conventional (Giemsa-staining, Ag-NOR and C-banding) and molecular (Fluorescent in situ hybridization (FISH) with 18S and 5S rDNA biotinylated probes) techniques. Both sexes had invariably diploid chromosome number 2n = 50 while karyotypes of males and females differed. That of male consisted of 32 metacentric + 18 submetacentric chromosomes and that of female consisted 31 metacentric + 18 submetacentric + 1 subtelocentric chromosomes. The Z chromosome was medium-sized metacentric, while W was highly heterochromatinized subtelocentric element. NORs as revealed by Ag-staining were situated at 2-7 telomeric regions while FISH with 18S probes showed consistently 10 signals at telomeric regions. FISH with 5S rDNA probe showed constantly signals at one metacentric pair. Distribution of centromeric heterochromatin was mostly in all chromosome pairs, besides some telomeric sites. The common origin of the sex chromosome system of ZZ/ZW type in the karyotypes of other representatives of the genus analyzed so far might be hypothesized based on biogeography and partial phylogeny of the group.