A New Report of the ZZ/ZW Sex Chromosome System in the Genus Triportheus (Pisces, Triporthinae).

In the present study six specimens of Triportheus paranaensis from the Paraná river, in Corrientes (Argentina) were analyzed. All the specimens showed 2n=52 chromosomes with a karyotype formula composed of 24 M +22 SM +4 ST +ZZ (M/M) in males or ZW (M/SM) in females. The constitutive heterochromatin pattern was similar to those of other Triportheus species previously analyzed. Two chromosome pairs with NORs were detected. One of these is located in the telomeric region of a metacentric chromosome pair. This position was unreported for any other Triportheus species, constituting a chromosome marker for the species. The comparative analysis between our results and the available in the literature suggests that the karyotype diversification of the group would have mainly produced by chromosome pericentric inversions and translocations. These structural changes in addition to duplications and translocations of the nucleolar organizer regions, could have modified the karyotype formula preserving the diploid chromosome number.