An uncommon condition for a sex chromosome system in Characidae fish. Distribution and differentiation of the ZZ/ZW system in Triportheus

Triportheus is a neotropical freshwater Characidae fish that has a well-dif ferentiated ZZ/ZW sex chromosome system. The W chromosome of this genus con tains a large amount of heterochromatin and is smaller than the Z chromosom e. This contrasts with other ZW fish systems where the W chromosome is larg er in size due to increased heterochromatin. All species of Triportheus tha t have been studied cytologically (about 50% of the known species for this genus, from some of the major South American hydrographic basins) share thi s sex chromosome system, indicating a probable synapomorphic condition not present in other genera of the large Characidae family. However, while the Z chromosome appears to be largely conserved, the W chromosome shows a diff erential evolution with morphological differentiations not only among speci es, but also among populations from the same hydrographic basin, and with s ome species presenting a greater homology between the W and the Z chromosom es than others.