Evolutionary aspects of the ZZ/ZW sex chromosome system in the Characidae fish, genus Triportheus. A monophyletic state and NOR location on the W chromosome.

Four species/populations of Triportheus, T. guentheri, T. cf. elongatus and T. paranense from different Brazilian hydrographic basins, were studied cytogenetically. All the species showed a similar karyotypic macrostructure, with a diploid chromosome number 2n = 52 and a ZZ/ZW sex chromosome system. Besides silver- and fluorochrome-staining, the chromosome mapping of 18S rDNA was also investigated using a biotinylated probe. In spite of some variation in the number of the NORs, a major chromosome site was always present on the short arm of an autosomal pair. In addition, a characteristic rDNA site was also observed on the telomeric region of the W chromosome in the four species/populations. In Triportheus differential reduction in size and heterochromatin accumulation appear to be the main processes associated with the evolution of the sex W chromosome. The location of rRNA genes on this chromosome may correspond to a plesiomorphic condition in the genus and, if so, predates to the sex chromosome system differentiation, with a possible influence in the initial steps of this process.