Triportheus fish species present 2n = 52 chromosomes. The karyotypes show similar macrostructure and a ZZ/ZW sex chromosome system, which probably represents synapomorphy for the genus. A probe of the Z chromosome was obtained from T. nematurus through microdissection, followed by unspecific amplification via DOP-PCR. This probe was used for WCP (whole chromosome painting) through fluorescent in situ hybridization (FISH) in several other Triportheus species, to analyze the differentiation of the ZZ/ZW system. The homology between this probe and chromosomes of species from other genera, putatively related to Triportheus, was also examined to search for evidence about evolution of their sex chromosomes. Complete homology was found among the Z chromosomes of all Triportheus species, while only small positive signals were found on the W chromosomes. Hybridization signals were absent in species from other genera. The present results reinforce both the conservative nature of Z chromosomes and the hypothesis that the ZZ/ZW sex chromosome system is a synapomorphic feature of Triportheus. On the other hand, besides reduction of size, W chromosomes have undergone accentuated composition changes in relation to Z chromosomes, since only a small region, usually located in the short arm, kept homology with the Z chromosomes.
Copyright 2008 S. Karger AG, Basel.