Highly conserved Z and molecularly diverged W chromosomes in the fish genus Triportheus (Characiformes, Triportheidae).

The main objectives of this study were to test: (1) whether the W-chromosome differentiation matches to species' evolutionary divergence (phylogenetic concordance) and (2) whether sex chromosomes share a common ancestor within a congeneric group. The monophyletic genus Triportheus (Characiformes, Triportheidae) was the model group for this study. All species in this genus so far analyzed have ZW sex chromosome system, where the Z is always the largest chromosome of the karyotype, whereas the W chromosome is highly variable ranging from almost homomorphic to highly heteromorphic. We applied conventional and molecular cytogenetic approaches including C-banding, ribosomal DNA mapping, comparative genomic hybridization (CGH) and cross-species whole chromosome painting (WCP) to test our questions. We developed Z- and W-chromosome paints from T. auritus for cross-species WCP and performed CGH in a representative species (T. signatus) to decipher level of homologies and rates of differentiation of W chromosomes. Our study revealed that the ZW sex chromosome system had a common origin, showing highly conserved Z chromosomes and remarkably divergent W chromosomes. Notably, the W chromosomes have evolved to different shapes and sequence contents within ~15-25 Myr of divergence time. Such differentiation highlights a dynamic process of W-chromosome evolution within congeneric species of Triportheus.

First cytogenetic studies of the genus Heptapterus (Actinopterygii, Siluriformes): karyotype differentiation and review of cytogenetic data on the Heptapteridae family.

The Neotropical fish Heptapterus mustelinus, collected in the Pindorama stream of the upper Paraná River basin (Brazil), was studied cytogenetically, verifying 54 chromosomes (26m + 18m + 4st + 6a). This diploid number has not been reported among the Heptapteridae that have been studied to date. Unlike most species of the family, there were multiple Ag-nucleolar organizer regions (NOR) and heterochromatin present in the centromeric region of most of the chromosomes of the complement, being Chromomycin A(3) (CMA(3)(+))/4',6-diamidino-2-phenylindole (DAPI(-)) in nine chromosomal pairs, besides the one that coincides with the Ag-NORs. The data presented in this work reveal a different path in the karyotypic evolution of H. mustelinus when compared to the others Heptapteridae genera.