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Abstract
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.