Independent sex chromosome evolution in lower vertebrates: a molecular cytogenetic overview in the Erythrinidae fish family.

The Erythrinidae fish family is an excellent model for analyzing the evolution of sex chromosomes. Different stages of sex chromosome differentiation from homomorphic to highly differentiated ones can be found among the species of this family. Here, whole chromosome painting, together with the cytogenetic mapping of repetitive DNAs, highlighted the evolutionary relationships of the sex chromosomes among different erythrinid species and genera. It was demonstrated that the sex chromosomes can follow distinct evolutionary pathways inside this family. Reciprocal hybridizations with whole sex chromosome probes revealed that different autosomal pairs have evolved as the sex pair, even among closely related species. In addition, distinct origins and different patterns of differentiation were found for the same type of sex chromosome system. These features expose the high plasticity of the sex chromosome evolution in lower vertebrates, in contrast to that occurring in higher ones. A possible role of this sex chromosome turnover in the speciation processes is also discussed.