Chromosomal painting and ZW sex chromosomes differentiation in Characidium (Characiformes, Crenuchidae)

A W-specific probe for efficient chromosome painting was isolated by microdissection and degenerate oligonucleotide primed-polymerase chain reaction (DOP-PCR) amplification of W chromosomes from C. gomesi. The W probe generated weak signals dispersed on the proto sex chromosomes in C. zebra, dispersed signals in both W and Z chromosomes in C. lauroi and, in C. gomesi populations revealed a proximal site on the long arms of the Z chromosome and the entire W chromosome. All populations showed small terminal W probe sites in some autosomes. The 18S rDNA revealed distinctive patterns for each analyzed species/population with regard to proto sex chromosome, sex chromosome pair, and autosome location.The results from dual-color fluorescence in situ hybridization (dual-color FISH) using W and 18S rDNA probes allowed us to infer the putative evolutionary pathways for the differentiation of sex chromosomes and NORs, from structural rearrangements in a sex proto-chromosome, followed by gene erosion and heterochromatin amplification, morphological differentiation of the sex chromosomal pair, and NOR transposition, giving rise to the distinctive patterns observed among species/populations of Characidium. Biogeographic isolation and differentiation of sex chromosomes seem to have played a major role in the speciation process in this group of fish.The Crenuchidae is widespread in freshwater systems of the South and Central Americas [1]. The Characidium is the most representative group in this family, comprising 50 valid species [1]. This is a particularly interesting Neotropical fish group for cytogenetic studies because it presents a diversified model of sex chromosomes [2]. In the Characidium, a diploid number of 50 chromosomes is observed in all studied species and a karyotype of 32 m + 18 sm is most commonly reported [2-10].Although the diploid number is conserved, the Characidium exhibit remarkable interspecific and interpopulation differences, such as (i) inter- and intrain