Idiograms showing main characteristics already identified for the ZZ/ZW sex chromosome system in Characidium species. It was highlighted the position of the centromere, distribution of euchromatin and heterochromatin, W-specific probes, and rDNA sites. The a column detaches the species carrying 18S rDNA sites on the short and long arms of the Z and W chromosomes, respectively; the b column highlights the species bearing 18S rDNA sites on the long arms of both Z and W chromosomes; the c column shows the species that do not present 18S rDNA sequences on either Z or W chromosomes; the d column presents the species bearing Z and W chromosomes with unusual characteristics, including morphology, 18S and 5S rDNA sites, and W-specific probe distribution.