ABSTRACT
The development of cytogenetics, in particular of modern molecular techniques, has allowed a deeper knowledge on genomes structure and organization, providing a useful tool for understanding the mechanisms of evolution. Before presenting the state-of-the-art of the cytogenetics of the case of evolution-in-action by hybridization and polyploidy, it is important to briefly mention the problems related to nomenclature faced by taxonomists when dealing with non-Mendelian organisms. The possibility of finding either chromosome rearrangements or genome mutations like polyploidy in distinct animal groups, namely in those more plastic with numerical variations, will also certainly increase. Although major ribosomal genes have been insufficiently analyzed in Eurasian cyprinids, one single small sm chromosome pair bearing the major rDNA sites in its short arms is the most common situation for the studied Leuciscinae species and has been considered as a plesiomorphic condition.
