Comparative cytogenetics in Apareiodon affinis (Pisces, Characiformes) andconsiderations regarding diversification of the group

Comparative cytogenetic studies in Apareiodon affinis (Pisces, Characiforme s) from two hydrographic Brazilian basins showed significant divergences re lated to the general karyotype structure, C-banding and nucleolar organizer region (NOR) bearing chromosomes. In the upper Parana basin population, di stinct diploid numbers were observed among sexes, the females showing 2n = 55 and the males 2n = 54 chromosomes, characterizing a multiple sex chromos ome system of the ZZ/ZW(1)W(2) type. A diploid number equal to 54 chromosom es was found for the Cuiaba river population, without a sex chromosome hete romorphism. However, the occurrence of acrocentric chromosomes represents a n unique character for this population. These karyotypic differences indica te that the analyzed populations must represent distinct Apareiodon species .