Cytogenetic studies on Apareiodon affinis (Pisces, Characiformes) from Parana river basin: sex chromosomes and polymorphism

Comparative cytogenetic studies were carried out on Apareiodon affinis from an important hydrographic system at South America, the Parana river basin. Two distant regions were chosen, which were separated by Guaira Falls (for merly Sete Quedas); the region in the upper part of the hydrographic basin is called Upper Parana (Brazil), whereas and the other in the lower part is called Lower Parana (Argentina). Individuals from Upper Parana have diploi d numbers of 2n = 54 (NF = 108) for males and 2n = 55 (NF = 110) for female s, showing female heterogamety with a ZZ/ZW(1)W(2) multiple sex chromosomes system that is endemic for the region. In different localities at Lower Pa rana, the specimens presented diploid number of 2n = 54 for both sexes, wit hout any sex chromosomes heteromorphism. However, they have an accentuated polymorphism characterized by variation in number of acrocentric chromosome s, constituting something new for family Parodontidae. The most likely hypo thesis to explain the origin of such polymorphism is based on successive pe ricentric inversions giving rise to acrocentric chromosomes. Thus, it was p ossible to detect 10 cytotypes along the Lower Parana basin. Such chromosom al variations possibly are the consequence of an adaptative process. Our da ta probably indicate the occurrence of distinct species in each region that share the same denomination.