EXTENSIVE NUCLEOLUS ORGANIZER REGION POLYMORPHISM IN GYMNOTUS-CARAPO (GYMNOTOIDEI, GYMNOTIDAE)

An investigation was conducted on six populations of Gymnotus carapo f rom tho Upper Parana river system in Brazil. A wide variability of the NOR-bearing chromosomes was found involving paracentric inversions in ;be heterochromatic regions associated with NORs, resulting in 5 NOR c hromosome phenotypes. The distribution of the distinct NOR phenotypes varied both inter- and intrapopulationally. Results obtained using an exact test revealed that the six populations are in Hardy-Weinberg equ ilibrium. Although NOR size polymorphism is quite a common occurrence in fish karyotypes, this is the first description of a balanced polymo rphism involving 3 different NOR chromosomes in 5 combinations.