Sex-ratio distortion associated with the presence of a B chromosome in Astyanax scabripinnis (Teleostei, Characidae)

Cytogenetic studies were conducted on 154 specimens of Astyanax scabripinnis collected at three localities in the Campos do Jordao region (State of Sao Paulo, Brazil). The C-banding pattern suggested that the metacentric B chromosome found in most of the specimens is an isochromosome derived from chromosome 24, the only chromosome in the standard complement that carries interstitial C-bands similar to those present in each arm of the B chromosome. The sex ratio was biased toward females in the Corrego das Pedras and Ribeirao do Casquilho streams and toward males in the Ribeirao das Perdizes stream. In all three populations analyzed, the B chromosome was more frequent in females than in males. In the most exhaustively sampled population (those from Corrego das Pedras), there was a highly significant association between B-chromosome frequency and sex-ratio distortion, with a disproportionately high number of males without B chromosomes and females with one B chromosome.