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 scabri pinnis collected at three localities in the Campos do Jordao region (S tate of Sao Paulo, Brazil). The C-banding pattern suggested that the m etacentric B chromosome found in most of the specimens is an isochromo some derived from chromosome 24, the only chromosome in the standard c omplement that carries interstitial C-bands similar to those present i n each arm of the B chromosome. The sex ratio was biased toward female s in the Corrego das Pedras and Ribeirao do Casquilho streams and towa rd 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-chromoso me frequency and sex-ratio distortion, with a disproportionately high number of males without B chromosomes and females with one B chromosom e.