Cytogenetic studies were conducted on 154 specimens of AstyΑnΑx scabripinnis collected at three localities in the Campos do JordÃo region (State of SÃo 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 CÓrrego das Pedras and RibeirÃo do Casquilho streams and toward males in the RibeirÃo 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 CÓrrego 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.

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