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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A satellite DNA sequence of Parodon hilarii (named pPh2004) was isolated, cloned and sequenced. This satellite DNA is composed of 200bp, 60% AT rich. In situ hybridization (FISH) results revealed that the satellite DNA pPh2004 is located in the terminal regions of several chromosomes, forming highly evident blocks in some and punctual marks in others. The comparison between the FISH and C-banding results showed that the location of this satellite DNA coincides with that of most terminal heterochromatins. However, some regions are only marked by FISH whereas other regions are only marked by C-banding. The possible existence of more than one satellite DNA family could explain these partial differences. The in situ hybridization with the satellite DNA and the G- and C-bandings confirmed the presence of a sex chromosome system of the ZZ/ZW type in P. hilarii, as well as the correct identification of the Z chromosome in the karyotype. This chromosome displays a segment of terminal heterochromatin in the long arm, similar to the segment observed in the short arm of the W chromosome, also showing a G-banding pattern similar to that of the short arm and part of the long arm of the W chromosome. A hypothesis on the origin of the W chromosome from an ancestral chromosome similar to the Z chromosome is presented.

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