Altitudinal variation for B chromosome frequency in the characid fish Astyanax scabripinnis

The analysis of three populations of the characid fish Astyanax scabripinni s located at different altitudes along the same stream has revealed the pre sence of a macro B chromosome in two high-altitude populations (1800 m and 1920 m) but its absence from a low-altitude population (700 m). Because the stream flows through very rugged mountains, with numerous falls ensuring t hat any gene flow occurs downstream only, the absence of B chromosomes from the low-altitude populations is best interpreted in the light of the paras itic theory of B chromosome evolution. Under this theory, we would expect B chromosomes to be more frequent where environmental conditions are more fa vourable for the species, because Bs are best tolerated there. The widespre ad presence of these B chromosomes in numerous Brazilian river headwaters, which are the preferred habitat for this species, support this possibility.