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

The analysis of three populations of the characid fish Astyanax scabripinnis located at different altitudes along the same stream has revealed the presence 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 that any gene flow occurs downstream only, the absence of B chromosomes from the low-altitude populations is best interpreted in the light of the parasitic theory of B chromosome evolution. Under this theory, we would expect B chromosomes to be more frequent where environmental conditions are more favourable for the species, because Bs are best tolerated there. The widespread presence of these B chromosomes in numerous Brazilian river headwaters, which are the preferred habitat for this species, support this possibility.