title: Altitudinal variation for B chromosome frequency in the characid fish Astyanax scabripinnis.
abstract: 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.
authors.affiliation: Depto. de Genética e Evolução, Universidade Federal de São Carlos, CP: 676, 13565-905 São Carlos, SP, Brazil.
authors.forename: D M
authors.initials: DM
authors.lastname: Néo
authors.forename: O M
authors.initials: OM
authors.lastname: Filho
authors.forename: J P
authors.initials: JP
authors.lastname: Camacho
mesh_terms: D000531:Altitude; D000818:Animals; D002875:Chromosomes; D005260:Female; D005399:Fishes / Q000235:genetics*; D007621:Karyotyping; D008297:Male; D013045:Species Specificity