Morphological differentiation and possible origin of B chromosomes in natural Brazilian population of Astyanax scabripinnis (PISCES, CHARACIDAE)

Specimens of Astyanax scabripinnis from three different altitudes (1920, 18 00 and 700 m) along the Ribeirao Grande stream in the Campos do Jordao regi on (Sao Paulo State, Brazil) were investigated. The same diploid number, 2n = 50, was detected in the three populations, with the following karyotypic constitution: 6M, 22SM, 10ST and 12A. The populations located at 1920 and 1800 m altitude presented a high incidence of B chromosomes varying in numb er (0-2), shape (meta- and submetacentrics), size (large and small) and sex -related frequency (they were more frequent among females). The two morphol ogically variant B chromosomes probably evolved from a metacentric macrochr omosome, which is the most commonly observed B chromosome in several A. sca bripinnis populations.