Sympatric occurrence of two cytotypes of Astyanax scabripinnis (Characiformes, Characidae)

Among four specimens of Astyanax scabripinnis sympatrically collected from the Tamandua stream (State of Sao Paulo, Brazil), three, named cytotype I, had 2n = 50 chromosomes (6M + 26SM + 4ST + 14A) and a fundamental number (F N) of 86, and one specimen, named cytotype II, had 2n = 48 chromosomes (6M + 28SM + 4ST + 10A) and FN = 86. Besides the difference in diploid number, the cytotypes showed a clear difference in the distribution of constitutive heterochromatin. One NOR-bearing chromosome pair was detected in both cyto types and one specimen of cytotype I had multiple NORs (tree chromosome pai rs). Robertsonian translocations and an increase or loss of heterochromatin are proposed to explain the karyotypic divergence observed. Some aspects r elated to the chromosome evolution of Astyanax scabripinnis are discussed.