SUPERNUMERARY-B CHROMOSOMES RESTRICTED TO MALES IN ASTYANAX-SCABRIPINNIS (PISCES, CHARACIDAE)

Specimens of Astyanax scabripinnis (36 males and 19 females) from the Jucu river, Vitor Hugo county, State of Espirito Santo, Brazil, were a nalyzed cytogenetically and a system of supernumerary B chromosomes re stricted to males was detected for the first time in fish. The basic c hromosome number and karyotype for both males and females was 2n - 50 (NF - 64), with six metacentrics, eight submetacentrics and 36 acrocen trics. In testis cells, the basic modal number was 25 bivalents. Super numerary microchromosomes ranging in number from zero to four were obs erved in somatic cells of 34 of the 36 males; in three males the B chr omosomes were restricted to the germ cells and in two males and all fe males they were absent. Nucleolus organizer regions were identified by silver staining and the NORs varied in number, size and location, bot h inter and intraindividually. The maximum number of active NORs per c ell was 15, but 13 morphologically distinct chromosome pairs may be in volved in the nucleolus organization in this population.Small heteroch romatin blocks were detected in the pericentromeric regions of all chr omosomes and faint bands were detected in the telomeric regions of som e chromosomes. The Bs presented intermediate staining when submitted t o C banding. The presence of supernumerary B chromosomes restricted to males in the A. scabripinnis complex strongly suggests a role for the se chromosomes in the evolutionary history of this group.