First report of a B chromosome in a natural population of

Several species of the genus Astyanax have already been genetically studied, and B-chromosomes have been con-sidered to be an interesting feature in some species of this group. In the present paper we report, for the first time, the occurrence of a B microchromosome in a natural population of A. altiparanae. This additional genomic element was identified as an acrocentric chromosome, similar in size to the smallest chromosomal pairs of the standard karyotype. Analysis of the constitutive heterochromatin pattern by C-banding evidenced heterochromatic blocks lo-cated on centromeric, pericentromeric, and interstitial regions of some chromosomes, and also positive marks in a subtelocentric chromosomal pair that presented the short arms entirely heterochromatic. The application of this methodology also revealed a heterochromatic pattern in the extra chromosome, a typical feature of supernumerary chromosomes. Key words: B chromosomes, fish cytogenetics, Astyanax altiparanae. Received: September 27, 2006; Accepted: May 22, 2007. Several species of the genus Astyanax have already been genetically studied, mainly under a cytogenetical fo-cus. The chromosomal number ranged from 2n = 36 in A. schubarti to 2n = 50 in A. scabripinnis paranae (Oliveira et al., 1988), which demonstrates the high karyotype diversity related to chromosomal morphology and structure in this fish group. Cytogenetic studies have been performed in dis-tinct populations of A. altiparanae, evidencing karyotypes composed of 50 chromosomes (Daniel-Silva and Almei