Nucleolar organizing regions, 18S and 5S rDNA in Astyanax scabripinnis (Pisces, Characidae): populations distribution and functional diversity

Astyanax scabripinnis specimens from four distinct populations in Brazil we re studied with respect to their karyotype macrostructure, nucleolar organi zer regions, and 18S and 5S rRNA genes. The four populations showed a 2n = 50 chromosomes (3 M + 11 SM + 5 ST + 6 A pairs) and 1-2 B chromosomes. No c hromosomal differentiations were observed between sexes. Although a karyoty pic diversity has been characterized in this fish group, the populations no w analyzed presented the same macrokaryotypic pattern. Chromosome mapping o f 5S rDNA showed a total of eight sites located in four distinct chromosoma l pairs, with no apparent differences among populations. A comparative stud y on 18S rDNA locations and Ag-NORs showed some secondary NOR sites that ar e not usually expressed in karyotypes and a probable differential NOR activ ity among populations. Correlations between these data, environmental condi tions and B chromosomes are discussed.