Heterochromatin polymorphism associated with 18S rDNA: a differential pathway among Hoplias malabaricus fish populations.

The distribution of constitutive heterochromatin has showed a relevant association with karyotypic evolution in several animal groups. In the present paper, a polymorphic condition for a heterochromatic segment overlapping a nucleolar organizer region in the fish species Hoplias malabaricus from the Iguaçu river, Southern Brazil, was analyzed. In accordance with previous studies, this population belongs to cytotype A, showing 2n = 42 meta-submetacentric chromosomes, without differentiation of sex chromosomes. Three phenotypes were established with regard to the heterochromatin located near the centromere of chromosome pair 16, and named rr for homomorphic reduced bands, dd for homomorphic duplicated bands and rd for the heteromorphic condition. The rr phenotype was predominant (70.6%), followed by rd (23.5%) and dd (5.9%), showing Hardy-Weinberg equilibrium in the population. In addition, the 18S rDNA sites also show the polymorphic condition. A possible role for this NOR/heterochromatin segment in the evolution of sex chromosomes in other H. malabaricus cytotypes is discussed. Copyright 2003 S. Karger AG, Basel