Non-random Distribution of the GC-rich Heterochromatin and Nucleolar rDNA Sites on Astyanax scabripinnis Chromosomes

Abstract

A close relationship has been verified between presumedly GC-rich conspicuous segments and the nucleolus organizer regions on chromosomes of cold-blooded vertebrates. C-banding, and chromomycin A₃ (CMA) or mithramycin A (MM) staining as well as rDNA in situ hybridization were performed in mitotic chromosomes of a population of the characid fish Astyanax scabripinnis in order to study GC-rich heterochromatin and the pattern of nucleolar rDNA location. The results showed a close relationship between large telomeric heterochromatic segments and GC-specific fluorochrome bands. Some CMA⁺ (or MM⁺) bands presented no correspondence with nucleolar rDNA sites detected by fluorescence in situ hybridization (FISH). Although GC specific fluorochromes have been widely used to infer nucleolus organizer regions location in fish chromosomes, a detailed FISH analysis indicates that, in this population, rDNA clusters are not always located on CMA⁺ (or MM⁺) bands, and some GC-rich bands do not bear rDNA clusters. Based on some available models we discussed some possible mechanisms for the non-random nucleolar rDNA and CMA⁺ heterochromatin distribution in A. scabripinnis chromosomes.