Gene-rich and gene-poor chromosomal regions have different locations in the interphase nuclei of cold-blooded vertebrates.

In situ hybridizations of single-copy GC-rich, gene-rich and GC-poor, gene-poor chicken DNA allowed us to localize the gene-rich and the gene-poor chromosomal regions in interphase nuclei of cold-blooded vertebrates. Our results showed that the gene-rich regions from amphibians (Rana esculenta) and reptiles (Podarcis sicula) occupy the more internal part of the nuclei, whereas the gene-poor regions occupy the periphery. This finding is similar to that previously reported in warm-blooded vertebrates, in spite of the lower GC levels of the gene-rich regions of cold-blooded vertebrates.

Avian genomes: different karyotypes but a similar distribution of the GC-richest chromosome regions at interphase.

The chicken karyotype, like that of the vast majority of avian species, shows a large number of dot-shaped microchromosomes that are characterized, like most telomeric regions of the macrochromosomes, by the highest GC levels and the highest gene densities. In interphase nuclei, these gene-dense regions are centrally located, and are characterized by an open chromatin structure (a similar situation also exists in mammals). Avian species belonging to the Accipitridae family (diurnal raptors) show a karyotype with no very large chromosomes, and with only a very small number of microchromosomes.