Compositional mapping of chicken chromosomes and identification of the gene-richest regions

'Compositional chromosomal mapping', namely the assessment of the GC level of chromosomal bands, led to the identification, in the human chromosomes, of the GC-richest H3(+) bands and of the GC-poorest L1(+) bands, which were so called on the basis of the isochore family predominantly present in the bands. The isochore organization of the avian genome is very similar to th ose of most mammals, the only difference being the presence of an additiona l, GC-richest, H4 isochore family. In contrast, the avian karyotypes are ve ry different from those of mammals, being characterized, in most species, b y few macrochromosomes and by a large number of microchromosomes. The 'comp ositional mapping' of chicken mitotic and meiotic chromosomes by in-situ hy bridization of isochore families showed that the chicken GC-richest isochor es are localized not only on a large number of microchromosomes but also on almost all telomeric bands of macrochromosomes. On the other hand, the GC- poorest isochores are generally localized on the internal regions of macroc hromosomes and are almost absent in microchromosomes. Thus, the distinct lo calization of the GC-richest and the GC-poorest bands observed on human chr omosomes appears to be a general feature of chromosomes from warm-blooded v ertebrates.