The dynamics of chromosome evolution in birds and mammals

Comparative mapping, which compares the location of homologous genes in dif ferent species, is a powerful tool for studying genome evolution(1). Compar ative maps suggest that rates of chromosomal change in mammals can vary fro m one to ten rearrangements per million years(1-4). On the basis of these r ates we would expect 84 to 600 conserved segments in a chicken comparison w ith human or mouse. Here we build comparative maps between these species an d estimate that numbers of conserved segments are in the lower part of this range. We conclude that the organization of the human genome is closer to that of the chicken than the mouse and by adding comparative mapping result s from a range of vertebrates, we identify three possible phases of chromos ome evolution. The relative stability of genomes such as those of the chick en and human will enable the reconstruction of maps of ancestral vertebrate s.