Changes in chromosome number during vertebrate karyotype evolution. The reconstructed proto-chromosomes in Fig. 4 allow us to discuss how the number of chromosomes has changed in individual vertebrate lineages. (Left) The phylogenetic tree of vertebrates, (right) the distribution of chromosome number in individual lineages (Gregory et al. 2007; Animal Genome Size Database, http://www.genomesize.com). Considering the numbers of proto-chromosomes in Fig. 4, two ancient whole-genome duplication events almost quadrupled the number of chromosomes; subsequently, chromosome numbers in individual lineages tended to decrease in many lineages, although not in the avian lineage.
