Reconstructed ancestral chromosomes. Ancestral vertebrate chromosomes A, B, and F had two alternative scenarios, fusions or fissions, between the 2R WGD events, as shown in Fig. 3. Thus, the number of proto-chromosomes ranges from 10 to 13 depending on the choice of two alternatives. The figure illustrates the scenario in which only fissions took place. Ten reconstructed proto-chromosomes in the vertebrate ancestor shown at the top are assigned distinct colors, and their daughter chromosomes in the gnathostome ancestor are distinguished by their respective vertical bars. In the genomes of the osteichthyan, teleost, and amniote ancestors, and human, chicken, and medaka genomes, genomic regions are assigned colors and vertical bars that represent correspondences of individual regions to the proto-chromosomes in the gnathostome ancestor from which respective regions originated. Unassigned blocks are shown in the rightmost chromosome (Un) in the osteichthyan and amniote ancestors.
