A generalized model of vertebrate chromosomal evolution. The ancestral vertebrate karyotype has remained relatively stable over the last ∼370 million years, and large segments of ancestral chromosomes are still retained among lineages. Different combinations of ancestral chromosomes fused together as chromosome numbers decreased in parallel within anuran and salamander lineages. Gene orders defining ancestral chromosomal segments have been rearranged locally during evolution, but synteny has been maintained in the face of dramatic increases and decreases of interstitial genomic content and genome size. Some avian chromosomes are predicted to correspond to entire ancestral chromosomes, although fission and fusion played a role in shaping the generalized avian karyotype. Divergence dates were taken from the literature as follows: tetrapod ancestor (Ruta et al. 2003), amniote ancestor (Hedges and Kumar 2004; Riesz and Muller 2004), amphibian (salamander-frog) ancestor (Zhang and Wake 2009). (MYA) Million years ago.
