Vertebrate chromosome evolution scenario. (A) For simplicity, we illustrate two proto-chromosomes (red and blue bars) duplicated by the first round of WGD. Subsequently, fission divided one of the duplicated chromosomes. (B) The second round of WGD doubled the proto-chromosomes. Blocks in chromosomes are labeled with their respective chromosome positions in the human genome. (C) After the second WGD, early vertebrates underwent slow changes in karyotype over a long evolutionary process. (D) In the ancestral mammalian lineage, intensive interchromosomal rearrangements occurred and the ancestral chromosomes were broken into smaller segments that were distributed across many human chromosomes. (E) In the ancient ray-finned fish lineage, intensive chromosome fusions merged the ancestral chromosomal segments into ancestral teleost chromosomes. (F) Another round of WGD in the ancestral teleost doubled proto-chromosomes, but afterward, few global rearrangements shaped the present medaka genome.
