Fig. 2.

Ancestral karyotypes of amphioxus, chordates, and vertebrates. (A) Bb probably best recapitulates the ancestral karyotype of Branchiostoma amphioxus, with Bj and Bf having undergone chromosomal fusions. (B) Genes on chr13, chr14, and chr17 of Bb have their homologous genes located on the same set of chicken chromosomes. Each line connecting chromosomes of Bb and chicken chromosomes is scaled to the proportion of Bb genes that are homologous to the genes of one chicken chromosome. (C) The inferred relationship between Bb chromosomes and CLG. (D) Composition of chicken chromosome by CLG homologous sequences. The colored bands represent the Bb-chicken synteny blocks. A different scale for macrochromosomes (20 Mb) and microchromosomes (2 Mb) was used. (E) Reconstructed 1R and 2R of three CLGs. One color represents one CLG, and when one chromosome is composed with more than one CLG, two or more CLG blocks are linked together. (F) The ohnolog genes were used to construct the phylogeny of ohno-chromosomes (ohno-A, B, C, D), which refer to gene groups derived from WGDs. Bb homologs were used as the outgroup. Bootstrapping values shown placed at the internal nodes. (G) 244 ohnolog gene groups were used to date 1R and 2R. Fossil calibration for the mouse-human nodes: 62 to 101 My, bird-mammal nodes: 306 to 332 My.