Possible evolutionary scenario for the origin of posterior Hox genes in amphioxus, ambulacrarians and vertebrates. The presence of the second intron splitting the homeobox into two exons is shown in those genes where it is present, and inferred in ancestral conditions. The red asterisk indicates that the amphioxus Hox12 gene intron is in a different position and thus was acquired secondarily. The green asterisk indicates that Hox13β of the lamprey L. japonicum has retained the ancestral intron, but that intron has been lost in all other vertebrate Hox13 genes. The black question marks indicate the unclear evolutionary origin of AmbP and PG11/14 ancestral genes: they could have been originated independently, or were present in the last common ancestor of deuterostomes. The existence of more Hox genes in the ancestral states cannot be excluded at this time.
Pascual-Anaya et al. BMC Developmental Biology 2013 13:26 doi:10.1186/1471-213X-13-26