by Aaron G. Filler, MD, PhD
Figure 7-5 Hox Specification of Axial High Order Modules
A – The linear arrangement of Hox genes along the chromosome is maintained across most of the existing types of animals. In most vertebrates, there are four slightly different copies of the Hox gene array.
B – Comparing birds and mammals, a shift in the expression boundaries is seen so that the cervical (neck) region and sacral region in birds are elongated, but the thoracic and lumbar regions are greatly shortened. The cervico-thoracic boundary is at somite number nineteen in birds but at number twelve in mammals.
A - From: Raven, P. H., G. B. Johnson, J. B. Losos, et al. (2005). Biology. Boston, McGraw Hill. © The McGraw-Hill Companies, Inc, drawing reproduced by their permission.
B - From: Kuratani, S. (2005a). Craniofacial development and the evolution of the vertebrates: the old problems on a new background. Zoological Science 22(1): 1-19. Reproduced by permission of Zoological Society of Japan)