Hox expression in the American alligator and evolution of archosaurian axial patterning.

The avian body plan has undergone many modifications, most associated with adaptation to flight and bipedal walking. Some of these modifications may be owing to avian-specific changes in the embryonic Hox expression code. Here, we have examined Hox expression in alligator, the closest living relative of birds, and an archosaur with a more conservative body plan. Two differences in Hox expression between chick, alligator, and other tetrapods correlate with aspects of alligator or bird-specific skeletal morphology. First, absence of a thoracic subdomain of Hoxc-8 expression in alligator correlates with morphological adaptations in crocodilian thoracic segments. Second, Hoxa-5, a gene required to pattern the cervical-thoracic transition, shows unique patterns of expression in chick, alligator, and mouse, correlating with species-specific morphological patterning of this region. Given that cervical vertebral morphologies evolved independently in the bird and mammalian lineages, the underlying developmental mechanisms, including refinement of Hox expression domains, may be distinct.