Evolutionary shifts of vertebrate structures and Hox expression up and down the axial series of segments: a consideration of possible mechanisms

The term 'transposition' describes how, during vertebrate evolution, anatom ical structures have shifted up or down the axial series of segments. For e xample, the neck/thorax junction and the position of the forelimb in the ch icken have shifted posteriorly, relative to mouse, by a distance of seven s omites or vertebrae. By examining the expression boundaries of some chick H ox genes not previously described, we provide new evidence that axial shift s in anatomical structures correspond with shifts in Hox expression domains . These shifts occur both in mesodermal components (somites, vertebrae, and lateral plate mesoderm) and neural components (spinal ganglia). We discuss morphogen gradient, timing, spreading, and growth models for the setting o f Hox expression boundaries, and consider how evolutionary shifts in bounda ry positions might have been effected in terms of these models.