Ectopic Hoxa2 induction after neural crest migration results in homeosis of jaw elements in Xenopus

Hox genes are required to pattern neural crest (NC) derived craniofacial an d visceral skeletal structures. However, the temporal requirement of Hox pa tterning activity is not known. Here, we use an inducible system to establi sh Hoxa2 activity at distinct NC migratory stages in Xenopus embryos. We un cover stage-specific effects of Hoxa2 gain-of-function suggesting a multist ep patterning process for hindbrain NC. Most interestingly, we show that Ho xa2 induction at postmigratory stages results in mirror image homeotic tran sformation of a subset of jaw elements, normally devoid of Hox expression, towards hyoid morphology, This is the reverse phenotype to that observed in the Hoxa2 knockout. These data demonstrate that the skeletal pattern of rh ombomeric mandibular crest is not committed before migration and further im plicate Hoxa2 as a true selector of hyoid fate, Moreover, the demonstration that the expression of Hoxa2 alone is sufficient to transform the upper ja w and its joint selectively may have implications for the evolution of jaws .