Axial Hox9 activity establishes the posterior field in the developing forelimb

Current models hold that the early limb field becomes polarized into anterior and posterior domains by the opposing activities ofHand2 andGli3 . This polarization is essential for the initiation ofShh expression in the posterior margin of the limb bud, but how this polarity is established is not clear. Here we show that initial anteroposterior polarization of the early forelimb field requires the function of all fourHox9 paralogs (Hoxa9 ,Hoxb9 ,Hoxc9 , andHoxd9 ). This is unexpected, given that onlyHoxA andHoxD AbdB group genes have been shown to play a role in forelimb patterning, regulating the activation and maintenance ofShh expression and subsequent proximal-distal patterning of the forelimb. Our analysis ofHox9 quadruple mutants demonstrates thatHox9 function is required for the expression ofHand2 in the posterior limb field. Subsequently,Gli3 expression is not repressed posteriorly,Shh expression is not initiated, and collinear expression ofHoxA/D10–13 is not established, resulting in severely malformed forelimbs lacking all posterior,Shh -regulated elements. ThisHox9 mutant phenotype is restricted to the forelimbs; mutant hindlimbs are normal, revealing fundamental differences in the patterning mechanisms governing the establishment of forelimb and hindlimb fields.