Reprogramming Hox expression in the vertebrate hindbrain: influence of paraxial mesoderm and rhombomere transposition.

The developing vertebrate hindbrain consists of segments known as rhombomeres, which express combinations of Hox genes implicated in specifying segmental identity. Using chick-chick and chick-transgenic mouse graftings, we show that anterior to posterior rhombomere transpositions result in a progressive posterior transformation and coordinate induction of new Hox expression. This shows that hindbrain plasticity is evolutionarily conserved and implies rhombomeres may be undergoing continual assessment of their identities. The nature of the changes is dependent on both the anteroposterior position of the graft and its origin. Transposed somites from specific axial levels and developmental stages have a graded ability to induce changes in Hox expression, indicating that paraxial mesoderm is a source of the environmental signal responsible for the plasticity.