Hox gene induction in the neural tube depends on three parameters: competence, signal supply and paralogue group

It has been previously shown that Hox gene expression in the rhombencephalon is controlled by environmental cues. Thus posterior transposition of anterior rhombomeres to the r7/8 level results in the activation of Hox genes of the four first paralog groups and in homeotic transformations of the neuroepithelial fate according to its position along the anteroposterior axis. We demonstrate here that although the anteroposterior levels of r2 to r6 express Hox genes they do not have inducing activity on more anterior territories. If transposed at the posterior rhombencephalon and trunk level, however, the same anterior regions are able to express Hox gene such as Hoxa-2, a-3 or b-4. We also provide evidence that these signals are transferred by two paths: one vertical, arising from the paraxial mesoderm, and one planar, travelling in the neural epithelium. The competence to express Hox genes extends up to the forebrain and midbrain but expression of Hox genes does not preclude Otx2 expression in these territories and results only in slight changes in their phenotypes. Similarly, rhombomeres transplanted to posterior truncal levels turned out to be able to express posterior genes of the first eight paralog groups to the exclusion of others located downstream in the Hox genes genomic clusters. This suggests that the neural tube is divided into large territories characterized by different Hox gene regulatory features.