Positional regulation of Krox-20 and mafB/kr expression in the developing hindbrain: potentialities of prospective rhombomeres.

Krox-20 and mafB/kr encode transcription factors involved in the control of hindbrain development and are expressed in rhombomeres (r) 3 and 5 and 5 and 6, respectively. To analyse the regulation of the expression of these genes by positional cues, focusing on the stages just preceding the formation of rhombomeres, we have performed ectopic grafts involving single prospective rhombomeres (pr) or couples of pr on 4-6 somite avian embryos. Transplantation of pr6 in the pr5 position leads to Krox-20 activation and grafting of pr7 in the pr5 position results in mafB/kr activation. Furthermore, pr6 grafted in the pr5 position develops an r5-like cytoarchitecture. These data establish that rostral transplantation can lead to anteriorization within the hindbrain. However, additional experiments indicate that the competence of the transplanted tissue for such anteriorization appears limited and that transformations corresponding to shifts of a single rhombomere are favoured. We also show that caudal transplantation of pr5 into the pr6 position can lead to a down-regulation of Krox-20 expression consistent with posteriorization, suggesting that caudalizing influences are present within the nonsomitic hindbrain after the 4- to 6-somite stage. Finally, combinations of extirpation and grafting experiments suggest that the regulation of mafB/kr expression in the r6-r7 region may involve anteriorizing influences in addition to previously identified posteriorizing signals from the somitic region.