PLASTICITY OF TRANSPOSED RHOMBOMERES - HOX GENE INDUCTION IS CORRELATED WITH PHENOTYPIC MODIFICATIONS

In this study we have analysed the expression of Hoxb-4, Herb-1, Hoxa- 3, Hoxb-3, Hoxa-4 and Hoxd-4 in the neural tube of chick and quail emb ryos after rhombomere (r) heterotopic transplantations within the rhom bencephalic area, Grafting experiments were carried out at the 5-somit e stage, i,e before rhombomere boundaries are visible. They were prece eded by the establishment of the precise fate map of the rhombencephal on in order to determine the presumptive territory corresponding to ea ch rhombomere. When a rhombomere is transplanted from a caudal to a mo re rostral position it expresses the same set of Hox genes as in situ, By contrast in many cases, if rhombomeres are transplanted from rostr al to caudal their Hox gene expression pattern is modified, They expre ss genes normally activated at the new location of the explant, as evi denced by unilateral grafting, This induction occurs whether transplan tation is carried out before or after rhombomere boundary formation, M oreover, the fate of the cells of caudally transplanted rhombomeres is modified: the rhombencephalic nuclei in the graft develop according t o the new location as shown for an r5/6 to r8 transplantation. Transpl antation of 5 consecutive rhombomeres (i.e. r2 to r6), to the r8 level leads to the induction of Hoxb-4 in the two posteriormost rhombomeres but not in r2,3,4. Transplantations to more caudal regions (posterior to somite 3) result in some cases in the induction of Hoxb-4 in the w hole transplant, Neither the mesoderm lateral to the graft nor the not ochord is responsible for the induction, Thus, the inductive signal em anates from the neural tube itself, suggesting that planar signalling and predominance of posterior properties are involved in the patternin g of the neural primordium.