Different levels of Hoxa2 are required for particular developmental processes.

Hoxa2 is required for a variety of developmental processes in the branchial arches and in the hindbrain. We have created a Hoxa2 allele that is about 45% as active in transcription as its wild-type counterpart. This allele, together with the Hoxa2 null and wild-type alleles, allowed the generation of embryos developing in the presence of different levels of Hoxa2 activity. Analysis of these embryos indicates that in general the hindbrain is more resistant to Hoxa2 deficiencies than the second branchial arch. Also, within the second arch, proximo-caudal areas are more sensitive than the rostro-distal. In the hindbrain, basic segmentation and patterning processes seem to occur normally at Hoxa2 levels as low as 20% of the normal. In addition, specific neuronal markers along the dorso-ventral axis of the hindbrain seem differentially affected by reduced Hoxa2 levels. These results provide new clues to understand the role of Hoxa2 in the different embryonic areas where it is required.