The Olig3 gene encodes a bHLH factor that is expressed in the ventricular zone of the dorsal alar plate of the hindbrain. We found that the Olig3+ progenitor domain encompassed subdomains that co-expressed Math1, Ngn1, Mash1 and Ptf1a. Olig3+ cells give rise to neuronal types in the dorsal alar plate that we denote as class A neurons. We used genetic lineage tracing to demonstrate that class A neurons contribute to the nucleus of the solitary tract and to precerebellar nuclei. The fate of class A neurons was not correctly determined in Olig3 mutant mice. As a consequence, the nucleus of the solitary tract did not form, and precerebellar nuclei, such as the inferior olivary nucleus, were absent or small. At the expense of class A neurons, ectopic Lbx1+ neurons appeared in the alar plate in Olig3 mutant mice. By contrast, electroporation of an Olig3 expression vector in the chick hindbrain suppressed the emergence of Lbx1+ neurons. Climbing fiber neurons of the inferior olivary nucleus express Foxd3 and require Olig3 as well as Ptf1a for the determination of their fate. We observed that electroporation of Olig3 and Ptf1a expression vectors, but not either alone, induced Foxd3. We therefore propose that Olig3 can cooperate with Ptf1a to determine the fate of climbing fiber neurons of the inferior olivary nucleus.
