Nkx6 homeodomain proteins are involved in the development of the central nervous system (CNS) in insects and vertebrates. Now, two papers in this issue conclude that Nkx6 proteins have retained their ancestral role in neuronal fate specification over the 800 million years since insects and vertebrates diverged, despite their genes being regulated differently in the two groups of animals. On p. ,Cheesman and colleagues report that Nkx6 proteins have similar CNS expression patterns in Drosophila and zebrafish, and regulate motoneuron formation in both animals. They identify a zebrafish nkx6.1 gene and show that ectopic expression of fly or fish Nkx6 generates extra motoneurons in both species. By contrast, knockdown of Nkx6.1 protein in zebrafish reduces motoneuron formation and increases ventral interneuron formation. On p. , Broihier and co-workers show that Nkx6 is a key factor in the specification of motoneuron subtype identity in Drosophila. By analysing hb9 Nkx6 double mutants, the researchers show that Nkx6 collaborates with the homeodomain protein Hb9 to specify ventrally projecting motoneuron fate and to repress dorsally projecting motoneuron fate. Nkx6 also promotes axonogenesis in Nkx6-positive motoneurons, a role that seems to be conserved in vertebrates.
