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Hindbrain patterning: Krox20 couples segmentation and specification of regional identity.

We have previously demonstrated that inactivation of the Krox20 gene led to the disappearance of its segmental expression territories in the hindbrain, the rhombomeres (r) 3 and 5. We now performed a detailed analysis of the fate of prospective r3 and r5 cells in Krox20 mutant embryos. Genetic fate mapping indicates that at least some of these cells persist in the absence of a functional Krox20 protein and uncovers the requirement for autoregulatory mechanisms in the expansion and maintenance of Krox20-expressing territories. Analysis of even-numbered rhombomere molecular markers demonstrates that in Krox20-null embryos, r3 cells acquire r2 or r4 identity, and r5 cells acquire r6 identity. Finally, study of embryonic chimaeras between Krox20 homozygous mutant and wild-type cells shows that the mingling properties of r3/r5 mutant cells are changed towards those of even-numbered rhombomere cells. Together, these data demonstrate that Krox20 is essential to the generation of alternating odd- and even-numbered territories in the hindbrain and that it acts by coupling the processes of segment formation, cell segregation and specification of regional identity.