LIM-homeodomain genes as territory markers in the brainstem of adult and developing Xenopus laevis.

We investigated expression patterns of the LIM-homeodomain (LIM-hd) genes x-Lhx1, x-Lhx2, x-Lhx5, and x-Lhx9 in the brainstem of Xenopus laevis during larval development and in the adult. The two groups of paralogous genes, x-Lhx1/x-Lhx5 and x-Lhx2/x-Lhx9, showed fundamentally different expression patterns, being expressed in ventral versus dorsal territories of the midbrain and hindbrain, respectively. Indeed, prominent expression of x-Lhx1/5 was found in the mesencephalic tegmentum and the hindbrain reticular formation, whereas conspicuous x-Lhx2/9 expression was observed in the torus semicircularis and isthmic nucleus. A few shared expression domains for the two pairs of paralogs included the optic tectum and the anterodorsal and pedunculopontine nuclei. In each structure, expression of the two paralogs was almost identical, indicating that the regulation of their expression in this part of the brain has evolved slightly since gene duplication occurred. Notable exceptions included the expression of x-Lhx1 but not x-Lhx5 in the Purkinje cells and the expression of x-Lhx9 but not x-Lhx2 in the lateral line nucleus. The analysis of LIM-hd expression patterns throughout development allowed the origin of given structures in early embryos to be traced back. x-Lhx1 and x-Lhx5 were relevant to locate the cerebellar anlage and to follow morphogenesis of the cerebellar plate and cerebellar nuclei. They also highlighted the rhombomeric organization of the hindbrain. On the other hand, x-Lhx2 and x-Lhx9 showed a dynamic spatiotemporal pattern relative to tectal development and layering, and x-Lhx9 was useful to trace back the origin of the isthmus in early development.