Expression patterns of Hoxb genes in the Xenopus embryo suggest roles in anteroposterior specification of the hindbrain and in dorsoventral patterning of the mesoderm.

Hox genes are thought to participate in patterning the anteroposterior (a-p) axis during vertebrate embryogenesis. In this investigation, the spatial expression of six Hoxb genes was analyzed in early embryos of Xenopus laevis by in situ hybridization. Hoxb gene expression was first detected in late gastrulae/early neurulae, by which stage, the characteristic spatially colinear Hoxb gene expression sequence was already apparent. Dissection experiments indicated that the establishment of these localized expression patterns coincides with the acquisition of anteroposterior positional information along the main body axis. The Hoxb genes continued to be expressed in similar domains along the anteroposterior axis at all developmental stages examined, although there were some changes in expression at the cellular level. Interestingly, the 3' genes, Hoxb-1, Hoxb-3, and Hoxb-4 were expressed in very restricted domains in the future hindbrain, while Hoxb-5, Hoxb-7, and Hoxb-9 transcripts were present along the entire presumptive spinal cord. It was thus notable that the 5' Hoxb genes exhibited different types of expression domain than the 3' Hoxb genes. These observations suggest that there may be different mechanisms regulating the expression of the 3' and 5' Hoxb genes. Expression of all of the Hoxb genes analyzed, except Hoxb-4, was predominantly detectable in the central nervous system or in neural crest-derived structures. Hoxb-4 mRNA was detected in the central nervous system, but interestingly, the major expression site for this gene was the somites. The other Hoxb genes tested failed to show significant expression in the somitic mesoderm, although transcripts from genes 5' from Hoxb-4 were detected in other mesodermal tissues. In the vertebrate trunk, anteroposterior patterning of the CNS is thought to be regulated by the somites. The results obtained here for Xenopus embryos did not explicitly support the idea of a Hoxb code for the somites, although we cannot rule this out. Instead, interestingly, the data were consistent with a role for Hoxb genes in dorsoventral patterning of the mesoderm.