EXPRESSION PATTERNS OF HERB GENES IN THE XENOPUS EMBRYO SUGGEST ROLESIN ANTEROPOSTERIOR SPECIFICATION OF THE HINDBRAIN AND IN DORSOVENTRALPATTERNING OF THE MESODERM

Hox genes are thought to participate in patterning the anteroposterior (a-p) axis during vertebrate embryogenesis. In this investigation, th e spatial expression of six Herb genes was analyzed in early embryos o f Xenopus laevis by in situ hybridization. Herb gene expression was fi rst detected in late gastrulae/early neurulae, by which stage, the cha racteristic spatially colinear Herb gene expression sequence was alrea dy apparent. Dissection experiments indicated that the establishment o f these localized expression patterns coincides with the acquisition o f anteroposterior positional information along the main body axis. The Herb genes continued to be expressed in similar domains along the ant eroposterior 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 restr icted domains in the future hindbrain, while Hoxb-5, Hoxb-7, and Hoxb- 9 transcripts were present along the entire presumptive spinal cord. I t was thus notable that the 5' Herb genes exhibited different types of expression domain than the 3' Herb genes. These observations suggest that there may be different mechanisms regulating the expression of th e 3' and 5' Herb genes. Expression of all of the Herb genes analyzed, except Hoxb-4, was predominantly detectable in the central nervous sys tem or in neural crest-derived structures. Hoxb-4 mRNA was detected in the central nervous system, but interestingly, the major expression s ite for this gene was the somites. The other Herb genes tested failed to show significant expression in the semitic mesoderm, although trans cripts from genes 5' from Hoxb-4 were detected in other mesodermal tis sues. In the vertebrate trunk, anteroposterior patterning of the CNS i s thought to be regulated by the somites. The results obtained here fo r Xenopus embryos did not explicitly support the idea of a Herb code f or the somites, although we cannot rule this out. Instead, interesting ly, the data were consistent with a role for Herb genes in dorsoventra l patterning of the mesoderm. (C) 1994 Academic Press, Inc.