The branchial region of the vertebrate head forms through complex interactions involving rhombomeric segments, neural crest and branchial arches. It is though that aspects of their patterning mechanisms are linked and involve Hox-2 genes, whose overlapping and spatially restricted expression domains represent a combinatorial code for generating regional diversity. Vertebrates possess four Hox clusters of Antennapedia class homeobox genes, related to each other by duplication and divergence from a common ancestral complex. In consequence, at equivalent positions in different clusters there are highly related genes known as subfamilies or paralogous groups. As Hox-2 genes cannot fully account for patterning individual rhombomeres, we investigated whether offsets in expression limits of paralogous genes could account for the generation of regional diversity. We report here that, with the exception of the labial subfamily, paralogues show identical expression limits in rhombomeres, cranial ganglia and branchial arches, providing a combinatorial Hox code for the branchial region that seems to be different in organization to that of the trunk.
In this study we have investigated the organization and regulation of the mouse Hox‐2.7 gene. There are several alternative transcripts some of which are conserved between mouse and humans. By Northern and in situ analysis we are able to identify at least three types of transcripts which are different in size and splicing pattern and have distinctly different boundaries of expression in the nervous system. One subset of the endogenous transcripts has a boundary of expression that corresponds to the adjacent Hox‐2.8 gene instead of Hox‐2.7. In another type of transcript there is an alternative reading frame which predicts a protein that has homology to an enzyme ATPase and suggests that a non‐homeobox containing gene may be located in the Hox‐2 cluster. A Hox‐2.7‐lacZ transgene is expressed in a similar pattern to the endogenous gene in that spatially‐restricted domains of expression are seen in the branchial arches, neural tube, paraxial mesoderm (somites), cranial ganglia, neural crest and gut. However, the anterior boundaries of transgene expression only correspond to the subset of Hox‐2.7 transcripts which map to the Hox‐2.8 boundary. The proximity of a Hox‐2.7 promoter to regions which regulate the adjacent Hox‐2.6 gene and the expression of transgenic and endogenous transcripts in a Hox‐2.8 pattern, suggest that regulatory elements may be shared by neighbouring genes to establish the complete expression pattern.
We have compared the ways in which vertebrate Hox genes are used in the patterning of three distinct embryonic contexts, the branchial region, the somites, and the limb. We have identified common features of the three systems, but have suggested on the basis of their differences (in both embryological properties and use of Hox genes) that it is better to consider each as an independent system for regional specification. Nevertheless, there are sufficient common features to expect that exploitation of the distinct experimental advantages of each system will provide important insights to the mode of operation of the others.
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