HOX GENES AND THE EVOLUTION OF DIVERSE BODY PLANS

Homeobox genes encode transcription factors that carry out diverse rol es during development. They are widely distributed among eukaryotes, b ut appear to have undergone an extensive radiation in the earliest met azoa, to generate a range of homeobox subclasses now shared between di verse metazoan phyla. The Hox genes comprise one of these subfamilies, defined as much by conserved chromosomal organization and expression as by sequence characteristics. These Hox genes act as markers of posi tion along the antero-posterior axis of the body in nematodes, arthrop ods, chordates, and by implication, most other triploblastic phyla. In the arthropods this role is visualized most clearly in the control of segment identity. Exactly how Hox genes control the structure of segm ents is not yet understood, but their differential deployment between segments provides a model for the basis of segment diversity. Within t he arthropods, distantly related taxonomic groups with very different body plans (insects, crustaceans) may share the same set of Hox genes. The expression of these Hox genes provides a new character to define the homology of different body regions. Comparisons of Hox gene deploy ment between insects and a branchiopod crustacean suggest a novel mode l for the derivation of the insect body plan.