HOX GENES AND THE DIVERSIFICATION OF INSECT AND CRUSTACEAN BODY PLANS

CRUSTACEANS and insects share a common origin of segmentation(1,2), bu t the specialization of trunk segments appears to have arisen independ ently in insects and various crustacean subgroups(3'4). Such macroevol utionary changes in body architecture may be investigated by comparati ve studies of conserved genetic markers(5-7). The Hox genes are well s uited for this purpose, as they determine positional identity along th e body axis in a wide range of animals. Here we examine the expression of four Hox genes in the branchiopod crustacean Artemia franciscana, and compare this with Hox expression patterns from insects. In Artemia the three 'trunk' genes Antp, Ubx and abdA are expressed in largely o verlapping domains in the uniform thoracic region, whereas in insects they specify distinct segment types within the thorax and abdomen. Our comparisons suggest a multistep process for the diversification of th ese Hox gene functions, involving early differences in tissue specific ity and the later acquisition of a role in defining segmental differen ces within the trunk. We propose that the branchiopod thorax may be ho mologous to the entire pregenital (thoracic and abdominal) region of t he insect trunk.