Molecules and the body plan: the Hox genes of Cirripedes (Crustacea)

Among arthropods, Cirripedia (barnacles) are remarkable in that they completely lack abdominal segments. This feature prompted us to study the Hox genes of three cirripede species, representing a wide array of the diversity of these organisms, a segmented sessile barnacle, Elminius modestus (Thoracica), the parasite of a crab, Sacculina carcini (Rhizocephala), and the burrowing barnacle Trypetesa lampas (Acrothoracica). Using PCR amplification of genomic DNA and cDNA and library probing, we have found seven clear cirripedian homologues of the eight homeotic Hox genes known in insects, including labial and proboscipedia homologues, that were not previously reported in crustaceans. In addition we have isolated a divergent Antp-like gene, named Diva, that we homologize to the ftz gene of insects. The homeotic gene abdominalA (abdA) was not retrieved from any of these three cirripede species. By contrast, we have found all eight homeotic homologue genes, including abdA, in Ulophysema oeresundense, a crustacean possessing a well-developed abdomen, belonging to the Ascothoracica, generally thought to be the sister group of Cirripedia. Since we have found in barnacles homeobox-containing genes that are more divergent from the Antennapedia type than the typical abdA, we believe that a bona fide abdA gene would not have escaped our search. Hence, the abdA gene has been lost or is profoundly derived in sequence during the evolution leading to the cirripedian lineage. If confirmed, the lack of abdA would represent the first case in which the loss of a homeotic gene is correlated with a change in body plan during the evolution of metazoans.