ABSTRACT
Cirripedia (barnacles) constitute a crustacean monophyletic taxon which is very well defined by several synapomorphies. In particular, all cirripedes are composed of six thoracic segments, but are devoid of any complete abdominal segment. This body plan is preserved in the adult in non-parasitic groups, while the parasitic rhizocephalan cirripedes completely lose arthropodian segmentation at the adult stage. These traits make them a particularly favourable model for studying the formation and maintenance of segmental identity. For the above reasons, it seemed worthwhile to look at the segmentation gene engrailed in a cirripede. A complete engrailed.a cDNA was isolated from larvae of the rhizocephalan cirripede Sacculina carcini. Its expression was monitored during larval development by use of the monoclonal antibody MAb4D9 directed against the Drosophila homologous proteins. The Sacculina engrailed.a gene is expressed during the second and third larval stages in stripes within a posterior area corresponding to the presumptive trunk segments. Surprisingly, these stripes appear in a posterior to anterior sequence. Six engrailed.a stripes characterize the thoracic segments of the cirripedean ground plan.
