The caudal gene of the barnacle Sacculina carcini is not expressed in its vestigial abdomen.

We report the characterization of a caudal gene from the rhizocephalan cirripede Sacculina carcini and its embryonic and larval expression patterns. Cirripedes are maxillopodan crustaceans that are devoid of any complete abdominal segment at the adult stage. We currently explore the genetic basis of this peculiar body plan. In a previous study we have shown that they probably lack the abdominalA gene, while possessing the other Hox genes shared by arthropods. However, at least a part of the genetic program might be conserved, since the engrailed.a and engrailed.b genes are expressed in a posterior region that we interpret as a relic of an ancestral abdomen. Here we show first that the Sacculina caudal gene is expressed early in embryogenesis, which makes it the earliest genetic marker evidenced in the development of Sacculina and of any other crustacean species. It is expressed later in the embryo in the caudal papilla, a posterior proliferating zone of cells. During the larval stages, the caudal gene is first expressed in the whole thoracic region; then its expression regresses to the posterior end of the larva. Surprisingly, it is never expressed in the vestigial abdomen. This lack of expression of the Sacculina caudal gene in a posterior region, at odds with what is known in all other studied metazoan species, might be correlated with the defective development of the abdomen.