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 cirr ipede Sacculina carcini and its embryonic and larval expression patterns. C irripedes are maxillopodan crustaceans that are devoid of any complete abdo minal 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 proba bly lack the abdominalA gene, while possessing the other Hox genes shared b y arthropods. However, at least a part of the genetic program might be cons erved since the engrailed.a and engrailed.b genes are expressed in a poster ior region that we interpret as a relic of an ancestral abdomen. Here we sh ow first that the Sacculina caudal gene is expressed early in embryogenesis , which makes it the earliest genetic marker evidenced in the development o f Sacculina and of any other crustacean species. It is expressed later in t he embryo in the caudal papilla, a posterior proliferating zone of cells. D uring the larval stages, the caudal gene is first expressed ill the whole t horacic region: then its expression regresses to the posterior end of the l arva. Surprisingly, it is never expressed in the vestigial abdomen. This la ck of expression of the Sacculina caudal gene in a posterior region, at odd s with what is known in all other studied metazoan species, might be correl ated with the defective development of the abdomen.