Possible implication of Hox genes Abdominal-B and abdominal-A in the specification of genital and abdominal segments in cirripedes

The crustaceans cirripedes (barnacles) are characterised by the lack of fully developed abdominal segments at any stage of their life cycle. However, in nauplius larvae of the cirripede Sacculina carcini, we detected five small engrailed stripes in a postero-dorsal region behind the sixth thoracic segment, that we interpreted as a vestigial abdomen. Here, we present additional morphological and genetic data on Sacculina to further characterise this structure. Scanning electron microscopy analysis confirms the existence of a segmented region in this part of the naupliar body. However, at the late naupliar stage, this structure stops its development and degenerates. This region expresses the Hox gene Abdominal-B, which may indicate that it actually corresponds to the posterior-most part of the Sacculina trunk. In addition, Abdominal-B expression differentiates two types of larvae that probably correspond to male and female larvae, respectively. In contrast, no abdominal-A expression can be detected in the vestigial abdomen. We discuss the possible implication of the loss or divergence of the Abdominal-A protein in the impaired development of abdominal segments in cirripedes.