From head to rootlet: comparative transcriptomic analysis of a rhizocephalan barnacle Peltogaster reticulata (Crustacea: Rhizocephala)

Rhizocephalan barnacles stand out in the diverse world of metazoan parasites. The body of a rhizocephalan female is modified beyond revealing any recognizable morphological features, consisting of the interna, the system of rootlets, and the externa, a sac-like reproductive body. Moreover, rhizocephalans have an outstanding ability to control their hosts, literally turning them into “zombies”. Here we performed the first comparative transcriptomic analysis of rhizocephalan ( Peltogaster reticulata ) adult female body parts and established the spatial heterogeneity of gene expression in different regions. This resulted in an unexpected discovery of germ cells in the lumen of interna, which was additionally validated by histological methods. Our finding of germ cells in interna deprives externa from the exclusive role in propagation and renders it a simple brooding chamber. Furthermore, we confirmed that the neurons of the host are attracted to the rootlets of the interna, indicating a great intimacy of host-parasite relationship. Finally, based on the continual expression of development-associated genes, we suggest that rhizocephalans “got stuck in the metamorphosis”, even in their reproductive stage.