Evolutionary Transition in the Regulation of Vertebrate Pronephros Development: A New Role for Retinoic Acid.

The anterior-posterior (AP) axis in chordates is regulated by a conserved set of genes and signaling pathways, including genes and retinoic acid (RA), which play well-characterized roles in the organization of the chordate body plan. The intermediate mesoderm (IM), which gives rise to all vertebrate kidneys, is an example of a tissue that differentiates sequentially along this axis. Yet, the conservation of the spatiotemporal regulation of the IM across vertebrates remains poorly understood. In this study, we used a comparative developmental approach focusing on non-conventional model organisms, a chondrichthyan (catshark), a cyclostome (lamprey), and a cephalochordate (amphioxus), to assess the involvement of RA in the regulation of chordate and vertebrate pronephros formation. We report that the anterior expression boundary of early pronephric markers ( and ), positioned at the level of somite 6 in amniotes, is conserved in the catshark and the lamprey. Furthermore, RA, driving the expression of genes like in amniotes, regulates the anterior pronephros boundary in the catshark. We find no evidence for the involvement of this regulatory hierarchy in the AP positioning of the lamprey pronephros and the amphioxus pronephros homolog, Hatschek's nephridium. This suggests that despite the conservation of and expressions in chordate pronephros homologs, the responsiveness of the IM, and hence of pronephric genes, to RA- and -dependent regulation is a gnathostome novelty.