Lophotrochozoa get into the game: the nodal pathway and left/right asymmetry in bilateria

Animals as diverse as humans, flies, crabs, and snails show overall bilateral symmetry, but each species has specific structures and organs that display left/right asymmetry, and the presence of these asymmetries is vital to the organism. Here, we review recent results showing that part of the molecular pathway that sets left/right asymmetry in vertebrates is also conserved in snails, suggesting that left/right asymmetry was present in the common ancestor of all bilaterians. More specifically, we can now predict that the signaling molecule Nodal and the transcription factor Pitx were expressed on the right side of the bilaterian ancestor. These results also allow us to understand how the direction of shell coiling (chirality) is regulated in snails and provides interesting insights into the possible inversion of the dorsoventral axis in the lineage leading to chordates.