Spiral cleavages determine the left-right body plan by regulating Nodal pathway in monomorphic gastropods, Physa acuta.

The handedness of gastropods is genetically determined, but the molecular nature of the gene responsible and the associated mechanisms remain unknown. In order to characterize the chiromorphogenesis pathway starting from the gene to the left-right asymmetric body plan, we have closely analyzed the cytoskeletal dynamics of the Physa (P.) acuta embryo, a fresh water non-dimorphic sinistral snail, during the early developmental stage by mechanically altering the handedness of the embryos at the critical spiral third cleavage. A fertile situs inversus was created and the nodal-Pitx gene expression patterns were completely mirror imaged to the wild type at the trochophore stage. Together with our previous work on Lymnaea (L.) stagnalis, we could show that chirality is established at the third cleavage, as dictated by the single handedness-determining gene locus, and then chirality information is transferred via subsequent spiral fourth and fifth cleavages to the later developmental stage, dictating the nodal-Pitx expression pathway. The cytoskeletal dynamics of manipulated and non-manipulated embryos of sinistral P. acuta and dextral dominant L. stagnalis are compared.