Body handedness is directed by genetically determined cytoskeletal dynamics in the early embryo.

Although substantial progress has been made recently in understanding the establishment of left-right asymmetry in several organisms, little is known about the initial step for any embryo. In gastropods, left-right body handedness is determined by an unknown maternally inherited single gene or genes at closely linked loci and is associated with the sense of spiral cleavage in early embryos. Contrary to what has been believed, we show that temporal and spatial cytoskeletal dynamics for the left- and right-handed snails within a species are not mirror images of each other. Thus, during the third cleavage of Lymnaea stagnalis, helical spindle inclination (SI) and spiral blastomere deformation (SD) are observed only in the dominant dextral embryos at metaphase-anaphase, whereas in the recessive sinistral embryos, helicity emerges during the furrow ingression. Actin depolymerization agents altered both cleavages to neutral. Further, we found a strong genetic linkage between the handedness-specific cytoskeletal organization and the organismal handedness, using backcrossed F4 congenic animals that inherit only 1/16 of dextral strain-derived genome either with or without the dextrality-determining gene(s). Physa acuta, a sinistral-only gastropod, exhibits substantial SD and SI levotropically. Thus, cytoskeletal dynamics have a crucial role in determination of body handedness with further molecular, cellular, and evolutionary implications.