Asymmetry with a twist

Abstract

In snails, manipulating the orientation of cells in the early embryo alters the left–right asymmetry of the shell and body. These findings refine the search for the symmetry-breaking event in this and other animals. The chirality of snail shell coiling — the 'handedness' that means a structure can't be superimposed on its mirror image — is determined genetically by a single locus and is maternally inherited. The gene responsible has not been identified. Now Reiko Kuroda et al. have found that a simple manipulation of cells in 8-cell stage embryos of the great pond snail, Lymnaea stagnalis, can reverse chirality in the adult. Remarkably, expression of nodal, a gene that imparts left–right asymmetry in many species, is reversed by the cellular rearrangement. They also show a strong genetic linkage between the handedness-determining gene or genes and the chiral cytoskeletal dynamics at the third cleavage that promotes the dominant-type blastomere arrangement. The availability of this tractable experimental system should make the mechanism of left–right symmetry more amenable to study.