Left-right asymmetry is pervasive in animals and known to be under tight genetic control. One conspicuous example is in snails: their shells coil either dextrally (as in most species or strains within a species) or sinistrally, and the shell reflects the chirality of the whole body, which is established in early embryogenesis. Previous work has implicated the formin diaphanous in the regulation of chirality in Lymnaea snails, but it was unclear whether Lsdia1 was truly the causative gene, or just linked to it. Now, Masanori Abe and Reiko Kuroda provide definitive proof of the role of Lsdia1 in shell coiling. CRISPR/Cas9 targeting of Lsdia1 leads to sinistral offspring in a dextral background, in a stably inherited manner. In the embryo, LsDia1 regulates the third cleavage, when definitive chirality is first displayed. Mothers with Lsdia1 mutations produced embryos that, at the one-cell stage, lacked detectable LsDia1 protein. Remarkably, reducing vitelline membrane tension reveals that the fertilised egg itself shows chirality, and that this depends on the Lsdia1 genotype of the mother. Finally, sinistral embryos show mirror-image patterns of nodal and Pitx expression, two genes known to regulate chirality across animals. This study thus reports the first CRISPR/Cas9 gene knockouts in molluscs, the earliest demonstration of chirality in animals, and decisively proves the contribution of LsDia1 to LR patterning in Lymnaea.
