This is another fascinating paper showing how little we know about the most basic questions, including ‘symmetry-breaking’ during animal development.
Animals tend to be outwardly symmetric but internally are asymmetric
Unlike other animals, snails show inherited variation in asymmetry
We found that both snails and frogs use a common gene to define left and right
Asymmetry is probably an ancient and conserved property of cells and animals
While components of the pathway that establishes left-right asymmetry have been identified in diverse animals, from vertebrates to flies, it is striking that the genes involved in the first symmetry-breaking step remain wholly unknown in the most obviously chiral animals, the gastropod snails. Previously, research on snails was used to show that left-right signaling of Nodal, downstream of symmetry breaking, may be an ancestral feature of the Bilateria [ 1, 2 ]. Here, we report that a disabling mutation in one copy of a tandemly duplicated, diaphanous-related formin is perfectly associated with symmetry breaking in the pond snail. This is supported by the observation that an anti-formin drug treatment converts dextral snail embryos to a sinistral phenocopy, and in frogs, drug inhibition or overexpression by microinjection of formin has a chirality-randomizing effect in early (pre-cilia) embryos. Contrary to expectations based on existing models [ 35 ], we discovered asymmetric gene expression in 2- and 4-cell snail embryos, preceding morphological asymmetry. As the formin-actin filament has been shown to be part of an asymmetry-breaking switch in vitro [ 6, 7 ], together these results are consistent with the view that animals with diverse body plans may derive their asymmetries from the same intracellular chiral elements [ 8 ].