Handed asymmetry in nematodes

Like most animals, C. elegans and other nematodes exhibit several internal left-right asymmetries with an essentially invariant (dextral) handedness. Handedness is established in early cleavage, resulting in a markedly left-right-asymmetric embryo on which bilateral symmetry must be superimposed later in embryogenesis. Some of the asymmetric cell interactions that accomplish this have been identified, but the mechanism that initially establishes dextral rather than sinistral handedness is not understood, in C. elegans or any other embryo. Analysis of mutations that result in reversal of handedness, such as spn-1 in C. elegans, should help elucidate this process. A model involving centriolar segregation is proposed as a possible mechanism for handedness choice.