Abstract
Two thirds of the 302 neurons in C. elegans form bilaterally symmetric pairs in its physical connectome, and similar gross morphological symmetries are seen in the nervous systems of many other animals. A central question is whether and how this morphological symmetry is broken to produce functional asymmetry. Addressing this question, in all but two cases, has been impossible because no promoters are known that can direct gene expression to a single cell within a symmetric pair. Here we develop an efficient genetic code expansion system in C. elegans and use this system to create a photo-activatable version of Cre recombinase. Using this system, we target single neurons within a bilaterally symmetric pair (PLMR and PLML) with a laser. This turns on Cre and thereby switches on expression of an optogenetic channel in a single cell. We hereby overcome the limitation that these neurons cannot be targeted by genetic means. Our approach enables the generation of large numbers of animals for downstream experiments. By globally illuminating groups of freely moving animals to stimulate the targeted neurons that express an optogenetic channel we dissect the individual contributions of PLMR and PLML to the C. elegans touch response. Our results reveal that the individual neurons make asymmetric contributions to this behaviour, and suggest distinct roles for PLMR and PLML in the habituation to repeated stimulation. Our results demonstrate how genetic code expansion and optical targeting can be combined to break the symmetry of neuron pairs in C. elegans and thereby dissect the contributions of individual neurons to behaviour.
Competing Interest Statement
The authors have declared no competing interest.