GENETIC-TRANSFORMATION OF THE SYNAPTIC PATTERN OF A MOTONEURON CLASS IN CAENORHABDITIS-ELEGANS

Caenorhabditis elegans possesses two classes of inhibitory locomotory neurons, the DD and VD motoneurons (mns), and they form complementary components of a cross-inhibitory neuronal network innervating dorsal a nd ventral body muscles. The DD and VD mns (collectively called the D mns) share a number of morphological and neurochemical features, and m utations in a number of different genes disrupt both cell types in ide ntical ways; however, the DD and VD mns have different lineal origins and different synaptic patterns. Given the number of phenotypic featur es shared by the D mns, it was of interest to determine what is respon sible for the synaptic patterns that distinguish them. An analysis of the locomotory defect along with a genetic epistasis test suggested th at unc-55 mutations alter the function of the VD but not the DD mns. C orrelated with the defective locomotory behavior of unc-55 mutants was an alteration in the distribution of varicosities, structures associa ted with presynaptic elements, on the VD mns. The pattern of varicosit ies of the unc-55 VD mns resembled that of the wild-type DD mns. Moreo ver, the selective removal of the DD mns revealed that unc-55 VD mns h ad adopted a functional role appropriate for the DD mns. Thus, unc-55 appears to be involved in producing the synaptic patterns that disting uish the two D mn classes from one another; when the gene is mutated t he VD and DD mns become structurally similar and functionally equivale nt.