Metamorphic-like changes in the nervous system of the nematode Caenorhabditis elegans

During postembryonic development of the nematode Caenorhabditis elegans, one class of embryonic motoneurons, the DD cells, respecifies its pattern of synaptic connections. At the same time, a closely related set of postembryonic motoneurons, the VD cells, complete differentiation and assume a pattern of connections equivalent to the original pattern of the DD cells. These types of changes are reminiscent of changes observed in the nervous systems of animals as they undergo metamorphosis. The DD and VD neurons arise through different lineage mechanisms and in the adult, receive different synaptic inputs and make different outputs. The embryonic DD motoneurons are clonally related to one another; whereas the postembryonic VD motoneurons are produced by a repeated sublineage in which each stem cell generates four or five cell types in addition to the VD cells. In spite of these differences, it has been possible to identify only one gene by mutation that effects one of the two motoneuronal classes. Mutations in the gene unc-55 (unc meaning uncoordinated) cause the VD cells to become essentially identical to the DD cells; thus the unc-55 gene product appears necessary and sufficient to transform homeotically the pattern of synaptic connections of an entire class of motoneuron.