Structural and functional analysis of the nervous system of \(C.\) \(elegans\) with a special focus on the dauer stage

Alternative development of the L1 larva into a long-term resilient dauer larva allows the otherwise fast reproductive nematode model organism Caenorhabditis elegans to arrest development and to survive harsh environmental conditions. Improvement of the environment allows recovery of reproductive development. In this thesis, I investigate the C. elegans dauer larva in a comparative manner to non-dauer stages in the following three steps. First, dauer development and behavior depends on sensory neuron endings of anterior sensilla, such as amphid neurons. In this thesis, I provide a high-resolution study of the precise morphology of amphid neurons that underlies variability (AWB, AWC) as well as precise measurements to analyze dauer-specific plasticity (AFD, AWC). Furthermore, I investigated the regulation of nictation which is a dauer-specific behavior to actively reach new habitats with better environmental conditions. By lifting and moving their anterior body, dauer larvae increase the chance to attach to transport hosts, allowing them to hitchhike. The active dauer nervous system includes inner labial “2” (IL2) neurons which end in the anterior lip sensilla. These neurons regulate nictation and extend a complex dauer-specific dendritic branching. In this inter-individual comparative study, I further provide the precise morphology of these dauer-specific dendrites on an IL2 sub-classes level (IL2L vs. IL2Q). The reconstructions are based on focused ion beam scanning electron microscopy (FIB-SEM) providing near-to-isotropic three-dimensional (3D) image stacks. The resulting high z-resolution allows following of finely branched neuronal structures of C. elegans dauer larvae. Next, how this dauer-specific branching of sensory IL2 neurons is reflected in the synaptic connectivity is of high importance to understand the processing in the nervous system and thus functionality. So-called connectome maps represent the total of synaptic connectivity of an animal. Such connectomes were created for the small and constant nervous system of the nematode C. elegans under enormous effort for different developmental stages that would also be very important for dauer development. To deliver a basis for a better understanding of dauer-specific regulative processes on the circuitry level, I here present an analysis of three mapped dauer connectomes. These maps encompass the nerve ring region only, similar to available non-dauer data. The nerve ring is most relevant for circuit analysis because it contains by far the most synapses and neuron types. Besides FIB-SEM data sets with high z-resolution, this study is enriched with one transmission electron microscopy data set, adding high lateral resolution. In the analysis, the connectivity of the IL2 neurons is revealed, showing one significant dauer-specific connection from IL2Q to RIG interneurons. The interneurons, such as the aforementioned RIG neurons, then pass neuronal signals to motor neurons which activate or inhibit body wall muscle (BWM) cells, representing the locomotor circuit. Furthermore, the BWM cells of this locomotor circuit are activated by cholinergic and inhibited by GABAergic motor neurons. The principal functionality of this network is comparatively well-known for non-dauers. To understand dauer-specific differences in the connectivity of this network on a synaptic level, additional information about the type and the location of receptor fields is important. The receptor fields can be localized in 3D with array tomography (AT) that can combine fluorescent immuno-staining of receptor subunits with the electron microscopy ultrastructure. To further develop this method in this thesis, cholinergic and GABAergic receptor fields were extra-cellularly localized in more easy-to-handle non-dauers. This allows a first precise 3D view of receptor fields at neuro-muscular junctions. Furthermore, the staining method was further adapted to stain the receptor fields in the more challenging dauer larvae. This allows systematic AT imaging of receptors and thus an enrichment of connectome mapping in the future.