Mind of a dauer: Comparative connectomics reveals developmental plasticity

A fundamental question in neurodevelopmental biology is how flexibly the nervous system can change during development. To address this question of developmental plasticity, we analyzed the connectome of dauer, an alternative developmental stage of nematodes with physiological and behavioral characteristics remarkably distinct from other developmental stages. We reconstructed the complete chemical connectome of a dauer by manual volumetric reconstruction and automated synapse detection using deep learning. While the basic architecture of the nervous system was preserved, there were also structural changes in neurons, large or small, that were closely associated with changes in the connectivity, some of which in turn evoked dauer-specific behaviors such as nictation. Combining the connectome data and optogenetic experiments were enough to reveal dauer-specific neural connections for the dauer-specific behavior. Graph theoretical analyses showed higher clustering of motor neurons and more feedback connections from motor to sensory neurons in the dauer connectome, suggesting that the dauer connectome allows a quick response to an ever-changing environment. We suggest that the nervous system in the nematode, which can be extended to animals in general, has evolved to obtain the ability to respond to harsh environments by reversibly developing a connectome quantitatively and qualitatively differentiated from other developmental stages.