Phoresy is a widespread form of commensalism that facilitates dispersal of one species through an association with a more mobile second species. Dauer larvae of the nematode Caenorhabditis elegans exhibit a phoretic behavior called nictation, which could enable interactions with animals such as isopods or snails. Here, we show that natural C. elegans isolates differ in nictation. We use quantitative behavioral assays and linkage mapping to identify a genetic locus (nict-1) that mediates the phoretic interaction with terrestrial isopods. The nict-1 locus contains a Piwi-interacting small RNA (piRNA) cluster; we observe that the Piwi Argonaute PRG-1 is involved in the regulation of nictation. Additionally, this locus underlies a trade-off between offspring production and dispersal. Variation in the nict-1 locus contributes directly to differences in association between nematodes and terrestrial isopods in a laboratory assay. In summary, the piRNA-rich nict-1 locus could define a novel mechanism underlying phoretic interactions.

Nictation is a behaviour in which a nematode stands on its tail and waves its head in three dimensions. This activity promotes dispersal of dauer larvae by allowing them to attach to other organisms and travel on them to a new niche. In this review, we describe our understanding of nictation, including its diversity in nematode species, how it is induced by environmental factors, and neurogenetic factors that regulate nictation. We also highlight the known cellular and signalling factors that affect nictation, for example, IL2 neurons, insulin/IGF-1 signalling, TGF-b signalling, FLP neuropeptides and piRNAs. Elucidation of the mechanism of nictation will contribute to increased understanding of the conserved dispersal strategies in animals.

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.

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In animals, cilia are important for the interaction with environments and the proper function of tissues and organs. Understanding the distinctive identities of each type of ciliated cell is essential for therapeutic solutions for ciliopathies, complex disorders with impairments of various organs caused by defective cilia development and function. Here, we report a regulatory module consisting of a cascade of transcription factors and their target genes that confer the cell type-specific ciliary identities on the IL2 ciliated neurons in C. elegans. We found that DAF-19M, isoform of the sole C. elegans RFX transcription factor DAF-19, through X-box promoter motif variants, heads a regulatory module in IL2 neurons, comprising the core target genes klp-6 (kinesin), osm-9 (TRP channel), and cwp-4 (novel); under the overall control of terminal selector proteins UNC-86 and CFI-1. Considering the conservation of this DAF-19M module in IL2 neurons for nictation, a dauer larva-specific behavior, and in male-specific neurons for mating behavior, we propose the existence of an evolutionarily adaptable, hard-wired genetic module for distinct behaviors that share the feature "recognizing the environment."

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