A longitudinal study of Caenorhabditis elegans larvae reveals a novel locomotion switch, regulated by G(αs) signaling

Functional Insights into Protein Kinase A (PKA) Signaling from C. elegans

Protein kinase A (PKA), which regulates a diverse set of biological functions downstream of cyclic AMP (cAMP), is a tetramer consisting of two catalytic subunits (PKA-C) and two regulatory subunits (PKA-R). When cAMP binds the PKA-R subunits, the PKA-C subunits are released and interact with downstream effectors. In Caenorhabditis elegans (C. elegans), PKA-C and PKA-R are encoded by kin-1 and kin-2, respectively. This review focuses on the contributions of work in C. elegans to our understanding of the many roles of PKA, including contractility and oocyte maturation in the reproductive system, lipid metabolism, physiology, mitochondrial function and lifespan, and a wide variety of behaviors. C. elegans provides a powerful genetic platform for understanding how this kinase can regulate an astounding variety of physiological responses.

Behavioral States

Caenorhabditis elegans' behavioral states, like those of other animals, are shaped by its immediate environment, its past experiences, and by internal factors. We here review the literature on C. elegans behavioral states and their regulation. We discuss dwelling and roaming, local and global search, mate finding, sleep, and the interaction between internal metabolic states and behavior.

A wake-active locomotion circuit depolarizes a sleep-active neuron to switch on sleep

Sleep-active neurons depolarize during sleep to suppress wakefulness circuits. Wake-active wake-promoting neurons in turn shut down sleep-active neurons, thus forming a bipartite flip-flop switch. However, how sleep is switched on is unclear because it is not known how wakefulness is translated into sleep-active neuron depolarization when the system is set to sleep. Using optogenetics in Caenorhabditis elegans, we solved the presynaptic circuit for depolarization of the sleep-active RIS neuron during developmentally regulated sleep, also known as lethargus. Surprisingly, we found that RIS activation requires neurons that have known roles in wakefulness and locomotion behavior. The RIM interneurons—which are active during and can induce reverse locomotion—play a complex role and can act as inhibitors of RIS when they are strongly depolarized and as activators of RIS when they are modestly depolarized. The PVC command interneurons, which are known to promote forward locomotion during wakefulness, act as major activators of RIS. The properties of these locomotion neurons are modulated during lethargus. The RIMs become less excitable. The PVCs become resistant to inhibition and have an increased capacity to activate RIS. Separate activation of neither the PVCs nor the RIMs appears to be sufficient for sleep induction; instead, our data suggest that they act in concert to activate RIS. Forward and reverse circuit activity is normally mutually exclusive. Our data suggest that RIS may be activated at the transition between forward and reverse locomotion states, perhaps when both forward (PVC) and reverse (including RIM) circuit activity overlap. While RIS is not strongly activated outside of lethargus, altered activity of the locomotion interneurons during lethargus favors strong RIS activation and thus sleep. The control of sleep-active neurons by locomotion circuits suggests that sleep control may have evolved from locomotion control. The flip-flop sleep switch in C. elegans thus requires an additional component, wake-active sleep-promoting neurons that translate wakefulness into the depolarization of a sleep-active neuron when the worm is sleepy. Wake-active sleep-promoting circuits may also be required for sleep state switching in other animals, including in mammals.

This study in nematodes shows that to understand sleep state switching, the flip-flop model for sleep regulation needs to be complemented by additional wake-active sleep-promoting neurons that activate sleep-active sleep-promoting neurons to induce sleep.

Powerful and interpretable behavioural features for quantitative phenotyping of Caenorhabditis elegans

Behaviour is a sensitive and integrative readout of nervous system function and therefore an attractive measure for assessing the effects of mutation or drug treatment on animals. Video data provide a rich but high-dimensional representation of behaviour, and so the first step of analysis is often some form of tracking and feature extraction to reduce dimensionality while maintaining relevant information. Modern machine-learning methods are powerful but notoriously difficult to interpret, while handcrafted features are interpretable but do not always perform as well. Here, we report a new set of handcrafted features to compactly quantify Caenorhabditis elegans behaviour. The features are designed to be interpretable but to capture as much of the phenotypic differences between worms as possible. We show that the full feature set is more powerful than a previously defined feature set in classifying mutant strains. We then use a combination of automated and manual feature selection to define a core set of interpretable features that still provides sufficient power to detect behavioural differences between mutant strains and the wild-type. Finally, we apply the new features to detect time-resolved behavioural differences in a series of optogenetic experiments targeting different neural subsets.

This article is part of a discussion meeting issue ‘Connectome to behaviour: modelling C. elegans at cellular resolution’.

PKA/KIN-1 mediates innate immune responses to bacterial pathogens in Caenorhabditis elegans

The genetically tractable organism Caenorhabditis elegans is a powerful model animal for the study of host innate immunity. Although the intestine and the epidermis of C. elegans that is in contact with pathogens are likely to function as sites for the immune function, recent studies indicate that the nervous system could control innate immunity in C. elegans. In this report, we demonstrated that protein kinase A (PKA)/KIN-1 in the neurons contributes to resistance against Salmonella enterica infection in C. elegans. Microarray analysis revealed that PKA/KIN-1 regulates the expression of a set of antimicrobial effectors in the non-neuron tissues, which are required for innate immune responses to S. enterica. Furthermore, PKA/KIN-1 regulated the expression of lysosomal genes during S. enterica infection. Our results suggest that the lysosomal signaling molecules are involved in autophagy by controlling autophagic flux, rather than formation of autophagosomes. As autophagy is crucial for host defense against S. enterica infection in a metazoan, the lysosomal pathway also acts as a downstream effector of the PKA/KIN-1 signaling for innate immunity. Our data indicate that the PKA pathway contributes to innate immunity in C. elegans by signaling from the nervous system to periphery tissues to protect the host against pathogens.

Food responsiveness regulates episodic behavioral states in Caenorhabditis elegans

Animals optimize survival and reproduction in part through control of behavioral states, which depend on an organism's internal and external environments. In the nematode Caenorhabditis elegans a variety of behavioral states have been described, including roaming, dwelling, quiescence, and episodic swimming. These states have been considered in isolation under varied experimental conditions, making it difficult to establish a unified picture of how they are regulated. Using long-term imaging, we examined C. elegans episodic behavioral states under varied mechanical and nutritional environments. We found that animals alternate between high-activity (active) and low-activity (sedentary) episodes in any mechanical environment, while the incidence of episodes and their behavioral composition depend on food levels. During active episodes, worms primarily roam, as characterized by continuous whole body movement. During sedentary episodes, animals exhibit dwelling (slower movements confined to the anterior half of the body) and quiescence (a complete lack of movement). Roaming, dwelling, and quiescent states are manifest not only through locomotory characteristics but also in pharyngeal pumping (feeding) and in egg-laying behaviors. Next, we analyzed the genetic basis of behavioral states. We found that modulation of behavioral states depends on neuropeptides and insulin-like signaling in the nervous system. Sensory neurons and the Foraging homolog EGL-4 regulate behavior through control of active/sedentary episodes. Optogenetic stimulation of dopaminergic and serotonergic neurons induced dwelling, implicating dopamine as a dwell-promoting neurotransmitter. Our findings provide a more unified description of behavioral states and suggest that perception of nutrition is a conserved mechanism for regulating animal behavior.NEW & NOTEWORTHY One strategy by which animals adapt to their internal states and external environments is by adopting behavioral states. The roundworm Caenorhabditis elegans is an attractive model for investigating how behavioral states are genetically and neuronally controlled. Here we describe the hierarchical organization of behavioral states characterized by locomotory activity, feeding, and egg-laying. We show that decisions to engage in these behaviors are controlled by the nervous system through insulin-like signaling and the perception of food.

Sleep and Development in Genetically Tractable Model Organisms

Sleep is widely recognized as essential, but without a clear singular function. Inadequate sleep impairs cognition, metabolism, immune function, and many other processes. Work in genetic model systems has greatly expanded our understanding of basic sleep neurobiology as well as introduced new concepts for why we sleep. Among these is an idea with its roots in human work nearly 50 years old: sleep in early life is crucial for normal brain maturation. Nearly all known species that sleep do so more while immature, and this increased sleep coincides with a period of exuberant synaptogenesis and massive neural circuit remodeling. Adequate sleep also appears critical for normal neurodevelopmental progression. This article describes recent findings regarding molecular and circuit mechanisms of sleep, with a focus on development and the insights garnered from models amenable to detailed genetic analyses.

Inactivity periods and postural change speed can explain atypical postural change patterns of Caenorhabditis elegans mutants

BACKGROUND

With rapid advances in genome sequencing and editing technologies, systematic and quantitative analysis of animal behavior is expected to be another key to facilitating data-driven behavioral genetics. The nematode Caenorhabditis elegans is a model organism in this field. Several video-tracking systems are available for automatically recording behavioral data for the nematode, but computational methods for analyzing these data are still under development.

RESULTS

In this study, we applied the Gaussian mixture model-based binning method to time-series postural data for 322 C. elegans strains. We revealed that the occurrence patterns of the postural states and the transition patterns among these states have a relationship as expected, and such a relationship must be taken into account to identify strains with atypical behaviors that are different from those of wild type. Based on this observation, we identified several strains that exhibit atypical transition patterns that cannot be fully explained by their occurrence patterns of postural states. Surprisingly, we found that two simple factors-overall acceleration of postural movement and elimination of inactivity periods-explained the behavioral characteristics of strains with very atypical transition patterns; therefore, computational analysis of animal behavior must be accompanied by evaluation of the effects of these simple factors. Finally, we found that the npr-1 and npr-3 mutants have similar behavioral patterns that were not predictable by sequence homology, proving that our data-driven approach can reveal the functions of genes that have not yet been characterized.

CONCLUSION

We propose that elimination of inactivity periods and overall acceleration of postural change speed can explain behavioral phenotypes of strains with very atypical postural transition patterns. Our methods and results constitute guidelines for effectively finding strains that show "truly" interesting behaviors and systematically uncovering novel gene functions by bioimage-informatic approaches.

HandKAchip - Hands-free killing assay on a chip

Small animals such as the roundworm C. elegans are excellent models for studying bacterial infection and host response, as well as for genetic and chemical screens. A key methodology is the killing assay, in which the number of surviving animals is tracked as a function of the time post infection. This is a labor-intensive procedure, prone to human error and subjective choices, and often involves undesired perturbation to the animals and their environment. In addition, the survival of animals is just one aspect of a multi-dimensional complex biological process. Here we report a microfluidic-based approach for performing killing assays in worms, compatible with standard assays performed on solid media. In addition to providing accurate and reproducible survival curves at a considerably reduced labor, this approach allows acquisition of a multitude of quantitative data with minimal undesired perturbations. These measurements are obtained automatically at a worm-by-worm resolution using a custom image processing workflow. The proposed approach is simple, scalable, and extendable, and is significantly more economical than standard manual protocols.

The Neuropeptides FLP-2 and PDF-1 Act in Concert To Arouse Caenorhabditis elegans Locomotion

During larval molts, Caenorhabditis elegans exhibits a sleep-like state (termed lethargus) that is characterized by the absence of feeding and profound locomotion quiescence. The rhythmic pattern of locomotion quiescence and arousal linked to the molting cycle is mediated by reciprocal changes in sensory responsiveness, whereby arousal is associated with increased responsiveness. Sensory neurons arouse locomotion via release of a neuropeptide (PDF-1) and glutamate. Here we identify a second arousing neuropeptide (FLP-2). We show that FLP-2 acts via an orexin-like receptor (FRPR-18), and that FLP-2 and PDF-1 secretion are regulated by reciprocal positive feedback. These results suggest that the aroused behavioral state is stabilized by positive feedback between two neuropeptides.

Serotonin promotes exploitation in complex environments by accelerating decision-making

Background

Fast responses can provide a competitive advantage when resources are inhomogeneously distributed. The nematode Caenorhabditis elegans was shown to modulate locomotion on a lawn of bacterial food in serotonin (5-HT)-dependent manners. However, potential roles for serotonergic signaling in responding to food discovery are poorly understood.

Results

We found that 5-HT signaling in C. elegans facilitates efficient exploitation in complex environments by mediating a rapid response upon encountering food. Genetic or cellular manipulations leading to deficient serotonergic signaling resulted in gradual responses and defective exploitation of a patchy foraging landscape. Physiological imaging revealed that the NSM serotonergic neurons responded acutely upon encounter with newly discovered food and were key to rapid responses. In contrast, the onset of responses of ADF serotonergic neurons preceded the physical encounter with the food. The serotonin-gated chloride channel MOD-1 and the ortholog of mammalian 5-HT1 metabotropic serotonin receptors SER-4 acted in synergy to accelerate decision-making. The relevance of responding rapidly was demonstrated in patchy environments, where the absence of 5-HT signaling was detrimental to exploitation.

Conclusions

Our results implicate 5-HT in a novel form of decision-making, demonstrate its fitness consequences, suggest that NSM and ADF act in concert to modulate locomotion in complex environments, and identify the synergistic action of a channel and a metabotropic receptor in accelerating C. elegans decision-making.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0232-y) contains supplementary material, which is available to authorized users.

Distinct Mechanisms Underlie Quiescence during Two Caenorhabditis elegans Sleep-Like States

Electrophysiological recordings have enabled identification of physiologically distinct yet behaviorally similar states of mammalian sleep. In contrast, sleep in nonmammals has generally been identified behaviorally and therefore regarded as a physiologically uniform state characterized by quiescence of feeding and locomotion, reduced responsiveness, and rapid reversibility. The nematode Caenorhabditis elegans displays sleep-like quiescent behavior under two conditions: developmentally timed quiescence (DTQ) occurs during larval transitions, and stress-induced quiescence (SIQ) occurs in response to exposure to cellular stressors. Behaviorally, DTQ and SIQ appear identical. Here, we use optogenetic manipulations of neuronal and muscular activity, pharmacology, and genetic perturbations to uncover circuit and molecular mechanisms of DTQ and SIQ. We find that locomotion quiescence induced by DTQ- and SIQ-associated neuropeptides occurs via their action on the nervous system, although their neuronal target(s) and/or molecular mechanisms likely differ. Feeding quiescence during DTQ results from a loss of pharyngeal muscle excitability, whereas feeding quiescence during SIQ results from a loss of excitability in the nervous system. Together these results indicate that, as in mammals, quiescence is subserved by different mechanisms during distinct sleep-like states in C. elegans.

Caenorhabditis elegans exhibit a coupling between the defecation motor program and directed locomotion

Distinct motor programs can be coupled to refine the repertoire of behavior dynamics. However, mechanisms underlying such coupling are poorly understood. The defecation motor program (DMP) of C. elegans is composed of a succession of body contraction and expulsion steps, performed repeatedly with a period of 50-60 sec. We show that recurring patterns of directed locomotion are executed in tandem with, co-reset, and co-terminate with the DMP cycle. Calcium waves in the intestine and proton signaling were shown to regulate the DMP. We found that genetic manipulations affecting these calcium dynamics regulated the corresponding patterns of directed locomotion. Moreover, we observed the initiation of a recurring locomotion pattern 10 seconds prior to the posterior body contraction, suggesting that the synchronized motor program may initiate prior to the DMP. This study links two multi-step motor programs executed by C. elegans in synchrony, utilizing non-neuronal tissue to drive directed locomotion.

A Generative Statistical Algorithm for Automatic Detection of Complex Postures

This paper presents a method for automated detection of complex (non-self-avoiding) postures of the nematode Caenorhabditis elegans and its application to analyses of locomotion defects. Our approach is based on progressively detailed statistical models that enable detection of the head and the body even in cases of severe coilers, where data from traditional trackers is limited. We restrict the input available to the algorithm to a single digitized frame, such that manual initialization is not required and the detection problem becomes embarrassingly parallel. Consequently, the proposed algorithm does not propagate detection errors and naturally integrates in a "big data" workflow used for large-scale analyses. Using this framework, we analyzed the dynamics of postures and locomotion of wild-type animals and mutants that exhibit severe coiling phenotypes. Our approach can readily be extended to additional automated tracking tasks such as tracking pairs of animals (e.g., for mating assays) or different species.

Sensory Neurons Arouse C. elegans Locomotion via Both Glutamate and Neuropeptide Release

C. elegans undergoes periods of behavioral quiescence during larval molts (termed lethargus) and as adults. Little is known about the circuit mechanisms that establish these quiescent states. Lethargus and adult locomotion quiescence is dramatically reduced in mutants lacking the neuropeptide receptor NPR-1. Here, we show that the aroused locomotion of npr-1 mutants results from the exaggerated activity in multiple classes of sensory neurons, including nociceptive (ASH), touch sensitive (ALM and PLM), and stretch sensing (DVA) neurons. These sensory neurons accelerate locomotion via both neuropeptide and glutamate release. The relative contribution of these sensory neurons to arousal differs between larval molts and adults. Our results suggest that a broad network of sensory neurons dictates transitions between aroused and quiescent behavioral states.

Animals switch between periods of behavioral arousal and quiescence in response to environmental, developmental, and circadian cues. Little is known about the circuit mechanisms that produce these behavioral states. During larval molts, C. elegans exhibits a sleep-like state (termed lethargus) that is characterized by the absence of feeding and profound locomotion quiescence. We previously showed that mutants lacking the neuropeptide receptor NPR-1 exhibit increased arousal during larval molts, which is in part mediated by increased secretion of an arousal peptide (PDF-1). Here, we compare the circuits regulating arousal in larval molts and adults. We show that a broad network of sensory neurons arouses locomotion but that the impact of each neuron differs between lethargus and adults. We propose that this broad sensory network allows C. elegans to adapt its behavior across a broad range of developmental and physiological circumstances.

Agarose Microchambers for Long-term Calcium Imaging of Caenorhabditis elegans

Behavior is controlled by the nervous system. Calcium imaging is a straightforward method in the transparent nematode Caenorhabditis elegans to measure the activity of neurons during various behaviors. To correlate neural activity with behavior, the animal should not be immobilized but should be able to move. Many behavioral changes occur during long time scales and require recording over many hours of behavior. This also makes it necessary to culture the worms in the presence of food. How can worms be cultured and their neural activity imaged over long time scales? Agarose Microchamber Imaging (AMI) was previously developed to culture and observe small larvae and has now been adapted to study all life stages from early L1 until the adult stage of C. elegans. AMI can be performed on various life stages of C. elegans. Long-term calcium imaging is achieved without immobilizing the animals by using short externally triggered exposures combined with an electron multiplying charge-coupled device (EMCCD) camera recording. Zooming out or scanning can scale up this method to image up to 40 worms in parallel. Thus, a method is described to image behavior and neural activity over long time scales in all life stages of C. elegans.

Homeostasis in C. elegans sleep is characterized by two behaviorally and genetically distinct mechanisms

Biological homeostasis invokes modulatory responses aimed at stabilizing internal conditions. Using tunable photo- and mechano-stimulation, we identified two distinct categories of homeostatic responses during the sleep-like state of Caenorhabditis elegans (lethargus). In the presence of weak or no stimuli, extended motion caused a subsequent extension of quiescence. The neuropeptide Y receptor homolog, NPR-1, and an inhibitory neuropeptide known to activate it, FLP-18, were required for this process. In the presence of strong stimuli, the correlations between motion and quiescence were disrupted for several minutes but homeostasis manifested as an overall elevation of the time spent in quiescence. This response to strong stimuli required the function of the DAF-16/FOXO transcription factor in neurons, but not that of NPR-1. Conversely, response to weak stimuli did not require the function of DAF-16/FOXO. These findings suggest that routine homeostatic stabilization of sleep may be distinct from homeostatic compensation following a strong disturbance.

DOI:http://dx.doi.org/10.7554/eLife.04380.001

The regenerative properties of sleep are required by all animals, with even the simplest animal, the nematode Caenorhabditis elegans, displaying a sleep-like state called lethargus. During development, nematodes must pass through four larval stages en route to adulthood, and the end of each stage is preceded by a period of lethargus lasting 2 to 3 hr.

Human sleep is divided into distinct stages that recur in a prescribed order throughout the night. Nematodes, on the other hand, simply experience alternating periods of activity and stillness as they sleep. Nevertheless, in both species, any disruptions to sleep automatically lead to adjustments of the rest of the sleep cycle to compensate for the disturbance and to ensure that the organism gets an adequate amount of sleep overall.

To date, it has been assumed that a single mechanism is responsible for adjusting the sleep cycle after any disturbance, regardless of its severity. However, Nagy, Tramm, Sanders et al. now show that this is not the case in C. elegans. Sleeping nematodes that were lightly disturbed by exposing them to light or to vibrations—causing them to briefly increase their activity levels—compensated for the disturbance by lengthening their next inactive period. By contrast, worms that were vigorously agitated by stronger vibrations showed a different response: the alternating pattern of stillness and activity was disrupted for several minutes, followed by an overall increase in the length of time spent in the stillness phase.

Experiments using genetically modified worms revealed that these two responses involve distinct molecular pathways. A signaling molecule called neuropeptide Y affects the response to minor sleep disruptions, whereas a transcription factor called DAF-16/FOXO is involved in the corresponding role after major disruptions. Given that neuropeptide Y has already been implicated in sleep regulation in humans and flies, it is not implausible that similar mechanisms may occur in response to disturbances of our own sleep.

DOI:http://dx.doi.org/10.7554/eLife.04380.002

Deep conservation of genes required for both Drosphila melanogaster and Caenorhabditis elegans sleep includes a role for dopaminergic signaling

OBJECTIVES

Cross-species conservation of sleep-like behaviors predicts the presence of conserved molecular mechanisms underlying sleep. However, limited experimental evidence of conservation exists. Here, this prediction is tested directly.

MEASUREMENTS AND RESULTS

During lethargus, Caenorhabditis elegans spontaneously sleep in short bouts that are interspersed with bouts of spontaneous locomotion. We identified 26 genes required for Drosophila melanogaster sleep. Twenty orthologous C. elegans genes were selected based on similarity. Their effect on C. elegans sleep and arousal during the last larval lethargus was assessed. The 20 most similar genes altered both the quantity of sleep and arousal thresholds. In 18 cases, the direction of change was concordant with Drosophila studies published previously. Additionally, we delineated a conserved genetic pathway by which dopamine regulates sleep and arousal. In C. elegans neurons, G-alpha S, adenylyl cyclase, and protein kinase A act downstream of D1 dopamine receptors to regulate these behaviors. Finally, a quantitative analysis of genes examined herein revealed that C. elegans arousal thresholds were directly correlated with amount of sleep during lethargus. However, bout duration varies little and was not correlated with arousal thresholds.

CONCLUSIONS

The comprehensive analysis presented here suggests that conserved genes and pathways are required for sleep in invertebrates and, likely, across the entire animal kingdom. The genetic pathway delineated in this study implicates G-alpha S and previously known genes downstream of dopamine signaling in sleep. Quantitative analysis of various components of quiescence suggests that interdependent or identical cellular and molecular mechanisms are likely to regulate both arousal and sleep entry.

Why do sleeping nematodes adopt a hockey-stick-like posture?

A characteristic posture is considered one of the behavioral hallmarks of sleep, and typically includes functional features such as support for the limbs and shielding of sensory organs. The nematode C. elegans exhibits a sleep-like state during a stage termed lethargus, which precedes ecdysis at the transition between larval stages. A hockey-stick-like posture is commonly observed during lethargus. What might its function be? It was previously noted that during lethargus, C. elegans nematodes abruptly rotate about their longitudinal axis. Plausibly, these “flips” facilitate ecdysis by assisting the disassociation of the old cuticle from the new one. We found that body-posture during lethargus was established using a stereotypical motor program and that body bends during lethargus quiescence were actively maintained. Moreover, flips occurred almost exclusively when the animals exhibited a single body bend, preferentially in the anterior or mid section of the body. We describe a simple biomechanical model that imposes the observed lengths of the longitudinally directed body-wall muscles on an otherwise passive elastic rod. We show that this minimal model is sufficient for generating a rotation about the anterior-posterior body axis. Our analysis suggests that posture during lethargus quiescence may serve a developmental role in facilitating flips and that the control of body wall muscles in anterior and posterior body regions are distinct.

Neurobiology of Caenorhabditis elegans Locomotion: Where Do We Stand?

Animals use a nervous system for locomotion in some stage of their life cycle. The nematode Caenorhabditis elegans, a major animal model for almost all fields of experimental biology, has long been used for detailed studies of genetic and physiological locomotion mechanisms. Of its 959 somatic cells, 302 are neurons that are identifiable by lineage, location, morphology, and neurochemistry in every adult hermaphrodite. Of those, 75 motoneurons innervate body wall muscles that provide the thrust during locomotion. In this Overview, we concentrate on the generation of either forward- or backward-directed motion during crawling and swimming. We describe locomotion behavior, the parts constituting the locomotion system, and the relevant neuronal connectivity. Because it is not yet fully understood how these components combine to generate locomotion, we discuss competing hypotheses and models.

Measurements of behavioral quiescence in Caenorhabditis elegans

The nematode Caenorhabditis (C.) elegans, a long time work horse for behavioral genetic studies of locomotion, has recently been studied for quiescent behavior. Methods previously established for the study of C. elegans locomotion are not well-suited for the study of quiescent behavior. We describe in detail two computer vision approaches to distinguish quiescent from movement bouts focusing on the behavioral quiescence that occurs during fourth larval stage lethargus, a transition stage between the larva and the adult. The first is the frame subtraction method, which consists of subtraction of temporally adjacent images as a sensitive way to detect motion. The second, which is more computationally intensive, is the posture analysis method, which consists of analysis of the rate of local angle change of the animal's body. Quiescence measurements should be done continuously while minimizing sensory perturbation of the animal.

Multi-well imaging of development and behavior in Caenorhabditis elegans

BACKGROUND

The nematode Caenorhabditis elegans is widely used as a model for understanding the neuronal and genetic bases of behavior. Recent studies have required longitudinal assessment of individual animal's behavior over extended periods.

NEW METHOD

Here we present a technique for automated monitoring of multiple worms for several days. Our method uses an array of plano-concave glass wells containing standard agar media. The concave well geometry allows worms to be imaged even at the edge of the agar surface and prevents them from burrowing under the agar. We transfer one worm or embryo into each well, and perform imaging of the array of wells using a single camera. Machine vision software is used to quantify size, activity, and/or fluorescence of each worm over time.

RESULTS

We demonstrate the utility of our method in two applications: (1) quantifying behavioral quiescence and developmental rate in wild-type and mutant animals, and (2) characterizing differences in mating behavior between two C. elegans strains.

COMPARISON WITH EXISTING METHOD(S)

Current techniques for tracking behavior in identified worms are generally restricted to imaging either single animals or have not been shown to work with arbitrary developmental stages; many are also technically complex. Our system works with up to 24 animals of any stages and is technically simple.

CONCLUSIONS

Our multi-well imaging method is a powerful tool for quantification of long-term behavioral phenotypes in C. elegans.