The Long-Rage Directional Behavior of the Nematode C. Elegans

Like any mobile organism, C. elegans relies on sensory cues to find food. In the absence of such cues, animals might display defined search patterns or other stereotyped behavior. The motion of C. elegans has previously been characterized as a sinusoid whose direction can be modulated by gradual steering or by sharp turns, reversals and omega bends. However, such a fine-grained behavioral description does not by itself predict the longrange features of the animals’ pattern of movement. Using large (24 cm x 24 cm) Petri dishes, we characterized the movement pattern of C. elegans in the absence of stimuli. To collect trajectories over such a large surface, we devised an imaging setup employing an array of consumer flatbed scanners. We have confirmed quantitatively the results obtained with the scanner-array setup with a camera imaging setup, in a more stringently homogeneous environment. Wild-type worms display striking behavior in the absence of food. The majority (~60%) of the animals’ paths displays persistence in the direction of motion over length scales that are 50-100 times the body-length of C. elegans. The overall direction of movement differs from animal to animal, suggesting that the directed motion we observe might not be interpreted as a taxis to an external cue in the experimental environment. Interestingly, animals appear to exhibit directionality at large scales despite nondirectional motion at smaller scales. We quantified the extent of local directional persistence by computing the autocorrelation function of the velocities. Unexpectedly, correlations in the direction of motion decay over time scales that are much faster than the scales over which directional persistence appears to be maintained. We sought to establish quantitatively that the worm motion is, in fact, biased. To determine whether a null, random walk-like model of locomotion could account for directional behavior, we generated synthetic trajectories drawing from the same angle and step distributions of individual trajectories, and quantified the probabilities of obtaining larger net displacements than the experimental. Such a model fails to reproduce the experimental results. Moreover, the mean square displacements computed for the data display non-diffusive behavior, further demonstrating that the observed directional persistence cannot be explained by a simple random-walk model. To corroborate the hypothesis of biased movement in a model-independent fashion, we employed a geometrical characterization of the trajectories. Isotropic, unbiased walks result in paths that display a random distribution of turning angles between consecutive segments. In contrast, parsing of the worm’s trajectories yields different results depending on the segmentation scale adopted. In fact, increasing the segment size results in increasingly narrow turning angle distributions, centered around the zero. This suggests the emergence of directional coherence at long time scales. In order to investigate whether directional persistence is attained by a sensory mechanism, we analyzed the paths displayed by animals with impaired sensory function. Animals mutant for che-2, which display disrupted ciliary morphology and pleiotropic behavioral defects, exhibited non-directional behavior. Surprisingly however, daf-19 mutants, which lack sensory cilia altogether, displayed residual directionality, albeit at a lower penetrance (~20%) than the wild-type. This result suggests that directionality might implicate sensory modalities that do not require ciliary function, such as AFD-mediated thermosensation or URX-mediated oxygen sensation. Alternatively, the behavior of daf-19 mutants might imply that neural activity, but not sensory inputs, are required to achieve directed motion. Mutations in osm-9, a TRPV channel implicated in several avoidance behaviors in the worm, did not result in an observable phenotype. In contrast, mutations in tax-2/tax-4, a cGMP-gated channel required to transduce a number of sensory stimuli, resulted in loss of directionality. However, specific mutations targeting the signal transduction pathways for thermotaxis, olfaction, phototaxis, and aerotaxis, upstream of TAX-4, did not disrupt directional behavior. To get further insight into the nature of the stimulus directing the animals’ behavior, if any, we performed rescue experiments of TAX-4 function in specific subsets of neurons. In agreement with the results obtained by genetic lesions in the signal transduction pathways for thermotaxis and odortaxis, no rescue of directional behavior was observed when expressing TAX-4 in the thermosensory neuron AFD, or in the olfactory neurons AWB and AWC. Partial rescue of wild-type behavior was obtained by expression of TAX-4 in a set of five cells, which comprised the oxygen-sensing AQR, PQR and URX neurons as well as the ASJ and ASK sensory neurons, which transduce chemical stimuli and responses to dauer pheromone. To address the concern that the animals’ motion might be directed to a chemosensory cue within the plate, we investigated the correlation between path directions displayed by animals that were assayed on a same plate. We did not observe a detectable correlation between path headings, indicating that the worm is not chemotaxing to a plate-specific cue. In conclusion, our results indicate that the motion of C. elegans cannot be assimilated to a random walk, and that directional persistence arises at long times despite local nondirectional behavior. In addition, although we have not conclusively ruled out a sensorybased explanation, the genetic and phenomenological evidence gathered foreshadows the intriguing possibility that C. elegans might be achieving directional motion by relying solely on self-based information

Glia Are Required For Sensory Neuron Morphology And Function In Caenorhabditis elegans

The nervous system emerges from the coordinated development of neurons and glia. To better understand the processes that enable nervous system development and function we have studied the sensory organs of Caenorhabditis elegans because their anatomy and function are well-characterized. Specifically, we have focused on two aspects of sensory organs: how do glia interact with neurons to enable proper development and function and how are sensory cilia generated. To uncover any glial roles, we ablated the major glial cell of the amphid sensilla. Embryonic glial ablation did not affect neuronal survival and resulted in sensory neuron dendrites that were far too short, revealing a glial role in anchoring sensory neuron dendrites. To examine post-developmental glial roles, we ablated glia after the amphid sensory organ was fully formed. These glia-ablated animals exhibited profound sensory deficits as determined by behavioral assays, failed to maintain the proper morphology of some modified sensory cilia, and had defects in neuronal uptake of lipophilic dyes. Further, animals lacking glia showed no Ca2+ responses in the ASH sensory neuron after stimulation with a high osmolarity solution. To understand the molecular bases of these glial activities, we characterized a sheath glia expressed gene, fig-1, that encodes a protein with thrombospondin type I domains. FIG-1 likely functions extracellularly, is essential for neuronal dye uptake, and also affects behavior. To characterize the molecular basis of cilia morphogenesis and function, we cloned the che-12 and dyf-11 mutants which have chemotaxis and dye uptake defects. CHE-12 and DYF-11 are conserved ciliary proteins required for maintenance of cilium morphology and function. Furthermore, DYF-11 undergoes intraflagellar transport (IFT) and may function at an early stage of IFT-B particle assembly. Our results suggest that glia are required for multiple aspects of sensory organ function. Moreover, as thrombospondin 1 is a glial-secreted protein required for synapse formation in mice, these results suggest that some of the molecular components underlying glia-neuron interactions in C. elegans might be conserved

Chemosensory Communication and Neural Substrates of Social Behavior in the African Cichlid Fish, Astatotilapia burtoni

Social animals must constantly assess their environment to make appropriate decisions. In several fish species, chemosensory signaling is crucial for social communication, conveying information on sex, reproductive state, and social status. Despite its importance in fishes, relatively little is known about how they employ chemical signaling, with only a few of the over 30,000 extant species investigated thus far. Further, there is a scarcity of information about where and how socially-relevant chemosensory signals are processed in the brain of fishes, or how chemosensory signals are integrated with other senses to elicit appropriate behaviors. For my doctoral research, I examined the role of chemosensory signaling during social interactions in a model cichlid fish, Astatotilapia burtoni. I also examined neural activation patterns in various social contexts to understand how socially-relevant visual and chemosensory signals are processed in the brain. Finally, I examined levels of putative pheromone-detecting receptors in the olfactory epithelium of juveniles and adults of varying social and reproductive status to gain insight into peripheral processing of sexually-relevant olfactory signals. I found that, similar to A. burtoni males, females use contextual urine release with higher urination rates in the presence of dominant males (reproductive context) and brooding females (aggressive context). Using in situ hybridization for the immediate early gene cfos as a proxy for neural activation, I identified key brain regions involved in mediating these context-specific behaviors. Further, I show that dominant males have altered behavioral and neural responses to visual and chemosensory signals from receptive females, which supports nonredundant signaling. Finally, using quantitative PCR I evaluated gene levels of all six putative pheromone detecting receptors, oras, in the olfactory epithelium and found that expression of some oras varies with reproductive status in females but not males. These data reveal the neural substrates mediating intra- and inter-sexual social behaviors in a single fish species, advancing our understanding of how socially-relevant chemosensory information is processed in the brain. It also provides the framework for examining how olfaction is integrated with other sensory modalities in the brain to mediate social communication and adaptive behavioral decisions across vertebrates

Comparison of electrophysiological auditory measures in fishes

© Springer International Publishing Switzerland 2016. Sounds provide fishes with important information used to mediate behaviors such as predator avoidance, prey detection, and social communication. How we measure auditory capabilities in fishes, therefore, has crucial implications for interpreting how individual species use acoustic information in their natural habitat. Recent analyses have highlighted differences between behavioral and electrophysiologically determined hearing thresholds, but less is known about how physiological measures at different auditory processing levels compare within a single species. Here we provide one of the first comparisons of auditory threshold curves determined by different recording methods in a single fish species, the soniferous Hawaiian sergeant fish Abudefduf abdominalis, and review past studies on representative fish species with tuning curves determined by different methods. The Hawaiian sergeant is a colonial benthic-spawning damselfish (Pomacentridae) that produces low-frequency, low-intensity sounds associated with reproductive and agonistic behaviors. We compared saccular potentials, auditory evoked potentials (AEP), and single neuron recordings from acoustic nuclei of the hindbrain and midbrain torus semicircularis. We found that hearing thresholds were lowest at low frequencies (~75–300 Hz) for all methods, which matches the spectral components of sounds produced by this species. However, thresholds at best frequency determined via single cell recordings were ~15–25 dB lower than those measured by AEP and saccular potential techniques. While none of these physiological techniques gives us a true measure of the auditory “perceptual” abilities of a naturally behaving fish, this study highlights that different methodologies can reveal similar detectable range of frequencies for a given species, but absolute hearing sensitivity may vary considerably

Robustness Enhancement of Sensory Transduction by Hair Bundles

How do biological systems ensure robustness of function despite developmental and environmental variation? Our sense of hearing boasts exquisite sensitivity, precise frequency discrimination, and a broad dynamic range. Experiments and modeling imply, however, that the auditory system achieves this performance for only a narrow range of parameter values. Although the operation of some systems appears to require precise control over parameter values, I describe how the function of the ear might instead be made robust to parameter perturbation. The sensory hair cells of the cochlea are physiologically vulnerable: small changes in parameter values could compromise hair cells\u27 ability to detect stimuli. Most ears, however, remain highly sensitive despite differences in their physical properties. I propose that, rather than exerting tight control over parameters, the auditory system employs a homeostatic mechanism that increases the robustness of its operation to variation in parameter values. To slowly adjust the response to sinusoidal stimulation, the homeostatic mechanism feeds back to its adaptation process a rectified version of the hair bundle\u27s displacement. When homeostasis is enforced, the range of parameter values for which the sensitivity, tuning sharpness, and dynamic range exceed specified thresholds can increase by more than an order of magnitude. Certain characteristics of the hair cell\u27s behavior might provide a means to determine through experiment whether such a mechanism operates in the auditory system. This homeostatic strategy constitutes a general principle by which many biological systems might ensure robustness of function

A small, computationally flexible network produces the phenotypic diversity of song recognition in crickets.

How neural networks evolved to generate the diversity of species-specific communication signals is unknown. For receivers of the signals, one hypothesis is that novel recognition phenotypes arise from parameter variation in computationally flexible feature detection networks. We test this hypothesis in crickets, where males generate and females recognize the mating songs with a species-specific pulse pattern, by investigating whether the song recognition network in the cricket brain has the computational flexibility to recognize different temporal features. Using electrophysiological recordings from the network that recognizes crucial properties of the pulse pattern on the short timescale in the cricket Gryllus bimaculatus, we built a computational model that reproduces the neuronal and behavioral tuning of that species. An analysis of the model's parameter space reveals that the network can provide all recognition phenotypes for pulse duration and pause known in crickets and even other insects. Phenotypic diversity in the model is consistent with known preference types in crickets and other insects, and arises from computations that likely evolved to increase energy efficiency and robustness of pattern recognition. The model's parameter to phenotype mapping is degenerate - different network parameters can create similar changes in the phenotype - which likely supports evolutionary plasticity. Our study suggests that computationally flexible networks underlie the diverse pattern recognition phenotypes, and we reveal network properties that constrain and support behavioral diversity

Engineering derivatives from biological systems for advanced aerospace applications

The present study consisted of a literature survey, a survey of researchers, and a workshop on bionics. These tasks produced an extensive annotated bibliography of bionics research (282 citations), a directory of bionics researchers, and a workshop report on specific bionics research topics applicable to space technology. These deliverables are included as Appendix A, Appendix B, and Section 5.0, respectively. To provide organization to this highly interdisciplinary field and to serve as a guide for interested researchers, we have also prepared a taxonomy or classification of the various subelements of natural engineering systems. Finally, we have synthesized the results of the various components of this study into a discussion of the most promising opportunities for accelerated research, seeking solutions which apply engineering principles from natural systems to advanced aerospace problems. A discussion of opportunities within the areas of materials, structures, sensors, information processing, robotics, autonomous systems, life support systems, and aeronautics is given. Following the conclusions are six discipline summaries that highlight the potential benefits of research in these areas for NASA's space technology programs