Ammonium-acetate is sensed by gustatory and olfactory neurons in Caenorhabditis elegans

P. aeruginosa controls both C. elegans attraction and pathogenesis by regulating nitrogen assimilation

Detecting chemical signals is important for identifying food sources and avoiding harmful agents. Like most animals, C. elegans use olfaction to chemotax towards their main food source, bacteria. However, little is known about the bacterial compounds governing C. elegans attraction to bacteria and the physiological importance of these compounds to bacteria. Here, we address these questions by investigating the function of a small RNA, P11, in the pathogen, Pseudomonas aeruginosa, that was previously shown to mediate learned pathogen avoidance. We discovered that this RNA also affects the attraction of untrained C. elegans to P. aeruginosa and does so by controlling production of ammonia, a volatile odorant produced during nitrogen assimilation. We untangle the complex regulation of P. aeruginosa nitrogen assimilation, which is mediated by a partner-switching mechanism involving environmental nitrates, sensor proteins, and P11. In addition to mediating C. elegans attraction, nitrogen assimilation is important for bacterial fitness and pathogenesis during C. elegans infection by P. aeruginosa . These studies define ammonia as a major mediator of trans-kingdom signaling, reveal the physiological importance of nitrogen assimilation for both bacteria and host organisms, and highlight how a bacterial metabolic pathway can either benefit or harm a host in different contexts.

Involvement of cyclic nucleotide-gated channels in soybean cyst nematode chemotaxis and thermotaxis

The soybean cyst nematode (SCN) is one of the most damaging pests affecting soybean production. SCN displays important host recognition behaviors, such as hatching and infection, by recognizing several compounds produced by the host. Therefore, controlling SCN behaviors such as chemotaxis and thermotaxis is an attractive pest control strategy. In this study, we found that cyclic nucleotide-gated channels (CNG channels) regulate SCN chemotaxis and thermotaxis and Hg-tax-2, a gene encoding a CNG channel, is an important regulator of SCN behavior. Gene silencing of Hg-tax-2 and treatment with a CNG channel inhibitor reduced the attraction of second-stage juveniles to nitrate, an attractant with a different recognition mechanism from the host-derived chemoattractant(s), and to host soybean roots, as well as their avoidance behavior toward high temperatures. Co-treatment of ds Hg-tax-2 with the CNG channel inhibitor indicated that Hg-tax-2 is a major regulator of SCN chemotaxis and thermotaxis. These results suggest new avenues for research on control of SCN.

The proton channel OTOP1 is a sensor for the taste of ammonium chloride

Ammonium (NH4+), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NH4Cl). Extracellular NH4Cl evoked large dose-dependent inward currents in HEK-293 cells expressing murine OTOP1 (mOTOP1), human OTOP1 and other species variants of OTOP1, that correlated with its ability to alkalinize the cell cytosol. Mutation of a conserved intracellular arginine residue (R292) in the mOTOP1 tm 6-tm 7 linker specifically decreased responses to NH4Cl relative to acid stimuli. Taste responses to NH4Cl measured from isolated Type III TRCs, or gustatory nerves were strongly attenuated or eliminated in an Otop1-/- mouse strain. Behavioral aversion of mice to NH4Cl, reduced in Skn-1a-/- mice lacking Type II TRCs, was entirely abolished in a double knockout with Otop1. These data together reveal an unexpected role for the proton channel OTOP1 in mediating a major component of the taste of NH4Cl and a previously undescribed channel activation mechanism.

Forkhead transcription factor FKH-8 cooperates with RFX in the direct regulation of sensory cilia in Caenorhabditis elegans

Cilia, either motile or non-motile (a.k.a primary or sensory), are complex evolutionarily conserved eukaryotic structures composed of hundreds of proteins required for their assembly, structure and function that are collectively known as the ciliome. Ciliome gene mutations underlie a group of pleiotropic genetic diseases known as ciliopathies. Proper cilium function requires the tight coregulation of ciliome gene transcription, which is only fragmentarily understood. RFX transcription factors (TF) have an evolutionarily conserved role in the direct activation of ciliome genes both in motile and non-motile cilia cell-types. In vertebrates, FoxJ1 and FoxN4 Forkhead (FKH) TFs work with RFX in the direct activation of ciliome genes, exclusively in motile cilia cell-types. No additional TFs have been described to act together with RFX in primary cilia cell-types in any organism. Here we describe FKH-8, a FKH TF, as a direct regulator of the sensory ciliome genes in Caenorhabditis elegans. FKH-8 is expressed in all ciliated neurons in C. elegans, binds the regulatory regions of ciliome genes, regulates ciliome gene expression, cilium morphology and a wide range of behaviors mediated by sensory ciliated neurons. FKH-8 and DAF-19 (C. elegans RFX) physically interact and synergistically regulate ciliome gene expression. C. elegans FKH-8 function can be replaced by mouse FOXJ1 and FOXN4 but not by other members of other mouse FKH subfamilies. In conclusion, RFX and FKH TF families act jointly as direct regulators of ciliome genes also in sensory ciliated cell types suggesting that this regulatory logic could be an ancient trait predating functional cilia sub-specialization.

Chemosensory detection of aversive concentrations of ammonia and basic volatile amines in insects

Basic volatiles like ammonia are found in insect environments, and at high concentrations cause an atypical action potential burst, followed by inhibition in multiple classes of olfactory receptor neurons (ORNs) in Drosophila melanogaster. During the period of inhibition, ORNs are unable to fire action potentials to their ligands but continue to display receptor potentials. An increase in calcium is also observed in antennal cells of Drosophila and Aedes aegypti. In the gustatory system, ammonia inhibits sugar and salt responses in a dose-dependent manner. Other amines show similar effects in both gustatory and olfactory neurons, correlated with basicity. The concentrations that inhibit neurons reduce proboscis extension to sucrose in Drosophila. In Aedes, a brief exposure to volatile ammonia abolishes attraction to human skin odor for several minutes. These findings reveal an effect that prevents detection of attractive ligands in the olfactory and gustatory systems and has potential in insect control.

The phosphatidylinositol transfer protein PITP-1 facilitates fast recovery of eating behavior after hypoxia in the nematode Caenorhabditis elegans

Among the fascinating adaptations to limiting oxygen conditions (hypoxia) is the suppression of food intake and weight loss. In humans, this phenomenon is called high-altitude anorexia and is observed in people suffering from acute mountain syndrome. The high-altitude anorexia appears to be conserved in evolution and has been seen in species across the animal kingdom. However, the mechanism underlying the recovery of eating behavior after hypoxia is still not known. Here, we show that the phosphatidylinositol transfer protein PITP-1 is essential for the fast recovery of eating behavior after hypoxia in the nematode Caenorhabditis elegans. Unlike the neuroglobin GLB-5 that accelerates the recovery of eating behavior through its function in the oxygen (O₂ )-sensing neurons, PITP-1 appears to act downstream, in neurons that express the mod-1 serotonin receptor. Indeed, pitp-1 mutants display wild-type-like O₂ -evoked-calcium responses in the URX O₂ -sensing neuron. Intriguingly, loss-of-function of protein kinase C 1 (PKC-1) rescues pitp-1 mutants' recovery after hypoxia. Increased diacylglycerol (DAG), which activates PKC-1, attenuates the recovery of wild-type worms. Together, these data suggest that PITP-1 enables rapid recovery of eating behavior after hypoxia by limiting DAG's availability, thereby limiting PKC activity in mod-1-expressing neurons.

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.

Assessment and Maintenance of Unigametic Germline Inheritance for C. elegans

The recent work of Besseling and Bringmann (2016) identified a molecular intervention for C. elegans in which premature segregation of maternal and paternal chromosomes in the fertilized oocyte can produce viable animals exhibiting a non-Mendelian inheritance pattern. Overexpression in embryos of a single protein regulating chromosome segregation (GPR-1) provides a germline derived clonally from a single parental gamete. We present a collection of strains and cytological assays to consistently generate and track non-Mendelian inheritance. These tools allow reproducible and high-frequency (>80%) production of non-Mendelian inheritance, the facile and simultaneous homozygosis for all nuclear chromosomes in a single generation, the precise exchange of nuclear and mitochondrial genomes between strains, and the assessments of non-canonical mitosis events. We show the utility of these strains by demonstrating a rapid assessment of cell lineage requirements (AB versus P1) for a set of genes (lin-2, lin-3, lin-12, and lin-31) with roles in C. elegans vulval development.

Analysis of Mutants Suggests Kamin Blocking in C. elegans is Due to Interference with Memory Recall Rather than Storage

Higher-order conditioning phenomena, including context conditioning and blocking, occur when conditioning to one set of stimuli interacts with conditioning to a second set of stimuli to modulate the strength of the resultant memories. Here we analyze higher-order conditioning in the nematode worm Caenorhabditis elegans, demonstrating for the first time the presence of blocking in this animal, and dissociating it from context conditioning. We present an initial genetic dissection of these phenomena in a model benzaldehyde/NH₄Cl aversive learning system, and suggest that blocking may involve an alteration of memory retrieval rather than storage. These findings offer a fundamentally different explanation for blocking than traditional explanations, and position C. elegans as a powerful model organism for the study of higher order conditioning.

The taste response to ammonia in Drosophila

Ammonia is both a building block and a breakdown product of amino acids and is found widely in the environment. The odor of ammonia is attractive to many insects, including insect vectors of disease. The olfactory response of Drosophila to ammonia has been studied in some detail, but the taste response has received remarkably little attention. Here, we show that ammonia is a taste cue for Drosophila. Nearly all sensilla of the major taste organ of the Drosophila head house a neuron that responds to neutral solutions of ammonia. Ammonia is toxic at high levels to many organisms, and we find that it has a negative valence in two paradigms of taste behavior, one operating over hours and the other over seconds. Physiological and behavioral responses to ammonia depend at least in part on Gr66a+ bitter-sensing taste neurons, which activate a circuit that deters feeding. The Amt transporter, a critical component of olfactory responses to ammonia, is widely expressed in taste neurons but is not required for taste responses. This work establishes ammonia as an ecologically important taste cue in Drosophila, and shows that it can activate circuits that promote opposite behavioral outcomes via different sensory systems.

A Gustatory Neural Circuit of Caenorhabditis elegans Generates Memory-Dependent Behaviors in Na+ Chemotaxis

Animals show various behaviors in response to environmental chemicals. These behaviors are often plastic depending on previous experiences. Caenorhabditis elegans, which has highly developed chemosensory system with a limited number of sensory neurons, is an ideal model for analyzing the role of each neuron in innate and learned behaviors. Here, we report a new type of memory-dependent behavioral plasticity in Na⁺ chemotaxis generated by the left member of bilateral gustatory neuron pair ASE (ASEL neuron). When worms were cultivated in the presence of Na⁺, they showed positive chemotaxis toward Na⁺, but when cultivated under Na⁺-free conditions, they showed no preference regarding Na⁺ concentration. Both channelrhodopsin-2 (ChR2) activation with blue light and up-steps of Na⁺ concentration activated ASEL only after cultivation with Na⁺, as judged by increase in intracellular Ca²⁺ Under cultivation conditions with Na⁺, photoactivation of ASEL caused activation of its downstream interneurons AIY and AIA, which stimulate forward locomotion, and inhibition of its downstream interneuron AIB, which inhibits the turning/reversal behavior, and overall drove worms toward higher Na⁺ concentrations. We also found that the Gq signaling pathway and the neurotransmitter glutamate are both involved in the behavioral response generated by ASEL.SIGNIFICANCE STATEMENT Animals have acquired various types of behavioral plasticity during their long evolutionary history. Caenorhabditis elegans prefers odors associated with food, but plastically changes its behavioral response according to previous experience. Here, we report a new type of behavioral response generated by a single gustatory sensory neuron, the ASE-left (ASEL) neuron. ASEL did not respond to photostimulation or upsteps of Na⁺ concentration when worms were cultivated in Na⁺-free conditions; however, when worms were cultivated with Na⁺, ASEL responded and inhibited AIB to avoid turning and stimulated AIY and AIA to promote forward locomotion, which collectively drove worms toward higher Na⁺ concentrations. Glutamate and the Gq signaling pathway are essential for driving worms toward higher Na⁺ concentrations.

Embryonic Methamphetamine Exposure Inhibits Methamphetamine Cue Conditioning and Reduces Dopamine Concentrations in Adult N2 Caenorhabditis elegans

Methamphetamine (MAP) addiction is substantially prevalent in today's society, resulting in thousands of deaths and costing billions of dollars annually. Despite the potential deleterious consequences, few studies have examined the long-term effects of embryonic MAP exposure. Using the invertebrate nematode Caenorhabditis elegans allows for a controlled analysis of behavioral and neurochemical changes due to early developmental drug exposure. The objective of the current study was to determine the long-term behavioral and neurochemical effects of embryonic exposure to MAP in C. elegans. In addition, we sought to improve our conditioning and testing procedures by utilizing liquid filtration, as opposed to agar, and smaller, 6-well testing plates to increase throughput. Wild-type N2 C. elegans were embryonically exposed to 50 μM MAP. Using classical conditioning, adult-stage C. elegans were conditioned to MAP (17 and 500 μM) in the presence of either sodium ions (Na+) or chloride ions (Cl-) as conditioned stimuli (CS+/CS-). Following conditioning, a preference test was performed by placing worms in 6-well test plates spotted with the CS+ and CS- at opposite ends of each well. A preference index was determined by counting the number of worms in the CS+ target zone divided by the total number of worms in the CS+ and CS- target zones. A food conditioning experiment was also performed in order to determine whether embryonic MAP exposure affected food conditioning behavior. For the neurochemical experiments, adult worms that were embryonically exposed to MAP were analyzed for dopamine (DA) content using high-performance liquid chromatography. The liquid filtration conditioning procedure employed here in combination with the use of 6-well test plates significantly decreased the time required to perform these experiments and ultimately increased throughput. The MAP conditioning data found that pairing an ion with MAP at 17 or 500 μM significantly increased the preference for that ion (CS+) in worms that were not pre-exposed to MAP. However, worms embryonically exposed to MAP did not exhibit significant drug cue conditioning. The inability of MAP-exposed worms to condition to MAP was not associated with deficits in food conditioning, as MAP-exposed worms exhibited a significant cue preference associated with food. Furthermore, our results found that embryonic MAP exposure reduced DA levels in adult C. elegans, which could be a key mechanism contributing to the long-term effects of embryonic MAP exposure. It is possible that embryonic MAP exposure may be impairing the ability of C. elegans to learn associations between MAP and the CS+ or inhibiting the reinforcing properties of MAP. However, our food conditioning data suggest that MAP-exposed animals can form associations between cues and food. The depletion of DA levels during embryonic exposure to MAP could be responsible for driving either of these processes during adulthood.

C. elegans chemotaxis assay

Many organisms use chemotaxis to seek out food sources, avoid noxious substances, and find mates. Caenorhabditis elegans has impressive chemotaxis behavior. The premise behind testing the response of the worms to an odorant is to place them in an area and observe the movement evoked in response to an odorant. Even with the many available assays, optimizing worm starting location relative to both the control and test areas, while minimizing the interaction of worms with each other, while maintaining a significant sample size remains a work in progress (1-10). The method described here aims to address these issues by modifying the assay developed by Bargmann et al.(1). A Petri dish is divided into four quadrants, two opposite quadrants marked "Test" and two are designated "Control". Anesthetic is placed in all test and control sites. The worms are placed in the center of the plate with a circle marked around the origin to ensure that non-motile worms will be ignored. Utilizing a four-quadrant system rather than one 2 or two 1 eliminates bias in the movement of the worms, as they are equidistant from test and control samples, regardless of which side of the origin they began. This circumvents the problem of worms being forced to travel through a cluster of other worms to respond to an odorant, which can delay worms or force them to take a more circuitous route, yielding an incorrect interpretation of their intended path. This method also shows practical advantages by having a larger sample size and allowing the researcher to run the assay unattended and score the worms once the allotted time has expired.

Dedicated olfactory neurons mediating attraction behavior to ammonia and amines in Drosophila

Animals across various phyla exhibit odor-evoked innate attraction behavior that is developmentally programmed. The mechanism underlying such behavior remains unclear because the odorants that elicit robust attraction responses and the neuronal circuits that mediate this behavior have not been identified. Here, we describe a functionally segregated population of olfactory sensory neurons (OSNs) and projection neurons (PNs) in Drosophila melanogaster that are highly specific to ammonia and amines, which act as potent attractants. The OSNs express IR92a, a member of the chemosensory ionotropic receptor (IR) family and project to a pair of glomeruli in the antennal lobe, termed VM1. In vivo calcium-imaging experiments showed that the OSNs and PNs innervating VM1 were activated by ammonia and amines but not by nonamine odorants. Flies in which the IR92a(+) neurons or IR92a gene was inactivated had impaired amine-evoked physiological and behavioral responses. Tracing neuronal pathways to higher brain centers showed that VM1-PN axonal projections within the lateral horn are topographically segregated from those of V-PN and DC4-PN, which mediate innate avoidance behavior to carbon dioxide and acidity, respectively, suggesting that these sensory stimuli of opposing valence are represented in spatially distinct neuroanatomic loci within the lateral horn. These experiments identified the neurons and their cognate receptor for amine detection, and mapped amine attractive olfactory inputs to higher brain centers. This labeled-line mode of amine coding appears to be hardwired to attraction behavior.

Chemosensory cue conditioning with stimulants in a Caenorhabditis elegans animal model of addiction

The underlying molecular mechanisms of drug abuse and addiction behaviors are poorly understood. Caenorhabditis elegans (C. elegans) provide a simple, whole animal model with conserved molecular pathways well suited for studying the foundations of complex diseases. Historically, chemotaxis has been a measure used to examine sensory approach and avoidance behavior in worms. Chemotaxis can be modulated by previous experience, and cue-dependent conditioned learning has been demonstrated in C. elegans, but such conditioning with drugs of abuse has not been reported. Here we show that pairing a distinctive salt cue with a drug (cocaine or methamphetamine) results in a concentration-dependent change in preference for the cue that was paired with the drug during conditioning. Further, we demonstrate that pairing of either drug with a distinctive food type can also increase preference for the drug-paired food in the absence of the drug. Dopamine-deficient mutants did not develop drug-paired, cue-conditioned responses. The findings suggest that, like vertebrates, C. elegans display a conditioned preference for environments containing cues previously associated with drugs of abuse, and this response is dependent on dopamine neurotransmission. This model provides a new and powerful method to study the genetic and molecular mechanisms that mediate drug preference.

Aversive olfactory learning and associative long-term memory in Caenorhabditis elegans

The nematode Caenorhabditis elegans (C. elegans) adult hermaphrodite has 302 invariant neurons and is suited for cellular and molecular studies on complex behaviors including learning and memory. Here, we have developed protocols for classical conditioning of worms with 1-propanol, as a conditioned stimulus (CS), and hydrochloride (HCl) (pH 4.0), as an unconditioned stimulus (US). Before the conditioning, worms were attracted to 1-propanol and avoided HCl in chemotaxis assay. In contrast, after massed or spaced training, worms were either not attracted at all to or repelled from 1-propanol on the assay plate. The memory after the spaced training was retained for 24 h, while the memory after the massed training was no longer observable within 3 h. Worms pretreated with transcription and translation inhibitors failed to form the memory by the spaced training, whereas the memory after the massed training was not significantly affected by the inhibitors and was sensitive to cold-shock anesthesia. Therefore, the memories after the spaced and massed trainings can be classified as long-term memory (LTM) and short-term/middle-term memory (STM/MTM), respectively. Consistently, like other organisms including Aplysia, Drosophila, and mice, C. elegans mutants defective in nmr-1 encoding an NMDA receptor subunit failed to form both LTM and STM/MTM, while mutations in crh-1 encoding the CREB transcription factor affected only the LTM.

Enhancement of chemotactic response to sodium acetate in the nematode Caenorhabditis elegans

In this study, we investigated the chemotactic response of a wild-type (N2) nematode (Caenorhabditis elegans) to a water-soluble attractant, sodium acetate, after pre-exposure to the chemical. The chemotactic response to 1.0 M sodium acetate of the non-exposed control nematodes was lower than that of the nematodes that were pre-exposed to 1.0 M sodium acetate for 90 min (p < 0.05). The increase in the response to sodium acetate was observed up to 6 hr, but not at 12 hr after exposure. To clarify the mechanism of this enhancement of the chemotactic response, several mutants were used. The chemotactic response of pre-exposed tph-1 and bas-1 mutants, whose main defect was serotonin secretion, was enhanced in comparison with that of the control mutants (p < 0.01). However, cat-1 and cat-2 mutants, which are respectively defective in serotonin and dopamine secretion and dopamine secretion only, showed no enhancement of the chemotactic response to sodium acetate, even when pre-exposed to this chemical. When the cat-1 and cat-2 mutants were pre-exposed to sodium acetate and bred in the presence of 40 mM dopamine, these mutants showed enhanced chemotactic response to sodium acetate (p < 0.05). These results suggest that the enhancement of chemotactic response to sodium acetate after pre-exposure to this chemical is modulated by dopaminergic neurotransmission.

Behavioural and genetic evidence for C. elegans' ability to detect volatile chemicals associated with explosives

BACKGROUND

Automated standoff detection and classification of explosives based on their characteristic vapours would be highly desirable. Biologically derived odorant receptors have potential as the explosive recognition element in novel biosensors. Caenorhabditis elegans' genome contains over 1,000 uncharacterised candidate chemosensory receptors. It was not known whether any of these respond to volatile chemicals derived from or associated with explosives.

METHODOLOGY/PRINCIPAL FINDINGS

We assayed C. elegans for chemotactic responses to chemical vapours of explosives and compounds associated with explosives. C. elegans failed to respond to many of the explosive materials themselves but showed strong chemotaxis with a number of compounds associated with commercial or homemade explosives. Genetic mutant strains were used to identify the likely neuronal location of a putative receptor responding to cyclohexanone, which is a contaminant of some compounded explosives, and to identify the specific transduction pathway involved. Upper limits on the sensitivity of the nematode were calculated. A sensory adaptation protocol was used to estimate the receptive range of the receptor.

CONCLUSIONS/SIGNIFICANCE

The results suggest that C. elegans may be a convenient source of highly sensitive, narrowly tuned receptors to detect a range of explosive-associated volatiles.

Cooperative and uncooperative cyclic-nucleotide-gated ion channels

Ion channels gated by cyclic nucleotides serve multiple functions in sensory signaling in diverse cell types ranging from neurons to sperm. Newly discovered members from bacteria and marine invertebrates provide a wealth of structural and functional information on this channel family. A hallmark of classical tetrameric cyclic-nucleotide-gated channels is their cooperative activation by binding of several ligands. By contrast, the new members seem to be uncooperative, and binding of a single ligand molecule suffices to open these channels. These new findings provide a fresh look at the mechanism of allosteric activation of ion channels.

Lateralized gustatory behavior of C. elegans is controlled by specific receptor-type guanylyl cyclases

BACKGROUND

Even though functional lateralization is a common feature of many nervous systems, it is poorly understood how lateralized neural function is linked to lateralized gene activity. A bilaterally symmetric pair of C. elegans gustatory neurons, ASEL and ASER, senses a number of chemicals in a left/right asymmetric manner and therefore serves as a model to study the genetic basis of functional lateralization. The extent of functional lateralization of the ASE neurons and genes responsible for the left/right asymmetric activity of ASEL and ASER is unknown.

RESULTS

We show here that a substantial number of salt ions are sensed in a left/right asymmetric manner and that lateralized salt responses allow the worm to discriminate between distinct salt cues. To identify molecules that may be involved in sensing salt ions and/or transmitting such sensory information, we examined the chemotaxis behavior of animals harboring mutations in eight different receptor-type, transmembrane guanylyl cyclases (encoded by gcy genes), which are expressed in either ASEL (gcy-6, gcy-7, gcy-14), ASER (gcy-1, gcy-4, gcy-5, gcy-22), or ASEL and ASER (gcy-19). Disruption of a particular ASER-expressed gcy gene, gcy-22, results in a broad chemotaxis defect to nearly all salts sensed by ASER, as well as to a left/right asymmetrically sensed amino acid. In contrast, disruption of other gcy genes resulted in highly salt ion-specific chemosensory defects.

CONCLUSIONS

Our findings broaden our understanding of lateralities in neural function, provide insights into how this laterality is molecularly encoded, and reveal an unusual multitude of molecules involved in gustatory signal transduction.