Phenomenon of swarming in nematodes

Kin-recognition and predation shape collective behaviors in the cannibalistic nematode Pristionchus pacificus

Kin-recognition is observed across diverse species forming an important behavioral adaptation influencing organismal interactions. In many species, the molecular mechanisms involved are difficult to characterize, but in the nematode Pristionchus pacificus molecular components regulating its kin-recognition system have been identified. These determine its predatory behaviors towards other con-specifics which prevents the killing and cannibalization of kin. Importantly, their impact on other interactions including collective behaviors is unknown. Here, we explored a high altitude adapted clade of this species which aggregates abundantly under laboratory conditions, to investigate the influence of the kin-recognition system on their group behaviours. By utilizing pairwise aggregation assays between distinct strains of P. pacificus with differing degrees of genetic relatedness, we observe aggregation between kin but not distantly related strains. In assays between distantly related strains, the aggregation ratio is frequently reduced. Furthermore, abolishing predation behaviors through CRISPR/Cas9 induced mutations in Ppa-nhr-40 result in rival strains successfully aggregating together. Finally, as Caenorhabditis elegans are found naturally occurring with P. pacificus, we also explored aggregation events between these species. Here, aggregates were dominated by P. pacificus with the presence of only a small number of predators proving sufficient to disrupt C. elegans aggregation dynamics. Thus, aggregating strains of P. pacificus preferentially group with kin, revealing competition and nepotism as previously unknown components influencing collective behaviors in nematodes.

Comparison of solitary and collective foraging strategies of Caenorhabditis elegans in patchy food distributions

Collective foraging has been shown to benefit organisms in environments where food is patchily distributed, but whether this is true in the case where organisms do not rely on long-range communications to coordinate their collective behaviour has been understudied. To address this question, we use the tractable laboratory model organism Caenorhabditis elegans, where a social strain (npr-1 mutant) and a solitary strain (N2) are available for direct comparison of foraging strategies. We first developed an on-lattice minimal model for comparing collective and solitary foraging strategies, finding that social agents benefit from feeding faster and more efficiently simply owing to group formation. Our laboratory foraging experiments with npr-1 and N2 worm populations, however, show an advantage for solitary N2 in all food distribution environments that we tested. We incorporated additional strain-specific behavioural parameters of npr-1 and N2 worms into our model and computationally identified N2's higher feeding rate to be the key factor underlying its advantage, without which it is possible to recapitulate the advantage of collective foraging in patchy environments. Our work highlights the theoretical advantage of collective foraging owing to group formation alone without long-range interactions and the valuable role of modelling to guide experiments. This article is part of the theme issue 'Multi-scale analysis and modelling of collective migration in biological systems'.

Multitrophic Effects of Belowground Parasitoid Learning

The ability to learn allows organisms to take advantage of dynamic and ephemeral opportunities in their environment. Here we show that learning in belowground entomopathogenic nematodes has cascading multitrophic effects on their hosts, other nematodes, and nematophagous fungal predators. In addition to quantifying these effects, we show that social behavioral plasticity in these belowground parasitoids can amplify signaling by plant defense pathways and results in an almost doubling of insect herbivore infection by entomopathogenic nematodes. Cumulatively, these effects point to the critical role of plant signaling in regulating community structure while suggesting an equally important role for behavioral plasticity in shaping community dynamics.

A novel flavivirus in the soybean cyst nematode

Heterodera glycines, the soybean cyst nematode (SCN), is a subterranean root pathogen that causes the most damaging disease of soybean in the USA. A novel nematode virus genome, soybean cyst nematode virus 5 (SbCNV-5), was identified in RNA sequencing data from SCN eggs and second-stage juveniles. The SbCNV-5 RNA-dependent RNA polymerase and RNA helicase domains had homology to pestiviruses in the family Flaviviridae, suggesting that SbCNV-5 is a positive-polarity ssRNA virus. SbCNV-5 RNA was present in all nematode developmental stages, indicating a transovarial mode of transmission, but is also potentially sexually transmitted via the male. SbCNV-5 was common in SCN laboratory cultures and in nematode populations isolated from the field. Transmission electron microscopy of sections from a female SCN showed virus particles budding from the endoplasmic reticulum and in endosomes. The size of the viral genome was 19 191 nt, which makes it much larger than other known pestiviruses. Additionally, the presence of a methyltransferase in the SbCNV-5 genome is atypical for a pestivirus. When cDNA sequences were mapped to the genome of SbCNV-5, a disproportionate number aligned to the 3' NTR, suggesting that SbCNV-5 produces a subgenomic RNA, which was confirmed by RNA blot analysis. As subgenomic RNAs and methyltransferases do not occur in pestiviruses, we conclude that SbCNV-5 is a new flavivirus infecting SCNs.

Directional locomotion of C. elegans in the absence of external stimuli

Many organisms respond to food deprivation by altering their pattern of movement, often in ways that appear to facilitate dispersal. While the behavior of the nematode C. elegans in the presence of attractants has been characterized, long-range movement in the absence of external stimuli has not been examined in this animal. Here we investigate the movement pattern of individual C. elegans over times of ∼1 hour after removal from food, using two custom imaging set-ups that allow us to track animals on large agar surfaces of 22 cm×22 cm. We find that a sizeable fraction of the observed trajectories display directed motion over tens of minutes. Remarkably, this directional persistence is achieved despite a local orientation memory that decays on the scale of about one minute. Furthermore, we find that such trajectories cannot be accounted for by simple random, isotropic models of animal locomotion. This directional behavior requires sensory neurons, but appears to be independent of known sensory signal-transduction pathways. Our results suggest that long-range directional behavior of C. elegans may not be driven by sensory cues.

Behavioral motifs and neural pathways coordinating O2 responses and aggregation in C. elegans

BACKGROUND

Simple stimuli can evoke complex behavioral responses coordinated by multiple neural circuits. O(2) is an important environmental variable for most animals. The nematode C. elegans avoids high O(2), and O(2) levels regulate its foraging and aggregation.

RESULTS

Here, we dissect aggregation and responses to O(2) gradients into behavioral motifs and show how O(2) responses can promote aggregation. To remain in a group, C. elegans continually modify their movement. Animals whose heads emerge from a group will reverse or turn, thereby returning to the group. Re-entry inhibits further reversal, aiding retention in the group. If an animal's tail exits a group during a reversal, it switches to forward movement, returning to the group. Aggregating C. elegans locally deplete O(2). The rise in O(2) levels experienced by animals leaving a group induces both reversal and turning. Conversely, the fall in O(2) encountered when entering a clump suppresses reversal, turning, and high locomotory activity. The soluble guanylate cyclases GCY-35 and GCY-36, which are expressed in head and tail neurons, promote reversal and turning when O(2) rises. Avoidance of high O(2) is also promoted by the TRP-related channel subunits OCR-2 and OSM-9, and the transmembrane protein ODR-4, acting in the nociceptive neurons ASH and ADL. Both O(2) responsiveness and aggregation can be modified by starvation, but this is regulated by natural variation in the npr-1 neuropeptide receptor.

CONCLUSIONS

Our work provides insights into how a complex behavior emerges from simpler behavioral motifs coordinated by a distributed circuit.

Molecular approaches to aggregation behavior and social attachment

In many animal species individuals aggregate to live in groups. A range of experimental approaches in different animals, including studies of social feeding in nematodes, maternal behavior in rats and sheep, and pair-bonding in voles, are providing insights into the neural bases for these behaviors. These studies are delineating multiple neural circuits and gene networks in the brain that interact in ways that are as yet poorly understood to coordinate social behavior.

Social feeding in Caenorhabditis elegans is induced by neurons that detect aversive stimuli

Natural Caenorhabditis elegans isolates exhibit either social or solitary feeding on bacteria. We show here that social feeding is induced by nociceptive neurons that detect adverse or stressful conditions. Ablation of the nociceptive neurons ASH and ADL transforms social animals into solitary feeders. Social feeding is probably due to the sensation of noxious chemicals by ASH and ADL neurons; it requires the genes ocr-2 and osm-9, which encode TRP-related transduction channels, and odr-4 and odr-8, which are required to localize sensory chemoreceptors to cilia. Other sensory neurons may suppress social feeding, as social feeding in ocr-2 and odr-4 mutants is restored by mutations in osm-3, a gene required for the development of 26 ciliated sensory neurons. Our data suggest a model for regulation of social feeding by opposing sensory inputs: aversive inputs to nociceptive neurons promote social feeding, whereas antagonistic inputs from neurons that express osm-3 inhibit aggregation.

Natural variation in a neuropeptide Y receptor homolog modifies social behavior and food response in C. elegans

Natural isolates of C. elegans exhibit either solitary or social feeding behavior. Solitary foragers move slowly on a bacterial lawn and disperse across it, while social foragers move rapidly on bacteria and aggregate together. A loss-of-function mutation in the npr-1 gene, which encodes a predicted G protein-coupled receptor similar to neuropeptide Y receptors, causes a solitary strain to take on social behavior. Two isoforms of NPR-1 that differ at a single residue occur in the wild. One isoform, NPR-1 215F, is found exclusively in social strains, while the other isoform, NPR-1 215V, is found exclusively in solitary strains. An NPR-1 215V transgene can induce solitary feeding behavior in a wild social strain. Thus, isoforms of a putative neuropeptide receptor generate natural variation in C. elegans feeding behavior.