In a groundbreaking study, researchers have unveiled the intricate neural mechanisms underpinning predatory aggression, demonstrating how evolutionary adaptations in noradrenergic circuits have fine-tuned aggressive behavior in nematodes. Through a meticulous combination of genetic, behavioral, and imaging analyses, the team has identified specific sensory neurons that play a pivotal role in detecting prey and driving predatory states, shedding light on the evolutionary biology of aggression.
The focus of this study lies in the predatory nematode Pristionchus pacificus, a model organism that exhibits aggressive predation distinct from its close relative Caenorhabditis elegans, which primarily consumes microbes. Central to this behavior are the IL2 sensory neurons, a cluster of six neurons strategically positioned around the mouth opening. These neurons are unique in their expression of the Ppa-ser-3 gene and their exposure to the environment, positioning them as prime candidates for mediating the detection of prey and initiating aggressive predatory responses.
Interestingly, although IL2 neurons in C. elegans contribute to sensory modulation and nictation—a behavior associated with dispersal—they lack expression of octopamine receptors and, consequently, do not participate directly in aggression modulation. Conversely, in P. pacificus, these neurons receive signals via octopamine, a neurotransmitter akin to noradrenaline in vertebrates, implying an evolutionary recruitment of this neural pathway to regulate predatory aggression. This finding suggests a significant divergence in neuromodulatory circuits that underpins behavioral specialization among closely related nematodes.
To probe the specific role of IL2 neurons in predation, the researchers exploited the Ppa-klp-6 promoter, previously shown to drive expression exclusively in these six sensory neurons. By constructing transgenic lines expressing histamine-gated chloride channels under the klp-6 promoter, the team achieved precise genetic silencing of IL2 neurons through histamine exposure. Behavioral analysis following neuronal silencing revealed a marked decline in the detection of predatory events, accompanied by a substantial decrease in the duration and frequency of predatory states and aggressive transitions.
Quantitative assessment through sophisticated tracking and computational modeling illustrated that the suppression of IL2 activity disrupted normal predatory behavior patterns without significantly affecting other locomotory or feeding behaviors. This underlines the specificity of the IL2 neurons in regulating aggression rather than general exploratory or feeding activities. The probabilistic state transitions mapped from velocity and pumping rates underscored alterations in behavioral dynamics, confirming that IL2 neurons are essential nodes in neural circuits orchestrating predation.
Extending beyond P. pacificus, comparative analyses across nematode species shed light on the evolutionary conservation and divergence of predatory traits. The study highlighted the predatory nematode Auanema sudhausi, which possesses mouthparts akin to P. pacificus featuring teeth-like structures crucial for prey capture. Functional disruption of octopamine biosynthesis in A. sudhausi mutants led to diminished aggressive predation, affirming the centrality of octopaminergic signaling in modulating aggression across diverse nematode lineages.
In contrast, microbial-feeding C. elegans lacks these specialized mouth structures and does not display predatory aggression, illuminating the tight correlation between morphological adaptations, sensory neuron function, and neuromodulatory pathways in the evolution of predation. These insights collectively support a model whereby ancient neuromodulatory circuits, particularly those involving octopamine and its precursors tyramine, have been co-opted and refined to drive complex social behaviors such as aggression.
The authors propose an integrative model positioning octopamine and tyramine signaling as master regulators balancing aggressive versus docile behavioral states in nematodes. This bipartite system allows flexible modulation of behavior based on environmental cues, physiological states, and neural circuit activity, facilitating survival strategies that range from cooperative feeding to active predation. Such versatility underscores the adaptive value of neuromodulatory plasticity in evolutionary contexts.
Moreover, the study employed cutting-edge scanning electron microscopy to visualize the ultrastructural arrangement of IL2 sensory endings encircling the nematode mouth, illustrating their potential to form the primary interface for environmental sensory input critical for predation. These morphological details, paired with genetic and behavioral data, provide a comprehensive framework elucidating how sensory input is integrated to drive complex behaviors.
This research exemplifies a powerful integration of neurogenetics, ethology, and comparative anatomy to decode behavioral evolution. By pinpointing the cellular and molecular nodes controlling predatory aggression, it sets the stage for further exploration into how neuromodulatory systems adapt to ecological pressures, potentially informing broader understandings of aggression and social behavior across taxa.
Ultimately, these findings highlight the neoteric concept that behaviorally significant traits, including aggression, can arise from the nuanced adaptation of conserved neural circuits rather than wholesale genetic innovation. Such principles may generalize to other systems, offering profound implications for neurobiology, evolutionary biology, and even the development of interventions targeting maladaptive aggressive behaviors in higher organisms.
In summary, the elucidation of IL2 sensory neuron function and octopamine-mediated neuromodulation in nematode predation not only deepens understanding of these organisms’ ecology but also advances fundamental knowledge of how aggression evolves at the neural circuit level. This work stands as a testament to the power of integrative biology in revealing the mechanistic substrates of complex behaviors that shape survival and evolutionary trajectories.
Subject of Research: Neural mechanisms underlying predatory aggression and the evolution of noradrenergic circuits in nematodes.
Article Title: Predatory aggression evolved through adaptations to noradrenergic circuits.
Article References:
Eren, G.G., Böger, L., Roca, M. et al. Predatory aggression evolved through adaptations to noradrenergic circuits. Nature (2026). https://doi.org/10.1038/s41586-025-10009-x
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41586-025-10009-x
Keywords: predatory aggression, nematodes, IL2 sensory neurons, octopamine, tyramine, neuromodulation, Pristionchus pacificus, Caenorhabditis elegans, neural circuits, behavioral evolution, noradrenergic systems, ethology
