EPISODE 48: Murder Mode: How a Worm Evolved the Urge to Kill

Welcome to the next episode of the WOrM Podcast 🪱

Today we're talking about a worm with teeth.

And a nervous system that has been rewired — by evolution — to become aggressive.

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🧬 The central idea

Pristionchus pacificus is a predatory nematode.

It kills C. elegans larvae.

Sometimes for food.

Sometimes just to remove a competitor.

But how does its brain decide to attack?

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🔬 What's actually going on?

This is not just predation.

It is a distinct behavioural state — aggression —

driven by a specific neurochemical system.

The researchers used machine learning to identify six distinct behavioural states:

• roaming and dwelling — shared with C. elegans
• predatory search, predatory biting, predatory feeding — unique to a predatory context

The worm doesn't attack randomly.

It switches modes.

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⚡ Two chemicals. Opposite effects.

The key twist is this:

• Octopamine pushes the worm into aggressive, predatory states
• Tyramine pulls it back into passive, docile states

They act antagonistically — like a switch.

Remove octopamine → the worm stops attacking.

Remove tyramine as well → aggression returns.

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🧠 The receptors tell the story

Two octopamine receptors are required: Ppa-ser-3 and Ppa-ser-6.

One tyramine receptor mediates the passive state: Ppa-lgc-55.

Crucially — these receptors are expressed in sensory neurons at the worm's nose.

Specifically, the IL2 neurons.

These are the first point of contact between predator and prey.

Silence the IL2 neurons → aggression drops.

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🧠 A rewired circuit

In C. elegans, octopamine and tyramine do completely different things — fasting signals, escape responses.

In P. pacificus, evolution has repurposed these same molecules to regulate aggression.

The neurons producing them are conserved.

The function has diverged.

This is circuit-level evolutionary innovation.

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🧠 Ancient and widespread

The same octopamine-aggression link was found in Allodiplogaster sudhausi —

a distant relative in the Diplogastridae family.

So this adaptation is not unique to P. pacificus.

It emerged early, in the predatory lineage — and stuck.

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🌍 The bigger picture

This paper shows that:

• new behaviours can evolve through repurposing of existing neurochemical systems
• the same molecules can serve completely different functions in closely related species
• sensory neurons are a key site of neuromodulatory innovation

Evolution doesn't always build from scratch.

Sometimes it just rewires what's already there.

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🧠 The take-home message

A predatory worm evolved aggression

not through new neurons,

but through new ways of using old chemistry.

Octopamine and tyramine — present across invertebrates —

were redeployed to gate an entirely new behavioural state.

That is elegant. And slightly terrifying.

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📄 Paper discussed

Eren, G. G.; Böger, L.; Roca, M.; Hiramatsu, F.; Liu, J.; Alvarez, L.; Goetting, D. L.; Cockram, L. A.; Zorn, N.; Han, Z.; Okumura, M.; Scholz, M.; Lightfoot, J. W. (2026)Predatory aggression evolved through adaptations to noradrenergic circuitsNature, Vol 651https://doi.org/10.1038/s41586-025-10009-x

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📩 More info:🔗 www.veerenchauhan.com📧 veeren.chauhan@nottingham.ac.uk