Mosaic Brain Evolution Drives Learning in Tropical Butterfly - Neuroscience News

Summary: Researchers have discovered that Heliconius butterflies, known for feeding on both nectar and pollen, show mosaic brain evolution with specialized neural expansions linked to enhanced learning and memory abilities. This expansion occurs in specific brain structures called mushroom bodies, which are key for long-term visual memory and spatial learning.

By analyzing these butterflies’ brain circuits, scientists found that certain cells, known as Kenyon cells, grew at different rates, helping the butterflies navigate complex feeding routes. These findings highlight how brain structure adaptations support cognitive innovations, offering new insights into neural evolution.

Key Facts:

Source: University of Bristol

A species of tropical butterfly with unusually expanded brain structures display a fascinating mosaic pattern of neural expansion linked to a cognitive innovation.

The study, published today in Current Biology, investigates the neural foundations of behavioural innovation in Heliconius butterflies, the only genus known to feed on both nectar and pollen.

As part of this behaviour, they demonstrate a remarkable ability to learn and remember spatial information about their food sources—skills previously connected to the expansion of a brain structure called the mushroom bodies, responsible for learning and memory.

Lead author Dr Max Farnworth from the University of Bristol’s School of Biological Sciences explained: “There is huge interest in how bigger brains may support enhanced cognition, behavioural precision or flexibility. But during brain expansion, it’s often difficult to disentangle effects of increases in overall size from changes in internal structure.”

To answer this question, the study authors delved deeper into the changes that occurred in the neural circuits that support learning and memory in Heliconius butterflies.

Neural circuits are quite similar to electrical circuits as each cell has specific targets that they connect with, and assembles a net with its connections. This net then elicits specific functions by constructing a circuitry.

Through a detailed analysis of the butterfly brain, the team discovered that certain groups of cells, known as Kenyon cells, expanded at different rates. This variation led to a pattern called mosaic brain evolution, where some parts of the brain expand while others remain unchanged, analogous to mosaic tiles all being very different from each other.

Dr Farnworth explained: “We predict that because we see these mosaic patterns of neural changes, these will relate to specific shifts in behavioural performance – in line with the range of learning experiments which show that Heliconius outperform their closest relatives in only very specific contexts, such as long-term visual memory and pattern learning.”

To feed on pollen, Heliconius butterflies need to have efficient routes of feeding, as pollen plants are quite rare.

Project supervisor and co-author, Dr Stephen Montgomery said: “Rather than having a random route of foraging, these butterflies apparently choose fixed routes between floral resources – akin to a bus route.

“The planning and memory processes needed for this behaviour are fulfilled by the assemblies of neurons inside the mushroom bodies, hence why we’re fascinated by the internal circuitry throughout.

“Our results suggest that specific aspects of these circuits have been tweaked to bring about the enhanced capacities of Heliconius butterflies.”

This study contributes to the understanding on how neural circuits change to reflect cognitive innovation and change. Examining neural circuits in tractable model systems such as insects promises to reveal genetic and cellular mechanisms common to all neural circuits, thus potentially bridging the gap, at least on a mechanistic level, to other organisms such as humans.

Looking ahead, the team plans to explore neural circuits beyond the learning and memory centres of the butterfly brain. They also aim to increase the resolution of their brain mapping to visualise how individual neurons connect at an even more granular level.

Dr Farnworth said: “I was really fascinated by the fact that we see such high degrees of conservation in brain anatomy and evolution, but then very prominent but distinct changes.”

“This is a really fascinating and beautiful example of a layer of biodiversity we don’t usually see, the diversity of brain and sensory systems, and the ways in which animals are processing and using the information provided by the environment around them” concluded Dr Montgomery.

Author: Laura ThomasSource: University of BristolContact: Laura Thomas – University of BristolImage: The image is credited to Max Farnworth

Original Research: Open access.“Mosaic evolution of a learning and memory circuit in Heliconiini butterflies” by Max Farnworth et al. Current Biology

Abstract

Mosaic evolution of a learning and memory circuit in Heliconiini butterflies

How do neural circuits accommodate changes that produce cognitive variation? We explore this question by analyzing the evolutionary dynamics of an insect learning and memory circuit centered within the mushroom body.

Mushroom bodies are composed of a conserved wiring logic, mainly consisting of Kenyon cells, dopaminergic neurons, and mushroom body output neurons.

Despite this conserved makeup, there is huge diversity in mushroom body size and shape across insects. However, empirical data on how evolution modifies the function and architecture of this circuit are largely lacking.

To address this, we leverage the recent radiation of a Neotropical tribe of butterflies, the Heliconiini (Nymphalidae), which show extensive variation in mushroom body size over comparatively short phylogenetic timescales, linked to specific changes in foraging ecology, life history, and cognition.

To understand how such an extensive increase in size is accommodated through changes in lobe circuit architecture, we combined immunostainings of structural markers, neurotransmitters, and neural injections to generate new, quantitative anatomies of the Nymphalid mushroom body lobe.

Our comparative analyses across Heliconiini demonstrate that some Kenyon cell sub-populations expanded at higher rates than others in Heliconius and identify an additional increase in GABA-ergic feedback neurons, which are essential for non-elemental learning and sparse coding.

Taken together, our results demonstrate mosaic evolution of functionally related neural systems and cell types and identify that evolutionary malleability in an architecturally conserved parallel circuit guides adaptation in cognitive ability.

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