Researchers have discovered how the structure of brain networks enables various species to adapt to new mating signals over long periods of evolution.
Male fruit flies use several strategies to find a mate, including detecting pheromones in the dark and utilizing visual cues in bright conditions.
A recent study reveals that these small insects employ a flexible network of modular brain circuits to rapidly adjust to different mating signals. This research, published in Nature, is the first to illustrate how various species of fruit flies integrate new sensory inputs, like pheromones, into a fundamental set of brain circuits without needing to create entirely new neural pathways.
The results provide a broader perspective on how changes in brain wiring can affect the evolution of behavior. “The variety of behaviors across the animal kingdom is immense, but understanding how nervous systems evolve has been quite challenging,” explains Vanessa Ruta, director of the Laboratory of Neurophysiology and Behavior. “We’ve identified what we believe is a critical neural mechanism that allows brain circuits to adapt and reorganize across different species.”
Flexible integration
One of the puzzles of behavioral evolution is how brain circuits keep up with the fast-paced changes in social signals as species evolve. For example, courtship behaviors rapidly evolve, making it hard to believe that the fly brain completely reconstructs itself each time a new pheromone is discovered within the Drosophila lineage.
Until now, pinpointing where evolution impacts the nervous system to modify behavior was not feasible, thus leaving the core aspects of circuit adaptability shrouded in mystery. Ruta’s team focused on fruit flies, where closely related species share similar brain structures yet depend on markedly different cues for mating. For instance, D. simulans primarily uses visual signals to find a partner, while D. yakuba has developed a new ability to use pheromones to locate mates even in total darkness. These differences provided a unique opportunity to explore how similar brain structures interpret various social signals.
“We started examining parts of the brain that might be geared for adaptability,” says Rory Coleman, the study’s lead author and a postdoctoral fellow in the Ruta lab. “We were on the lookout for traits that could make the circuit innately flexible and serve as potential evolutionary hotspots leading to behavioral diversity.”
By comparing pheromone-detecting circuits in various species through behavioral assays, genetic methods, neuroimaging, and CRISPR genome editing, they identified sensory neurons in the male forelegs and P1 neurons in the higher brain as crucial for regulating courtship behaviors across species. The research team found that the fundamental neural components of male mating actions, like the P1 neurons, are conserved across species, but different sensory inputs can be integrated into this core circuit. This allows fly species to formulate diverse mating strategies without having to overhaul their entire neural architecture.
For example, the researchers discovered that P1 neurons were activated by distinct pheromones in D. melanogaster and D. yakuba. Nonetheless, the function of P1 neurons in initiating courtship remained consistent in both species. “A significant finding from our research is that there are specific nodes within the brains of these species that can effectively incorporate new sensory modalities,” says Ruta. “This adaptability enables conserved nodes, such as the P1 neurons, to initiate courtship in various species while responding to the unique signals from their respective females.”
A brain for social interactions
This study is part of Rockefeller’s Price Family Center for the Social Brain, an initiative aimed at exploring the neuronal, cellular, and molecular foundations that drive social behavior. Beyond highlighting the brain’s adaptability to new sensory information, this research also provides a practical approach to investigating how social behaviors evolve across species. “Our findings show that Drosophila serves as a compelling model for studying behavioral evolution,” states Ruta.
By looking into how differences in neural circuits influence behaviors like mating, the lab aims to enhance our comprehension of the intricate relationship between brain functionality and social behaviors, laying the groundwork for understanding how social circuits are designed to support adaptive behaviors in humans. While the brain structures of flies and humans are quite different, it is probable that some essential principles governing the evolution and adaptation of neural circuits are shared among species.
“We anticipate that comparative evolutionary research like this will uncover the fundamental principles that guide how neural circuits are developed across the animal kingdom, including in humans,” Coleman adds. “Numerous neurological disorders are believed to result from circuits being incorrectly wired,” Ruta notes. “By exploring neural circuits from an evolutionary perspective, we aspire to illuminate which neural patterns can be modified and how they can change, not solely due to diseases but as a result of evolutionary selection.”
