Nested premotor circuits activity found to drive male flies’ production of distinct courtship songs

While they are courting, some animals produce distinct sound patterns that clearly convey their intentions to potential mates. These sounds are typically produced via a series of muscle movements, which are in turn planned and controlled by neural circuits.

Fruit flies (Drosophila) are among the many insects known to produce these unique sounds, also referred to as courtship songs. Their courtship songs are produced by characteristic wing vibrations that follow specific patterns.

Male fruit flies produce two key types of courtship songs: ‘pulse’ and ‘sine.’ These two songs convey different messages to female flies, influencing how they respond to the males’ courtship.

Past studies have identified the unique wing movements underpinning pulse and sine courtship songs in Drosophila, showing that these movements are produced by wing control muscles. During the pulse song, a fly’s wing control muscles are active, while during the sine song part of them becomes inactive.

While previous research uncovered the muscles involved in the production of Drosophila courtship songs, the neural processes responsible for controlling these muscles remain poorly understood. A better understanding of these processes could shed new light on the intricate mechanisms that allow insects and other animals to adapt to changing environments.

Researchers at Howard Hughes Medical Institute recently carried out a study exploring the neural circuits involved in controlling the production of distinct Drosophila courtship songs.

Their findings, published in Nature Neuroscience, suggest that the rapid switching of motor actions that ultimately produces pulse and sine songs is driven by the coordinated activity of premotor circuits (i.e., neural circuits responsible for generating specific patterns of muscle activation to produce specific movements).

“We aimed to understand how neural circuits control the same muscles to produce different motor actions,” Hiroshi M. Shiozaki, first author of the study, told Medical Xpress.

“During courtship, male flies use wing vibrations to produce two different songs to attract females. It was widely assumed, including by us, that these two songs are generated by separate populations of neurons in the premotor circuit, but this hypothesis had not been tested.”

To test this hypothesis, Shiozaki and his colleagues recorded calcium signals in the ventral nerve cord (i.e., insect equivalent of the spinal cord) of adult flies as they were producing their courting sounds. This allowed them to determine what populations of neurons were active during pulse and sine songs.

“To investigate the mechanisms of song production, we developed a novel method for recording neural activity in singing flies,” explained Shiozaki. “We performed calcium imaging of specific neurons in both the brain and the ventral nerve cord while the flies switched between the two songs.”

The researchers found that one population of neurons in the flies’ ventral nerve cord was active during the production of both pulse and sine songs. During pulse songs, however, additional neurons also became active, resulting in the engagement of broader population of neurons.

“Contrary to our hypothesis, we found that activation of nested neural populations, rather than separate ones, drives different courtship songs,” said Shiozaki. “This discovery suggests that the motor system generates diverse actions through the combinatorial activation of premotor networks.”

This recent study by Shiozaki and his colleagues gathered new insight into the neural underpinnings of distinct courtship song generation in flies. Their results could pave the way for further research exploring the nested premotor circuit activity patterns that they uncovered or investigating the neural circuits driving the courtship behaviors of other species.

“In our future studies, we plan to investigate how the song circuit has evolved to produce species-specific songs,” added Shiozaki. “Using this tractable model system, we aim to discover general principles of circuit evolution underlying complex behavior.”

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