Nested premotor circuit activity stimulates production of distinct courtship songs in male flies

During courtship, some animals produce distinct sounds that clearly convey their intentions to potential mates. These sounds are usually produced by a series of muscle movements, which are themselves planned and controlled by neural circuits.

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

Male fruit flies produce two types of courtship songs: “pulse” and “sine.” These two songs convey different messages to female flies, influencing how they respond to male courtship.

Previous studies have identified specific wing movements underlying pulsed and sinusoidal courtship songs in Drosophila, showing that these movements are produced by wing control muscles. During pulsed song, the fly’s wing control muscles are active, while during sinusoidal song, a portion of them becomes inactive.

Although previous research has uncovered the muscles involved in producing fruit fly 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 complex mechanisms that allow insects and other animals to adapt to changing environments.

Researchers at the Howard Hughes Medical Institute recently conducted a study exploring the neural circuits involved in controlling the production of distinct courtship songs in fruit flies.

Their conclusions, published in Neuroscience of Naturesuggest that the rapid switching of motor actions that ultimately produces impulse and sinus 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 wanted 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 generally thought, including in our study, that these two songs were generated by distinct 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 spinal cord (i.e., the insect equivalent of the spinal cord) of adult flies while they produced their courtship sounds. This allowed them to determine which populations of neurons were active during pulsed and sinusoidal songs.

“To study the mechanisms of song production, we developed a new method for recording neuronal activity in singing flies,” Shiozaki explains. “We performed calcium imaging of specific neurons in the brain and ventral spinal cord while the flies alternated between the two songs.”

The researchers found that a population of neurons in the flies’ ventral spinal cord was active during the production of both pulsed and sinusoidal songs. However, during pulsed songs, other neurons also became active, leading to the engagement of a broader population of neurons.

“Contrary to our hypothesis, we found that activating nested neuronal populations, rather than separate populations, results in different courtship songs,” Shiozaki said. “This finding suggests that the motor system generates diverse actions through combinatorial activation of premotor networks.”

This recent study by Shiozaki and colleagues has provided insight into the neural mechanisms underlying the generation of courtship songs in flies. Their findings could pave the way for future research exploring the activity patterns of the nested premotor circuits they discovered or studying the neural circuits that govern courtship behaviors in other species.

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

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