Female fruit flies expect great love songs

December, 2020
Have you ever wished your partner would treat you to a sweet serenade? If you’re a fruit fly, you don’t just wish for it, you expect it. A team of researchers at Howard Hughes Medical Institute’s Janelia Research Campus recently mapped out the neural networks that underlie female response to a potential mate’s song.

How to woo a female fruit fly

If a male fruit fly successfully woos a receptive female, the pair will mate. To entice his partner, the male fly “sings” by extending a wing and vibrating it to produce an acoustic signal. The fly song consists of two repeating verses: brief trains of shrill tones followed by continuous soft hums. Previously, the researchers had uncovered the neural networks behind the male’s courtship song, but how the female perceives the melody was largely unknown.

In the new paper, the team investigated how the female fruit fly brain integrates the song to respond to a potential suitor. If she accepts the advance, her vaginal plates will open to allow mating. The team found that female receptivity depends not only on a good male performance, but also on the intrinsic mating status of the female.

Brain circuitry

Inside the female fruit fly brain, the right song is transduced into sensory information, which feeds to a special class of neurons and integrates with information from a second set of neurons. The second set of neurons conveys information about the mating status of the female. In response to a male song or mating status, both sets of neurons will produce a series of “stop” or “go” signals that eventually connect to the muscles of the vaginal plate. If the female has not yet mated, and if she “hears” a good enough song, the neuronal circuitry in her brain will produce a net “go” signal to the vaginal plate muscles. The vaginal plate will then open to allow mating. Alternatively, if she’s not satisfied, she will reject the male.

The researchers used a series of elegant genetic and physical manipulations of transgenic female flies to establish the relationships between different neuron types and vaginal plate opening. Using genetic tricks (GAL4/UAS and optogenetics) analogous to operating a molecular switchboard, the researchers determined in what context neurons will fire in response to a male song or mating status, as well as where these signals integrate.

Removing the aristae

The team physically removed the “hearing” organs of the female flies, the aristae, or the wings of the male flies. In each case, they observed that neurons which respond to male song did not fire in a pattern that normally leads to the vaginal plate opening response.

Additionally, the song of a different species of fly could not woo the female.

These experiments showed how crucial it is for the female to perceive the right male’s song. The researchers also established that the receptivity of the female was governed by whether or not she had previously mated.

In other words, if a female fly “hears” the right song from the right male, and has not previously mated, she’s DTF.

Maps to set the mood

In certain conditions, the male sang and sang, but to no avail. The female would not open her vaginal plates. The team synthesized information of these particular experimental conditions to assemble a map of the neural pathway that governs female fly receptivity to sex. The paper is important because it establishes how three components of a fundamentally important mating behavior are wired together as a unit.

While the paper is fun to read (who doesn’t like learning about fruit fly sex?!), it’s important to understand the big picture of this research.

Understanding neural circuity behind female sexual receptivity in the fruit fly may help us better understand signal processing that influences behavioral decisions across a range of species, including humans.

The female fruit fly maintains high standards for a love song; she turns down potential suitors that can’t hit the right tune. From an evolutionary standpoint, it’s probably good to set the bar the high to get the best partners.

Keep your standards high, ladies and gents.

“If I cannot fly, let me sing” – Stephen Sondheim (American Composer)

Link to the original paper, published in the November issue of Nature: https://www.nature.com/articles/s41586-020-2972-7#Sec2