Video Monitoring and Genetic Manipulation Reveals Secrets of Fruit Fly Sex

macro photograph of blue fly on plant's stem

The nitty-gritty details of how flies mate and fertilize their eggs have remained somewhat obscure. But researchers recently used video monitoring and genetic manipulation to reveal some interesting details.

They found that male flies actually love the process of ejaculation. To confirm their finding, the team trained the flies to associate red light-induced ejaculation with a certain odor and then re-tested them for preference.

Sexual Behavior

Scientists are using video monitoring and genetic manipulation to study the love lives of fruit flies. They have shown that a single “switch gene” triggers complex behaviors, revealing how evolution ‘hardwires’ some animal responses.

Previous experiments suggested that flies find the most pleasure in sperm release, but no one had been able to measure this directly. So researchers led by Galit Shohat-Ophir of Bar-Ilan University in Israel used a special technology to target specific neurons in the fly brain and nerve cord. These neurons send messages about sex peptides to the insect’s reproductive tract, where it binds to cells and changes a female’s behavior.

Shohat-Ophir switched on a red light that triggered the ejaculation process in these genetically engineered flies. They then let the sex-starved critters choose whether to hang out in a zone that was associated with the odor of ejaculation or an unassociated area. As expected, flies that got it on found the experience pleasurable. Flies that didn’t, however, tended to drown their sorrows in alcohol.

Shohat-Ophir and colleagues also analyzed the brains of these sexy fruit flies to figure out how these signals are interpreted. They found that the neurons responsible for the sex peptide signals are connected to other neurons in the brain and nerve cord, which are thought to be involved in a female’s decision to mate.

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The nitty-gritty details of how a male fruit fly knows when a female is ready to mate and then what happens inside her reproductive organ have been largely mysterious. But after a painstaking series of genetic experiments, scientists have figured out what seems to be a molecular and neurobiological switch that kicks off a complex sequence of mating behaviors.

During courtship, the male flies tap their legs to send out a signal called an odorant that reaches the females’ brains via a long nerve. The odorant contains a compound called semen, which binds to receptors in the female’s reproductive organ to trigger an internal sense of readiness. Once the brain switches to the ‘pre-mated’ state, a flood of excitatory (‘go for it’) signals and inhibitory (‘Don’t bother’) ones flow into the courtship command center of the fly’s P1 neurons.

If the odorant reaches the P1 neurons in time, the fly will send a ‘Go for it’ message to her ovipositor — a tube-like structure that protrudes from the end of the abdomen. The ovipositor is the instrument by which a female lays her eggs, and the first male to reach her will father the vast majority, if not all, of her offspring.

The ‘go for it’ signal is triggered by an influx of dopamine into the brain. Researchers found that when the dopamine levels in a fly’s P1 neurons were low, the flies didn’t produce as much odorant. When the dopamine was high, however, the flies produced more odorant and rushed to mate.

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When a male and female fly are ready to mate, the female pushes her ovipositor into the genital opening of the male—an elongated body part that telescopes out of her abdomen. This typically takes 30 minutes to two hours. After mating, the female releases her eggs in warm, moist places such as animal faeces, carrion and rotting food. She can lay up to 500 fertilized eggs.

While scientists have studied the mechanics of fly mating, the role of the female, who makes the final decision to mate, has received little attention. Now, researchers have discovered a gene in the fly brain that controls this decision and, therefore, reproduction.

The new gene, a complex of three linked genes, is regulated by several other genes that also affect reproductive behaviors. It is the first time that such a complex of genes has been found to control a key behavioral decision in any animal.

The finding could have significant medical implications because it provides clues to fundamental neurobiological processes that may go awry in disease. In addition, the study reveals that a basic reward system is common to all animals—including ostensibly simple creatures like fruit flies. A previous study found that male fruit flies enjoy the process of ejaculation. Scientists trained them to associate a red light with the ejaculation process and then observed their behavior.

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Sperm Storage

When it comes to sex, animals seem to take it pretty seriously. Birds build nests, mice become aggressive, and even a simple fruit fly can lay hundreds of eggs at once. But what exactly triggers the mating behaviors in these creatures? Researchers have discovered a genetic switch in the brain that controls this behavior. The new study, published this week in the journal Current Biology, may shed light on how the brain regulates fundamental neurobiological processes — including those that go awry in disease.

Male flies are quite the Casanovas of the insect world, using their wings to woo females with wing vibrations and sperm that carry her scent. But despite all the effort they put into courtship, the nitty-gritty of how they mate and fertilize eggs has remained a mystery.

In a new series of experiments, Bar-Ilan University researchers Galit Shohat-Ophir and Shir Zer-Krispil used gene editing to create flies that ejaculate in response to red light. The longer wavelengths of that color penetrate into the flies’ abdomens and activate light-sensitive proteins in neurons called corazonin, which prompt the release of sperm. The flies’ brains also show elevated levels of the chemical neuropeptide F, which is involved in responses to rewards like food and sex.

When the flies were offered food or alcohol, those that had just experienced sex took the booze. The scientists believe that this demonstrates that sexual pleasure is a powerful reward, at least for flies.

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