Baker and his team discovered that a single gene determines whether a fly develops into male or female. This gene turns on a whole series of genes, telling cells to take on sex-specific characteristics.

To understand how this process works, they used video monitoring and genetic manipulation. First they made flies genetically engineered to ejaculate in response to red light.

The Male

A scientist can tell whether a fruit fly is male or female based on distinct pigment patterns on the body, a type of bristle found only on the legs of males and differences in genitalia. But the internal circuitry that guides courtship has remained mysterious.

For example, researchers knew that a brain chemical called dopamine plays an important role in fly mating, but they didn’t know how it worked. In the new study, Baker and his colleagues figured out that a single gene determines if the insect develops male or female characteristics. Baker calls it a “bureaucratic chain of command,” with the gene serving as the ultimate decision maker. “If you turn on that gene, it gives the go-ahead to develop female cells and male cell types,” he says. “If you turn off that gene, it stops the process.”

Male fruit flies put on quite a show when they come across a potential mate: They follow her around, exude pheromones, play a song with their wings and lick or tap at her genitalia. Then they decide whether to pursue a mate or run away.

The scientists wanted to understand what neurons in the fly’s brain processed these internal and external cues to produce a yes-or-no mating response. They used a range of techniques, including making individual neurons light up or silence them, to pinpoint the answer. They showed that neurons in two brain regions, pCd and pC1, respond to the male courtship song and the presence of pheromones. Silencing these neurons made virgin flies slow to mate, while increasing their activity caused them to respond more quickly.

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The Female

For many animals, sex is a big deal. Birds build nests, mice become aggressive and territorial, and even fruit flies take it seriously. After a male fly lays an egg, it has to go through a series of behaviors before the female can reject advances from other males and start laying her own eggs. Now researchers have outlined groups of neurons in the brain and nerve cord that control a female’s mating response, revealing the biological intricacies behind what looks simple enough on the outside.

For example, when a male approaches a female for the first time, a flood of excitatory (“Go for it!”) and inhibitory (“Don’t bother”) signals flow into the P1 courtship center in her brain. When the inhibitory signals are high, they prevent the male from pursuing her. But if the pheromones that signal sexual desire from the female are low, the neurons in the P1 center receive less inhibition and more dopamine, giving the male a green light to proceed.

Another important step in the process involves a component of fly semen that travels through a female’s reproductive tract to receptors in her brain. Normally, this chemical, called sex peptide, binds to cells in the reproductive tract and conveys a signal through a long nerve cord to reduce the female’s receptivity for other males. But researchers at Sloan Kettering Institute and other labs have discovered that a genetic mutation interferes with this process, so that when sex peptide arrives in a female’s uterus, it doesn’t trigger the correct response in her brain.

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The Ovipositor

Among Hymenoptera, the ovipositor is an organ of special interest. In particular, it has become a key component in the specialized modes of parasitoid behaviour. Parasitoid females use the ovipositor to deposit eggs in their hosts. This is possible because ovipositor bristles have sensory receptors at the end that can detect the presence of host eggs within the body. These sensors signal the neurons of the parasitoid that there are host eggs inside the body and that it needs to proceed with its oviposition process.

The ovipositor consists of tubular valves that project like jaws from the tip of the fly’s abdomen. It is a very elaborate structure that has evolved for specific purposes. For example, it can be used to pierce the skin of another animal to lay a parasitoid egg. It can also dig a hole for the burial of the egg or assist in capping it with a froth. Its specialised features make it an extremely versatile tool for the laying of flies’ eggs.

The ovipositor is controlled by ten pairs of muscles that produce rhythmical opening, closing and retraction movements. These muscles are supplied by the eighth and ninth segmental nerves of the terminal abdominal ganglion. The tergite of the ninth abdominal segment articulates with a triangular element known as the first or ventral valvifer (figure 2). This articulation allows rotation in the dorsal plane. The valvifer then articulates with a plate-like element called the second valvifer.

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The Sperm

As a rule, the first male to get to an egg fertilizes it. But a few years ago, researchers reported that flies with a specific genetic mutation could feel like they mated even though no sperm was actually involved.

These flies had a mutation that caused them to release neuropeptide F, which binds to a receptor in a fly’s reproductive tract and triggers the feeling of sex. This chemical signal is a bit like a key that opens the door to higher-level brain processing centers, and they found that the flies exhibited a similar response to the feeling of sex regardless of whether or not their partner’s sperm was present.

To test this, Shohat-Ophir’s team zapped a group of male flies with red light that was specifically tuned to stimulate ejaculation. The red light’s longer wavelengths penetrated the flies’ abdomens and reached light-sensitive proteins in their corazonin neurons, which control ejaculation. When triggered, these cells released sperm.

As the sperm flies flew, they got sucked up by the female’s ovipositor and then passed on their X and Y chromosomes to her eggs. Hundreds of millions of these sperm vie for each egg, and the one that makes it through the hardest part of the job – the acrosome, where the egg’s cap is located – gets to pass its genetic message on to the fertilized eggs inside the female.