Female flies' immune genes turned on by males' song

Fruit fly (Drosophila melanogaster) (c)Darren Obbard/ University of Edinburgh Fruit flies may have evolved to cope with the potential trauma of mating

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The courtship trill of a male fruitfly is an exciting sound for a female; it literally heightens her senses as she prepares to mate.

But a study has revealed that the sound also has an unexpected effect on the female's immune system.

Researchers have discovered that, for a female fly, preparation for mating involves the "rather unromantic" anticipation of potential infection.

The findings are reported in the Royal Society journal Proceedings B.

Elina Immonen and Mike Ritchie from the University of St Andrews, UK, carried out the study. They wanted to understand what genes were "switched on" when a fly prepares to mate.

The genetic snapshot of an amorous female helps build a picture of the basic biological building blocks that make a creature want to reproduce. And fruit flies (Drosophila melanogaster) are the perfect creatures for such a study; the function of almost every one of their genes has been documented.

"Basically, we wanted to know what [genetic] changes take place in the female when she's being stimulated by a sexy guy," said Prof Ritchie.

To investigate this, the team played female fruit flies a recording of the "song" that males produce by vibrating their wings. They then produced a read-out of the flies' active genes.

This showed that when the female heard the courtship song of a male of the same species, she would "generally get excited", Prof Ritchie explained.

For example, genes in the female's antennae - which are essentially her ears - were "switched on".

"But the big surprise," said Prof Ritchie, "was that genes involved in immune function were also switched on.

"It appears that if she hears a sexy song, she knows she's likely to mate soon, so she makes the physiological change to prepare for mating - that involves [increasing the activity of] immune genes."

Trauma of sex

Prof Ritchie explained that the female fruit flies had probably evolved to "cope" with the potential harm caused to them by mating.

This is something that Prof Mike Siva-Jothy from the University of Sheffield says makes evolutionary sense.

His work on bed bugs revealed that the insects deliberately damaged the females when they mated - stabbing her through the abdomen in order to inject sperm.

"This is an extreme example, but often, looking at extreme examples sheds light on what's going on in many species," he told BBC Nature.

INSECT SEX

  • Like many insects, fruit flies use their antennae to "sniff out" a potential mate. This chemical communication is vital for social insects to recognise and interact with each other
  • Scientists are able genetically "programme" the sexual behaviour of some fruit flies. In a recent study, scientists took control of flies' brains to make females behave just like males
  • To cope with their "traumatic" mating technique, male bed bugs protect their sperm against sexually transmitted infections by producing germ-busting ejaculates

He added that there was "lots of evidence" in insects that males routinely damage female genital tracts during mating, so he thinks that the immune switch-on that surprised the St Andrews team is probably an example of something that occurs throughout the insect world.

"My proposal is in any situation where you have predictable exposure to pathogens, you might expect females to anticipate the damage and... to respond.

"It's an idea that meshes very nicely with what [this study] has shown."

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