Promiscuous female fruit flies alter behaviour in males

Monday, 21 January, 2019

Promiscuous female fruit flies alter behaviour in males

New research led by the University of Oxford has demonstrated for the first time the effect female fruit flies having multiple partners has on sexual selection — before and after mating. Sexual selection is the branch of natural selection concerned with obtaining mates and fertility, rather than survival.

Over the last 50 years, biologists have realised that females in most animal species mate with multiple males during their lifetimes, in contrast to Darwin’s Victorian ideas of the monogamous female. However, it has previously been hard to work out how female promiscuity affects sexual selection. When females mate with more than one male, sexual selection can continue after mating because the sperm of rival males compete for eggs. But if females mate indiscriminately, do male adaptations for enticing choosy females become redundant?

Dr Juliano Morimoto, currently based at Macquarie University, collaborated with his colleagues in the UK to test the theory that increasing female promiscuity would reduce male competition before mating, while increasing their competition to fertilise the female’s eggs after mating. The results were published in the journal Nature Communications.

The researchers first genetically manipulated female Drosophila melanogaster fruit flies to increase their promiscuity. By deleting a sex peptide receptor, they reduced the time the females weren’t sexually receptive after mating and therefore led to them mating more frequently.

Hundreds of the more promiscuous females were then marked with paint and their interactions with male flies monitored. The researchers painstakingly counted the thousands of offspring produced and identified their fathers based on eye colour.

Once the female flies were genetically changed to become more promiscuous, sexual selection switched from favouring males who gain more mates (good at enticing) to favouring males who are better at post-mating competition (good at fertilising). While long theorised, this is the first time researchers have been able to show direct experimental proof of this theory.

“We found that when females mate promiscuously, male attractiveness is less important,” Dr Morimoto said. “Instead, having a large ejaculate might be what males need to win the war.”

The researchers also found that the male flies repeatedly mated with the same female in order to increase the chances of their sperm fertilising the female’s eggs. These males were particularly favoured by the females.

“This demonstrates that males adjust their sexual behaviour in response to the females’ promiscuity,” Dr Morimoto said.

Dr Stuart Wigby, whose lab at the University of Oxford hosted the research, said, “This work gives us new insights into the broad evolutionary principles that explain why males vary so much in nature. For example, why in some species males show spectacular displays or fight to the death for access to females, while in other species males invest in making lots of sperm or in pairing with one or a few females.”

Dr Morimoto added, “Because the gene we used to change female mating behaviour is very common among insects, our findings may also point to an important mechanism underpinning the evolution of insect reproductive patterns. This might either help in the development of improved ways to control insect pests or disease vectors by altering their reproduction, or at least help us understand the evolutionary consequences of attempting to do so.”

The researchers noted that they looked at closely confined small groups, which is a relevant model for many species but is also likely to explain why the males ended up repeatedly mating with the same females rather than seeking new mates. Seeing what happens in larger, more dispersed groups will be an important future step for understanding how promiscuity interacts with ecology to shape sexual selection.

Image credit: Dr Juliano Morimoto.

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