Promiscuous Female Flies Save Their Population
Promiscuous female files may help save their populations from extinction, a new study suggests. The study used fruit flies Drosophila pseudoobscura, shown mating above.
Credit: University of Exeter.

Why Are Most Animals Promiscuous? Sleeping around has its benefits for female files, and not just in the pleasure department. New research suggests promiscuity can help save their populations from extinction.

The findings could help to explain why females of most species, from insects to mammals, have multiple mates, despite this being more risky and costly for the individual. The process of breeding can increase the risk of predation and zap energy, time and nutrients for females.

The study suggests that polyandry, the phenomenon of having multiple mates, reduces the risk of giving birth to all-female broods, which could in turn lead to extinction.

Files can sometimes produce female-only broods when one member of the mating pair has a certain gene, called the SR, or sex-ratio, gene, which biases the genders of the resulting progeny. In males, the SR gene will kill off all of the sperm with a Y sex chromosome. As in humans, fly parents each contribute one sex chromosome to offspring, with males giving either an X or Y and females giving only X chromosomes. Offspring with XX become female and those with XY become males.

So offspring from males with the SR gene will be all-female. The all-female offspring will carry the SR gene, which will then be passed on to their sons, in turn, resulting in more all-female broods. Eventually there will be no males and the population will die out.

To test this scenario, the researchers gave some fly populations the opportunity to mate naturally, meaning that the females had multiple partners. The others were restricted to having one mate each. The scientists bred several generations of these populations to see how each fared over time.

Over 15 generations, five of the 12 populations that had been monogamous became extinct as a result of males dying out. The SR gene was far less prevalent in the polyandrous populations, and none of these populations became extinct.

The results show how having multiple mates can suppress the spread of the SR gene, making all-female broods a rarity, the researchers say. That's because males that carry the SR gene produce only half as many sperm as normal males. When a female mates with multiple males, their sperm will compete to fertilize her eggs. The few sperm produced by males carrying the SR gene are out-competed by the sperm from normal males, and the SR gene cannot spread.

"We were surprised by how quickly – within nine generations – a population could die out as a result of females only mating with one partner," said study author Nina Wedell of the University of Exeter, UK. "Polyandry is such a widespread phenomenon in nature, but it remains something of an enigma for scientists. This study is the first to suggest that it could actually save a population from extinction."

The results were published today in the journal Current Biology.