“Fruit flies are incredibly special because they are basically the first model organism and have been studied for over 100 years,” said Alexis Sperling, a researcher at the University of Cambridge and first author of the paper. In biology, model organisms are nonhuman species that are studied extensively to gain insight into particular biological phenomena.

“I think in any other animal, [this work] would have been a lot more difficult.”

While the vast majority of animals must breed to reproduce, a small minority can avoid dating and mating altogether. Virgin birth occurs when an egg develops into an embryo without needing fertilization from a male. When parthenogenesis occurs, the offspring are always female. And while their genes are similar to their mother’s, they are not exact clones.

Scientists have previously identified parthenogenesis in animals such as snakes, birds, fish, lizards and more recently, crocodiles. However, the new study, published Friday in Current Biology, is the first to provide a genetic basis for why it happens.

“So thing is, we basically have no idea of the genes that are involved in parthenogenesis, particularly in animals,” said Darren Parker, an evolutionary biologist at Bangor University in Wales who was not involved in the work. “This study actually starts to get at some of the genes that are involved, which is pretty cool.”

Fatherless flies are a way to survive

To understand the genetic basis for solo baby-making, the researchers turned to another species of fruit fly called Drosophila mercatorum. In this species, the female can reproduce with or without a mate. They sequenced the genomes of both sexual and asexual D. mercatorum individuals and identified three genes that differed between them.

Having identified these candidate genes responsible for virgin birth, the researchers then altered the corresponding genes in the model fruit fly, D. melanogaster.

After examining 220,000 fruit flies over the course of six years, the researchers declared victory: Altering those three genes gave D. melanogaster the ability to reproduce without mating.

“I couldn’t believe it,” said Sperling. “We saw the development of the embryos, and they looked pretty sketchy, but eventually they kind of sorted out and developed into adult flies.”

Whenever males were around, females with the ability for virgin birth mated and reproduced the normal way. But when males weren’t available, one to two percent of the second generation of female flies with this ability produced offspring asexually.

This form of parthenogenesis is thought to occur when a species faces unfavorable conditions, like a lack of mates. Switching to virgin birth can be a survival strategy to keep the species alive.

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“[Virgin births] usually happen in old flies that are definitely halfway through their life,” said Sperling, who added that the midlife mark for fruit flies is around 40 days. “This is something they do as a last-ditch effort, similar to zoo animals that have been isolated for 10 years.”

Sperling suspects that modern agricultural practices are creating the right kinds of pressures to spur virgin births in insects that attack crops, which could make the pests harder to eliminate.

“There are many pest species that are parthenogenetic,” she said, “and there are potentially things that humans could be doing to contribute to this through the use of pesticides or just having very homogeneous crops.”