Scientists discover secret of virgin birth, and switch on the ability in female flies

Scientists have pinpointed a genetic cause for virgin birth for the first time, and once switched on the ability is passed down through generations of females.

For the first time, scientists have managed to induce virgin birth in an animal that usually reproduces sexually: the fruit fly Drosophila melanogaster.

Once induced in this fruit fly, this ability is passed on through the generations: the offspring can reproduce either sexually if there are males around, or by virgin birth if there aren't.

For most animals, reproduction is sexual -- it involves a female's egg being fertilised by a male's sperm. Virgin birth, or 'parthenogenesis', is the process by which an egg develops into an embryo without fertilisation by sperm -- a male is not needed.

The offspring of a virgin birth are not exact clones of their mother but are genetically very similar, and are always female.

"We're the first to show that you can engineer virgin births to happen in an animal -- it was very exciting to see a virgin fly produce an embryo able to develop to adulthood, and then repeat the process," said Dr Alexis Sperling, a researcher at the University of Cambridge and first author of the paper.

She added: "In our genetically manipulated flies, the females waited to find a male for half their lives -- about 40 days -- but then gave up and proceeded to have a virgin birth."

In the experiments, only 1-2% of the second generation of female flies with the ability for virgin birth produced offspring, and this occurred only when there were no male flies around. When males were available, the females mated and reproduced in the normal way.

Switching to a virgin birth can be a survival strategy: a one-off generation of virgin births can help to keep the species going.

The study is published today in the journal Current Biology.

To achieve their results, researchers first sequenced the genomes of two strains of another species of fruit fly, called Drosophila mercatorum. One strain needs males to reproduce, the other reproduces only through virgin birth. They identified the genes that were switched on, or switched off, when the flies were reproducing without fathers.

With the candidate genes for virgin birth ability identified in Drosophila mercatorum, the researchers altered what they thought were the corresponding genes in the model fruit fly, Drosophila melanogaster. It worked: Drosophila melanogaster suddenly acquired the ability for virgin birth.

The research involved over 220,000 virgin fruit flies and took six years to complete.

Key to the discovery was the fact that this work was done in Drosophila melanogaster -- the researchers say it would have been incredibly difficult in any other animal. This fly has been the 'model organism' for research in genetics for over 100 years and its genes are very well understood.

Sperling, who carried out this work in the Department of Genetics, has recently moved to Cambridge Crop Science Centre to work on crop pests and hopes to eventually investigate why virgin birth in insects may be becoming more common, particularly in pest species.

"If there's continued selection pressure for virgin births in insect pests, which there seems to be, it will eventually lead to them reproducing only in this way. It could become a real problem for agriculture because females produce only females, so their ability to spread doubles," said Sperling.

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Size does matter: Group size and mating preferences drive deeper male voices

Deeper male voices in primates, including humans, offer more than sex appeal -- they may have evolved as another way for males to drive off competitors in large groups that favored polygyny, or mating systems where a male has multiple mates, according to researchers. The research is the most comprehensive investigation of differences in vocal pitch between sexes to date and has the potential to help to shed light on social behavior in humans and their closest living relatives.

The average speaking pitch of an adult male human is about half the average pitch, an octave lower, than that of an adult female human, said David Puts, professor of anthropology at Penn State and study co-author.

"It's a sex difference that emerges at sexual maturity across species and it probably influences mating success through attracting mates or by intimidating competitors," he said. "I thought it has to be a trait that's been subjected to sexual selection, in which mating opportunities influence which traits are passed down to offspring. Humans and many other primates are highly communicative, especially through vocal communication. So it seems like a really relevant trait for thinking about social behavior in humans and primates in general."

The researchers used specialized computer software to visualize vocalizations and measure voice pitch in recordings from 37 anthropoid primate species, or those most closely related to humans, including gorillas, chimpanzees and recordings of 60 humans evenly divided by sex. Samples for each species included at least two male and two female vocal recordings, for a total of 1,914 vocalizations. The team then calculated average male and female vocal fundamental frequency for each species to see how pronounced the difference was between sexes.

The scientists collected additional information for each species to help identify correlations between male versus female voice pitch and factors that could have contributed to the trait's evolution. Additional variables included body size and body mass differences between males and females, habitat type, adult sex ratios, mating competition intensity and testes size. They also categorized each species by mating system -- monogamous, in which males and females have one mate at a time; polygynandrous, in which males and females have multiple mating partners; and polygynous, in which some males have several mates.

The researchers used these data to test five hypotheses simultaneously to identify which factors may have played the strongest roles in driving sex differences in vocal pitch. The hypotheses were: intensity of mating competition, large group size, multilevel social organization, trade-off against the intensity of sperm competition, and poor acoustic habitats. Previous research has looked at one or two of these hypotheses at a time. The current study is the first to test multiple hypotheses simultaneously for vocal pitch differences using a robust dataset, ensuring data consistency and garnering convincing results, according to Puts.

The team found that fundamental frequency differences by sex increased in larger groups and those with polygynous mating systems, especially in groups with a higher female-to-male ratio. They reported their findings today (July 10) in Nature Communications.

"Our findings highlight the important role of sexual selection and offer possible evolutionary explanations for why males and females differ in voice pitch across primates," said Toe Aung, first author and assistant professor of psychology and counseling at Immaculata University, who worked on the study as part of his doctoral dissertation at Penn State. "This research also provides insight into sex differences in voice pitch in our common ancestors who lived millions of years ago."

Deeper male voices may act as an additional way to fend off mating competitors without having to engage in costly fighting by making males sound bigger, in addition to other physical traits like height and muscle size, according to the researchers. In adult humans, for instance, males vocalize at an average of 120 hertz whereas females vocalize at an average of about 220 hertz, putting humans right in the middle of polygynous societies, the researchers reported.

"Although social monogamy is really common in humans, mating and reproduction in our ancestors was substantially polygynous," Puts said. "Our findings help us to understand why male and female voices of our species differ so drastically. It may be a product of our evolutionary history, particularly our history of living in large groups in which some males reproduced with multiple females."

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Male animals are subject to stronger evolutionary pressures than females

Male animals are subject to stronger selection pressures than females, which may allow populations to adapt to environmental change more efficiently, according to a report published in the open-access journal eLife.

The study supports one of the long-standing assumptions underpinning the idea that sexual selection bolsters adaptation: that stronger selection on males allows them to purge the population of genetic mutations that reduce survival fitness.

Sexual selection is selection arising from competition for mating partners and/or their reproductive cells (their eggs or sperm). For almost a century, researchers have thought that sexual selection is the ultimate selective force that generates the differences we see between male and female animals in terms of reproductive fitness and life history. Yet, little is known about how sexual selection combines with other environmental pressures to impact population demography and adaptive ability.

Living organisms accumulate mutations throughout life - some of which help them become fitter for survival, and some of which provide no benefit and may even cause a disadvantage (called deleterious mutations). Sexual selection is thought to promote evolutionary adaptation if it gives rise to stronger net selection - that is, the total purifying selection against deleterious mutations - in males rather than females. This is because a population's productivity relies on females' ability to reproduce, so that stronger net selection on males allows a population to get rid of the deleterious mutations quickly and adapt to their environment with a lower cost to the population, which may eventually reduce the risk of extinction.

"Our knowledge on whether such stronger sexual selection on males translates into stronger net selection to females is still limited," says first author Lennart Winkler, a PhD student at TU Dresden, Germany. "Previous studies have used the phenotypic variance of fitness to measure net selection, but its relevance has been questioned. An alternative measure is the organism's genetic variance of fitness. We used both measures to show whether net selection is generally stronger on males across a broad range of species."

The team ran a systematic literature search and compiled 101 paired estimates of male and female genetic variances across 26 species for two important components of an organism's fitness: reproductive success and lifespan.

They then tested whether the phenotypic variances were aligned to the genetic variances, and whether genetic variances show consistent sex differences. They predicted that males would show larger genetic variance in reproductive success but not in lifespan.

They found that the phenotypic variance of lifespan but not of reproductive success predicted the genetic variance in either males or females. Importantly, however, the phenotypic variance of reproductive success was larger in males than females, and this translated into a male bias in genetic variance. This sex difference could be detected in polygamous but not monogamous species. By contrast, there were no consistent sex differences in phenotypic or genetic variance for lifespan.

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