Female Flies’ Previous Lovers Affect Size of Later Children

A female fly’s previous sexcapades can have a profound effect on how her future children look, redefining the way scientists think about inheritance in insects.

Researchers at the University of New South Wales in Sydney, Australia, discovered that a mother’s first sexual partner can determine the size of her later offspring, even if he didn’t sire them. This odd evolutionary twist is caused by a secret compound in sperm. (See “Why Female Flies Eat Sperm.”)

Neriid flies. Photograph by Russell Bonduriansky

“It is strange and certainly unexpected,” said Angela Crean, an evolutionary ecologist at the university who co-authored the study. “We thought genetics is how inheritance works, but that’s just one mechanism of inheritance.”

Everyone knows the story of how babies are made. Sperm meets egg and creates a new life-form, which is half father and half mother. However, there are environmental factors that affect the development of the fetus, like smoking (in humans) and other chemical exposures in the womb.

In the case of the fly, semen is an environmental factor that holds the key to a baby fly’s size—whether or not the baby is related to the fly that supplied the semen, according to a new study published in Ecology Letters.

To test this, Crean manipulated the size of males by feeding them a diet that was either low in nutrients, creating smaller males, or high in nutrients, which made the males bigger. (Also see “Why Genetically Modified Flies Don’t Want Sex.”)

Then she let female flies get it on with their first sexual partners and deprived them of egg-laying spots. In flies, eggs don’t get fertilized until they’re laid. This means that females can potentially have sex with multiple partners without actually creating any baby flies.

Better Males, Better Offspring

After two weeks Crean mated the females with their second sexual partners and let them lay eggs. When the lady flies were exposed to the semen of a small male and then had children with a big male, their offspring ended up looking like the small male. The opposite was also true. And you thought you had daddy issues.

“Better quality males tend to produce better quality offspring,” said Fiona Clissold, a nutritional ecophysiologist at the University of Sydney who was not involved with the paper. “[This study] means if you’re not a good quality male, you can piggyback off someone else [and pass on better traits].”

The study looked at more than 500 flies, but no one knows why these insects have evolved this ability or how it works.

The next step is to pinpoint the influential compound and figure out whether the same principle applies to other animals. Crean says this finding could potentially impact human health—but there’s no way to test whether the same thing happens in our own species.

“It seems unbelievable, so you want to have a ‘this is what causes that,’” Crean said. “That can be a bit of a needle-in-the-haystack approach at the moment [because] there are hundreds of compounds in semen.”

The study is exciting because it suggests that there are more external factors that influence the evolution of a species, Clissold said.

“Nothing is simple in biology,” she added.

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Mollie Bloudoff-Indelicato is a science journalist who loves em dashes, ’80s music and parasites. She has a master’s degree from the Columbia University Graduate School of Journalism with concentrations in science journalism, photography, and radio reporting. Contact her at news@mbloudoff.com, and follow her on Twitter at @mbloudoff.