How Previous Sexual Partners Affect Offspring

It’s a long-held belief among animal breeders that pure-bred progeny are best produced by females who have never mated before. Call it puritanical or ridiculous, but in breeding, it’s been a long-standing practice—even though there has never been much science to back it up. Now, however, researchers at University of New South Wales in Australia believe they may finally have some evidence to give that notion some scientific support.

Working with flies, Angela Crean, a research fellow at the evolution and ecology research center, picked up on her mentor’s work of looking at how male factors can influence offspring outside of the DNA in his semen.

“The genetic tests showed that even though the second male fertilized the eggs, the offsprings’ size was determine by the condition of the first male,” she says of her findings, published in the journal Ecology Letters. “The cool thing is that the non-genetic effects we are seeing are not necessarily tied to the fertilization itself.”

Cool, or really disturbing. The implications of the study are that any mates a female has had may leave some legacy—in the form of physical or other traits that are carried in the semen (but not the DNA-containing sperm)—that could show up in her future offspring with another mate.

While there’s a growing body of work showing that a mother’s diet, her smoking status, and other lifestyle habits can have an influence on her offspring, the data on similar factors on the father’s side is just emerging. With flies it’s known, for example, that males who eat a maggot-rich diet while they’re mere larvae, develop into larger than average adults, and on top of that, sire larger than average offspring as well. Males fed a meager maggot diet tend to be smaller have have smaller progeny.

Eager to learn how this was happening, Crean conducted a series of mating experiments with female flies when their eggs were immature. At that stage, the eggs are more receptive to absorbing factors in semen, but because they aren’t fully developed, they can’t be fertilized and won’t result in baby flies. When she and her colleagues “mated” these females with males who were larger, then allowed the females to actually mate with smaller males once they were mature, the offspring turned out to be large, just like the first males the females had sexual contact with. Genetically, they were the offspring of the second, smaller male, but physically, they resembled the larger males.

The same was true when they reversed the experiment and first exposed the females to smaller flies and then mated them with the larger ones.

To be sure that the was indeed due to something in the semen, Crean repeated the studies with an unfortunate group of male flies who had their genitalia glued down so they could not pass on any semen during their encounters. (“It’s horrifying but seemed nicer than cutting them off,” she says.) When these males, both large and small, were the first “mates” for females, their size did not have an effect on the offspring when the female mated with her second mate and had offspring. In other words, those offspring were large if the second male was large, and small if the second male was small.

Crean says the idea of a female’s previous mates having an effect on their offspring isn’t unheard of. In fact, this very idea, called telegony, was proposed by ancient scholars such as Aristotle but dismissed with the advent of genetics. But new findings about epigenetics — how our behaviors, such as diet, smoking and drinking — can affect our genes and how those changes can be passed on, make the idea of such non-genetic inheritance possible. “This could be seen as a maternal effect [such as diet or smoking] where the mother’s environment are her previous mating partners,” she says. “We have to realize that it’s not just DNA that gets passed on. It opens up the opportunity for all these other pathways that we had excluded.”

And while flies aren’t people, what are the chances that the same phenomenon is occurring in human reproduction? “It’s something we definitely don’t want to speculate about yet with humans,” she says. “There is no direct scientific evidence for that at all.” At least, for now.