Can Former Sex Partners Influence Offspring's Traits?

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A laboratory assistant sorts out fruit flies at the International Atomic Energy Agency laboratories in Seibersdorf, Austria, on October 14, 2009. Herwig Prammer/Reuters

New research has demonstrated that the offspring of flies can have physical traits passed on by their mothers' past lovers and not necessarily their "biological" fathers. The study, which appeared Wednesday in the Ecology Letters journal, confounded researchers, who say that the discovery entangles the traditional scientific understanding of genetic variation across generations.

Scientists from Australia's University of New South Wales discovered that what they fed male Neriid flies in their lab directly correlated with their offspring's size. It wasn't clear how this size trait was transmitted to the offspring, though; it didn't appear to have a genetic component.

Researchers then began to look into male flies' semen to see if there was something in there carrying the size trait to offspring. In an interview with Popular Science, the study's lead author, Angela Crean, said that fly's ejaculate consisted of only 5 percent sperm and contained a multitude of proteins, sugars and other fluids, like peptides—all of which could possibly influence a female fly's sexual behavior and the resulting offspring.

To solve the size trait transmission mystery, researchers created two groups of flies, one large-sized and one small, by feeding them high-nutrient and low-nutrient rich diets, respectively, during the larva stage of their growth. Researchers then mated both groups of flies with different immature female flies. They repeated the process once the females had matured, then investigated whether the resulting offspring bore traits of the first sexual partner or the most recent one.

They discovered that although the second male fly technically fathered the offspring, the little ones' size was determined by what their mother's first sexual partner ate when it was a larva. So if the fly had eaten a high-nutrient diet as a larva, it would go on to produce large offspring. The researchers hypothesized that this is due to molecules in the first mate's semen, which are then absorbed by some of the female's immature eggs. Those molecules determine the traits of offspring when the female's eggs are inseminated by other males later on.

Aristotle's concept of telegony predicted this long ago: His theory postulates that males leave a mark on their mate's body that influences the offspring's traits—even if the child's father is actually someone else. This theory was dismissed in the early 20th century, however, because it conflicted with pioneering genetics studies.

But this discovery doesn't necessarily contradict Gregor Mendel's theories of genetic variation; rather, it means there is more to discover about how traits are inherited. Researchers still don't know why size is a trait passed on by a first mate, as opposed to other traits, for instance. It's also unclear if this variation rings true for other species. Don't worry, though: Crean says it's doubtful that it exists in humans.