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.