Some 80 proteins in seminal fluid that can affect the likelihood of producing offspring have been identified by scientists at the University of Washington (UW).
Previously not known to have a role in reproduction, all the proteins were transferred from male fruit flies to females at mating.
The researchers have revealed that about 20 of the genes encoding these proteins were not even known to exist before their study.
They also say that their study confirms the presence of more than 70 additional proteins that other scientists had predicted could be found.
Michael J. MacCoss, assistant professor of genome sciences whose lab designs and tests new proteomic technologies, said that it was surprising to observe how rapidly seminal fluid proteins evolve in fruit flies.
"They change with the quickness we would expect for the immune system, which has to respond fast to new pathogens," MacCoss said.
The rapid evolution of the proteins that chaperone sperm may be due to the high-stakes competition between the many males that mate with each female fly.
Geoffrey D. Findlay, a doctoral candidate in the UW Department of Genome Sciences, says that each male fruit fly has an evolutionary advantage if he can increase the competitive ability of his sperm.
When the female retreats to lay eggs, according to the researcher, he wants them to be his offspring.
Findlay says that the male's seminal fluid proteins aim to ensure that his sperm are successfully stored in the female's reproductive tract, cue the female to lay eggs immediately after receiving his sperm, and make the female less likely to mate again with another male.
The proteins may also attempt to "disarm" the seminal proteins transferred to the female by other males, he says.
The researcher says that if a male's seminal proteins can outperform his competitors', he'll be more successful in passing on his genes to the next generation.
"Don't forget the female. She's not a passive participant in the chemical struggle," Findlay said.
The first male she mates with may not be the best father for her offspring, so it may not be in her interest to lay all of her eggs with his sperm.
It is widely suspected that proteins in the female reproductive tract are co-evolving with their male counterparts to look out for the female's own reproductive interests.
"There is cooperation and conflict between the male and female," Findlay said.
Writing about their study in the journal PLoS Biology, the researchers said that each is pushing the envelop to serve his or her own reproductive interests.
They say that the, when the interests of males and females don't match, the sexes undergo an evolutionary struggle for control of the outcome.
The competition among males, and the conflict between the sexes, may be driving the evolutionary patterns of their respective reproductive proteins.
The researchers say that this constant interplay between male and female proteins has caused seminal fluid content to differ between closely related species of fruit flies.
They have shown this in two ways. First, by making DNA sequence comparisons, they found that when the same proteins appear in different species, the molecules often have different sequences and have diverged more quickly than would be expected by chance. Second, using proteomics, they identified proteins that are found only in certain species' seminal fluid.
Other studies have shown that female fruit flies that get too many shots of seminal fluid may pay with their lives.
As the number of matings increase for a female, her behaviour is more constantly under the influence of male seminal proteins, and may move further away from the optimal for the female's physiological well-being.
Findlay says that the males are not trying to kill the female, but the toxic effect of mating is potentially a by-product of protein manipulation.
Previous studies of seminal fluid proteins took years because each protein had to be painstakingly culled out.
"Separating transferred proteins from an animal's own proteins is like searching for a needle in a haystack. We modified a technique to label the females' proteins by feeding them yeast carrying a stable isotope. This made the female proteins in the specimens invisible to our mass spectrometer. We then could pick out the transferred male proteins," said MacCoss.
The researcher believes that the same isotope labeling method could be used to detect other proteins transferred from one organism to another, such as from a nursing mother to her baby or from a pathogen to the animal it infects.
As to how does the knowledge of fruit fly seminal fluid proteins help improve understanding of fertility and infertility in other living things or in human couples, Willie J. Swanson, associate professor of genome sciences whose lab studies the evolution and function of reproductive proteins, said: "The specific genes and proteins might be different, but it's likely that other genes and proteins fulfilling similar reproductive functions will be found in other species. Seminal fluid proteins are of critical importance in reproductive fitness, but it's not as easy as saying, 'If we find this in fruit flies, this means it's in people.'"