Semen has controlling power over female genes and behaviour

Semen says turn those genes on (Image: CNRI/SPL)

THERE’S more to semen than sperm. In many animals, seminal fluid alters both the bodies and sometimes even the behaviour of females. Human semen, too, triggers changes in the uterus, and might have wider effects on women, aimed at just one goal.

“It’s all about maximising the chances of the male reproducing,” says Sarah Robertson of the University of Adelaide in Australia. The effects are most striking in fruit flies: seminal fluid can make the females eat more, lay more eggsand be less receptive to other males.

Now a team led by Tracey Chapman at the University of East Anglia in Norwich, UK, has found that male fruit flies selectively alter the chemical make-up of their seminal fluid. In the presence of rivals, the males produce more seminal proteins. “It came as a real surprise,” says Chapman. “It’s a sophisticated response to the social and sexual situation.”

Some of their findings were presented at the Society for Molecular Biology and Evolution conference in Vienna, Austria, last week, including their discovery that one of these proteins is a “master regulator” of genes. Females exposed to it show a wide range of changes in gene expression.

Chapman thinks this kind of seminal signalling is widespread in the animal world. The semen of people, pigs and mice affects the female reproductive tract, and the question is whether it can also produce behavioural responses in female mammals similar to those seen in fruit flies.

There have been claims that semen can do everything from making women sleepy after sex to strengthening the emotional bond with their partner. One 2002 study, based on a survey of 300 students, even found that women whose partners did not use condoms scored lower on a measure of depression.

If that effect is real, depression in some people might be treatable with artificial-semen suppositories. Gordon Gallup of the State University of New York at Albany, who carried out the study, says a PhD student of his has replicated the finding in a survey of 1000 women, but the results were never published.

In flies, seminal proteins can directly affect behaviour because they enter the circulatory system, travelling throughout the body to the brain. “They rapidly get to many places in the female,” Chapman says.

From the female’s perspective, seminal signalling is usually nothing sinister. According to Chapman, it’s an efficient way of getting a female’s body ready to produce offspring as soon as possible.

It’s not clear whether any components of human semen get into the bloodstream, but it could be possible, particularly for small molecules like hormones, says Robertson. She has shown that seminal fluid induces expression of a range of genes in the cervix, including ones that affect the immune system, ovulation, the receptivity of the uterus lining to an embryo, and even the growth of the embryo itself.

As for seminal signalling, she thinks it’s more likely to be indirect, with semen causing the cervix to produce molecules that influence the rest of the body. Her team is studying the effect of three microRNAs – RNA fragments that affect gene expression – released by the cervix in response to semen.

Whatever the mechanism, both Chapman and Robertson say it’s plausible that semen could have effects on women well beyond their reproductive tract.