Human eggs from biological females may be picky in the sperm they allow in for fertilization. According to new research, different chemical signals are more apt to attract sperm from some biological males than others, allowing the eggs to essentially “choose” their sperm – and it’s not always that of their partners.
"Human eggs release chemicals called chemoattractants that attract sperm to unfertilized eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract," said John Fitzpatrick, an associate professor at Stockholm University, in a statement.
Before mating, animals pick up on a variety of visual, acoustic, and chemical sexual signals that assist in choosing a mate, but the process doesn’t stop after fornication. Turns out, eggs may have more of a choice in the sperm that they allow in than previously believed. The underlying mechanisms in these processes is largely unknown, particularly whether chemical signals released from eggs allow females to “exert cryptic female choice” to favor sperm from specific males.
To determine how mating preference may be influenced by these processes, researchers from Stockholm University and Manchester University NHS Foundation Trust examined how sperm responds to follicular fluid, which surrounds eggs and contains chemoattractants for sperm. Follicular fluid and sperm samples were obtained from couples undergoing assisted reproductive treatment (IVF) to investigate how sperm responded to the follicular fluid from either their partner or a female who was not their partner.
"Follicular fluid from one female was better at attracting sperm from one male, while follicular fluid from another female was better at attracting sperm from a different male," said Fitzpatrick. "This shows that interactions between human eggs and sperm depend on the specific identity of the women and men involved."
Follicular fluid was shown to influence sperm behavior, but swimming behavior did not differ depending on the follicular fluid. Eggs from a female do not always attract the sperm from their partner and have more to do with the preference of their eggs than the capability of the sperm. It is thought that sperm has the sole responsibility of fertilizing an egg and the researchers argue that it wouldn’t make sense for the sperm to be picky. On the other hand, eggs can benefit by choosing high-quality sperm or that which is perhaps genetically compatible.
“Research on the way eggs and sperm interact will advance fertility treatments and may eventually help us understand some of the currently 'unexplained' causes of infertility in couples,” said Professor Daniel Brison, the scientific director of the Department of Reproductive Medicine at Saint Marys' Hospital.
The findings are “novel in human fertility” and may have implications for fertilization treatments.