An analysis of the microscopic dynamics between sperm and egg has revealed a fascinating twist to a phenomenon called female "cryptic choice." According to a new study, eggs use various chemical signals to attract different men's sperm — and not necessarily the sperm of their chosen partners.
For a baby to be born, one egg has to be fertilized by one of 250 million sperm. This research suggests that mate choice continues after sex and that human eggs can favor some sperm over others.
These findings were published Tuesday in the journal Proceedings of the Royal Society B.
Historically, the chemical communication between sperm and egg has been understudied, explains co-author John Fitzpatrick, a reproductive behavior researcher. Prior to his study, the sexually selective role of chemoattractants has been seen in animals like marine invertebrates, but not in humans.
Illuminating these chemical communications may lead to new treatments for infertility issues, which affect more than one in ten couples in the United States, the researchers say.
The study — At the start of the study, Fitzpatrick and his colleagues focused on understanding how sperm interacts with follicular fluid, and whether that dynamic is influenced by who the sperm belongs to.
Follicular fluid surrounds eggs and contains chemicals called chemoattractants, which attract sperm to unfertilized eggs. Sperm alter their swimming behavior to orient toward and accumulate in follicular fluid, based on chemoattractants.
Scientists have known chemoattractants play a powerful role in conception. What they haven't known is whether eggs use them to "pick" sperm based on human mate choice.
The team collected sperm and follicular fluid samples from participating couples undergoing assisted reproductive treatment. The researchers also gathered data about the couples' fertility-related histories including the number of eggs retrieved or fertilized, sperm density in ejaculate samples, embryo quality, and pregnancy outcomes.
Then, in the lab, scientists exposed sperm to follicular fluid from two females either simultaneously or non-simultaneously. One follicular fluid sample was derived from their chosen partner, while the other belonged to another participant. Each combination was repeated twice.
To quantify how follicular fluid influenced sperm behavior, researchers counted the number of sperm that accumulated in the follicular fluid of each female.
The results were striking, suggesting the identity of the creator of the sperm matters — but in a counterintuitive way.
"Because of these costs, eggs should be choosy about which sperm fertilize them."
"Human eggs are better at attracting sperm from some men more than others," Fitzpatrick says. “When comparing sperm from two men, eggs attract between 18 to 40 percent more sperm from the preferred man.”
Time and time again, experiments showed interactions between human eggs and sperm depend on the specific identity of the women and men involved. But eggs do not always agree with the women's choice of partner, nor attract more sperm from their partner compared to sperm from another male.
It was surprising that this happened across the study samples, Fitzpatrick says.
"Because humans, and many other animals, spend a lot of time and energy choosing their partners it wasn't at all clear that the eggs would continue being choosy," he explains.
Interestingly, eggs preferred sperm from their donor's partner about as often as they preferred sperm from a man who wasn’t their donor's partner. Fitzpatrick and his team hypothesize that egg choice would back up partner choice, but it didn't. The team hopes to figure out why in future research.
What egg preference means for the future of fertility — Based on these findings, it seems like chemoattractants enable eggs to have their own independent mate preferences, regardless of human partner choice. On the flip side, Fitzpatrick suspects that sperm aren't likely to be as choosy. That's because the costs for sperm and men are usually pretty low after fertilization relative to women.
Once sperm is inside the female's reproductive tract they either fertilize the egg or die, he explains. It doesn't make sense for sperm to be choosy at that point.
"But for eggs, and women, there are a lot of extra costs that come after fertilization such as the costs of pregnancy," Fitzpatrick says. "Because of these costs, eggs should be choosy about which sperm fertilize them."
Fitzpatrick stresses that this work does not show that infertility is caused by incompatibility between partners. This experiment was set up to assess this, and they found no evidence that this was the case.
In about one-third of infertility cases, there is not a clear cause, Fitzpatrick explains. Still, he reasons, these previously unexplored chemical dynamics may help explain these mysterious cases — perhaps eventually leading to new ways of diagnosing and treating infertility.
"We weren't considering how chemical signals might influence egg-sperm interactions before," Fitzpatrick says. "Our work helps open the door to consider this in the future.
Abstract: Mate choice can continue after mating via chemical communication between the female reproductive system and sperm. While there is a growing appreciation that females can bias sperm use and paternity by exerting cryptic female choice for preferred males, we know surprisingly little about the mechanisms underlying these post-mating choices. In particular, whether chemical signals released from eggs (chemoattractants) allow females to exert cryptic female choice to favour sperm from specific males remains an open question, particularly in species (including humans) where adults exercise pre-mating mate choice. Here, we adapt a classic dichotomous mate choice assay to the microscopic scale to assess gamete-mediated mate choice in humans. We examined how sperm respond to follicular fluid, a source of human sperm chemoattractants, from either their partner or a non-partner female when experiencing a simultaneous or non- simultaneous choice between follicular fluids. We report robust evidence under these two distinct experimental conditions that follicular fluid from different females consistently and differentially attracts sperm from specific males. This chemoattractant-moderated choice of sperm offers eggs an avenue to exercise independent mate preference. Indeed, gamete-mediated mate choice did not reinforce pre-mating human mate choice decisions. Our results demonstrate that chemoattractants facilitate gamete-mediated mate choice in humans, which offers females the opportunity to exert cryptic female choice for sperm from specific males.