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Developmental Biology - Eggs & Sperm

Human Eggs Prefer Some Sperm Over Others

Human eggs use chemical signals to attract sperm...

New research from Stockholm University and Manchester University NHS Foundation Trust shows that eggs use chemical signals to "choose" sperm. Different women's eggs attract different men's sperm - and not necessarily their partners.
Humans spend a lot of time and energy choosing their partner. However, a new study by researchers from Stockholm University and Manchester University NHS Foundation Trust (MFT) reveals choosing your partner continues even after sex - as human eggs can "choose" sperm.

"Human eggs release chemicals called chemoattractants to attract sperm to unfertilised eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract," explains John Fitzpatrick, an Associate Professor at Stockholm University.
So the researchers examined how sperm respond to follicular fluid, which surrounds eggs and contains sperm chemoattractants. Their goal was to find out if follicular fluids from different females attract sperm from some males more than others.

Microscopic mate choice

"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," observed Fitzpatrick. "This shows that interactions between human eggs and sperm depend on the specific identity of the women and men involved."
The egg does not always agree with a woman's choice of partner. Researchers found that eggs did not always attract more sperm from their partner compared to sperm from another male.

Is this egg or sperm choice? Professor Fitzpatrick explains that sperm have only one job - to fertilize eggs. It doesn't make sense for them to be choosy. Eggs, however, can benefit by picking more genetically compatible sperm.
"The idea that eggs are choosing sperm is really novel in human fertility. 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."

Daniel Brison PhD, Professor, Scientific Director, Department of Reproductive Medicine, Saint Marys' Hospital, and senior author.

The research is published in the scientific journal Proceedings of the Royal Society B.

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.

John L. Fitzpatrick, Charlotte Willis, Alessandro Devigili, Amy Young, Michael Carroll, Helen R. Hunter and Daniel R. Brison.

The research was supported by the Manchester University NHS Foundation Trust, the University of Manchester, the National Institutes of Health Research, a Knut and Alice Wallenberg Academy Fellowship (2016-0146) and Swedish Research Council Grant (2017-04680) to J.L.F., and a Wenner Gren Postdoctoral Fellowship to A.D. The authors thank the patients who consented to their sperm and follicular fluid being used in this research and the staff at the Department of Reproductive Medicine, St Mary's Hospital, Manchester for making this study possible, particularly Claudette Wright and Chelsea Buck. We also thank Andrea Pilastro for use of his sperm tracker.

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Jun 11 2020   Fetal Timeline   Maternal Timeline   News

Microscopic Mate Choice. Figure 1 is an overview of an experiment to assess variation in sperm response to fluid from different females, from both their partner and a non-partner. In a simultaneous experiment, sperm presented with a choice of follicular fluid from two females while in the non-simultaneous experimental design sperm were presented with a choice of follicular fluid from one female (either the partner or non-partner) and a control medium. The number of sperm that successfully entered the microcapillary tube were counted by light microscopy at 300 magnification to quantify sperm accumulation in and responsiveness to that fluid.

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