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Your Sperm Are So Competitive They Literally Poison Each Other to Win

It’s the genetic equivalent of throwing a banana peel in ‘Mario Kart’

After the abundance of sperm-related studies that have been published within the last year, I feel rather confident in saying that we have absolutely no freaking clue what is going on with reproduction. Sure, we’ve learned a lot — about the way sperm swim, about the selectivity of the egg, about the function of the sperm tail — but with each new discovery, a completely conflicting piece of evidence emerges. 

While I’d just about thrown away the concept of the sperm race entirely, a study from the Max Planck Institute for Molecular Genetics in Berlin in February is forcing me to reconsider. As the research claims, sperm does in fact race to the egg, and races so fiercely that some sperm can actually poison the competition. 

According to the study, the success of sperm cells in mice depends almost entirely upon a DNA segment called t-haplotype. Sperms that carried t-haplotype were found to move faster and more directly to the egg than sperm that didn’t carry it, who ended up moving slowly or in circles. It might not be that t-haplotype provides the sperm with greater motility, though: Instead, researchers believe that the success of t-haplotype sperm may be because it essentially poisons the rest. 

“Sperm with the t-haplotype manage to disable sperm without it,” study author Bernhard Herrmann said in a press release. “The trick is that the t-haplotype ‘poisons’ all sperm, but at the same time produces an antidote, which acts only in t-sperm and protects them. Imagine a marathon, in which all participants get poisoned drinking water, but some runners also take an antidote.”

The way this “poison” works is by sending out gene variants that distort regulatory signals. Basically, these signals confuse the other sperm, making it difficult for them to find the egg. However, this advantage for t-haplotype sperm only occurs when they’re in mixed company. In mice, this genetic detail is typically only found on one of their two 17th chromosomes. Mice with t-haplotype on both chromosomes were infertile. It’s thought that a protein called RAC1 might be responsible for this — two chromosome t-haplotype sperm produced too much of it, while non-t-haplotype sperm might produce too little of it. 

What does any of this mean for humans? 

That’s unclear. While mice are regularly used to help understand human reproduction, they’re still mice. If future studies in humans find a similar discrepancy with t-haplotype, genetic testing and manipulating RAC1 could help improve fertility in men. For now, we can mostly just say that mice sperm is particularly ruthless.