It takes just one sperm to fertilize a woman’s egg and for each sperm that reaches the egg, there are millions that don’t. You probably knew that already, but here’s the thing: not all sperm cells are equal. Some have mutations in their DNA sequence that allow them to swim straighter, rather than in circles, and faster on average than their competition. What’s more, sperm cells can even employ gruesome tactics, such as poisoning their neighbors in order to enhance their odds of fertilizing the egg.

It’s not just about luck

The difference between a ‘loser’ and a ‘winner’ sperm cell could be down to a protein: RAC1. In a new study, researchers at the Max Planck Institute for Molecular Genetics (MPIMG) in Germany studied mouse sperm cells under the microscope, finding that this protein is responsible for guiding the sperm in the right direction by chemically signaling from the outside and activating other proteins.

The RAC1 protein plays a critical role in controlling the motility of sperm, in particular the average path velocity and linearity. This protein is produced in sperm that carry a particular DNA sequence known as the t-haplotype.

The researchers in Germany knew from previous research that it is thanks to this genetic sequence that some sperm swim in a straighter path and at a faster velocity than sperm lacking the t-haplotype. However, they were shocked to learn that t-haplotype sperm can also ‘poison’ their competition by injecting them with certain genes that inhibit movement.

“Sperm with the t-haplotype manage to disable sperm without it,” study co-author Bernhard Herrmann, director at the MPIMG, said in a statement. “The trick is that the t‑haplotype ‘poisons’ all sperm, but at the same time produces an antidote, which acts only in t-sperm [those with the t-haplotype] and protects them.”

In other words, it literally is a race for life (or death) for the millions of sperm cells on a quest to fertilize egg cells — and luck seems to play a minor role.

“Imagine a marathon, in which all participants get poisoned drinking water, but some runners also take an antidote,” said Herrmann, who is also the director of the Institute of Medical Genetics at Charité – Universitätsmedizin Berlin. That’s the same hospital where Kremlin critic and Russian opposition leader Alexei Navalny was treated after being poisoned, allegedly by the Russian government.

According to experiments, the vast majority of sperm cells that made little progress on their paths were genetically “normal”, whereas those that moved in a straight and optimal path mostly had the t-haplotype genetic factor. Poisoned cells literally swam in circles until they died. Meanwhile,  t-haplotype sperm that had the antidote that inhibited the effects of the “poison” charge straight ahead.

“Our data highlight the fact that sperm cells are ruthless competitors,” says Herrmann. “Genetic differences can give individual sperm an advantage in the race for life, thus promoting the transmission of particular gene variants to the next generation,” says the scientist.

The findings were reported in the journal PLOS Genetics.