Mice Sperm Sabotage Other Swimmers With Poison

A study in mice found that poison-spewing sperm make others swim in circles, but carry the antidote for themselves

An illustration shows one sperm in a loop while another sperm is a line stretching toward a green circle, indicating an egg, on the right side of the screen
The protein RAC1 can cause some sperm to spin in circles until they die MPI f. Molecular Genetics/ Alexandra Amaral

Sperm are simple cells with a straightforward job: swim until they reach an egg, then fertilize it. But in mice, some sperm resort to divisive tactics in order to gain the advantage.

A study published on February 4 in the journal PLOS Genetics shows that a genetic variation in mouse sperm, called the “t-type,” can give a swimmer the upper hand. These t-type sperm are able to spread a protein called RAC1 that essentially poisons other sperm. T-type sperm plant the seeds of destruction early in their development, then fortify themselves against RAC-1, Brandon Specktor reports for Live Science. When it comes time to race for the egg, the t-type sperm can swim in a straight line while poisoned sperm swim in hapless circles until they die.

“We found out that the level of this protein… can be more or less active,” depending on whether the sperm have the gene to make it, and whether that gene is flipped on like a light switch, says biologist Alexandra Amaral of the Max Planck Institute for Molecular Genetics to Kassidy Vavra at Inverse. “The level of protein that is on has to be quite well regulated. If it is too much, sperm don't move well. And if it’s too low, it also doesn’t move well… they’re kind of in circles.”

T-type sperm produce the RAC1 protein at full throttle.

If all of the sperm in a group are t-type, and they’re all making RAC1, they will all struggle because there is so much of the poisonous protein going around, Sara Rigby reports for Science Focus magazine. On the other hand, if there are no t-type sperm present, then all the other sperm remain relatively healthy and swim well because there’s no overabundance of RAC1. However, if a cohort has a mix of t-type and normal sperm, then t-type will have the advantage.

"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," says Bernhard Herrmann, director of the Max Planck Institute for Molecular Genetics, in a statement. "Imagine a marathon, in which all participants get poisoned drinking water, but some runners also take an antidote."

The t-type sperm do the equivalent of poisoning the drinking water early in sperm development, affecting both themselves and their non-variant peers. All of the sperm inherit genes that make it difficult to interpret the chemical signals around them. But in the final cell division of sperm development, when half of a cell’s genes go to one sperm and the other half to another, only the sperm that inherit the t-type variation have an extra set of genes that reverses the poison’s effect, per Live Science.

The “poisoned” sperm end up swimming in circles, unable to advance in their quest. But the impervious t-type sperm swim ahead. In this case, there’s a 99 percent chance that the sperm that fertilizes the egg first will have the t-type variation. The research shows the importance of small genetic variations in sperms’ success, Amaral tells Inverse.

The study was conducted in about 100 mouse sperm cells, but not all species’ sperm behave the same way, University of California, Berkeley, cell biologist Polina Lishko tells Inverse. The study is preliminary, but future research could illuminate the specific molecular mechanism behind RAC1 that makes it damaging to sperm at high levels.

An earlier study showed a similar effect of RAC1 on bull sperm, which is more similar to human sperm than a mouse’s is. Amaral says that the team plans to conduct future research with human sperm, to see if RAC1 might be involved with some cases of male infertility.