Reproduction: Some sperm cells POISON each other in the race to get to the egg first

To win the race to fertilize an egg, some sperm don’t play fair and poison their opposition, a study finds.

Experts from Germany found in mouse studies that sperm with a genetic factor called a ‘t-haplotype’ can move more quickly towards their target.

Meanwhile, without this group of genes, their competition is less effective at progressing and they are more likely to wander aimlessly in circles.

The researchers linked the difference in motility (the ability to move) to the protein RAC1, which sends signals in from outside the sperm cells.

In white blood cells and cancer cells, RAC1 is known to play a role in directing its host cells to others that emit chemical signals.

Given this, and the findings of the new study, it’s possible that the protein may play a similar role in sperm cells, helping them make their way to their egg target.

The findings may even shed new light on the reasons behind certain forms of male infertility, increasing the potential for future treatments.

“Sperm with the t haplotype manages to knock out sperm without the t haplotype,” said paper author Bernhard Herrmann, who is the director of the Max Planck Institute for Molecular Genetics in Berlin.

‘The trick is that the t haplotype’ poisons ‘all sperm, but at the same time produces an antidote that only works in t sperm and protects them.’

“Imagine a marathon where all participants are given poisoned drinking water, but some runners also take an antidote.”

The team explained that the t-haplotype contains certain gene variants that are spread across all of the sperm and disrupt regulatory signals so that they would prevent the so-called ‘progressive’ moment by themselves.

Half of the sperm – the ones that also end up with the t haplotype when the animal’s chromosomes are evenly spaced between them – can produce another factor that counteracts this signal disruption, allowing them to swim straight ahead.

In their study, the researchers first looked at the sperm of those male mice that had the t haplotype on only one of their two ‘chromosomes 17’ and found that some of the cells (with the t haplotype) could swim straight ahead. , while the others were not.

When treating all sperm with a substance that inhibits RAC1, the team noted that the cells without the t haplotype regained the ability to swim correctly.

This, they explained, confirms that the t haplotype interferes with RAC1 activity, which in turn stops progressive movement.

In fact, according to the researchers, any abnormal level of RAC1 activity is bad for the sperm. Mice with two copies of the t haplotype, for example with one on each of their chromosomes 17, have high levels of RAC1 and are almost unable to move.

On the other hand, sperm from mice that have no t-haplotype at all also lose their ability to swim well when given the RAC1 inhibitor – indicating that low RAC1 levels are also bad for male fertility.

“The competitiveness of individual sperm appears to depend on an optimal level of active RAC1,” said author and molecular geneticist Alexandra Amaral.

“Both reduced and excessive RAC1 activity interfere with effective forward movement,” she added.

Abnormal RAC1 activity, the team speculated, could also explain some forms of male fertility in humans – meaning the findings could pave the way for new forms of fertility treatment.

“Our data underscores the fact that sperm cells are relentless competitors,” said Professor Herrmann, adding that the t-haplotype also provides a demonstration of how some genes use ‘dirty tricks’ to be passed on.

“Genetic differences can give individual sperm an advantage in the race for life, promoting the transfer of certain gene variants to the next generation.”

The full findings of the study are published in the journal PLOS Genetics.