It is literally a race for life when millions of sperm swim towards the egg cells to fertilize them. But does pure luck decide which sperm succeeds? As it turns out, there are differences in competitiveness between individual sperm. In mice, a “selfish” and naturally occurring DNA segment breaks the standard rules of genetic inheritance–and awards a success rate of up to 99 percent to sperm cells containing it.
A team of researchers at the Max Planck Institute for Molecular Genetics in Berlin describes how the genetic factor called “t-haplotype” promotes the fertilization success of sperm carrying it. The researchers for the first time showed experimentally that sperm with the t-haplotype move faster forward than their “normal” peers, and thereby establish their advantage in fertilization. The researchers analyzed individual sperm and revealed that most of the cells that made only little progress on their paths were genetically “normal,” whereas straight moving sperm mostly contained the t-haplotype.
Most importantly, they linked the differences in motility to the molecule RAC1, which might also play a role in directing sperm cells towards the egg, “sniffing” their way to their target.
“Sperm with the t-haplotype manage to disable sperm without it,” says Bernhard Herrmann, corresponding author of the study. “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,” he explains. “Imagine a marathon, in which all participants get poisoned drinking water, but some runners also take an antidote.”
“Our data highlight the fact that sperm cells are ruthless competitors,” says Herrmann. Furthermore, the example of the t-haplotype demonstrates how some genes use somewhat dirty tricks to get passed on. “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,” he says.