Might older human sperm produce healthier offspring? SCIEPRO / SCIENCE PHOTO LIBRARY

Sperm with stamina sire the healthiest, longest-lived offspring, at least in zebrafish. The finding challenges the prevailing orthodoxy about what determines the physical traits of sperm, which could have important evolutionary implications.

It also suggests that the methods fertility clinics use to select sperm – which instead favour the sprinters – could be improved.

“I definitely do think this is relevant,” says team leader Simone Immler at the University of East Anglia in the UK. “We miss out on a lot of steps during artificial fertilisation technologies.”


Longer lived

Half of zebrafish sperm stop swimming just 25 seconds after entering water, although some fare better and survive for about 1 minute.

To see if there was any difference between these short and relatively longer-lived sperm, Immler’s team split zebrafish ejaculate into two parts. One part was mixed with both eggs and water. With the other part, the eggs were added 25 seconds after the water, meaning that only the longer-lived sperm had a chance of fertilising them.

The results were striking. The offspring sired by longer-lived sperm were fitter, says Immler. “They not only reproduced more throughout life, they also lived longer.” However, the effects were less pronounced in female offspring than male ones.

Allowing only longer-surviving sperm to fertilise eggs might act as a form of quality control, weeding out sperm with harmful mutations, says Immler. But, surprisingly, this challenges conventional wisdom.

The stem cells that give rise to sperm have two slightly different copies of the genome. But sperm themselves have just one copy, containing a mix of the parental genomes.

Overturning the orthodoxy

For many decades, starting in the 1960s, it was thought that the physical traits of sperm were fully controlled by the diploid genetics of the male producing them, rather than the haploid genetics of the individual sperm cells, says biologist Richard Borowsky of New York University. If true, this means selection – natural or artificial – at the sperm stage cannot have much effect on the DNA of the resulting offspring.

But recent work by Borowsky and others suggests the orthodoxy is wrong, with implications for both evolution and fertility treatments. Immler’s study adds to the evidence. “This is potentially very important,” says Borowsky.

The study looks robust, says geneticist Anne Goriely at the University of Oxford, who studies another kind of sperm selection. It is very likely to have implications for humans and IVF, she says.

Immler is already working with two IVF clinicians to confirm and apply the findings to people. “I can’t tell you more at this very moment,” she says.

A far stretch

The basic finding appears valid, says Denny Sakkas of Boston IVF, who works on improving fertility treatments. “But to expand that to mammals is a pretty far stretch. They don’t have the data to indicate that.”

In humans, it is thought that the sperm that are first to reach the fallopian tubes – down which the eggs travel – are the best ones, says Sakkas. IVF clinics typically select the strongest swimmers and use them to fertilise eggs within hours.

“It’s very, very rare we will leave them overnight,” he says, because there is plenty of evidence that keeping the sperm of humans and other mammals in culture for too long is harmful.

However, this could be due to the artificial conditions. Inside women, sperm can survive up to a week.

Journal reference: Evolution Letters, DOI: 10.1002/evl3.101