By Michael Le Page
The Eliud Kipchoges of the sperm world sire healthier, longer-lived offspring than the Usain Bolts, at least in zebrafish. The findings challenge the prevailing orthodoxy about what determines the physical traits of sperm, which could have important evolutionary implications.
It also suggests the sperm selection methods used by fertility clinics – which favour the sprinters – could be improved.
“I definitely do think this is relevant,” says team leader Simone Immler of the University of East Anglia in the UK. “We miss out on a lot of steps during artificial fertilisation technologies.”
Half of Zebrafish sperm stop swimming just 25 seconds after after entering water, although some fare better and survive for about one 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 spermhad 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 not as pronounced in female offspring as in male ones.
Immler thinks allowing only longer surviving sperm to fertilise eggs acts as a form of quality control, weeding out sperm with harmful mutations. 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 60s, it was thought that the physical traits of sperm were controlled fully 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 of the University of Oxford, who studies another kind of sperm selection. It is very likely to have implications for humans and IVF, Goriely thinks.
Immler says she 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.”
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 get into the fallopian tubes first – the sprinters – 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’s 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: Evolutionary Letters, DOI: 10.1002/evl3.101
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