Selecting sperm with greater stamina results in offspring that are healthier and live longer, according to a recent study in zebrafish.

Previous research has revealed that individual sperm vary in their swimming stamina or 'lifespan', and that this directly influences their ability to fertilise an egg. The current findings now suggest that these differences may also have longer-term consequences for the resulting offspring.

Scientists from the University of Uppsala in Sweden and the University of East Anglia (UEA) in Norwich collected the sperm of 26 zebrafish and split them into two parts. One batch was mixed with water and eggs, allowing any sperm that were motile the opportunity to fertilise an egg. The remaining sperm were mixed with water but the eggs were instead introduced after 25 seconds, by which time around half of the sperm had already lost motility. This ensured only the longer-lived sperm had a chance to fertilise the eggs.

The resulting offspring were then reared until adulthood and their lifespan, and the number and survival of offspring they produced were measured. Offspring produced by longer-living sperm were fitter said group leader Dr Simone Immler, based at UEA.

'They not only reproduced more throughout life, they also lived longer,' she said. The effects were less pronounced in female than male offspring however.

It is a long-standing belief that all sperm capable of fertilising an egg are equivalent and therefore 'natural or artificial' selection at the sperm stage will have little effect on the DNA and health of the resulting offspring, she said.

A growing body of work, however, looks set to challenge this theory. The current findings build on an earlier study by Dr Immler and her team showing that sperm with varying longevity have differing genetic makeup. She speculated that allowing only longer-surviving sperm to fertilise eggs may act as a form of quality control, weeding out lower quality sperm or those with harmful mutations.

The data may have implications for fertility clinics. At the moment IVF clinics select the strongest swimming sperm and use them to fertilise eggs. However, such artificial selection removes many of the steps of natural sperm selection, said Dr Immler.

She told New Scientist that she was already working with two IVF clinicians to confirm and apply the current findings to people.

Dr Denny Sakkas of Boston IVF in Waltham, Massachusetts, who works on improving fertility treatments, agreed that the basic finding appears valid. 'But to expand that to mammals is a pretty far stretch,' said Dr Sakkas. 'They don't have the data to indicate that.'

The study was published in Evolution Letters.

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Reproduced from BioNews with permission, a web- and email-based source of news, information and comment on assisted reproduction and human genetics, published by Progress Educational Trust.