AUSTRALIAN researchers have found a way to cut off the fuel supply to the ''motor'' that drives human sperm, greatly reducing their swimming ability and opening a new avenue to developing a male contraceptive pill.
The discovery, published in the journal PLoS Genetics, also throws new light on the little-understood reasons for infertility in men.
The research, led by Moira O'Bryan, of Monash University's school of biomedical sciences, used mice in laboratory tests with scientists engineering mutation in a gene called RABL2 that delivers protein fuel to the engine in a sperm's tail.
The mutation resulted in sperm tails that were 17 per cent shorter than normal and a 50 per cent reduction in sperm production.
The most striking result was that all mice with the mutated gene were rendered infertile and their sperm incapable of swimming.
''They weren't wriggling or going anywhere, they were just twitching,'' Professor O'Bryan said.
''With this mutation, we get motors that don't work properly. To be fertile, sperm need motility … or swimming ability.''
The research was conducted with scientists from the University of Newcastle, John Curtin School of Medical Research, Garvan Institute of Medical Research and the University of Cambridge in Britain.
Professor O'Bryan said a future male pill might work to inhibit the RABL2 gene rather than change it permanently.
''The challenge with developing the male pill isn't rendering the sperm infertile but turning them back on again,'' she said.
However, as RABL2 is also found, although in lower concentrations, in other tissues such as the brain, kidneys and liver, an inhibitor specific to the testes would need to be developed.
One in 20 men are infertile and Professor O'Bryan said the reasons for this are not understood.
''People don't talk about it very often … In our research, we've had brothers who have been infertile and neither one knows about the other. And we can't tell them because of privacy issues,'' she said.
The fertility research was one part of an immense project involving experimental mutagenesis - the deliberate engineering of mutations on various genes - in laboratory mice.
Researchers looked at a variety of issues, including the immune system, hearing, facial abnormalities, diabetes and obesity.
Professor O'Bryan said the project was like a big clinic where hundreds of mice were sorted into various projects. ''The ones that were fat were sent off in one direction, those with funny faces were sent elsewhere. A group in Sydney looked at lactation. Fertility was the last thing we tested,'' she said.