Animals with a double dose of the mutant gene have 20 per cent more muscle and less fat. But they also grow so large in the womb that the calves sometimes cannot be born normally, often requiring surgery. The gene is found in a number of cattle breeds, including the Belgian Blue amd Charolais.
Now teams in the US and the Antipodes are investigating whether the same genes occur naturally in other farmyard animals, and whether transgenic animals incorporating the genes could be created to raise weightier animals, raising farm yields.
The discovery arose from work investigating muscle-wasting diseases such as muscular dystrophy. A team at Johns Hopkins University in Maryland found that removing a gene called GDF8 - for Growth Differentiation Factor 8 - from mice caused them to have huge muscles.
Researchers at the US Agricultural Research Service in Nebraska, together with scientists at the Ruakura Research Institute in New Zealand, then began searching for GDF8 - also known as myostatin - in cattle breeds such as the Belgian Blue, which is found in the US and Australia as well as Belgium.
"The mutation exists in several breeds," said Dr Tim Smith, whose work is reported in the latest edition of the US publication Genome Research. "They have to have a double copy of the gene to be heavily muscled. If they only have one then they're just a bit heavier but there aren't the associated birth problems." Cattle breeders would like to be able to test for it so they could eliminate the double copy.
Exactly what function the GDF8 protein performs is unclear. "It's a growth factor signalling protein which seems to bond to the surface of muscle molecules, and tells them to stop making fibres," said Dr Smith. "It's a negative molecule - telling the cells to stop. The loss of that control leads to increased muscle fibre."