Scientists have identified an altered version of a gene that leads to infertility in female sheep when present in two copies. But in a single copy, the same genetic mutation appears to increase fertility, causing the birth of non-identical twins or triplets. The researchers, based at the University of Otego, Dunedin, New Zealand published their results in this month's Nature Genetics.
Sheep, like people, usually produce one egg at a time, leading to single pregnancies (or identical twins if a single egg splits into two). But the Inverdale breed of sheep is unusual - 50 per cent of the ewes produce more than one lamb at a time, while the other 50 per cent are infertile. The altered gene involved in this strange reproduction pattern, located on the X chromosome, appears to increase the number of mature eggs produced (when present in one copy).
The human version of the gene - BMP15 - is known to make a protein that is involved in the egg ripening process. In sheep with two copies of the altered gene, the cells that surround and nourish the egg fail to multiply, so the immature egg dies of starvation. But in sheep with one copy of the altered gene, the egg-nourishing cells, though fewer in number than in normal sheep, may be more sensitive to the hormones that trigger ovulation. This in turn could increase the number of mature eggs released from the ovary each month.
The scientists say their discovery may have implications for research into human contraceptives and infertility treatments. 'The big question is whether sisters of infertile women are having twins' says team leader Dr Susan Galloway.
Sources and References
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Double trouble
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Scientists find gene that causes twins
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