Ovulation-the periodic release of an egg cell from the ovary-is an essential process in mammalian reproduction. The action of the hormones involved in ovulation, such as luteinizing hormone and the ever-popular oestrogen and progesterone, is well understood, but how other factors influence it is less clear. In humans, as well as cattle and sheep, only one egg is normally released - which can sometimes split and give rise to identical twins. But if multiple ovulations take place simultaneously, several eggs can be fertilized and fraternal twins or even triplets can be conceived. Genetic factors have long been suspected to contribute to this process, but precisely what they might be has remained obscure.
Years of animal breeding have led to the implication of genetic factors in the regulation of ovulation. For example, a tendency to produce fraternal twins and triplets at higher-than-normal frequency has been observed in specific breeds of sheep, such as the 'Inverdale' and the 'Hanna'. From breeding experiments, it is known that something on the X chromosome predisposes these sheep to multiple ovulations, but only when present in one copy-it causes infertility when an animal has two copies.
Susan Galloway (of AgResearch and the University of Otago) and colleagues have now identified the factor that underlies twinning in these sheep: BMP15 (Nature Genetics, Vol. 25, No. 3, pp. 279-293). This protein is expressed in the ovary, and it is similar to a class of growth factors that are known to be involved in the formation and growth of a variety of tissues. The researchers found that the gene encoding BMP15 is mutated in both the Inverdale and Hanna breeds, and they present several models to explain how the partial and complete loss of BMP15 activity might lead to multiple ovulation and infertility. These findings should lead to better understanding of the genetic mechanisms that influence ovulation.
Dr. Susan Galloway
AgResearch Molecular Biology Unit
University of Otago
Telephone: +64 64 3 479 7681
Fax : +64 3 477 5413
(C) Nature Genetics press release.
Nature Genetics, Vol. 25, No. 3
Message posted by: Trevor M. D'Souza