快猫短视频

IT SOUNDS impossible, but infertility can be inherited. Reproductive
biologists have discovered a gene that makes female sheep sterile when they
carry two defective copies. When they carry only one defective copy, however,
they are superfertile and give birth to twins or triplets, keeping the gene in
circulation.

No one knows whether a similar genetic disorder could affect humans. 鈥淭he big
question is whether sisters of infertile women are having twins,鈥� says team
leader Susan Galloway of the University of Otago in Dunedin, New Zealand.

Left to their own devices, sheep usually produce a single lamb. But selective
breeding has created breeds that produce twins and even triplets. Inverdale
sheep, a strain of Romney sheep, are unusual in that roughly 50 per cent of ewes
produce more than one lamb at a time, while the other 50 per cent are
infertile.

鈥淚t鈥檚 fantastic. They breed sheep for prolificacy, and they find that they
are infertile,鈥� says team member Olli Ritvos of the University of Helsinki.
Because the gene is carried on the X chromosome, farmers can easily get around
that problem. They cross Inverdale rams with breeds that don鈥檛 carry the
defective gene so that each daughter produces lots of lambs. The lambs from the
next generation are slaughtered for meat to prevent infertility being passed
on.

The biologists have discovered that the sheep gene responsible for the
strange fertility pattern is the same as the human gene BMP15. This
gene is active in human ovaries, but its exact role was a mystery.

In the sheep, BMP15 is essential for reproduction. It makes a
protein in the ripening egg, or oocyte, which seems to instruct the granulosa
cells around the egg to multiply. These cells supply the egg with nutrients and
their numbers need to be increased to keep the egg well nourished as it grows.
In sheep with two defective copies of BMP15, the granulosa cells do not
multiply and the oocytes die of starvation.

鈥淚t shows that there鈥檚 a factor made by the oocyte in sheep and humans that
influences its local surroundings,鈥� says Jock Findlay of the Prince Henry鈥檚
Institute of Medical Research in Melbourne. 鈥淭he suspicion had been building,
because a similar factor is found in rodents. This shows it鈥檚 a general
辫谤颈苍肠颈辫濒别.鈥�

But Findlay admits to being mystified by how one defective copy of
BMP15 can make a sheep superfertile. 鈥淚 frankly find it difficult to
understand,鈥� he says.

Galloway鈥檚 team has a hunch, though. Sheep with one defective copy of the
gene produce enough granulosa cells to nourish the developing oocytes, but fewer
than in a normal sheep. These granulosa cells are also known to be more
sensitive than the cells of normal sheep to pituitary hormones LH and FSH, which
trigger ovulation. That increased sensitivity could be responsible for the
increase in the number of mature eggs that are released from the ovary.