PIG breeders face a dilemma: people want lean pork, but lean sows don鈥檛
produce enough milk for their piglets. Now researchers in the Netherlands say
they鈥檝e discovered how to make fat sows produce lean offspring. They claim their
findings will change the way farm animals are bred.
Animals inherit two copies of each gene, and usually it doesn鈥檛 matter which
parent a copy comes from. But with a few 鈥済enetically imprinted鈥� genes, only the
copy that comes from your mother, or from your father, is expressed. If the
father鈥檚 copy is active, for example, it will be expressed in his daughters but
turned off in their children
(New 杏吧原创, 28 November 1998, p 26).
Such genes tend to control growth, perhaps because of the conflicting
interests of mothers and fathers, says Gavin Kelsey of the Babraham Institute
near Cambridge. Dad wants a big baby who is more likely to survive. Mum wants a
little one so she is more likely to survive.
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In mice, just 0.1 per cent of genes are imprinted, says Kelsey. But a team at
Wageningen Agricultural University has found an unprecedented number of
imprinted genes in pigs. Four out of five sets of genes associated with back fat
thickness, fat in muscle and muscle thickness in pigs are genetically imprinted.
鈥淚t surprised us,鈥� says team member Johan van Arendonk.
Armed with this knowledge, he says, breeders can breed a sow with a 鈥渇at鈥�
version of a gene from her father. This gene will make the sow fat but it will
be inactivated when it is passed to her offspring. So if the sow is crossed with
a boar carrying a lean version of the gene, all the piglets will be lean, as
they will only express the gene inherited from their father.
Breeders haven鈥檛 noticed that it matters whether traits come from mothers or
fathers, says van Arendonk, because each of the four gene complexes only
controls up to a tenth of the variation in each characteristic. Unless you look
for them, the imprinted effects do not stand out from the variation caused by
other normally inherited genes.
Van Arendonk suspects that animals have many more sexually imprinted genes,
as most inherited traits affect energy consumption, which in turn influences
growth. Kelsey, however, questions whether there are more imprinted genes than
have been found in mice. 鈥淔ive sets of genes is a small sample,鈥� he says.
But van Arendonk points out that, because there is so much genetic uniformity
in experimental mice, crosses have to be carefully designed to reveal any
genetic imprinting. 鈥淲e don鈥檛 do this, because we don鈥檛 think there is much
there,鈥� he says. 鈥淏ut because we think there isn鈥檛 much, we don鈥檛 look.鈥�
The Dutch results, which will appear shortly in Proceedings of the
National Academy of Sciences, might make people look more carefully, he
says. If a lot of economically important genes are imprinted, it could change
the way animal breeding is done.