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TRYING to grow bigger farm animals by genetically engineering them to produce
more growth hormone may be harder than we thought, say researchers in Canada.
Animals that have been bred to grow fast have hit a metabolic glass ceiling, and
any attempts to boost their growth further might only result in physical
deformities, they say.

Robert Devlin of Fisheries and Oceans Canada in West Vancouver and his
colleagues wanted to find out whether domestication affected an animal鈥檚
response to being genetically modified in this way. They decided to study fish,
because they could easily get hold of both wild and domesticated strains. They
started by engineering a slow-growing wild strain of rainbow trout. The
transgenic trout grew much faster than their non-engineered siblings, and were
over 17 times as heavy and over twice as long at 14 months of age.

However, spectacular as this seems, the transgenic wild trout still lagged
behind their domesticated counterparts鈥攁 strain bred for fast growth that
is commonly used in aquaculture. When the team introduced the gene into the
domesticated strain, they found that it had no effect on the fish鈥檚 growth.

Worryingly, both sets of engineered fish developed skull abnormalities, and
the GM domesticated fish all died before reaching sexual maturity. 鈥淕enetic
selection has pushed metabolic systems to their max already,鈥� says Devlin.
Although growth hormone is in abundant supply, other factors are limiting
growth. For instance, animals are finding it impossible to churn out enough
proteins to continue growing healthily.

Intriguingly, when Devlin and his team added the gene to a partly
domesticated strain of salmon, it did result in extra growth. He suspects this
is because the salmon strain had not yet reached its growth 鈥減lateau鈥�.

Researchers tinkering with the metabolic pathways of different species and
breeds will need to be careful. 鈥淕enetic background is a big issue,鈥� says
Devlin, and scientists need to think twice before diving in with transgenes.
鈥淚deally, the physiology should be done before the genetics.鈥�