Fat Fish

Traditional domestication of livestock appears more effective than genetic modification

New Scientist 14feb01

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's 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.

Die young

However, spectacular as this seems, the transgenic wild trout still lagged behind their domesticated counterparts - a 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's growth.

Worryingly, both sets of engineered fish developed skull abnormalities, and the GM domesticated fish all died before reaching sexual maturity. "Genetic selection has pushed metabolic systems to their max already," 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 "plateau".

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