The modified fish are bigger because they have three sets of chromosomes, rather than two sets, as is normal. Rainbow trout with the extra chromosomes--the long strands of DNA that determine genetic makeup--grow faster because they are unable to reproduce. The energy from the food they eat is shifted from reproduction to growth, said William Hershberger at the agency's Center for Cool and Cold Water Aquaculture.
Additionally, the flesh of these modified fish, which are called triploids, doesn't become soft with age, and their skin doesn't become dark--common traits in normal fish. The trout's prominent hook nose doesn't develop so rapidly either. While these characteristics aren't problems for the trout, consumers tend to shy away from ugly fish, Hershberger said. Ugly, unsold fish, of course, means wasted food and lower profits.
Under a strategy used by some fish hatcheries, embryos are exposed to heat or pressure about 20 minutes after fertilization to add the third set of chromosomes.
But under the technique created at the Agricultural Research Service, scientists apply heat or shock to actually add two extra sets of chromosomes, creating a tetraploid. When they grow, the four-chromosome-set fish breed with normal fish and give birth to a three-chromosome-set generation. The addition of the two extra chromosome sets can occur up to five hours after fertilization.
A three-chromosome-set trout grown in a hatchery will weigh 10 to 12 pounds after two years, he said, while a normal, two-chromosome-set fish from a hatchery will weigh 8 to 10 pounds. In the wild, a trout might make it to 5 pounds.
The technique promises a couple of advantages. One, all of the fish bred in the new generation will have three sets of chromosomes. Only about 70 percent of the fish created using the earlier technique ended up with three sets. The fish produced from the breeding of tetraploids with diploids also grow faster.
Genetic engineering remains. Opponents say it creates hazards for the ecosystem and that tomatoes and other genetically modified foods don't taste nearly as good as natural ones. Proponents assert that genetic modification, in the form of selective breeding, has been around since the dawn of agriculture at the end of the last ice age. Genetic modification also curbs pesticide consumption and increases yields.
Hershberger added that trout with three sets of chromosomes do not present a danger to the ecosystem if they get out of the lab because they are sterile. Thus, they can't breed with wild stocks.
However, the lab and breeders will have to take careful measures to make sure none of the four-chromosome-set fish get out.
"The tetraploids would breed with the duploids and create sterile triploids," he said.