Copies of the human gene for growth hormone have been transplanted into rabbits, sheep and pigs and have successfully carried out their biological function in the rabbits and pigs, a team of biologists reports this week.
The experiments were done by the group that transplanted the gene into mice in 1982 and 1983, creating a strain of giant mice that passed their new gene to offspring.
The animals in the recent experiment have not yet grown appreciably larger than normal, but the fact that they accepted the foreign gene and began using it to make human growth hormone confirms that gene transplants can be a useful method of producing new varieties of farm animals.
Through gene transfer, biotechnologists hope one day to be able to produce livestock varieties that grow faster or larger, producing more meat, milk or eggs on less feed and in less time.
It also may be possible to create varieties of animals that have resistance to common diseases or tolerance to extremes of heat or cold.
These are traits that livestock breeders have been working for centuries to produce through the slow methods of conventional selective breeding.
They have been limited, however, by the need to find examples of the desired trait in individuals of the same species.
Different species are rarely able to mate and produce fertile offspring. The resulting hybrids may then carry the desired combination of genes from two breeds.
Gene transplants broaden the range of livestock breeders' options by making it possible to transfer desirable genes from one species to another.
The new report, published in this week's issue of Nature, the British scientific journal, is by eight scientists working under the leadership of Ralph L. Brinster of the University of Pennsylvania and Richard D. Palmiter of the University of Washington in Seattle.
Also collaborating were scientists at the U.S. Department of Agriculture's Agricultural Research Service laboratory at Beltsville.
The transplants were done by injecting copies of the gene into fertilized ova of the recipient species.
The ova were fertilized in a laboratory dish using much the same procedure as is routine with in vitro, "test tube" fertilization of human ova. The genes were injected through a microscopically thin glass tube that pierced the ovum and squirted into the ovum.
In all, some 5,000 ova were injected and implanted in the uteri of the appropriate species. About 500 ova developed into fetuses or newborn animals.
Most of the animals failed to take up the foreign gene and integrate it into their chromosomes, the structures that contain genes. However, 11 to 13 percent of the rabbits' and pigs' offspring did take up the genes. The success rate for the sheep was 1.3 percent.
Although only a few embryos accepted the gene, they may be enough to establish new breeds if they behave as the mice did. Since the giant mouse strain was established, the foreign genes have been passed on through normal modes of inheritance in several successive generations.
The new experiments have not been continued long enough to see whether the genes will be passed on by the rabbits, sheep or pigs.
As with the mice, the growth hormone gene was linked to a regulatory gene that controls its rate of action. The regulator gene is one that responds to zinc in the animals' diet.
When the mice were fed trace amounts of zinc, the regulator gene spurred the growth hormone gene to a high rate of activity, causing it to direct the animals' cells to make large amounts of growth hormone.
The researchers have not yet given the rabbits, sheep or pigs extra zinc. They did, however, establish that the foreign gene was operating at a low level.
Before agriculturally useful gene transplants can be done, scientists will need to identify appropriate genes from other species and make quantities of the genes, using standard methods of genetic engineering.