Gene ‘Clusters’ Help Scientists Unlock Secret of Plants
Researchers today reported the first success with a new genetic technique that may markedly change the way strains of commercial crops are bred.
The researchers said in the British journal Nature that they have used the new technique to identify clusters of genes that are responsible for specific traits of tomatoes, such as the weight of the fruit or the amount of solids it contains. In the past, researchers have been able to accomplish this identification only for traits that are caused by single genes, such as color or herbicide resistance.
The results represent a significant advance, because most traits in both plants and animals are the result of the combined action of several genes, and scientists have not had a useful way to identify them. These traits include such simple things as height and weight in humans, as well as more complex characteristics, such as susceptibility to disease.
More than a dozen other research groups in the United States are also working to apply the new technique, which has been called “quantitative genetics” or “DNA-directed breeding,” to agricultural crops. The approach is expected to soon help provide tomatoes that give a much higher yield of tomato sauce and paste, and corn that has bigger and more nutritious kernels.
In the longer term, however, quantitative genetics may make a bigger contribution to animal and human physiology. Researchers are confident that the approach will make it possible for the first time to identify clusters of genes that produce susceptibility to such human diseases as atherosclerosis, hypertension and diabetes.
The new results represent a major contribution to the development of tomatoes that can be used in canned foods, said plant scientist Richard W. Michelmore of UC Davis. “But what is much more exciting and more important than what they have found is how they do it . . . and what it represents about our ability to do this with other systems. Their work will be greatly imitated.”
Although the technique could be applied to any plant, the researchers studied tomatoes because plant geneticists Steven D. Tanksley and his colleagues at Cornell University in Ithaca, N.Y., had developed a so-called “genetic map” of a common domestic tomato. This map is a series of 400 fragments of deoxyribonucleic acid (DNA) that encompass the entire genetic blueprint of the tomato.
The team’s goal was to identify the genes responsible for the size of the tomatoes produced, the amount of solids they contain, and their acidity. The greater the amount of solids a tomato contains, the more useful it is for producing tomato sauce and paste. Acidity is important for optimal preservation of tomato products.
To help identify the genes, botanist John D. Hewitt of UC Davis crossbred the domestic variety with a Peruvian tomato whose fruit is much smaller but which has twice the solid content. Geneticist Eric S. Lander and colleagues at the Whitehead Institute of the Massachusetts Institute of Technology then used sophisticated genetic engineering techniques to link specific DNA fragments in the progeny to specific traits.
By studying about 270 plants, they found DNA segments containing genes that contributed to fruit size on six of the tomato’s 12 chromosomes, segments correlating with solid content on four chromosomes and segments correlating with acidity on five chromosomes.
Specific Genes
The researchers hope eventually to identify the specific genes involved on each segment, but that may not be necessary for the production of better tomatoes. By monitoring the presence of the DNA segments in progeny obtained during conventional cross-breeding experiments, researchers will easily be able to tell if desired characteristics are being transferred, without waiting for fruit to develop.
The genetic knowledge will also identify which progeny should be further bred to ensure that the desired traits become established.
But what is more important, Lander said, “is that this is a perfectly generic experiment. The exact same thing could be done with mice to identify the genes for hypertension, for example.”
If five such genes were identified for hypertension, it would then be possible to produce five separate mouse strains, each containing one of the crucial genes, and thereby identify the specific contribution of each. “This would tell us a lot about hypertension in humans,” he said.
