Members of a research group from the Faculty of Agriculture at the Hebrew University, led by Prof. Danny Zamir. succeeded in isolating one of the genes that determine the degree of sweetness of the tomato
By Marit Selvin
Mendel, who formulated the laws of heredity in 1866, referred to the traits he tested in peas, such as height, flower color, seed shape, and more. All the traits he tested are simple traits dictated by a single gene. However, most human traits (body weight, intelligence, tendency to get certain diseases) and traits of animals and plants (the amount of the crop, the size of the fruit, the degree of sweetness) are complex traits that are dictated by many genes that are in reciprocal relationships with each other, and are also affected by external factors such as Environmental conditions.
For more than a century, geneticists have been trying to locate the genetic components of complex traits - how the various genes that make up each trait work together and how, in the end, the external expression of a particular trait is determined. Until now, the only way to learn about this was a statistical description of the variation obtained in the offspring population, but this did not lead to unequivocal results due to the complexity of the tested traits. For example, two people with the same weight can have a different composition of genes that participate in determining body weight; On the other hand, individuals with the same genetic composition, for example identical twins, can differ from each other if they are raised in different conditions. So far no way has been found to locate the genes involved in the expression of complex traits.
The first to isolate a gene responsible for complex traits are members of a research group from the Faculty of Agriculture at the Hebrew University, led by Prof. Danny Zamir. The group was able to isolate one of the genes that determine the degree of sweetness of the tomato. Their work was published about two months ago in the journal "Academy of Sciences Proceedings of the National". A week ago another work was published in the journal "Science": a research group from Cornell University in the state of New York succeeded in isolating one of the genes responsible for the size of the fruit in the tomato.
500 years ago Columbus brought the wild tomato from South America to Europe. The fruits of the wild tomato are small and contain a lot of sugar, but are not edible. Over the years, they bred the wild variety and cultivated the varieties with the large fruit. The cultivation of the tomato did create varieties adapted to modern agriculture, but the cultivated traits were replaced by other traits, such as sweetness or disease resistance, which can only be found in the wild varieties.
Zamir started the research 15 years ago. He tested thousands of tomato plants, from which he isolated the varieties that provide the information related to the sweetness trait. The research team, which included Eyal Friedman and Tsili Flavan, created tomato varieties identical to each other, except for a small segment of the chromosome, which was replaced by the corresponding segment in the wild species. When these strains were grown side by side under identical environmental conditions, any difference between them was due to the replaced segment originating from the wild strain. Out of the new varieties they created, they found 25 segments in the different chromosomes that affect the sugar percentage in the tomato. "We decided to focus on the segment on chromosome 9, because it increases the percentage of sugar in the fruit by 20% compared to an identical variety that does not contain the same gene. We isolated this segment, analyzed it and were able to locate the gene that increases the sugar percentage," says Zamir. "Now that we have the gene, we can transfer it to tomato plants through hybridization and get much sweeter varieties."
Zamir and his group paved the way for the isolation of additional genes in plants that control additional complex traits, such as resistance to diseases and stress conditions. The working methods they developed can be used to study the complex traits in animals as well. An accompanying article published last week in the journal "Science" reads: "Revealing the secrets of complex traits is probably the greatest challenge for geneticists in the 21st century. Even after the genome is mapped and the role of most genes is revealed, there is still not enough knowledge about the great variability of the genes that determine why one person is more prone to illness than another, or why one variety of tomato yields a larger crop than another. Identifying additional genes that participate in the expression of the complex traits will advance us to these understandings."
{Appeared in Haaretz newspaper, 19/7/2000}
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