Heinz 1706 tomatoes |
By Nicholas Wade, The New York Times, May 30, 2012
The tomato, whose
genome has just now been decoded, turns out to be one well-endowed vegetable,
possessing 31,760 genes. This rich legacy, possibly a reflection of the
disaster that killed off the dinosaurs, is some 7,000 more than that of a
person, and presents a complex puzzle to scientists who hope to understand its
secrets.
Graham
Seymour, left, and Gerard Bishop, both of Britain, were members of an
international team of plant geneticists that decoded the tomato genome in the
hope of producing better ones.
A consortium of plant geneticists from 14 countries has spent nine
years decoding the tomato genome in the hope of breeding better ones. The
scientists sequenced the genomes of both Heinz 1706, a variety used to make
ketchup, and the tomato’s closest wild relative, Solanum pimpinellifolium, which lives in the highlands of
Peru, where the tomato’s ancestors originated. Their results were published online Wednesday
in the journal Nature.
The tomato, though a fruit to botanists, has been decreed a vegetable by the United States
Supreme Court. The verdict is not so unreasonable given that the tomato has a
close cousin that is a vegetable, namely the potato. The genomes of the two
plants have 92 percent of their DNA in common, the tomato researchers report.
The main difference is that the potato is thought to have a handful of genes
that direct the plant’s energy away from producing fruit and into the
generation of tubers. But even with the genomes of the two plants deciphered,
those genes have not yet been identified, said Daniel Zamir, a plant geneticist
at Hebrew University in Jerusalem and one of the report’s two principal
authors.
The tomato genome is both of intrinsic interest and a key to
understanding the very versatile family of plants to which it belongs. Besides
the potato, the Solanaceae family, as it is known, includes the tobacco plant,
the pepper, the eggplant and deadly nightshade.
That the tomato and potato contain so many genes does not mean that
they are more sophisticated than people but that they have chosen a different
stratagem for managing their cells’ affairs. Humans make heavy use of a
technique called alternative splicing, which allows the components of each gene
to be assembled in many different ways, so that one gene can produce many
products.
The Solanaceae family, by contrast, has developed its genetic
complexity through gaining more genes. About 70 million years ago, some lucky
mishap in the process of cell division led to a triplication of the Solanum
genome. The two spare copies of each gene were free to change through mutation.
Many were useless and got dropped from the genome, but others developed useful
new functions.
The tomato genome team has been able to visualize the result of this
triplication by comparing the tomato’s genome with that of the grapevine, a
distant relative from which it parted company about 100 million years ago, well
before the triplication event. Some of the grape’s genes have a single
counterpart in the tomato genome, some have two counterparts and some have
three.
Usually the triplication of a genome would be a considerable handicap,
saddling a plant with three times as much DNA as it needs. But this event
occurred around the time of the catastrophe in which the dinosaurs perished,
and the extra genetic versatility may have been a lifesaver. “It’s easy to
think that in that period, with a lot of volcanic activity and little sunlight,
the reservoir of a lot of additional genes would be useful to a plant,” said
Jim Giovannoni, a plant geneticist at the Boyce Thompson Institute for Plant
Research in Ithaca, N.Y., who led the American contribution to the tomato
genome report.
Plant
breeders have had more success breeding tomatoes with features of interest to
producers, like long shelf life, than with the traits that matter to consumers,
like taste and quality, Dr. Giovannoni said. The tomato genome sequence may
help redress the balance, since plant breeders can now rely on DNA as well as
physical traits to govern their breeding programs, he said.
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