After years of frustrating science with its genetic complexity, wild wheat has finally been sequenced for the first time. This achievement could help engineer a better strain of wheat in the future, claims the international team of scientists behind the breakthrough, reported in the July 7 issue of Science.
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It was no great surprise to discover that the wheat genome is about three times the size of the human genome. The same is true of the tomato and many other plants. The DNA of wild wheat contains 12 billion nucleotides (the acid molecules comprising DNA), domestic wheat has 17 billion and we humans have 3.5 a mere billion nucleotides, which is somewhat embarrassing.
“Humans are believed to have about 20,000 genes, while wheat seems to have about 65,000 genes, probably a little more,” Tel Aviv University’s Raz Avni, a member of the team, tells Haaretz. He goes on to qualify that the sequencing – while by far the most complete ever done for wheat – is not entirely finished.
It bears noting that we have zero idea of what most of these genes do.
Making the sequencing all the more challenging is the fact that plants tend to have things like doubled chromosomes or even doubled whole genomes, which animals do not to have, says Avni, of TAU’s Department of Molecular Biology & Ecology of Plants.
Why, actually, would witless plants be genetically more complex than us animals and our big brains? Pressed on the point, Avni hazards a theory: Animals can flee. Plants can’t.
“The most basic difference between plants and animals is that they cannot move. They are stuck in one place and have to contend with all environmental conditions. If conditions turn ugly, animals can leave. Plants need more tools,” he speculates.
Lost and found
The whole point of sequencing wild wheat is the assumption that some genetic variation – and desirable characteristics – were lost in time. The hope is that by identifying “lost” genes, we can engineer them back into domestic wheat species.
In fact, we know of at least one desirable quality that was lost: protein content. In 2006, Dr. Assaf Distelfeld of the Institute for Cereal Crops Improvement, in TAU’s department of molecular biology, proved that wild wheat has higher protein content than domestic. Moreover, when he implanted the appropriate genes in domestic wheat, protein content was boosted by 10 to 15 percent. So the idea of improving future wheat is more than just theoretical.
One of the discoveries made by the sequencing team is that two key mutations rendered wild wheat more convenient for farming – although where and when real agriculture began, as opposed to sporadic cultivation, is fiercely debated (anywhere between 20,000 to 10,000 years ago; clearly, a protracted process was involved).
In wild wheat, the ripened grains merrily scatter with the wind, and as far as the plant is concerned, the farther the better. The two mutations keep the wheat seeds attached to the sheaf, which meant that prehistoric man with his stone sickles could reap more grains per stalk. (Avni says that the seeds do not stay stuck on the stalk unless both of the mutations in question exist.)
To verify that these were indeed the significant genes, TAU doctoral student Moran Nave compared 113 wild wheat varieties in the Middle East, including those from Israel, Turkey, Iran, Iraq and Syria, with 94 strains of cultivated wheat from around the world. The results were unequivocal: Those two mutations were found in every variety of cultivated wheat but in none of the varieties of wild wheat.
“Our guess is that early farmers found the varieties with the mutations occurring naturally and were smart enough to collect them,” says Avni.
If this is the first-ever full DNA sequencing of wheat (more or less), how exactly did the team know about these two mutations? Because previous stabs at sequencing domesticated wheat used in bread and pasta had produced fragments of gene sequences over the years. Comparing the wild sequence with information relating to those fragments (some unpublished), Avni explains, he and the other scientists were able to track down the two key mutations that enabled the fruitful cultivation of wheat.
The new study was led by Distelfeld in collaboration with several dozen scientists from institutions around the world and the Israel-based company NRGene, which developed the bioinformatics technology used in accelerating the research.
Ultimately, the team is hopeful that they can do something to improve global food security. Team member Dr. Curtis Pozniak of the University of Saskatchewan points out that wheat now accounts for almost 20 percent of the calories people consume worldwide, so protecting and improving the stock is crucial. For his part, Dr. Zvi Peleg of the Hebrew University of Jerusalem notes that wild emmer is peculiarly resistant to water stress.