Trying to find the origins of the modern plant, researchers came upon the ancestor of the best kind of bread wheat
Jodie Nicotra
It starts with a love story, as much genomic research does. This one is from 10,000 years ago, and involved a cultivated wheat and a wild, weedy grass. These gene-crossed lovers – and millennia of careful selection and nurturing by human farmers – resulted in an “elite wheat” beloved by its cultivators, but with one major issue. A disease that affects one could spread fast because there was so little genetic diversity in the new wheat.
A research team of international collaborators set out recently to investigate the history of this union. Specifically, they considered ways to reintroduce diversity into contemporary wheat by studying the genes of its ancestors. That way, they could ensure the wheat retained its best characteristics while also increasing its resistance to plant pathogens.
Along the way, they found a surprise: one of the ancient couple that gave rise to the contemporary bread wheat exists only in present-day Georgia. In the agriculture world, that’s like finding a previously unknown human ancestor.
A troubled romance of grain
The Fertile Crescent was a happening place about 10,000 years ago. Richly fueled by the Euphrates, Tigris, between which lay Mesopotamia (“between the rivers”), and the Nile, this curved area is known for its development of agriculture. That is the place and time Triticum turgidum, or durum wheat, a hard wheat favored for making semolina and pasta, was originally cultivated. T. turgidum is a tetraploid wheat, meaning that each cell has four copies of each chromosome. By comparison, humans are diploid, with two copies of each chromosome – one from the father and the other from the mother.
Farmers of the time apparently liked T. turgidum‘s characteristics, and so not only grew it themselves but also shared it with neighbors, and took it with them when they migrated. This expanded T. turgidum‘s range further east and north, eventually bringing it in contact with a local wild grass, Aegilops tauschii, also known as Tausch’s goat grass.
Ae. tauschii is diploid, meaning that it has two sets of chromosomes, as we’d said. It tends to occupy disturbed soil, like that tilled for wheat farming. Occasionally, the diploid Ae. tauschii would exchange genes with the tetraploid T. turgidum, and a new hexaploid (six-chromosome) wheat was created, Triticum aestivum.
Whether it was taste, higher gluten content, or better durability, something about the hexaploid hybrid T. aestivum drew farmers’ attention. Hexaploid wheats can adapt to many soils and environments, so it was likely just overall a more successful variety – so successful that it’s still one of the main bread wheats cultivated today.
But as Brande Wulff, one of the corresponding authors on the study points out to Truly Curious, the success of this new hexaploid vareity came at a cost. In each instance of hybridization, only one possible chromosome combination from each parent could unite to form the new organism.
“You get something new and wonderful, but you leave behind a lot of that genetic diversity,” Wulff said.
Wulff, who has spent his career working on disease resistance in plants, said that two things gradually winnowed out genetic diversity in the new species: the process of genetic union between the diploid and tetraploid plants (called polyploidization), and the fact that farmers bred only the few varieties of wheat they liked.
Combined with the current agricultural trend of monocultures, this resulted in plants with very little resistance to pathogens. Once a pathogen like stem rust or powdery mildew takes hold in a field full of a single variety, it can spread like wildfire. The resultant epidemic can spell disaster for human food supplies. This kind of damage is what we saw in the Irish potato famine and the Victoria blight.
“Crossing a racehorse with a donkey”
Ultimately, researchers want to create elite hybrids with the best of both worlds: namely, disease resistance from the wild relatives like Ae. tauschii with more wheat-like traits like taste, high yield, standing power, and ability to control the production of massive amounts of food that ultimately sustains the civilization we have today.
But creating viable hybrids is difficult, time-consuming work. To do so, they need to weed out (pun intended) all the undesirable feral traits from the wild relative, which takes a lot of back-crossing and cleaning up.
Wulff describes the project as something akin to crossing a racehorse with a donkey.
“It’s fascinating, that genetic diversity, but it’s also a really tough challenge,” he said.
Wulff’s lab is working now to clone multiple disease-resistant genes from the wild Ae. tauschii and introduce them to modern plants, essentially creating a “cocktail” of disease-resistant genes that pathogens would have a hard time overcoming.
Over the past decade, Wulff’s lab has developed a number of technologies to speed up the discovery and cloning of disease-resistant genes. To find genes that correspond to disease-resistant traits in wild types, for instance, Wulff realized they could use a system already provided by nature: namely, finding the sequence of genes in a collection of wild wheats, and seeing the external traits created by their gene sequences. This led to the lab’s decision to sequence the entire genome of all the Ae. tauschii lines.
At the time, such a massive project was both technical difficult and extremely expensive. And so the Open Wild Wheat Consortium was born, now an international research group of 31 scientists in Iran, the UK, China, Tajikistan, the US, Israel, and more.
“We phoned up our friends around the world who work on wild wheats and are just as crazy as we are, and said, ‘Let’s do this.’” Wulff said. “And everyone put a little money into the hat.”
Wulff’s lab raised over $200,000 purely for sequencing, an unusually high amount for this work. With the money, they sequenced not only the original 150 wild grass genetic lines, but expanded it to 242 unique lines.
But Wulff has bigger ideas.
He uses an analogy to joke, “Ten years ago I was driving around in a Škoda or something. Now I’m driving a Ferrari. But we want to build a spaceship.”
For the lab’s next project, Wulff wants to create better, cleaner gene assemblies to speed up gene discovery. Disease resistance genes are quite rare in populations, so the 242 sequences yielded only four or five examples of disease-resistant genes. Wulff’s lab plans to go to all the public genetic information banks and private collections. Sequencing all of those may allow researchers to increase the power of their studies and find rarer genes.
A Denisovan Wheat?
In the process of searching for a way to increase genetic diversity in wheat, the researchers found an unexpected surprise: a previously unknown wheat ancestor, something akin to a “Neanderthal” wheat line.
When the researchers sequenced the Ae. tauschii genes for the wheat genome study, they found two major lineages (L1 and L2) that they knew about already. But then they discovered another one (L3).
“At first we were wondering if this was like the Denisovan lineage of hominids,” Wulff said, referring to the recent discovery of an archaic “third line” of humans distinct from both modern humans and Neanderthals.
L3, the “Denisovan wheat,” can be traced back to a population in what is now Georgia, in the Caucasus region. Though the vast majority (97%) of contemporary strains of the plant descended from Ae. tauschii L2, a tiny footprint comes from L3. A paper published at the same time as this one, and authored by another member of the Open Wild Wheat Consortium, shows that the bread with the best quality gluten allele for breadmaking comes from L3.
“It’s our own little wheat evolution story,” said Wulff.
Jodie Nicotra writes about science and technology. She has taught science writing and many other types of writing at the University of Idaho.
The original report appeared in Nature Biotechnology