BioEd Online

By re-enacting an evolutionary event that happened to wheat thousands of years ago, researchers are producing new plant varieties that could save lives in regions where drought causes food shortages.

Bread wheat (Triticum aestivum), a staple food for millions of people around the world, is the product of two rare genetic events that happened during the Stone Age in a region of the Middle East known as the 'fertile crescent'.

Two different species can't usually breed to produce hybrid offspring, because their chromosomes don't match and can't pair properly during the process that produces sex cells such as eggs and sperm. But sometimes a genetic blip can produce sex cells with double the normal number of chromosomes, side-stepping the problem. If two sex cells of this type combine, a whole new fertile species with double the number of chromosomes is produced.

Doubling up

This rare 'duplication followed by fertilization' event has happened twice in the history of modern, common wheat. Around 30,000 years ago, a wild wheat (Triticum monococcum) hybridized with a species of goat grass (Aegilops speltoides) to generate primitive wheat called emmer, which had four sets of chromosomes. Then about 9,000 years ago, emmer wheat grown south of the Caspian Sea crossed with another wild goat grass (Aegilops tauschii) to produce a plant with six sets of chromosomes.

This hybrid had larger seeds than its ancestors, thanks to the bonus chromosomes, and so became a popular breed for early farmers. The descendents of these plants now cover more farmland globally than any other crop, filling more than 500 million acres worldwide.

But this genetic triumph came with a downside: the wheat was so popular that no one farmed anything else, leading to a very low genetic diversity and limiting the options for plant breeders hoping to develop varieties resistant to drought or pests. To counter this, researchers at the International Maize and Wheat Improvement Center (CIMMYT) in Mexico have developed a way to top up bread wheat's shallow gene pool.

Something old, something new

"We've been re-enacting in the lab what took place in nature nine thousand years ago," says Richard Trethowan, a specialist in wheat breeding at CIMMYT. Researchers collected wild goat grass from the Middle East and crossed it with modern versions of emmer wheat to create bread wheat all over again. They used chemicals in the lab to induce the rare chromosome doubling that makes hybrids fertile.

The technique helps to introduce new genes in the same way as genetic engineering, but without requiring the researchers to know which genes they are on the lookout for beforehand.

The new bread wheats are not themselves suitable for farming, since most of the new hybrids have qualities that are more advantageous to grasses than to wheat. "They’re ugly things," says Trethowan. But he adds that it is easy to use traditional breeding methods to get the few useful genes into common bread wheat strains.

Food for thought

The genetic input has allowed improvements to wheat's drought resistance, for example. One wheat strain developed by the team produces between 20 and 40% more grain under dry conditions than traditional bread wheat, the researchers told an international symposium of plant breeders in December.

CIMMYT has sent seeds produced by the research out to centres worldwide for local testing and development, and initial results have been promising. Farmers in Ecuador are racing to switch to one test strain that significantly outperforms the established local wheat, Trethowan says. He predicts that in five or six years time the new genes found by reinventing wheat will be dramatically improving yields everywhere. "We're on the brink of quite a big genetic revolution for wheat breeding," says Trethowan.

John Snape, a cereal geneticist at the John Innes Centre, Norwich, UK, adds that rich countries will probably benefit from this revolution too. "It is likely that climates in Europe will get hotter and drier thanks to climate change, and this will put new stresses on crops," he says. One fungal wheat disease, Fusarium head blight, has already started to plague European fields thanks to warmer, more humid summers, he points out. "Being able to reach out into wild species for new genes to tackle these problems is very valuable," he says.