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Taking a Slice at the Wheat Genome
28 November 2012 2:25 pm
Considered by some to be the Mount Everest of crop genomes, the challenging wheat genome is close to being scaled. An international team has produced a draft of wheat's DNA sequence, one that identifies many of its genes and has made possible the identification of thousands of potential genetic changes that could improve this key crop.
"A tremendous resource for wheat improvement and plant genetics has been developed," says Jeffrey Bennetzen, a plant geneticist at the University of Georgia, Athens, who was not involved with the work.
Wheat is the world's most widely grown crop, and it feeds a substantial portion of the world's population. But scientists have struggled to get a grip on its complex genetics. One complication is that the two kinds of wheat—bread wheat and pasta wheat—have different DNA makeups. Pasta wheat (durum), which is a hybrid of two wild grasses, has two genomes, one from each of its ancestors.
Bread wheat is even more complex: It has three genomes, the result of pasta wheat hybridizing with a third grass species. The new study, published online today in Nature, focuses on the bread wheat genome.
The bread wheat has almost six times as much DNA as the human genome. Unlike corn, which melded two ancestral genomes into one, bread wheat passes each of its three genomes to the next generation intact. "Since wheat has three related genomes, a big problem has been to work out which gene comes from which genome," says Peter Langridge, a plant geneticist at the University of Adelaide in Australia who is not involved with the study.
To begin to decipher the DNA, Michael Bevan, a plant geneticist at the John Innes Centre in Norwich, U.K., and his colleagues sequenced millions of DNA fragments from one variety of bread wheat; they then pieced together just the fragments containing genes. They also sequenced the genomes of two of wheat's ancestors and used them to assign two-thirds of the 95,000 genes they found to each of the three genomes. Genes are grouped into classes based on their similarity; in some cases, the classes expanded since hybridization, while in others, they shrank. As a result, bread wheat has more storage, defense response, and energy metabolism proteins than its ancestors.
As part of the project, the researchers also roughly sequenced several other bread wheat varieties and pinpointed the differences in their genes. Those differences represent potential sources of improved traits, such as drought resistance, that can be bred into new lines of wheat.
This draft genome didn't pin down the precise location of the genes or determine the identity of bases in between genes. "This is not a complete sequence," Langridge notes. "If the target genome sequence of wheat is Everest, then this very useful analysis is equivalent to a fly-by with a very good camera," Bennetzen says. "Nearly all the features can be seen, but understanding their connectedness will require a real effort to scale this genome by a [different] approach."