November 28, 2012 | 4
Common wheat (Triticum aestivum) might seem as boring as the sliced bread it is baked into. But genetically, it is vexingly complex.
Its genome is about six times as big as our own, and its genes are distributed among six sets of chromosomes (we humans have just two). In fact, the T. aestivum genome contains chunks of genomes from three different “parent,” ancestral grasses that were bred to create wheat.
This convolution and wheat’s high level of repeating sequences (some 80 percent of the plant’s DNA appears in duplicate or triplicate) have foiled early attempts to sequence its full genome, which has long been seen as a key to improving its cultivation to feed a swelling human population. (About one fifth of all the calories the human population eats come from wheat.) Now a new research effort has reaped an important swath of the sequence. The findings were published online November 28 in Nature (Scientific American is part of Nature Publishing Group).
The genetic complexity of wheat stems in large part from humanity’s long history of domesticating the crop. This species as we now know it emerged some 8,000 years ago as a cross of goat grass (Aegilops tauschii) and emmer wheat (Triticum dicocoides), which was itself a hybrid that contained two parent genomes on four sets of chromosomes.
To harvest the common wheat’s genome, researchers needed a quick and efficient sequencing technology that could plow through the 17 gigabases of genetic code. The team selected shotgun sequencing, in which random segments of a genome are broken into chunks, copied and then reassembled where overlapping patterns are detected.
To help parse the morass of genetic code, researchers compared the wheat genetic data to that of other grains, such as corn and rice. They also mapped the new sequences to those from the closest-known relatives for the three different parent genomes: A. tauschii, Aegilops speltoides and Triticum urartu, as well as Triticum durum (drum wheat), which contains both T. urartu and A. speltoides genomes. Being able to assign more than two thirds of genes to the three respective ancestral genomes “is particularly valuable to wheat researchers because it allows them to differentiate genes and DNA markers,” Peter Langridge of the Australian Center for Plant Functional Genomicsat the University of Adelaide wrote in an essay appearing in the same issue of Nature. This matching can be “a difficult and time-consuming process,” he noted.
With these methods, the researchers estimate that the common wheat genome contains some 94,000 to 96,000 individual genes. Many of the gene groups that have expanded with time and breeding are related to growth and energy use. Better understanding the location of these genes might help crop scientists make further improvements on different traits to improve yield, drought and disease tolerance, or nutritional profiles.
Scientists have yet to completely crack the wheat genome. “This is just one step in the global effort to produce a high-quality draft of the bread wheat genome sequence,” said Jan Dvorak, a professor of plant sciences at the University of California, Davis and co-author of the new study, in a prepared statement. Still, the analysis represents a a major advance that should yield practical benefits. “The identification of genetic markers in the genome will help breeders accelerate the wheat breeding process and integrate multiple traits in a single breeding program,” said study co-author Anthony Hall, also at Liverpool’s Institute of Integrative Biology, in a prepared statement. “This research is contributing to ongoing work to tackle the problem of global food shortage.”