Above: Bread wheat (Triticum aestivum) growing in the Czech Republic (Petr Filipov)
Did you know? Wheat is a global grain superstar! it’s grown on more land than any other crop, tolerates diverse climates, and has more protein than corn or rice.If you love eating bread as much as I do, you should be ecstatic that a draft sequence of the bread wheat genome was published in 2014. Bagel connoisseurs might even consider it the scientific feel-good story of the year!
Scientists from the International Wheat Genome Sequencing Consortium (IWGSC), including some from Canada, have cleared major technical hurdles to decipher wheat’s diabolically big and complex genome. Their success will help plant breeders identify agriculturally important genes and improve crops. This is significant because bread wheat (Triticum aestivum) accounts for 95% of the wheat grown worldwide and is a staple food crop for 30% of the world’s growing population.
Genomes and sequencing
All living things—from bacteria to bread wheat to humans—have a genome.
In general terms, your genome is the unique recipe that makes you. It contains all the instructions required to build and maintain your body. Your genome is over 99% identical to every other person’s genome. So less than 1% of your genome is all that makes you gentically different from the rest of the world's population.
The “human genome” is actually a composite from several different people, but the fact that 99% of the information is shared means it’s representative of the entire human species. While the genomes of individuals within a species are nearly identical, the representative genomes of different species get more and more different the farther apart they are on the evolutionary tree.
Did you know? There is no correlation between and organism’s biological complexity and the size of its genome. For example, the bread wheat genome (17 billion bases) is five times bigger than the human genome (3 billion bases).At a molecular level, the genome recipes of all living things are written in the language of nitrogenous bases, usually just called bases. Bases are the “information” part of DNA (deoxyribonucleic acid), which is organized into chromosomes inside cells. A species’ genome is the identity and order (sequence) of bases in all the DNA of all its chromosomes.
Sequencing a genome makes it possible to identify genes, which are sequences of bases that have a specific role in development, growth, or behavior. In other words, genes dictate many of an organism’s characteristics.
For plant breeders, being able to identify the gene(s) responsible for an agriculturally desirable characteristic makes the job of breeding better plants much easier. For example, sequencing the bread wheat genome could help improve the crop’s disease and pest resistance, drought tolerance, and yield.
The work of sequencing genomes has been going on since the mid-1990s. The first draft of the human genome was published in 2001, and the genomes the world’s other top food staples—rice and corn—were revealed in 2002 and 2009, respectively.
So why did the wheat genome take so long? It turns out that the genome of bread wheat is a beast, in terms of both size and complexity.
For sequencing, size matters
What kind of size are we talking about? The rice genome’s 470 million bases might sound like a lot, but the corn genome boasts 2.5 billion bases. The human genome is a bit bigger than corn, with 3 billion bases. However, the bread wheat genome is more than five times bigger than the human genome, with a cool 17 billion bases.
The bread wheat genome is so big because, genetically speaking, it’s an allohexaploid organism. “Allo” means other or different and “hexa” means six. "Ploidy" refers to the number of sets of chromosomes in an organism. Humans have two sets of chromosomes, one from the male parent and one from the female parent, so we’re diploid (“di” meaning two).
During the evolution of bread wheat, three diploid genomes combined to make one big hexaploid genome (2+2+2=6). This genome mash-up is a technological nightmare for sequencing. Scientists had to develop new approaches and tools to tackle the wheat genome, such as isolating smaller pieces of individual chromosomes.
Repetitive DNA stymies sequencing
Did you know? One of the largest genomes belongs to the marbled lungfish. It has 133 billion bases, which is 44 times bigger than the human genome! Size wasn’t the only challenge the scientists faced. The bread wheat genome is also very complex because about 80% of it consists of highly repetitive sequences that are not part of genes. Repetitive DNA is pretty much what it sounds like: long stretches in which a short sequence of bases repeats over and over again.
The usual approach is to sequence many small DNA fragments and then reassemble the whole sequence like a jigsaw puzzle. But when so much of the sequence is repetitive, it’s like trying to put together a puzzle with pieces that are all the same colour and shape. There are many possible solutions, but only one is correct! This sticky problem is one of the reasons that the bread wheat genome is still a “draft” and not a complete sequence.
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Still, the draft sequence is a treasure trove of information about the location and order of genes on all the bread wheat chromosomes. IWGSC scientists did figure out the complete sequence of one bread wheat chromosome, and their technical innovations will be applied to the rest of the genome.
I for one am grateful for the innovation and hard work of scientists who are tackling the tough bread wheat genome. They are fighting the good fight, for bread and bagel lovers everywhere.
IWGSC Press Kit (2014)
International Wheat Genome Sequencing Consortium
Link to addtional information about the IWGSC (the organization responsible for sequencing the bread wheat genome) and its activities in PDF format.
Maize genome mapped (2009)
Elie Dolgin, Nature News
Article discussing the mapping of the maize (corn) genome.
Eukaryotic Genome Complexity (2008)
Leslie A. Pray, Nature Education 1(1), 96
Article explaining why a larger genome doesn’t necessarily mean a more complex organism.
A Guide to Your Genome (2007)
National Human Genome Research Institute, US National Institutes of Health
General introduction to genomes, genes, mutations, genetic testing, and genetic research in PDF format. Includes links to additional resources.