Why the genome of wheat is so massive

https://www.economist.com/blogs/economist-explains/2018/01/economist-explains-18

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It has over five times as much DNA as the human genome (基因组；染色体组)

Jan 24th 2018by G.F.

THE domestication of wheat and other staple crops (主要农作物) in the Levant some 10,000 years ago allowed for persistent settlement above a level of mere subsistence—one possible definition of the beginning of civilisation. Early farmers grew naturally occurring hybrids of wheat, and over time tamed them into a robust, easy-to-harvest and high-yielding species, the history of which is revealed in the genome of modern bread wheat. It is an enormously dense, complicated genome. And unlike the genetic codes of staples like rice, soya (大豆) and maize (玉米), scientists struggled until 2017 to crack it. Why was it so hard to decipher (译解) and was it worth the effort?

The genomes of ancient wheats, such as wild emmer (野生二粒小麦), contain more of the DNA base pairs required to create proteins than that of humans. Domesticated hybrids, like bread wheat, are even larger. Bread wheat has nearly six times the number of DNA base pairs as humans (about 17bn compared with humans’ 3bn). That is in part because humans are diploid ([遗] 二倍体), with two sets of chromosomes (染色体), whereas the chromosomes of bread wheat come in sets of six (which correspond to the three ancient wheats of which bread wheat is a hybrid). Furthermore, the DNA of ancient wheat contained a huge amount of duplication. This means that bread wheat not only contains an enormous amount of genetic information, but that much of it is repeated. That makes decoding its genome complex. With fewer unique pieces, it is harder to fit the jigsaw together.

Other staple food crops had their genomes sequenced long before bread wheat. But then they are much simpler: popular strains (种族) of maize, soya and rice have 2.3bn, 1.1bn and 420m DNA base pairs respectively. The breakthrough with bread wheat came last year, when several different academic and industry projects matured. Both the International Wheat Genome Sequencing Consortium (IWGSC) [国际小麦基因组测序协会], which includes wheat farmers, breeders (饲养员) and scientists, and an independent group led by Johns Hopkins University managed to sequence it. Others decoded wild emmer, an ancestor of both bread and durum wheat [（做通心粉等用的）硬质小麦], and Aegilops tauschii, another of bread wheat's ancestors.

The value of the decoding is two-fold. First, it allows researchers to manipulate wheat without recourse to so much trial and error. Second, it allows them to insert attractive traits from ancient wheats into modern ones, rather than introduce genes from other organisms altogether (a process known as transgenics [转基因学]). These may have better resistance to pests or better tolerance of drought, but yet have poorer yields and quality, says Catherine Feuillet, head of trait research at Bayer, a German pharmaceuticals firm and an important part of IWGSC’s gene project. Without the genome operating as a sort of index of the wheat’s positive traits, it would take ten years using conventional techniques to cross (杂交) a new wheat with an ancient one and recover all the desired traits. With the genome to hand, and in the public domain (the IWGSC is eschewing [避免] patents), iteration and improvement can be done rather faster. More researchers can get involved. Help may even come from unexpected corners. Ms Feuillet talks of finding “a high-school student who may finally be able to find a key resistance gene for a fungal (真菌的) disease.”