渗入杂交与小麦杂种优势

Introgressive hybridization and wheat heterosis

杂种优势利用是实现小麦增产稳产的重要途径.渗入杂交为小麦杂种优势利用提供了发展动力.小麦具有大量可渗入利用的近缘属种,同时基因组结构有利于外源基因渗入.目前常用的基因渗入有两种方式.一是借助染色体工程,通过不同基因组之间的部分同源染色体重组,将单个外源染色体区段以易位系为载体的方式渗入.易位系在引入外源染色体区段的同时,丢失了对应的小麦染色体区段,因此在常规育种中应用受限.但当其应用于杂交小麦,既保留了外源区段的优良基因,也不存在小麦基因缺失问题.二是通过同源染色体遗传重组,将不同染色体区段上的基因同时渗入,可实现小麦基因组遗传结构的重塑.小麦基因组缓冲性强,单个基因拷贝变化常常仅有较小的遗传效应,但多个基因的同时导入可能产生明显效果.因此,从基因组的角度丰富小麦遗传变异对杂种优势利用尤为重要.小麦3个亚基因组的遗传多样性不对称,其中D基因组的遗传多样性远低于A和B.因此,将未参与小麦进化的节节麦亚种的遗传多样性导入小麦,尽快丰富小麦D基因组的遗传多样性,是当前同源渗入的优先任务.

To continue feeding the increasing global population,the amount of wheat produced in the next 50 years will need to exceed the total amount of wheat produced over the past 10000 years.This challenge is coupled with a shrinking resource base and the likely onset of climate change.Moreover,a wheat yield plateau seems to have been reached in many regions,leading to concerns that the increasing global demand will not be met without breeding innovations.Heterosis may be exploited to enhance and stabilize wheat yields,but the commercial application of hybrid wheat has been limited by the low level of heterosis.Although the genetic mechanism underlying wheat heterosis is still uncharacterized,strong heterosis is often related to the high genetic diversity of the parents.Introgressive hybridization is a powerful method for expanding the germplasm base by transferring alien genes into wheat populations,with considerable implications for advancing wheat heterosis.In addition to increasing genetic diversity,introgressed alien genes may exhibit distinct spatiotemporal expression patterns or regulatory characteristics in the receptor genetic background,which may affect heterosis.Wheat has rich wild relatives and an allohexaploid genome structure,which is conducive to gene introgression.Generally,there are two ways to introgress alien genes.First,individual chromosomal segments may be introgressed through homoeologous recombination to generate translocations that are mediated by chromosome engineering.The resulting translocations have limited use in classical wheat breeding because the introgression is accompanied by the loss of the corresponding wheat chromosomal region.However,the introgression may be beneficial for the application of heterosis because it and the corresponding wheat chromosome are maintained.Second,homologous-directed recombination enables the simultaneous introgression of multiple chromosomal regions to substantially reshape the wheat genome structure.The triplication of many genes in wheat(i.e.,three homeologs)provides a buffering effect,such that the allelic variation of one of the copies may have only a minor phenotypic effect.In contrast,combining a number of novel introgressed segments in a single genotype produces a significant positive effect in classical wheat breeding.The utility of homologous-directed introgression for the commercial application of hybrid wheat remains unclear.However,the genetic needs vary between classical breeding and the application of heterosis.A high rate of alien introgression may be harmful for classical breeding,but may be beneficial for generating strong heterosis.The genetic base of the three wheat subgenomes can be simultaneously restructured by using re-synthetic hexaploid wheat,which is derived from a cross between tetraploid wheat and Aegilops tauschii,as the introgression bridge.Alternatively,the three wheat subgenomes can be independently improved by using the three diploid progenitors and the closely related diploids.The resulting hybrid genomes are named A′,B′and D′,which may serve as foundational materials for promoting the application of wheat heterosis.Increasing the use of the untapped genetic diversity of A.tauschii should be prioritized to diversify the wheat D-genome,which is the least diverse wheat subgenome.