Chromosome Engineering Techniques for Targeted Introgression of Rust Resistance from Wild Wheat Relatives

Hexaploid wheat has relatively narrow genetic diversity due to its evolution and domestication history compared to its wild relatives that often carry agronomically important traits including resistance to biotic and abiotic stresses. Many genes have been introgressed into wheat from wild relatives using various strategies and protocols. One of the important issues with these introgressions is linkage drag, i.e., in addition to beneficial genes, undesirable or deleterious genes that negatively influence end-use quality and grain yield are also introgressed. Linkage drag is responsible for limiting the use of alien genes in breeding programs. Therefore, a lot of effort has been devoted to reduce linkage drag. If a gene of interest is in the primary gene pool or on a homologous chromosome from species in the secondary gene pool, it can be introgressed into common wheat by direct crosses and homologous recombination. However, if a gene of interest is on a homoeologous chromosome of a species belonging to the secondary or tertiary gene pools, chromosome engineering is required to make the transfer and to break any linkage drag. Four general approaches are used to transfer genes from homoeologous chromosomes of wild relatives to wheat chromosomes, namely, spontaneous translocations, radiation, tissue culture, and induced homoeologous recombination. The last is the method of choice provided the target gene(s) is not located near the centromere where recombination is lacking or is suppressed, and synteny between the alien chromosome carrying the gene and the recipient wheat chromosome is conserved. In this chapter, we focus on the homoeologous recombination-based chromosome engineering approach and use rust resistance genes in wild relatives of wheat as examples. The methodology will be applicable to other alien genes and other crops.