Synthetic Hexaploids: Harnessing Species of the Primary Gene Pool for Wheat Improvement

Incorporation of genetic diversity into elite wheat (Triticum aestivum L., 2n1⁄4 6x1⁄4 42, AABBDD) cultivars has long been recognized as a means of improving wheat productivity and securing global wheat supply. Synthetic hexaploid wheat (SHW) genotypes recreated from its two progenitor species, the tetraploid, Triticum turgidum (2n1⁄4 4x1⁄4 28, AABB) and its diploid wild relative, Aegilops tauschii (2n1⁄4 2x1⁄4 14, DD) are a useful resource of new genes for hexaploidwheat improvement. These include many productivity traits such as abiotic (drought, heat, salinity/sodicity, andwaterlogging) and biotic (rusts, septoria, barley yellow dwarfvirus (BYDV), crownrot, tan spot, spot blotch, nematodes,powderymildew, and fusarium head blight) stress resistance/tolerances as well as novel grain quality traits. Numerous SHWs have been produced globally by various institutions including CIMMYT-Mexico, ICARDA-Syria, Department of Primary Industries (DPI), Victoria-Australia, IPK-Germany, Kyoto University-Japan, and USDAARS. This review examines the varied aspects in the utilization of synthetics for wheat improvement including the traits and genes identified, mapped, and transferred to common wheat. It has also been demonstrated that synthetic backcross-derived lines (SBLs, i.e., when SHW is crossed to adapted local bread varieties) show significant yield increases and thus, enhanced yield performance across a diverse range of environments, demonstrating their potential for improving wheat productivity worldwide. This is particularly evident in moisturelimited environments. The use of SBLs, advanced backcross QTL analysis, chromosome introgression lines, and whole genome association mapping is contributing to the elucidation of the genetic architecture of some of the traits. The contribution of transgressive segregation to enhanced phenotypes and the mechanisms including its genetic and physiological basis are yet to be elucidated. 36 FRANCIS C. OGBONNAYA ET AL.