Ogbonnaya, F.C.;Abdalla, O.; Mujeeb-Kazi, A.; Kazi, A.G.; Xu, S.S.; Gosman, N.; Lagudah, E.S.; Bonnett, D.G.; Sorrells, M.E.; Tsujimoto, H.
Incorporation of genetic diversity into elite wheat (Triticum aestivum L., 2n¼6x¼42, 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 (2n¼4x¼28, AABB) and its diploid wild relative, Aegilops tauschii (2n¼2x¼14, DD) are a useful resource of new genes for hexaploid wheat improvement. These include many productivity traits such as abiotic (drought, heat, salinity/sodicity, and waterlogging) and biotic (rusts, septoria, barley yellow dwarf virus (BYDV), crownrot, tanspot, 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 moisture limited 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. Understanding these would further enhance the utility of SBLs. Considerable progress has beenmade in the identification of useful quantitative trait loci (QTL) and genes; however the transfer of such rich genetic diversity into elite wheat cultivars is still quite limited. Gaps still exist in data cataloging; and access to such information could serve as an important community resource. Future production of new SHWshould extend to under-exploited AB genome tetraploids such as T. turgidum ssp. carthlicum, T. turgidum ssp. dicoccum, and T. turgidum ssp. dicoccoides and identifying gaps in the Ae. tauschii germplasm used for existing SHW. Identifying geographical areas where the progenitor species of the existing SHW were collected would assist in guiding future collection missions. The recent advances in molecular technologies with whole genome sequencing becoming affordable will provide researchers with opportunities for more detailed analysis of traits and the deployment of more efficient strategies in the use of the unique exotic alleles derived from SHW for common wheat improvement. Thus, the contribution of SHW and the derived SBLs to wheat cropping systems worldwide is likely to grow in significance. However, these potential benefits are only realizable if phenotyping is equally extensive and effective.
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