Triticeae Genomics and Genetics, 2024, Vol.15, No.4, 196-205 http://cropscipublisher.com/index.php/tgg 197 the goal of developing targeted breeding strategies that utilize the genetic diversity of hexaploid wheat to improve food security and crop resilience. 2 Understanding Hexaploid Wheat Genetics 2.1 Evolution and origin of hexaploid wheat 2.1.1 Ancestral Species and Hybridization Events The formation of hexaploid wheat (Triticum aestivum, AABBDD) underwent two crucial allopolyploidization events, involving three diploid progenitors. The initial hybridization event between tetraploid wheat (Triticum turgidum, AABB) and diploid goatgrass (Aegilops tauschii, DD) led to the creation of hexaploid wheat^5^6^9. This process facilitated the transfer of genetic diversity from the parental species to hexaploid wheat, thereby enhancing its adaptability and agronomic traits^8^9. Figure 1 intricately depicts the complex journey of forming hexaploid wheat from multiple ancestor species through hybridization and polyploidization events, involving the dynamics of genetic variation and natural selection (Liu et al., 2021). Figure 1 Evolution and domestication of crop wheats with different ploidy levels (Adopted from Liu et al., 2021) Image caption: BTR1, Tg, Sog, and 5Aq are the wild-type alleles of four domestication genes; btr1, tg, and sog are recessive mutant alleles; and 5AQis the dominant domesticated form of 5Aq. The black boxes show materials that have not yet been identified or validated (Adopted from Liu et al., 2021) 2.1.2 Genetic composition of hexaploid wheat (AABBDD) The genetic composition of hexaploid wheat includes three distinct genomes: A, B, and D. This complex genome structure provides a rich source of genetic diversity, which is crucial for breeding and improving wheat varieties. The A and B genomes were derived from tetraploid wheat, while the D genome was contributed by Aegilops tauschii (Yuan et al., 2020; Liu et al., 2021; Li et al., 2021). This combination of genomes has endowed hexaploid wheat with unique traits, such as increased yield potential and resilience to environmental stresses (Wan et al., 2020; Wan et al., 2023) 2.2 Genetic and genomic characteristics 2.2.1 Chromosome structure and organization Hexaploid wheat exhibits a complex chromosome structure, with 21 pairs of chromosomes (2n = 6x = 42). The chromosomes are organized into three homologous sets corresponding to the A, B, and D genomes. This polyploid nature has led to frequent numerical and structural chromosome changes, which play a significant role in expanding genetic diversity and facilitating wheat evolution (Yuan et al., 2020; Zhang et al., 2021). Additionally, polyploidization has been shown to enhance genetic recombination, particularly in the D genome, further contributing to the evolutionary potential of hexaploid wheat (Wan et al., 2020).
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