Triticeae Genomics and Genetics, 2024, Vol.15, No.1, 56-65 http://cropscipublisher.com/index.php/tgg 57 agricultural production. At the same time, it provides reference and inspiration for the domestication and genetic diversity research of other crops. 1 The History and Background of Domestication of Barley 1.1 Origin and distribution of barley The origin of barley is closely linked to the Fertile Crescent, where wild progenitors of several key agricultural cereal species, including barley, are endemic. Archaeological evidence suggests that barley was domesticated in this region, particularly in the Jordan Valley. This domestication process led to a reduction in genetic diversity, as evidenced by the comparison of haplotypes between wild barley (Hordeum. vulgare subsp. spontaneum) and domesticated barley (Hordeum vulgare). For instance, domesticated barley lines show a significant reduction in the number of haplotypes and nucleotide diversity compared to their wild counterparts (Kilian et al., 2006). Further studies have shown that the domesticated gene pools of barley were derived from multiple wild ancestors, indicating a complex domestication process. The distribution of recombination events along barley chromosomes is uneven, with reduced diversity in the pericentromeric regions of both cultivars and landraces compared to wild barley. This pattern suggests that domesticated barley underwent significant genetic bottlenecks, which reduced its genetic diversity (Chen et al., 2020). 1.2 The historical process of domestication of barley The domestication of barley involved a series of genetic and evolutionary changes that were driven by both natural and human selection. Initially, wild barley exhibited a brittle rachis, which allowed seeds to disperse naturally. However, for early farmers, this trait was undesirable as it made harvesting difficult. Mutations in two adjacent, dominant, and complementary genes led to the development of a non-brittle rachis, which retained grains on the inflorescence at maturity, thus enabling effective harvesting (Pourkheirandish et al., 2015). Genetic studies have shown that domesticated barley lines exhibit significantly reduced nucleotide diversity compared to their wild counterparts. For instance, the number of haplotypes and average nucleotide diversity were markedly lower in domesticated barley, indicating a loss of genetic variation due to domestication bottlenecks (Kilian et al., 2006). This reduction in diversity was observed across multiple loci, with some loci becoming monomorphic in domesticated lines (Kilian et al., 2006). Interestingly, the domestication process was not uniform across all regions. Independent selections of germplasm with non-brittle rachis were made in different parts of the Levant, suggesting that barley domestication occurred in multiple locations within the Fertile Crescent (Pourkheirandish et al., 2015). This regional variation in domestication events contributed to the genetic diversity observed in modern barley cultivars. Furthermore, the domestication of barley also involved changes in the pericentromeric regions of chromosomes, which are typically low in recombination events. These regions showed dramatically reduced diversity in domesticated barley compared to wild barley, indicating that domesticated gene pools were derived from multiple wild ancestors (Figure 1) (Chen et al., 2022). The evolutionary patterns of these regions were shaped by linkage disequilibrium and domestication, highlighting the complex genetic landscape of barley domestication (Chen et al., 2022). Chen et al. (2022) demonstrated the variation of barley varieties, local varieties, and wild barley on different genomic segments (Figure 1). By analyzing the positive sweep signal, they revealed that specific regions of the genome were strongly selected during domestication. The study found that wild barley displayed strong positive sweep signals in different chromosomal regions, indicating that domestication gene pools may have originated from multiple wild ancestors. 1.3 Genetic structural changes in barley during domestication process Domestication has led to a significant reduction in genetic diversity in barley. Studies comparing wild and domesticated barley have shown a marked decrease in nucleotide diversity and haplotype number in domesticated lines (Kilian et al., 2006). For instance, the number of haplotypes and average nucleotide diversity (π) were
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