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Tibetan weedy barleys reside at the edges of qingke (hulless barley) fields in Tibet (Xizang). The spikes of these weedy barleys contain or lack a brittle rachis, with either two- or six-rowed spikes and either hulled or hulless grains at maturity. Although the brittle rachis trait of Tibetan weedy barleys is similar to that of wild barley (Hordeum vulgare ssp. spontaneum Thell.), these plants share genetic similarity with domesticated barley. The origin of Tibetan weedy barleys continues to be debated. Here, we show that most Tibetan weedy barleys originated from cross-pollinated hybridization of domesticated barleys, followed by hybrid self-pollination and recombination between Non-brittle rachis 1 (btr1) and 2 (btr2). We discovered the specific genetic ancestry of these weedy barleys in South Asian accessions. Tibetan weedy barleys exhibit lower genetic diversity than wild and Chinese landraces/cultivars and share a close relationship with qingke, genetically differing from typical eastern and western barley populations. We classified Tibetan weedy barleys into two groups, brittle rachis (BR) and non-brittle rachis (NBR); these traits align with the haplotypes of the btr1 and btr2 genes. Whereas wild barleys carry haplotype combinations of Btr1 and Btr2, each showing lower proportions in a population, the recombinant haplotype BTR2H8+BTR1H24 is predominant in the BR group. Haplotype block analysis based on whole-genome sequencing revealed two recombination breakpoints, which are present in 80.6% and 16.8% of BR accessions according to marker-assisted analysis. Hybridization events between wild and domesticated barley were rarely detected. These findings support the notion that Tibetan weedy barleys originated via recombination between Btr1 and Btr2 in domesticated barley.

Tibetan weedy barleys reside at the edges of qingke (hulless barley) fields in Tibet (Xizang). The spikes of these weedy barleys contain or lack a brittle rachis, with either two- or six-rowed spikes and either hulled or hulless grains at maturity. Although the brittle rachis trait of Tibetan weedy barleys is similar to that of wild barley (Hordeum vulgare ssp. spontaneum Thell.), these plants share genetic similarity with domesticated barley. The origin of Tibetan weedy barleys continues to be debated. Here, we show that most Tibetan weedy barleys originated from cross-pollinated hybridization of domesticated barleys, followed by hybrid self-pollination and recombination between Non-brittle rachis 1 (btr1) and 2 (btr2). We discovered the specific genetic ancestry of these weedy barleys in South Asian accessions. Tibetan weedy barleys exhibit lower genetic diversity than wild and Chinese landraces/cultivars and share a close relationship with qingke, genetically differing from typical eastern and western barley populations. We classified Tibetan weedy barleys into two groups, brittle rachis (BR) and non-brittle rachis (NBR); these traits align with the haplotypes of the btr1 and btr2 genes. Whereas wild barleys carry haplotype combinations of Btr1 and Btr2, each showing lower proportions in a population, the recombinant haplotype BTR2H8+BTR1H24 is predominant in the BR group. Haplotype block analysis based on whole-genome sequencing revealed two recombination breakpoints, which are present in 80.6% and 16.8% of BR accessions according to marker-assisted analysis. Hybridization events between wild and domesticated barley were rarely detected. These findings support the notion that Tibetan weedy barleys originated via recombination between Btr1 and Btr2 in domesticated barley., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Wild barley forms a two-rowed spike with a brittle rachis whereas domesticated barley has two- or six-rowed spikes with a tough rachis. Like domesticated barley, 'agriocrithon' forms a six-rowed spike; however, the spike is brittle as in wild barley, which makes the origin of agriocrithon obscure. Haplotype analysis of the Six-rowed spike 1 (vrs1) and Non-brittle rachis 1 (btr1) and 2 (btr2) genes was conducted to infer the origin of agriocrithon barley. Some agriocrithon barley accessions (eu-agriocrithon) carried Btr1 and Btr2 haplotypes that are not found in any cultivars, implying that they are directly derived from wild barley through a mutation at the vrs1 locus. Other agriocrithon barley accessions (pseudo-agriocrithon) carried Btr1 or Btr2 from cultivated barley, thus implying that they originated from hybridization between six-rowed landraces carrying btr1Btr2 and Btr1btr2 genotypes followed by recombination to produce Btr1Btr2. All materials we collected from Tibet belong to pseudo-agriocrithon and thus do not support the Tibetan Plateau as being a center of barley domestication. Tracing the evolutionary history of these allelic variants revealed that eu-agriocrithon represents six-rowed barley lineages that were selected by early farmers, once in south-eastern Turkmenistan (vrs1.a1) and again in the eastern part of Uzbekistan (vrs1.a4)., (© 2018 The Authors The Plant Journal © 2018 John Wiley & Sons Ltd.)