A novel mutation conferring the nonbrittle phenotype of cultivated barley

Abstract

The nonbrittle rachis, resulting in a seed head which does not shatter at maturity, is one of the key phenotypes that distinguishes domesticated barley from its wild relatives. The phenotype is associated with two loci, Btr1 and Btr2, with all domesticated barleys thought to have either a 1 bp deletion in Btr1 or an 11 bp deletion in Btr2. We used a PCR genotyping method with 380 domesticated barley landraces to identify those with the Btr1 deletion and those with the Btr2 deletion. We discovered two landraces, from Serbia and Greece, that had neither deletion. Instead these landraces possess a novel point mutation in Btr1, changing a leucine to a proline in the protein product. We confirmed that plants carrying this mutation have the nonbrittle phenotype and identified wild haplotypes from the Gaziantep region of southeast Turkey as the closest wild relatives of these two landraces. The presence of a third mutation conferring the nonbrittle phenotype of domesticated barley shows that the origin of this trait is more complex than previously thought, and is consistent with recent models that view the transition to agriculture in southwest Asia as a protracted and multiregional process.

Keywords: Hordeum vulgare; agricultural origins; barley; brittle rachis; nonbrittle phenotype.

Figure 1

Structure of the novel btr1 allele in domesticated barley. (a) Alignment between the amino acid sequences of the wild‐type brittle allele (Btr1), the previously reported nonbrittle allele (here called btr1A), and the novel nonbrittle allele that we report (btr1B). The Btr1 and btr1A sequences are from Hordeum spontaneum OUH602 and Hordeum vulgare cv KNG, respectively (Pourkheirandish et al., 2015). The BTR1 protein has two hydrophobic regions indicated by green shading. Changes giving rise to the two btr1 alleles are highlighted in yellow. (b) Transmembrane orientation of the BTR1 protein predicted by sosui (Hirokawa et al., 1998). The hydrophobic regions and the Leu–Pro substitution in the btr1B product are indicated using the same colours as in (a).

Figure 2

Comparison of the seed head of barley landrace PI 374426, possessing the btr1b allele, with a wild barley seed head. (a) Seed head of PI 374426, displaying a nonbrittle, six‐row phenotype. (b) After forcibly removing spikelets (right) from the PI 374426 ear, the rachis remains intact (left). (c) The mature seed head of wild barley accession PI 662202. (d) With PI 662202, the mature rachis disarticulates into segments that remain attached to the spikelets. It is impossible to remove spikelets without breaking the rachis. Bars, 2 cm.