Abstract
Triticum turgidum and T. timopheevii are two tetraploid wheat species sharing T. urartu as a common ancestor, and domesticated accessions from both of these allopolyploids exhibit nonbrittle rachis (i.e., nonshattering spikes). We previously described the loss-of-function mutations in the Brittle Rachis 1 genes BTR1-A and BTR1-B in the A and B subgenomes, respectively, that are responsible for this most visible domestication trait in T. turgidum. Resequencing of a large panel of wild and domesticated T. turgidum accessions subsequently led to the identification of the two progenitor haplotypes of the btr1-A and btr1-B domesticated alleles. Here, we extended the haplotype analysis to other T. turgidum subspecies and to the BTR1 homologues in the related T. timopheevii species. Our results showed that all the domesticated wheat subspecies within T. turgidum share common BTR1-A and BTR1-B haplotypes, confirming their common origin. In T. timopheevii, however, we identified a novel loss-of-function btr1-A allele underlying a partially brittle spike phenotype. This novel recessive allele appeared fixed within the pool of domesticated Timopheev’s wheat but was also carried by one wild timopheevii accession exhibiting partial brittleness. The promoter region for BTR1-B could not be amplified in any T. timopheevii accessions with any T. turgidum primer combination, exemplifying the gene-level distance between the two species. Altogether, our results support the concept of independent domestication processes for the two polyploid, wheat-related species.
Highlights
Originating via hybridization among different progenitor species, allopolyploid wheat species are widespread due to their enhanced capacity for genetic adaptation relative to their diploid ancestors [1]
BTR1-A-hapT7 (Figure 2), the closest wild haplotype to the domesticated BTR1A-hapT8 haplotype, consists only of T. araraticum accessions of Iraqi provenance. These results suggest that the T. araraticum founder stock carried the BTR1-A-hapT7 haplotype and pinpoints Iraq as the likely place of T. timopheevii domestication, conflicting with previous results that point to northern Syria and southern Turkey as the domestication regions [9], but supporting relatively new evidence of a larger geographical distribution of domesticated timopheevii than previously thought [12,13]
We suggest that the semi-brittle accession WTW–102 from Iraq, carrying the BTR1-A-hapT8 haplotype, may represent a missing link in T. timopheevii domestication, analogous to domesticated emmer wheat” (DEW)–10489 and DEW–10516 within the T. turgidum lineage
Summary
Originating via hybridization among different progenitor species, allopolyploid wheat species are widespread due to their enhanced capacity for genetic adaptation relative to their diploid ancestors [1]. As the common ancestor of all economically important domesticated wheat species, T. turgidum drew much scientific attention; the genetic and morphological characteristics of this polyploid lineage were intensely investigated in an effort to understand the process of wheat evolution and domestication. Turgidum (Desf.) Husn.), Khorasan wheat (TRN—T. turanicum), Polish wheat (POL—T. polonicum), Persian wheat (CRT—T. carthlicum), and Georgian emmer (PLC— T. paleocolchicum). Possessing nonbrittle spikes but tough glumes, domesticated emmer served as an important evolutionary step toward the development of the various fully domesticated (free-threshing) tetraploid wheat subspecies: Durum wheat Whether they evolved from a single, common, domesticated emmer ancestor or emerged independently from separate wild or domesticated emmer accessions is still unknown [8]. As members of the T. turgidum lineage, they all share the same BBAA genomic composition
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