The brittle rachis character, which causes spontaneous shattering of spikelets, has an adaptive value in wild grass species. The loci Br₁ and Br₂ in durum wheat (Triticum durum Desf.) and Br₃ in hexaploid wheat (T. aestivum L.) determine disarticulation of rachides above the junction of the rachilla with the rachis such that a fragment of rachis is attached below each spikelet. Using microsatellite markers, the loci Br₁, Br₂ and Br₃ were mapped on the homoeologous group 3 chromosomes. The Br₂ locus was located on the short arm of chromosome 3А and linked with the centromeric marker, Xgwm32, at a distance of 13.3 cM. The Br₃ locus was located on the short arm of chromosome 3B and linked with the centromeric marker, Xgwm72 (at a distance of 14.2 cM). The Br₁ locus was located on the short arm of chromosome 3D. The distance of Br₁ from the centromeric marker Xgdm72 was 25.3 cM. Mapping the Br₁, Br₂, and Br₃ loci of the brittle rachis suggests the homoeologous origin of these 3 loci for brittle rachides. Since the genes for brittle rachis have been retained in the gene pool of durum wheat, the more closely linked markers with the brittle rachis locus are required to select against brittle rachis genotypes and then to avoid yield loss in improved cultivars.
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