Comparative Mapping of Genes for Brittle Rachis in Triticum

The brittle rachis phenotype is of adaptive value in wild grass species because it causes spontaneous spike shattering. The genes on the homoeologous group 3 chromosomes determine the brittle rachis in Triticeae. A few genotypes with brittle rachis have also been found in the cultivated Triticum. Using microsatellite markers, the homoeologous genes for brittle rachis were mapped in hexaploid wheat (Triticum aestivum L.), durum wheat (Triticum turgidum L. conv. durum /Desf./) and Aegilops tauschii Coss. On chromosome 3AS, the gene for brittle rachis, Br2, was linked with the centromeric marker, Xgwm32, at the distance of 13.3 cM. Br3 was located on chromosome 3BS and linked with the centromeric marker,Xgwm72 (14.2 cM). Br1 was located on chromosome 3DS. The distance from the centromeric marker Xgdm72 was 23.6 cM. The loci Br1, Br2 and Br3 determine disarticulation of rachides above the junction of the rachilla with the rachis so that a fragment of rachis is attached below each spikelet. The rachides of Ae. tauschii are brittle at every joint, so that the mature spike disarticulates into barrel type. The brittle rachis was determined by a dominant gene, Brt, which was linked to the centromeric marker, Xgdm72 (19.7 cM), on chromosome 3DS. A D-genome introgression line, R-61, was derived from the cross Bet Hashita/Ae. tauschii, whose rachis disarticulated as a wedge type. The gene for brittle rachis of R-61, tentatively designated as Br61, was distally located on chromosome 3DS, and was linked with the centromeric marker, Xgdm72 (27.5 cM). We discussed how the brittle rachis of R-61 originated genetically.