Genes Involved in the Domestication of Wheat

Brittle rachis (Br)

The mature spike rachis of wild emmer [Triticum turgidum L. ssp. dicoccoides (Korn. ex Asch. and Graebner) Thell.] disarticulates spontaneously between each spikelet leading to the dispersion of wedge-type diaspores. By contrast, the spike rachis of domesticated emmer (Triticum turgidum L. ssp. turgidum) fails to disarticulate and remains intact until it is harvested. This major distinguishing feature between wild and domesticated emmer wheat is controlled by two major genes, brittle rachis 2 (Br-A1) and brittle rachis 3 (Br-B1) on the short arms of chromosomes 3A and 3B, respectively. Because of their biological and agricultural importance, we have deternied a more precise location of Br-A1 and Br-B1 on microsatellite marker-based linkage maps of chromosomes 3A and chromosome 3B, respectively (Nalam et al. 2006).

Nalam, V.J., M.I. Vales, C.J.W. Watson, S.F. Kianian and O. Riera-Lizarazu. 2006. Map-based analysis of genes affecting the brittle rachis character in tetraploid wheat (Triticum turgidum L.). Theor. Appl. Genet. 112: 373–381.

Compactum (C)

Club wheat (Triticum aestivum ssp. compactum) carries the dominant form of the compactum (C) gene resulting in a compact spike relative to spikes of common wheat (T. aestivum ssp. aestivum). The C locus, with its effect on spike compactness, rachilla morphology, seed size and number also impacts yield. This may explain its presence in cultivated hexaploid forms. Toward the eventual isolation and characterization of this gene, we undertook a project to locate C on genetic linkage maps of chromosome 2D (Johnson et al. 2008). The C locus was completely linked to markers placed in two different bins flanking the centromere. We were therefore unable to map C to a precise bin or chromosome arm. A cytogenetically-based study performed by Rao (1972: Wheat Information Service 35:9) placed C on the long arm of chromosome 2D. Thus, we suspect that C is located in a chromosomal segment near the centromere in the long arm of chromosome 2D. Due to C’s proximity to the centromere, the map-based isolation of this gene will be challenging because proximal segments of chromosomes are characterized by large physical distance with greatly reduced levels of recombination.

Johnson, E.B., V.J. Nalam, R.S. Zemetra, and O. Riera-Lizarazu. 2008. Mapping the compactum locus in wheat (Triticum aestivum L.) and its relationship to other spike morphology genes of the Triticeae. Euphytica 163:193-201.

Tenacious glumes 1 (Tg1)

Major and minor mutations were involved in the evolution of the free-threshing habit in hexaploid wheat (Triticum aestivum). A major gene or gene, Q, located on the long arm of chromosome 5A inhibits speltoidy but also has pleiotropic effects on rachis fragility and glume tenacity. All non-free-threshing wild wheats carry the recessive q allele and all free-threshing tetraploid and hexaploid wheats carry the dominant Q allele. The tenacious glumes 1 (Tg1) locus on chromosome 2D also governs the free-threshing habit in hexaploid wheat (Kerber and Rowland 1974: Can. J. Genet. Cytol., 16:145-154). Synthetic hexaploids (2n=6x=42, AABBDD) produced by hybridization between a free-threshing tetraploid Triticum turgidum (2n=4x=24, AABB) with Ae. tauschii (2n=2x=14, DD) are non-free-threshing despite being homozygous for dominant Q allele (see Figure 1). These observations suggest that a recessive tg allele as well as a dominant Q allele must be present for the full expression of the free-threshing character.

Threshed wheat

Figure 1. Threshed spikes and components from the common wheat (T. aestivum) cultivar Opata 85 (A) and the synthetic hexaploid W-7984 (B). Eight randomly chosen mature spikes of each line were processed through a gasoline-powered thresher. Spike and spike fragments (Spk), threshed seeds (Sd), unthreshed seed in spikelets (Spl), and chaff (Ch) were collected for analysis. Percent threshability was calculated as the percentage of completely threshed seeds out of all seeds harvested. Opata 85 is fully threshable (100%) whereas the synthetic hexaploid is not (34%).

Genetic analysis showed that a minimum of two genes controlled the free-threshing character in crosses involving synthetic wheats (Villareal et al. 1996: Plant Breeding 115:407-409). When we studied the free-threshing habit (Jantasuriyarat et al. 2004) in a recombinant inbred line population developed from a cross between a spring wheat, Opata-85 and the W-7984 synthetic hexaploid, various QTL on chromosomes 2A, 2B, 2D, 5A, 6A, 6D and 7B were found to significantly affect the free-threshing characteristic. However, the free-threshing habit was predominantly affected by a quantitative trait locus (QTL) on the short arm of chromosomes 2D (corresponding to the tenacious glumes 1, Tg1, gene) and to a lesser extent by a QTL on the long arm of chromosomes 5A (corresponding to the Q factor). Intense research on the nature of Q has shown that its is an APETALA2-like transcription factor (Faris et al. 2003: Genetics 164:311-321). On the other hand, there have been few studies concerning Tg1. The aim of our research efforts is to fill this void by using map-based methods to characterize Tg1. Toward this goal, we have determined a more precise map location for Tg1 on the short arm of chromosome 2D (Nalam et al. 2007).

Nalam, V.J., M.I. Vales, C.J.W. Watson, E.B. Johnson, and O. Riera-Lizarazu. 2007. Map-based analysis of genetic loci on chromosome 2D that affect glume tenacity and threshability, components of the free-threshing habit in common wheat (Triticum aestivum L.). Theor. Appl. Genet. 116:135–145

Jantasuriyarat, C., M. I. Vales, C.J.W. Watson, and O. Riera-Lizarazu. 2004. Identification and mapping of genetic loci affecting free-threshing habit and spike compactness in wheat (Triticum aestivum L.). Theor. Appl. Genet. 108: 261-273.