Localization of genes involved in the domestication of hexaploid (Triticum aestivum) wheat

Tenacious glumes (Tg) - 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 (Tg) 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.

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 glumes1, 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 Tg.  The aim of our research efforts is to fill this void by using map-based methods to characterize Tg.  Toward this goal, we are working to better define Tg's location on the short arm of chromosome 2D (Nalam et al. 2003).