Identification and independent validation of a stable yield and thousand grain weight QTL on chromosome 6A of hexaploid wheat (Triticum aestivum L.)

Abstract

Background: Grain yield in wheat is a polygenic trait that is influenced by environmental and genetic interactions at all stages of the plant's growth. Yield is usually broken down into three components; number of spikes per area, grain number per spike, and grain weight (TGW). In polyploid wheat, studies have identified quantitative trait loci (QTL) which affect TGW, yet few have been validated and fine-mapped using independent germplasm, thereby having limited impact in breeding.

Results: In this study we identified a major QTL for TGW, yield and green canopy duration on wheat chromosome 6A of the Spark x Rialto population, across 12 North European environments. Using independent germplasm in the form of BC2 and BC4 near isogenic lines (NILs), we validated the three QTL effects across environments. In four of the five experiments the Rialto 6A introgression gave significant improvements in yield (5.5%) and TGW (5.1%), with morphometric measurements showing that the increased grain weight was a result of wider grains. The extended green canopy duration associated with the high yielding/TGW Rialto allele was comprised of two independent effects; earlier flowering and delayed final maturity, and was expressed stably across the five environments. The wheat homologue (TaGW2) of a rice gene associated with increased TGW and grain width was mapped within the QTL interval. However, no polymorphisms were identified in the coding sequence between the parents.

Conclusion: The discovery and validation through near-isogenic lines of robust QTL which affect yield, green canopy duration, thousand grain weight, and grain width on chromosome 6A of hexaploid wheat provide an important first step to advance our understanding of the genetic mechanisms regulating the complex processes governing grain size and yield in polyploid wheat.

Figure 1

QTL analysis for yield and thousand grain weight. Genome-wide QTL analyses for yield (red) and thousand grain weight (blue) in the Spark x Rialto DH population, across five environments (Norwich UK, Sandringham UK, Scotland, France, Germany) and in three years (2001, 2002 and 2003). The threshold value for significance is set at 2.5 LOD. Consistent, significant effects for both yield and thousand grain weight were observed on chromosomes 4D (Rht-D1) and 6A. Inset: the box and whisker plot exhibits the percentage difference between DH lines fixed for the QTL region on chromosome 6A, with the median and mean denoted by the black and yellow line, respectively.

Figure 2

Genetic map of chromosome 6A for the Spark x Rialto DH population. Markers coloured green correspond to the marker with the highest LOD score for yield (wPt-7063), TGW (BE497701) and Green Canopy Duration after heading (Xwmc256) from MTME analysis. Markers coloured red represent those used for marker assisted selection during the development of Near Isogenic Lines.

Figure 3

Multi Environment QTL Mapping. Co-localisation of QTL for yield (red solid line), thousand grain weight (blue dash line), and green canopy duration (black dash line) on chromosome 6A across multiple environments and years. Flanking markers used for selection during the production of Near Isogenic Lines are indicated in red. Genetic distances (cM) correspond to those shown in Figure 2.

Figure 4

Effect of 6A introgression on grain morphometric parameters. Percentage increase conferred by the Rialto introgression in the 6A BC₂ NILs (2010–2012) and BC₄ NILs (2013). Significant differences are represented by * (P <0.05), ** (P <0.01) and *** (P <0.001).

Figure 5

Field grown wheat grains across environments. Twenty representative grains from selected BC₂ NILs (2010–2012) and BC4 NILs (2013) carrying the Spark (S) or Rialto (R) introgression showing differences in grain width between NIL pairs and across environments.

Figure 6

Relative chlorophyll content (SPAD units) of flag leaves from BC ₂ NILs. SPAD units of NILs carrying the Rialto (red) or Spark (blue) 6A interval in 2010 (solid lines) and 2011 (dashed lines) after anthesis. Significant differences are represented by * (P <0.05), ** (P <0.01) and *** (P <0.001).