Understanding the genetics of wheat yield to deploy high and stable yielding wheat varieties across UK environments (PhD)

Abstract

There is an urgent need to increase crop yields to address food insecurity. Grain weight, determined by grain length and width, is an important component of final grain yield component. However, our understanding of the mechanisms that control grain weight in polyploid wheat is limited. The overall aim of this thesis was to understand the mechanisms that control grain length and width in hexaploid wheat through the characterisation of two previously identified grain weight quantitative trait loci (QTL) on chromosomes 5A and 6A.

Using near isogenic lines (NILs) we found that the 5A and 6A QTL act through different mechanisms to increase grain weight. The 5A QTL acts post-fertilisation, primarily to increase grain length (4.0%) through increased pericarp cell size. The 5A QTL also has a pleiotropic effect on grain width (1.5%) during late grain development. The 6A QTL acts during very early grain development, perhaps pre-fertilisation, and specifically increases final grain width (2.3%).

Fine-mapping reduced the QTL mapping intervals and revealed complex underlying genetic architectures. The 6A QTL mapped to a large linkage block in the centromeric region of chromosome 6A containing the known grain size gene, TaGW2-A, although we provide evidence to suggest that this is not the causal gene underlying the 6A QTL. Fine-mapping of the 5A QTL suggests that two tightly linked genes with an additive effect on grain length underlie the locus. A haplotype analysis suggests that the 5A QTL is not fixed in UK germplasm.

The corresponding physical intervals for both the 6A and 5A QTL remain large and contain several hundred genes, making speculation on candidates for the causal genes difficult. A transcriptomics study with the 5A NILs provided insight into the genes and pathways that are differentially regulated and hence may play a role in controlling the differences in grain weight. The markers and germplasm developed within this thesis had been published and have been made available to UK breeding companies to ensure quick uptake of this knowledge and transfer into improved wheat varieties.