Hybrid breeding, where two inbred lines are crossed to create superior F1 plants, has the potential to increase yield and yield stability in wheat. However, commercialisation of hybrid wheat is limited by high production costs but could be offset by increasing the yield. Introducing new diversity into hybrid breeding programmes could increase yield through improved traits and by increasing the genetic distance between hybrid parents which is hypothesised to increase heterosis. In this project, synthetic hexaploid-derived wheat (SHW) and tetraploid x hexaploid wheat (THW) pre-breeding lines containing novel diversity were used to assess their potential for hybrid breeding.

Hybrid wheat is sown at a lower density than conventional line-bred wheat and so traits that increase yield at lower sowing densities may be valuable in a hybrid context. Thirty ‘large eared’ SHW and THW lines were sown at typical line-bred (100%) and hybrid (70%) sowing densities in a multi-year field trial to see if the large ear trait resulted in yield advantages at the lower density. Most ear and seed yield component traits were higher at the hybrid sowing density, with yields maintained or only slightly reduced at the hybrid density. However, this was mostly influenced by tiller number and ear traits often showed a trade-off with tiller number.

In a second trial series, 99 hybrids were created and tested in five field trials across UK, France, and Germany over two years to test whether increased genetic diversity could lead to improved hybrid performance. Hybrids were developed by crossing diverse and commercial elite male parents with three male-sterile testers as female parents. Results were variable with some pre-breeding hybrids performing well and hybrids generally showing better yield stability than inbreds.

A separate trial investigated the optimal sowing density for yield in hybrid wheat. Across five different hybrids, the optimal sowing density ranged from 80-100% relative to inbreds, which is above the typical hybrid sowing rate of 70%.

Crosses were also made between different pre-breeding lines used as males in the hybrid trials to create new material with recombined diversity and to reduce their relatedness to their backcrossed parent, Robigus. Lines from 15 crosses have been taken through to the F6 plant stage after being selected for beneficial traits and will be used in forward breeding.

A transcriptomics study was carried out to investigate how gene expression differed between two parents and their reciprocal F1 hybrids. Two parents contrasting for yellow-rust (YR) resistance and their reciprocal hybrids were inoculated with YR to look at their transcriptional differences at 0, 3, and 5 days post inoculation (DPI). Large numbers of differentially expressed genes were found between all four genotypes with the hybrids mostly showing more similar expression patterns to their YR-susceptible parent, whilst the YR resistance appeared to be dominant. 95.3-99.9% of genes exhibited additive expression and the genes that exhibited non-additive expression were mostly overdominant at 0 and 3 DPI, and partially dominant at 5 DPI. In conclusion, these findings illustrate the complexity of hybrid breeding in hexaploid wheat. They show that diverse material does show potential for hybrid improvement; however, targeting single traits such as the large-eared character in isolation will probably be insufficient to drive sustained improvements in yield and yield stability.