TY  - THES
T1  - Design and assessment of breeding strategies for hybrid wheat in Europe
A1  - Boeven,Philipp Hans Günter
Y1  - 2019/10/08
N2  - Wheat is one of the top three global staple crops, possesses the largest global cultivation area, and plays a key role for the world’s future food security. However, its projected yield increase is insufficient to meet the future food and feed demand of an ever-growing world population. Consequently, the rate of breeding progress and productivity of wheat must be increased. Unfortunately, current wheat line breeding has a low return on investment mainly due to high levels of farm saved seeds, which makes wheat less attractive for the plant breeding industry and leads to lower investments and progress compared to other crops where the hybrid technology is established. Hybrid breeding is a worldwide success story in many crops but is not yet established in wheat. Hybrid wheat promises increased yield gain due to the exploitation of heterosis, higher yield stability and stabilized return on investments for breeding companies which warrants further investment and breeding progress in this important stable crop.

The self-pollinating nature of wheat is a major bottleneck for hybrid seed production and efficient hybrid wheat breeding requires the redesign of the wheat floral architecture to enhance cross-pollination. Furthermore, the longterm success of hybrid wheat is crucially dependent on the establishment of heterotic groups, on the identification of a high yielding heterotic pattern, and finally, on the realized amount of heterosis and hybrid performance. Therefore, the main objectives of my thesis research were to: (i) analyze the genetic diversity and adaptation in a global winter wheat collection and evaluate how diversity trends could be used to support the development of heterotic groups in wheat; (ii) assess the relationship between heterosis and genetic distance under maximized diversity and evaluate the usefulness of exotic germplasm for hybrid wheat breeding; (iii) dissect the genetic architecture underlying male floral traits in wheat to enable genomics-assisted breeding approaches and investigate the trait seed set which is most crucial for an efficient hybrid seed production.

The analyses of genetic diversity in a large worldwide panel of 1,110 winter wheat varieties released during the past decades showed no major population structure but revealed genetically distinct subgroups. Most of the global diversity trends could be explained by breeding history and were associated with geographical origin and long-term domestication. We found that the frequency of the copy number variants at the Photoperiod-B1 (Ppd-B1) and the Vernalization-A1 (Vrn-A1) loci reflect wheat adaptation to the environmental conditions of the different regions of origin. Thus, adaptation issues add an additional layer of complexity and hamper the direct introgression of genetic diversity to support the genetic divergence between heterotic pools. Based on all these analyses, we proposed HyBFrame, a unified framework illustrating how global wheat genetic diversity can be used to support and accelerate reciprocal recurrent selection for the development of genetically distinct heterotic groups in wheat.

In a second experiment, we produced 2,046 wheat hybrids by crossing elite with elite lines as well as elite with exotic lines and performed multi-environmental field trials. Interestingly, we found an average midparent heterosis of about 10% in elite crosses as well as in exotic crosses and observed no evidence for a breakdown of heterosis under maximized genetic distance among the hybrid parents. Genetic distance based on genome-wide molecular markers revealed only a very weak association with midparent heterosis for grain yield. Here, we elaborated a functional Rogers’ distance giving weight to heterosis loci and observed a strong positive association between heterosis and this novel distance measure. Hence, considering the genetic architecture of heterosis revealed a more accurate picture of the relationship between heterosis and genetic distance. In addition, the genetic architecture of heterosis in wheat is crucially dependent on the genetic background. We found that a higher number of negative dominance and dominance-by-dominance epistatic effects can reduce the level of absolute heterosis in wide crosses between exotic lines and elite testers. Moreover, hybrid performance in wheat is mainly driven by parental per se performance. Thus, elite lines are favorable for hybrid breeding and should be employed as the starting material for heterotic grouping.

Hybrid seed production is the major bottleneck for hybrid wheat breeding and explains the low market share of hybrid wheat varieties. Seed set on the female plants in crossing blocks is the most crucial trait for hybrid seed production in wheat. We tested 31 male lines and evaluated the hybrid seed set on two female tester lines in crossing blocks. Seed set showed a large genotypic variation and a high heritability suggesting that recurrent selection for increased seed set is feasible. The major problem is the synchronized flowering between male and female lines, making the evaluation of seed set in large panels very complex and difficult. Hence, indirect male floral traits with high correlation to the trait seed set would be promising to breed for improved hybrid seed production. We found a strong association between seed set and visual anther extrusion, underscoring that indirect male floral traits have a high potential for preliminary male screenings. We also dissected the genetic architecture underlying promising male floral traits and assessed the potential of genomics-assisted approaches for their improvement. We employed a panel of 209 diverse wheat lines and found a complex genetic architecture underlying all male floral traits. The Reduced height gene Rht-D1 was identified as the only major QTL, for which the commonly used height-reducing allele showed negative effects on male floral traits. This genetic architecture with many moderate- or small-effect QTL limits classical marker-assisted selection. In contrast, genomic prediction yielded moderate to high prediction abilities for anther extrusion. Finally, we proposed a breeding scheme to increase cross-pollination in wheat based on a combination of phenotypic and genomics-assisted selection.

Taken together, hybrid breeding in wheat is a very promising approach and the next years will show if all of the current issues can be solved. This thesis research contributed to breeding strategies for hybrid wheat breeding and to the general understanding of heterosis in crops.
KW  - Weizenzüchtung
KW  - Hybridzüchtung
KW  - Heterosis
KW  - Saatgutproduktion
KW  - Fremdbefruchtung
CY  - Hohenheim
PB  - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
AD  - Garbenstr. 15, 70593 Stuttgart
UR  - http://opus.uni-hohenheim.de/volltexte/2019/1660
ER  -