Exploiting heterosis in perennial ryegrass (Lolium perenne) through development of inbred lines, and the impact on population variability : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science (MSc) in Agricultural Science, Massey University

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Genetic improvements in the dry matter yield of perennial ryegrass via plant breeding are typically achieved through recurrent selection, delivering rates of genetic gain estimated to be in the range of 0.25 - 0.76% per year. Hybrid breeding is commonly used in self- compatible species (e.g. maize) to achieve significant yield increases through the exploitation of heterosis. However, hybrid breeding has not been used to a large extent in perennial ryegrass, due to its self-incompatibility (SI) system. However, using marker assisted selection (MAS), the alleles responsible for SI in perennial ryegrass can now be manipulated. A method has been developed which uses MAS to develop parent lines with controlled SI alleles, which are inbred for two cycles and are then crossed to create hybrids. This method provides the opportunity to exploit heterosis in perennial ryegrass breeding and for significant gains in dry matter yield. The first experiment in this thesis aimed to investigate the expression of heterosis in F1 hybrid plants produced by this proposed novel SI hybrid breeding method. It was expected that the hybrid offspring would at least display mid-parent heterosis. Experiment one also investigated the variability in key morphological traits, in the expectation that the cycles of inbreeding would have increased genetic uniformity in the parent lines and hybrids. The hybrids did display mid-parent heterosis throughout the experiment, providing evidence that the proposed method successfully captures heterosis in the perennial ryegrass breeding cycle. Evidence of high-parent heterosis were also observed throughout the experiment, which indicates the potential to develop F1 hybrids with significant yield increases compared to current cultivars. Therefore, the method may be commercially viable. No consistent changes in the morphological variation of the parent lines or hybrids was observed, which is a positive outcome for the ecology of perennial ryegrass in grazed pasture communities. The second experiment investigated expression of heterosis in F1 hybrid offspring from pairs crosses with different genetic backgrounds. The amount of variation in heterosis within each F1 hybrid population was also investigated. It was expected that expression of heterosis would vary dependent on the genetic background and that there would be significant variation in expression of heterosis within each F1 population. The expression of, and variation in, heterosis was of interest because with the advent of the SI hybrid breeding method, breeders may benefit from quantifying the combining ability of their perennial ryegrass breeding pools. This would enable better selection of plants for entry into the hybrid breeding pipeline. Mid-parent and high-parent heterosis were detected, but the levels of expression were variable within, and between, the two genetic backgrounds. This supports the hypothesis that there is variation in the performance of hybrids with differing genetic backgrounds, and therefore, there would be value in quantifying the combining ability of perennial ryegrass breeding pools.
The following Figures have been removed for copyright reasons but may be accessed via their source listed in the References: 2.7 (Wang et al., 2014 Fig. 1); 2.10-2.12 (Conaghan & Casler, 2011 Fig. 1-3 respectively); 2.14 (Pembleton et al., 2015 Fig. 1).
Lolium perenne, Breeding, New Zealand, Genetics, Heterosis