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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Date
2018
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Massey University
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Abstract
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.
Description
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).
Keywords
Lolium perenne, Breeding, New Zealand, Genetics, Heterosis