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Assessment of wheat x maize doubled haploid technology for genetic improvement of New Zealand wheat : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University, Canterbury, New Zealand

Campbell, Anna Wendy
Fields of Research
New Zealand must continue to produce superior wheat cultivars to retain competitive advantage within New Zealand and contribute in terms of global wheat breeding. Doubled haploid (DH) technology allows the production of homozygous wheat lines in a single generation. The integration of DH technology into New Zealand wheat breeding and genetics programmes has the potential to reduce the breeding time of new cultivars and improve our understanding of agronomically important genetic traits. The main goal of this thesis was to increase the efficiency of the development of genetically improved New Zealand wheat cultivars by inducing direct homozygosity through wheat x maize crosses. To achieve this goal, a number of objectives needed to be met. The first of these was to develop a method of wheat DH production through wheat x maize crosses using New Zealand germplasm. A number of New Zealand wheat cultivars were crossed with different maize genotypes. A successful method for producing wheat DHs was achieved with all genotypes. Several factors were identified as being important in the efficiency of wheat DH production. These included: seasonal constraints, time of embryo excision, method of auxin application and media composition. Of these factors, seasonal constraints were the major limitation in the use of the wheat x maize method for producing DHs in New Zealand. The second objective was to determine optimal environmental conditions for the production of wheat DHs through wheat x maize crosses. New Zealand wheat cultivars were grown in a glasshouse until they reached the booting stage when they were transferred to differing temperature and light intensity conditions. Results showed that both temperature and light intensity significantly affected haploid embryo numbers. In a further examination, it was found that light intensity acted at the level of pollen tube growth. This was a maternal plant effect and measurements of electron transport rate and quantum yield showed that photosynthesis may influence the maternal plant in such ways as to promote and/or inhibit pollen tube growth in wheat x maize crosses. The third objective was to evaluate whether wheat DHs produced through the wheat x maize method are 'normal' and genetically stable. DHs were made from homozygous wheat cultivars and compared with their parent cultivar in a field trial. Results showed that a small amount of aberrant genetic variation was introduced into DH lines. It was concluded that the variation was minimal and unlikely to have an effect in breeding programmes, verifying the use of wheat x maize crosses over anther culture. The fourth objective was to determine where in a wheat breeding programme DHs could be used for maximum genetic gain. A model was tested which integrated DH and marker assisted selection (MAS) into a breeding programme. When genetic variation of the MAS selected DH plants was compared to the genetic variation of an unselected F₃ population, five of the six measured traits were normally distributed and had no significant loss of genetic variation. The sixth trait, height, was bimodally distributed and further analysis of the data showed that the inheritance of major gene/s and factors such as linkage and/or epistasis may have been involved in its distribution. Overall the model was shown to be a valid breeding tool, allowing a breeder to estimate minimum DH population sizes required for minimal loss of genetic variation. Results from the research outlined in this thesis indicate that DH technology can be used to reduce the time involved in producing new wheat cultivars and for producing useful populations for genetical studies.
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