The genetics of oil yield in Melaleuca alternifolia and Eucalyptus loxophleba

Genetic improvement of essential Myrtaceae based oil crops is vital to the long-term health and profitability of Australian essential oil industries. Improving oil yield and efficiency is an important part of maintaining the competitive advantage that Australian industries currently hold over international competitors. Over the last 20 years, gains in oil yield have been made through the tea tree breeding programme, which has helped the industry get to where it is today. This work needs to continue to advance, for the health of the industry into the future. Using molecular techniques to facilitate this is the best way forward for the industry. To better understand how oil yield is controlled in both tea tree (Melaleuca alternifolia) and York gum (Eucalyptus loxophleba) within Australia, I first investigated whether gene expression had an effect on oil yield in M. alternifolia. In this study I investigated the transcript abundance of 13 genes (dxr, dxs2, dxs3, cmk, mcs, mct, hds, mvk, hmgs1, pmd1, ippi1, ippi2 and gpps) in 48 individuals that varied in oil yield. The expression of all genes in the pathway explained 87% (R2 = 0.87) of variation in oil yield, These result are important in that they show that oil yield isnʼt just controlled by one or two genes in the biosynthetic pathway, but is influenced by multiple genes and that interactions between these genes is an important determinant of oil yield. Two genetic association studies were performed to identify variation within candidate genes that can be used as molecular markers in the respective breeding programmes of M. alternifolia and E. loxophleba. Both these studies identified single nucleotide polymorphisms (SNPs) within candidate genes that have an effect on economically important traits and in M. alternifolia a number of these SNPs were then successfully validated within the breeding programme genetic stock. The next step is incorporating the information generated in this thesis about the genetics of oil yield into the respective breeding programmes of both species. Doing this is likely to increase the gains in oil yield made through selective breeding and help secure the long-term future of these iconic Australian industries