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Molecular markers are being applied increasingly to various aspects of tree improvement. This thesis reports two such applications. The first involves tracing the natural origins and assessing the genetic diversity of the Portuguese Landrace of E. globulus, which is a core component of the Portuguese breeding program. In order to achieve this, variation in chloroplast DNA (cpDNA) sequence and microsatellite allele frequency in 47 individuals of the Portuguese Landrace were compared with native stand material. The native gene pool of E. globulus is genetically diverse, with substantial geographically structured genetic variation in molecular markers, which makes it possible to trace the origin of germplasm of unknown pedigree, such as the Portuguese Landrace. Similar affinities suggested by the two independent marker systems provided strong evidence that the Portuguese Landrace was predominantly derived from south-eastern Tasmania and to a lesser extent south-eastern Victoria. Ascertaining the origin of and diversity in such populations will help to avoid inbreeding and will capture the most favourable traits from native populations when they are used in breeding programs. The second application of molecular markers addressed in this thesis involved the construction of a genetic linkage map and searching for molecular markers associated with quantitative trait loci (QTL) in E. globulus. This study focussed on identifying QTL for susceptibility to Mycosphaerella leaf blotch (MLB), a fungal disease affecting the growth of eucalypt plantations worldwide, and other potentially correlated traits. In order to search for QTL, parental and consensus linkage maps were constructed in an F2 inter-provenance cross of Eucalyptus globulus with clonal replication. The consensus map used for QTL detection contained 169 markers (34 SSR and 135 AFLP loci) in 11 linkage groups. The inclusion of SSR markers allowed comparison of linkage and QTL information to other mapping and QTL studies conducted in different mapping pedigrees of E. globulus and other species in the subgenus Symphyomyrtus. The F2 was genetically variable, with significant differences detected between genotypes for all recorded traits. Significant QTL were detected for susceptibility to MLB, herbivory by the autumn gum moth, growth and vegetative phase change. In the case of MLB susceptibility, two major unlinked QTL explained over 70% of the phenotypic variance. This study reports the first QTL affecting height to vegetative phase change and disease resistance in E. globulus. These findings will help to elucidate the genetic control of these complex traits in Eucalyptus, and provide information that will be useful in attempts to incorporate them into breeding programs. |
