Quantitative genetics and breeding of Eucalyptus globulus

This thesis studies quantitative genetic variation in the forest tree, Eucalyptus globulus. It examines the effects of inbreeding and hybridisation on fitness, as well as genetic variation in breeding traits using an extensive base population and a diallel of intra- and inter-race reciprocal crosses. Marked post-zygotic barriers to hybridisation occurred between E. globulus and E. ovata, with F\\(_1\\) hybrids exhibiting high field mortality. Inbreeding similarly had deleterious effects, but by 10 years F\\(_1\\) survival was less than the selfs. Relative hybrid fitness is dependent on inbreeding levels of parental population. Open-pollinated (OP) progenies also exhibited growth depression compared with outcrosses, clearly indicating significant inbreeding is occurring under this mating system. Genetic variation was examined in a base population of E. globulus established on four sites in Argentina. This population included OP progenies from native stands and land races. Thirty variables were studied, representing growth, bark thickness, form, transition to adult foliage and Pilodyn penetration, and used to determine population affinities. Significant genetic differentiation occurred between populations in most traits. Land races had closer affinities to native stands from southern Tasmania. Many native populations were superior in growth to land races, but land races had better form. Significant genetic variation was also detected within E. globulus populations. Heritabilities were low for forking, survival and form; intermediate for growth and bark thickness; and high for Pilodyn penetration and transition to adult foliage. There was strong positive genetic correlation between the same trait measured at different sites and ages. Growth traits were genetically independent from other key breeding traits. Genetic parameter estimates from OP progenies may be biased by inbreeding depression, as well as specific combining (SCA), maternal and carry-over effects. The effects of such factors on growth were examined using a diallel mating amongst eight trees from two E. globulus races. Rapid germination and large seeds resulted in larger seedlings in the nursery, but these carry-over effects disappeared after two-years field growth at two Tasmanian sites. Additive and SCA effects were insignificant at this age, but the interaction between males and females was significant, with reciprocal inter-race hybrids differing in performance. Parents differed in chloroplast haplotypes and this is the first evidence for cyto-nuclear interactions affecting growth of a eucalypt species. Such reciprocal effects could bias genetic parameters and mean the cross direction may be important to maximise gains from mass controlled pollinated seed production systems being developed.