A molecular analysis of the population structure, mating system and demography of Eucalyptus

Abstract

This thesis aimed to study the evolutionary forces affecting populations of Eucalyptus, with special emphasis on the compounds implicated in plant defence, notably terpenes. In Chapter 1, I explored the genetic structure in Eucalyptus globulus to understand how processes such as gene flow and genetic drift have shaped the genetic distribution of the species. Using microsatellite-derived genotypes of individuals covering the range of the species, I separated E. globulus into five geographically distinct regions. Furthermore, local-scale spatial genetic analysis using two of the regions detected spatial genetic structure of over 40 km, indicating long-distance gene flow and a larger population than previously thought. The five regions provided the units of study in Chapters 2 and 3. This chapter also provided data on population structure for the association studies of candidate genes from secondary metabolic pathways (Appendix 5). In Chapter 2, I turned my attention to genes of known function - dxs and dxr genes from the non-mevalonate terpene biosynthesis pathway because these had been previously implicated as bottlenecks to terpene production in other species. The aim was to determine whether the genes revealed the variation we observed in the concentrations of foliar terpenes. Using gene sequences from 104 E. globulus individuals, I showed that the dxr and two copies of dxs genes were under purifying selection but there was no evidence that these genes cause the variation we see in foliar terpene production. I also found that different enzymatic domains encoded by the genes have taken different evolutionary pathways. In Chapter 3, I reconstructed the demographic history of E. globulus using information from introns and third coding sites of exons of dxr and two copies of dxs, with the aim of estimating the timing of major demographic events for the entire species and for the five regions from Chapter 1. To do this, I applied a novel analysis using Bayesian Skyline plots. The demographic reconstruction of the regions showed two trends of exponential expansion, which started around the early-mid Pleistocene transition. These trends suggested that the island populations expanded earlier while those on the mainland expanded faster. These appeared as two continuous expansions when the entire species was analysed. The results of this study excluded early human activity as an important cause of expansion of Eucalyptus globulus. In Chapter 4, I examined the genetic variation in families within populations over shorter and contemporary evolutionary time-frames. The aim was to examine the correspondence between mating systems and heritability of foliar terpenes and to test if all populations of a species are equally suited for inferring marker-based heritability. Using microsatellite genotypes and the foliar terpene profile from three disparate populations of E. tricarpa, I found that the estimates of the heritability of foliar terpenes differed among populations and were not correlated with outcrossing rates or pollen heterogeneity among females. The variable mating systems and structure of the pollen pool resulted in some populations providing more reliable heritability estimates, which is important for most studies of community genetics. I concluded my thesis by discussing the value of next generation sequencing technology in expanding the population genetic aspects covered in this project. Show more