Dean Williams

A latitudinal cline in disease resistance of a host tree

The possible drivers and implications of an observed latitudinal cline in disease resistance of a host tree were examined. Mycosphaerella leaf disease (MLD) damage, caused by Teratosphaeria species, was assessed in five Eucalyptus globulus (Tasmanian blue gum) common garden trials containing open-pollinated progeny from 13 native-forest populations. Significant population and family within population variation in MLD resistance was detected, which was relatively stable across different combinations of trial sites, ages, seasons and epidemics. A distinct genetic-based latitudinal cline in MLD damage among host populations was evident. Two lines of evidence argue that the observed genetic-based latitudinal trend was the result of direct pathogen-imposed selection for MLD resistance. First, MLD damage was positively associated with temperature and negatively associated with a prediction of disease risk in the native environment of these populations; and, second, the quantitative inbreeding coefficient (QST) significantly exceeded neutral marker FST at the trial that exhibited the greatest MLD damage, suggesting that diversifying selection contributed to differentiation in MLD resistance among populations. This study highlights the potential for spatial variation in pathogen risk to drive adaptive differentiation across the geographic range of a foundation host tree species.

Stiffness and checking of Eucalyptus nitens sawn boards: genetic variation and potential for genetic improvement

A trial was undertaken to assess the extent to which variation in sawn-board quality traits of plantation-grown Eucalyptus nitens is under genetic control and amenable to genetic improvement. Five hundred and sixty trees from 129 families and three central Victorian races were sampled from an open-pollinated progeny trial in Tasmania, Australia. Acoustic wave velocity (AWV) was assessed on standing trees and sawlogs. Wedges from disks extracted from sawlogs were assessed for basic density and checking. Processed boards from 496 of the trees were assessed for board stiffness (static modulus of elasticity, MOE), and internal and surface checking. Genetic differences among races were significant for AWV and MOE traits. The Southern race had the highest mean values for these traits. Significant additive genetic variation within races was observed in all traits, demonstrating that the quality of plantation-grown E. nitens boards could be improved through breeding. Estimated narrow-sense heritabilities were 0.85 for standing-tree AWV, 0.71 for log AWV, 0.37 for board MOE, and ranged from 0.20 to 0.52 for checking traits. A strongly positive genetic correlation (rg = 1.05) was observed between standing-tree AWV and board MOE, indicating that AWV could be used as a selection trait to improve E. nitens board stiffness. The genetic correlation between basic density and board MOE was also positive (rg = 0.62). However, a significant and adverse genetic correlation (rg = 0.61) was identified between basic density and surface check length. Wood stiffness and checking traits were more-or-less genetically independent, and genetic correlations between surface and internal checking were positive but only moderate (rg = 0.48–0.52).

Achievements in forest tree improvement in Australia and New Zealand 9. Genetic improvement of Eucalyptus nitens in Australia

Summary Eucalyptus nitens is the second most widely planted eucalypt species in Australia. The species is principally grown for the production of pulpwood but substantial areas are also managed for solid-wood production. The first large-scale E. nitens progeny trials were established in the 1970s and up to two cycles of breeding have since been completed. Estimates of genetic gains achieved through breeding are not routinely published, but numerous genetic gain trials have been established. Advances in the understanding of E. nitens genetic architecture and reproductive biology have been integrated into operational breeding and deployment programs. Despite extensive research into alternative deployment strategies, improved E. nitens genotypes are almost universally deployed as seedlings derived from open-pollinated seed-orchards.

Chemical Variation in a Dominant Tree Species: Population Divergence, Selection and Genetic Stability across Environments

Understanding among and within population genetic variation of ecologically important plant traits provides insight into the potential evolutionary processes affecting those traits. The strength and consistency of selection driving variability in traits would be affected by plasticity in differences among genotypes across environments (G×E). We investigated population divergence, selection and environmental plasticity of foliar plant secondary metabolites (PSMs) in a dominant tree species, Eucalyptus globulus. Using two common garden trials we examined variation in PSMs at multiple genetic scales; among 12 populations covering the full geographic range of the species and among up to 60 families within populations. Significant genetic variation in the expression of many PSMs resides both among and within populations of E. globulus with moderate (e.g., sideroxylonal A h2op = 0.24) to high (e.g., macrocarpal G h2op = 0.48) narrow sense heritabilities and high coefficients of additive genetic variation estimated for some compounds. A comparison of Qst and Fst estimates suggest that variability in some of these traits may be due to selection. Importantly, there was no genetic by environment interaction in the expression of any of the quantitative chemical traits despite often significant site effects. These results provide evidence that natural selection has contributed to population divergence in PSMs in E. globulus, and identifies the formylated phloroglucinol compounds (particularly sideroxylonal) and a dominant oil, 1,8-cineole, as candidates for traits whose genetic architecture has been shaped by divergent selection. Additionally, as the genetic differences in these PSMs that influence community phenotypes is stable across environments, the role of plant genotype in structuring communities is strengthened and these genotypic differences may be relatively stable under global environmental changes.

Association of Allelic Variation in Xylem Genes with Wood Properties in 'Eucalyptus nitens'

Summary We used association studies to identify allelic variation in genes that influence wood fibre development in Eucalyptus nitens (Deane & Maiden). Genes selected for analysis were differentially expressed in wood with contrasting properties such as cellulose and lignin content, pulp yield and microfibril angle (MFA). Single nucleotide polymorphisms (SNPs) were identified by sequencing the candidate genes in a number of unrelated individuals. Selected SNPs were genotyped across 420 unrelated E. nitens trees from central Victorian populations and growing in a provenance trial at Meunna in north-western Tasmania. Significantly associated SNPs were genotyped across two other populations in northern Tasmania in order to validate associated SNPs. We have compiled a database of phenotypic information relating particularly to wood fibre properties for each individual in the association and validation populations. Associations between SNPs and wood properties were identified by comparing trait means in different SNP genotype classes. Several significantly associated SNPs identified in the Meunna population were validated in the other populations. The direction of the allele effect was reversed for two SNPs that were associated with kraft pulp yield. DNA markers identified in this research may be used to complement existing selection methods in breeding programs.

The effect of tree spacing on the production of flowers in Eucalyptus nitens

Summary The effect of spacing on the production of flowers and capsules in Eucalyptus nitens was studied in two spacing trials located in northern Tasmania. Tree density in these trials ranged from 468 to 4216 stems ha−1. Reproductive structures were collected in litter traps and these data were used to calculate reproductive output on a per tree and per hectare basis. Between 8735 (5-y-old site, 1333 stems ha”1) to 234098 (13-y-old site, 1082 stems ha”1) flowers were produced per hectare over a single flowering season at these two study sites at the planting densities expected of a pulpwood plantation. This represented 8 and 211 flowers per tree respectively. As tree density decreased, the production of flowers and capsules increased on both a per-tree and per-hectare basis. It is estimated that the number of flowers per hectare is likely to be anywhere between between 1.4 and 10-fold greater under the spacing expected in sawlog regimes (250 trees ha−1). This difference in reproductive output between plantations of E. nitens that use different spacing regimes is one of the many factors that need to be considered in assessing the risk of wilding establishment or hybridisation with adjacent native eucalypts.

Genetic correlations in multi-species plant/herbivore interactions at multiple genetic scales: implications for eco-evolutionary dynamics

In plant/herbivore systems, elucidating the hierarchical genetic correlations that exist between enemies to a host plant (e.g., in the magnitude of damage) and determining how stable these effects are across environments is crucial for our understanding of potential eco-evolutionary dynamics in these systems. This sort of information would allow us to better know how plant populations have evolutionarily diverged in their phenotypic traits, which organisms are driving the evolutionary change and how rapid evolutionary change in one enemy or plant species can feedback to affect other herbivore and pathogen species showing genetically correlated responses to the host plant. Here, we investigate consistency in patterns of the genetic correlations within and among populations in preferences among multiple enemies for a globally planted species, Eucalyptus globulus, and explore how stable these genetically based correlations are across environments. We show plant enemies respond to underlying host genetic variation at two genetic hierarchical scales and that relationships between plant enemies are both independent and correlative. Our finding of a significant positive genetic correlation in damage to the host plant between a sawfly and a fungal leaf pathogen species suggests a potential indirect eco-evolutionary feedback loop mediated by the genetic correlation. We also demonstrate that among population divergence patterns are not constrained by within population correlations, and this decoupling between among and within population patterns suggests that diffuse relationships between two enemies are not constraining the evolutionary diversification in resistance of populations of their host plant. In addition, when genetic effects are present, there is clear stability in the genetic influences of this host tree on its enemies across trials. This indicates that in an eco-evolutionary dynamic setting, the strength and consistency of any selective force would be maintained across environments.

Improved Methods for Achieving Traceability of Tree and Log Identities in Timber Processing Studies

We successfully tracked the identities of more than 550 selected 13-year-old trees from a Eucalyptus nitens (Deane & Maiden) progeny trial through tree felling, harvesting of butt logs, and sawmill processing to finished sawn boards using a two-stage approach. To track log identity from the standing tree to the log yard, we used a numbered under-bark wooden identification plug, glued into a hole drilled in the trunk prior to harvesting. To track tree identity for individual sawn boards, log-end templates with corresponding tree identification numbers were glued to the log ends before milling. Materials and methods used withstood harvesting and debarking, log transportation, milling, air and kiln drying, steam reconditioning, and final machining. A second study confirmed the success of the under-bark plug method to successfully track 548 selected standing trees through harvesting and transportation to the mill.