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

Abstract

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.

Heterosis May Result in Selection Favouring the Products of Long-Distance Pollen Dispersal in Eucalyptus

“…Common-environment field trials based on OP progenies collected from native populations indicated that there is marked quantitative genetic differentiation between northern and southern races, with northern populations having greater bark thickness [27]; wood with more extractives and lignin content, and a higher syringyl/guaiacyl ratio [70]; later transition to adult foliage [71]; more resistance to leaf disease [72] and drought [28]; and tending to be generally slower growing than southern races [73], [74]. There are also differences in foliar chemistry [42], [75], [76] and marsupial herbivory [77] that could partly explain the fitness differences we have observed. A specific study undertaken on the Geeveston trial at age 3 years showed that the northern trees (♀ N ♂ N ) were more susceptible to marsupial herbivory than southern trees (♀ S ♂ S ) in the field and in artificial feeding trials, most likely due to their foliage containing more tannins and essential oils [78].…”

Genetic control of cuticular wax compounds inEucalyptus globulus

“…Asterisks indicate significance ( P < 0.05), with boldface indicating significance after a Bonferroni adjustment to P = 0.0035. Q st values significantly different from the reported average F st (0.09) between sub‐races for microsatellite markers are shown in boldface, with asterisks indicating Q st values significantly different from the recorded maximum F st (0.201) (see O'Reilly‐Wapstra et al ., ). Statistical significance was tested using a Wald F ‐test for the fixed sub‐race effect.…”

“…Genetic‐based variation in wax chemistry at the sub‐race and family within sub‐race levels was studied by growing families with known female pedigrees in a replicated common environment field trial (Hamilton et al ., ; O'Reilly‐Wapstra et al ., ); located at Salmon River (41°01′S, 144°52′E) in northwest Tasmania, Australia. The trial was established in 2006 using progeny derived from open‐pollinated seed (families) from 140 trees sampled across the natural range of E. globulus .…”

“…The species is heteroblastic (James & Bell, ; Hudson et al ., ), and part of a complex of four related species (Jones et al ., ). Natural patterns of genetic variation in E. globulus have been well studied, including wood chemistry (Stackpole et al ., ) and foliar secondary metabolites (O'Reilly‐Wapstra et al ., ). There is significant quantitative genetic variation across the geographic range of the species in virtually all traits studied, which has been summarized by classification of the native range into 13 geographic races and 20 sub‐races (Dutkowski & Potts, ).…”

Fifty years of genetic studies: what to make of the large amounts of variation found within populations?

“…In this case, the rationale is that variance in such traits may indicate different coexisting optima-a sign of spatially or temporally variable selection and adaptation. Some pioneering studies have shown that natural forest populations studied in situ display heritability for fitnessrelated traits, such as chemical traits associated to herbivore resistance in eucalypts (O'Reilly-Wapstra et al 2013) and white spruce (Mageroy et al 2014), serotiny and growth traits in Mediterranean pines (Budde et al 2014;Alía et al 2014) and leaf functional traits in cork oak (Ramírez-Valiente et al 2014). More results of the same kind are available in beech (Bontemps et al, in preparation).…”