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The phenotypic and genetic basis of browsing resistance of Eucalyptus globulus to marsupial herbivores


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O'Reilly-Wapstra, J ORCID: 0000-0003-4801-4412 2004 , 'The phenotypic and genetic basis of browsing resistance of Eucalyptus globulus to marsupial herbivores', PhD thesis, University of Tasmania.

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Significant phenotypic variation in plant resistance to herbivores occurs between and
within many plant species. Plant secondary metabolites play a major role in conferring
resistance to many herbivores, and intraspecific variation in plant resistance can be
influenced by both genetic and environmental variation. The evolutionary and
mechanistic processes behind variation in plant resistance in many systems is still
unknown, particularly in relation to plant/mammalian herbivore interactions. In this
thesis, intraspecific variation in resistance of Eucalyptus globulus to mammaJian
herbivore browsing was investigated and the mechanistic and genetic basis of this
variation examined.
There was significant genetic variation in resistance of E. globulus to browsing by the
common brushtail possum (Trichosurus vulpecula) and the red-bellied pademelon
(Thylogale billardierii). Browsing damage was assessed on coppiced juvenile foliage of
trees of known genetic stock grown in a common environment field trial. There was
significant genetic variation in plant resistance amongst geographically distinct
populations at the race, locality and family levels. These results were confirmed in a
captive feeding trial where there was significant genetic variation in resistance among E.
globulus localities (populations).
The characteristics responsible for the intraspecific variation in resistance of juvenile
coppiced E. globulus foliage were then investigated. There was significant phenotypic
variation in foliage chemistry between individual plants. Sideroxylonals, a group of
formylated phloroglucinol compounds (FPCs), accounted for the greatest amount of the
observed variation in intake of foliage by T vulpecula. Variation in these compounds
explained the significant genetic based differences in browsing resistance between
different populations within E. globulus. These results may offer some evidence for the
potential importance of mammalian herbivores as selective agents on the evolution of
plant resistance within Eucalyptus.
Hybridisation in plants offers an ideal opportunity to investigate evolutionary processes
influencing plant/herbivore interactions. including, for example, the mode of inheritance
of plant resistance. The inheritance of resistance to browsing by T. vulpecula and the
inheritance of the defensive plant compounds of three year old juvenile E. globulus
foliage were investigated using known hybrid progeny (pure parental intra-race hybrids
and inter-race F1 hybrids) of two genetically differentiated geographical races (north
eastern Tasmanian race and south-eastern Tasmanian race) of E. globulus. The results
indicate that the F 1 hybrid was intermediate in resistance, compared to the parental
hybrids, suggesting an additive pattern of inheritance of browsing resistance to T.
vulpecula. In this foliage the concentration of FPCs was low and the combined effects of
condensed tannins and essential oils appeared to explain the pattern of resistance of
these hybrids.
Effects of variability in plant genotype and in environmental conditions within a
population can influence resistance of plants independently and/or can exhibit a
genotype-by-environmental interaction. The effects of environmental variability (two
nutrient treatments; high and low) on the genetic based expression of defensive
chemistry and resistance of two E. globulus genotypes (St. Helens and Blue Gum Hill)
to browsing by T. vulpecula was examined. While no significant independent genotype
and environmental effects on intake of E. globulus seedling foliage were evident, there
was a genotype-by-environment interaction. Genetic differences between the two
populations was only expressed under the low nutrient regime. Significant independent
effects of genotype and nutrient treatment were observed for primary and secondary leaf
Finally, the feasibility of using near infrared reflectance spectroscopy (NIR.S) as an
accurate tool to rapidly predict the nitrogen content and the resistance to browsing by T.
vulpecula of individual E. globulus trees was investigated. Two successful calibration
equations were developed (one for nitrogen and one for intake (resistance)) !hat met all
statistical requirements, suggesting that these models were useful in predicting nitrogen
content and resistance of new E. globulus trees. Independent validation of the nitrogen
calibration equation showed that the model was successful at predicting nitrogen content
of new E. globulus foliage. However, independent validation of the calibration equation
for intake was unsuccessful. We suggest that a change in the defensive chemical profile
across seasons, resulted in poor independent validation of the intake calibration model.
We discuss the importance of using independent validation when assessing the
predictive power of a calibration model and argue that with refinement in specific
systems, NIRS can be used as a powerful, rapid tool in predicting resistance of trees to
browsing herbivores.

Item Type: Thesis - PhD
Authors/Creators:O'Reilly-Wapstra, J
Keywords: Eucalyptus, Marsupials
Copyright Holders: The Author
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Copyright 2004 the author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2004. Includes bibliographical references

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