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Abstract

Interspecific hybridisation is potentially a very useful tool for tree breeders,
particularly as a source of variation for important genetic traits. However, hybrids in
several tree species have been shown to be highly susceptible to damage by some
pests. Therefore, the primary aim of this study was to determine if hybrids in
Eucalyptus were more susceptible to pests than pure species. To achieve this, the
responses of a number of vertebrate, invertebrate and fungal pests to hybrid eucalypts
and their parent species, were examined in several experimental field trials and in a
natural hybrid zone.

Firstly, susceptibility of hybrids to the fungal leaf disease caused by Mycosphaerella
spp. was examined in an experimental field trial, including first generation (F1)
hybrids and controlled crosses of E. globulus and E. nitens. Both the host preference
and genetic variation in resistance were investigated. Heritabilites were low to
moderate and disease severity was greatest on the F1 hybrids. Hybrids were more
susceptible to damage by Mycosphaerella than both E. globulus and E. nitens.

Secondly, the host species preference and hybrid susceptibility was examined for
chrysomelid leaf beetles (Chrysophtharta spp. and Paropsis spp.), the gum leaf
skeletoniser (Uraba lugens) and for brush tail possums (Trichosurus vulpecula) in a
number of experimental hybrid trials. Heritability estimates were very low for possum
damage on E. globulus, and high for E. nitens. The proportion of dominance
variation for possum damage was consistantly higher than heritability estimates in E.
globulus, and at least as high as heritability estimates in E. nitens. E. morrisbyi and
E. gunnii consistently had the most possum damage and E. globulus, E. nitens and E.
johnstonii the least. Responses of the pest species to the different hosts and hybrids
varied but hybrids were generally intermediate in their susceptibility to the different
herbivores when compared with their parent species.

Thirdly, this thesis examines the host species preference of a number of insect taxa
and the host susceptibility of E. amygdalina, E. risdonii and E. amygdalina x E.
risdonii hybrids. In a previous study by Whitham et al. (1995) in a natural hybrid
zone, hybrids were found to be more susceptible to both insect and fungal taxa than
either E. amygdalina or E. risdonii . The genetic basis of this observed hybrid
susceptibility was examined by determining the distribution of a number of insect taxa
in an experimental field trial, where the pedigree of the majority of hybrids and pure
species was known. Species richness was found to be greater on hybrids than pure
parent species. Furthermore, F1 eucalypt hybrids tended to be more susceptible to
attack than advanced generation hybrids, arguing against hybrid breakdown being the cause of the greater susceptibility of the hybrids. Mechanisms contributing to the
observed responses on the E. amygdalina x E. risdonii hybrids were also examined.
Leaf toughness and the oil content and composition of the parent species and hybrids
was determined and the effect of the different oil components on the distribution of the
insect taxa was discussed.
Finally, the cause of preferential weevil damage on hybrids between E. amygdalina
and E. risdonii in a field trial was examined. While other factors such as larval
survival and egg loss were examined, oviposition by the eucalypt weevil Gonipterus
scutellatus was highly biased towards the hybrid phenotypes. This lead to much
larger larval numbers on hybrids and was therefore the primary factor contributing to
the higher damage observed on hybrids in the field trial.

Results are discussed in terms of the susceptibility of hybrids and their usefulness in
temperate hardwood forestry. The possible nature of hybrid susceptibility in
Eucalyptus is also examined.

Item Type:	Thesis - PhD
Authors/Creators:	Dungey, HS
Keywords:	Eucalyptus, Eucalyptus, Eucalyptus
Copyright Holders:	The Author
Copyright Information:	Copyright 1997 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 would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references
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