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Inbreeding in three forest eucalypts


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Hardner, CM 1996 , 'Inbreeding in three forest eucalypts', PhD thesis, University of Tasmania.

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In eucalypts, inbreeding may occur under open-pollination (OP) or when
effective population sizes are reduced by intensive selection. Breeding strategies
employing inbreeding have also been suggested. This Thesis examines inbreeding
in three widespread and commercially important temperate eucalypts: Eucalyptus
globulus, E. nitens and E. regnans.

Inbreeding depression was severe for growth traits in all species. Flowering
was also depressed in E. nitens, whereas pilodyn penetration and relative bark
thickness were unaffected. In a long term E. regnans study, the survival of inbred
progenies was significantly lower than outcrosses. However, the mortality of selfs
obscured an increase in inbreeding depression with age for growth traits. In most
traits examined, inbreeding depression was associated with levels of quantitative
dominance, however, some traits exhibited inbreeding depression but no
dominance, suggesting the action of rare alleles with major effect.

Inbred progenies were virtually absent from OP families from closed native E.
globulus forests, however, outcrossing rates were lower in families from isolated
parents. Biparental inbreeding may also be important under OP in native stands, as
mating nearest neighbour depressed growth equivalent to half-sib inbreeding.
However, the selection against selfs observed in E. regnans, suggested most
inbreds are eliminated by reproductive maturity thereby reducing the level of
inbreeding in natural populations and the ability to purge genetic load. The fitness
of parents under OP did not appear to be related to additive differences but may be
correlated with outcrossing rate. However, it is argued that mixed mating is
evolutionary stable, favoured by the reproductive ecology of the species.

Heritabilities estimated from OPs were greatly inflated compared to
heritabilities estimated from outcross progenies in E. regnans and E. nitens,
particularly for traits that exhibited inbreeding depression. A reduction in
outcrossing rate from 1.0 to 0.8 in E. globulus was associated with lower breeding
value estimates, suggesting variability among families in outcrossing rates may
inflate heritabilities estimates and bias breeding values. In the E. regnans study,
inflated heritabilities and additive variances estimated from OPs approximated
outcross parameters as inbreds were selected against. However, OP heritabilities
were also reduced as stand development enhanced of variation within OP families.
In addition, OPs overestimated age-age correlations and poorly predicted outcross

Mixed model methods were developed to account for dominance and
inbreeding, and were applied to self and outcross progenies of E. regnans.
Ignoring inbreeding depression biased breeding values for diameter, but dominance
was unimportant. Accounting for rare deleterious alleles improved the model,
however, breeding values predicted under a simple model were almost perfectly
correlated with those under a more complex model.

Item Type: Thesis - PhD
Authors/Creators:Hardner, CM
Keywords: Eucalyptus, Eucalyptus
Copyright Holders: The Author
Copyright Information:

Copyright 1996 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 (leaves 150-177)

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