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Molecular evidence for reticulate evolution in the Tasmanian eucalypts

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McKinnon, Gay Elizabeth (2003) Molecular evidence for reticulate evolution in the Tasmanian eucalypts. PhD thesis, University of Tasmania.

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Abstract

In 1971, eucalypt biologists Pryor and Johnson proposed that natural
hybridisation had played a significant role in the evolutionary history of the
Australian eucalypts. Their proposal was based on the fact that
interspecific F rhybrids and regions of intergradation between species are
quite common in Eucalyptus today, and contribute to difficulties in species
delimitation. The past movements of hybridising eucalypt taxa through
areas of contact, in response to climatic and environmental changes, may
therefore have enabled the interspecific exchange of genes and possibly
adaptations. This work uses a variety of phylogeographic markers to
investigate evolutionary processes, including interspecific gene flow, among
species of Eucalyptus subgenus Symphyomyrtus from the Australian island
of Tasmania. Since much of the evidence presented in this thesis relies on the rp12-trnHpsbA
marker region of chloroplast (cp) DNA, heritability of this region was
checked in 425 progeny of Eucalyptus, comprising 194 progeny of 25
different intraspecific crosses of Eucalyptus globulus, and 231 interspecific
hybrids (Fi , F2and backcrosses) between E. globulus and E. nitens. The
results supported the use of this marker to trace the maternal lineage.
An overview of cpDNA variability in Tasmanian species of section
Maidenaria was obtained by sequencing the rp12-trnH region for multiple
samples of all 17 species across their natural geographic ranges in Tasmania. The results showed that in Tasmanian Maidenaria, cpDNA
correlates with geography, far more than with taxonomy. Many widespread
species demonstrate intraspecific cpDNA variability, and this is coupled
with extensive haplotype sharing between species within regions. A history
of gene flow between species, in addition to shared ancestry, is the most
likely explanation. Overall haplotype diversity across all species is lowest in
central areas of Tasmania formerly occupied by alpine vegetation during
glacial intervals, and in northern regions that were periodically linked to
continental Australia by land bridges. The eastern region of Tasmania is
significantly richer in haplotype diversity, consistent with the former
existence of one or more glacial refugia. This finding is in agreement with
both climate modelling and patterns of endemism. To further investigate the hypothesis of gene flow among species, a detailed
phylogeographic analysis of two species was undertaken using cpDNA,
nuclear ribosomal DNA, and introns of the cinnamoyl coA reductase (CCR)
gene. The widespread forest tree, Eucalyptus globulus, and its rare
congener, Eucalyptus cordata, share a cpDNA lineage in their region of
sympatry in southern Tasmania. To clarify the reasons for cpDNA sharing,
mixed and allopatric populations of the two species were sampled across
their full geographic ranges. To distinguish between hybridisation and
lineage sorting, the analysis considered both the geographic distribution of
shared cpDNA haplotypes and their positions in the haplotype network for
each species. The results supported at least five occurrences of cpDNA
introgression from E. cord ata to E. globulus. Introgression appeared not to
be mediated by the co-occurring species Eucalyptus vimirtalis/dalrympleana/rubida. Unlike cpDNA haplotypes, nuclear
ribosomal DNA sequences showed local differentiation between species.
Data from the CCR gene were consistent with cpDNA data, although some
differences were noted. Southern Tasmanian populations of E. globulus
were differentiated from adjacent northern populations and had high levels
of a CCR haplotype that grouped with the dominant haplotype in E. cord ata.
Within southern Tasmania, levels of this haplotype were highest in
individuals with the same cpDNA lineage as E. cordata. However, this
result could not be unambiguously attributed to introgression, as opposed
to lineage sorting. This is the first use of a single copy nuclear gene to
investigate evolutionary processes in Eucalyptus, and highlights some
advantages and problems inherent in the use of nuclear genes for
phylogenetic and phylogeographic research.
The overall picture emerging from this research is that a complex
combination of historical factors has contributed to geographically
structured genetic variation in Tasmanian Eucalyptus. Despite the
complexity of the pattern, it has been possible to partially resolve it, and to
show that hybridisation is one of the contributing factors.

Item Type: Thesis (PhD)
Keywords: Eucalyptus, Transgenic plants, Hybridization
Copyright Holders: The Author
Copyright Information:

Copyright 2003 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, 2004. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:46
Last Modified: 29 Sep 2017 06:40
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