Justin A Bloomfield1, Paul Nevill2,3,
René E Vaillancourt1, Dorothy A
Steane1,4, Brad M Potts1
1School of Plant Science and Cooperative Research Centre
for Forestry, University of Tasmania, Private Bag 55, Hobart,
Tasmania 7001, Australia
2School of Forest and Ecosystem Science and Cooperative
Research Centre for Forestry, University of Melbourne, Parkville,
Victoria 3010, Australia
3Current address: Botanic Gardens and Parks Authority,
Kings Park and Botanic Gardens and School of Plant Biology, The
University of Western Australia, Western Australia, Australia
4Author for correspondence: Dorothy.Steane@utas.edu.au
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Figure 1. This statistical parsimony tree shows
relationships between chloroplast haplotypes found in E.
obliqua in Tasmania. The size of a circle represents the
relative frequency of a haplotype; each haplotype is represented by
a unique colour; haplotype numbers correspond to haplotypes
identified by Nevill et al. (unpubl.; Haplotypes 19 - 42)
and new haplotypes determined in this study (Haplotypes 45 - 55);
each line joining two haplotypes indicates a putative cpDNA
mutation event (i.e., the insertion or deletion of a single
basepair at a microsatellite locus).
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Eucalyptus obliqua dominates much of the wet sclerophyll
forest managed for forestry purposes by Forestry Tasmania (Neyland
et al. 2009). Clearfelling followed by high intensity
burning and aerial sowing has been the dominant silvicultural
system used for
E. obliqua forests in Tasmania since the
1960s (Hickey and Wilkinson 1999). However, aggregated
retention systems, where patches of intact forest are retained
within a harvested coupe, have been adopted recently as standard
practice for
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Figure 2. Nineteen chloroplast haplotypes were
found in 65 Tasmanian E. obliqua populations. Pie charts
indicate the haplotype composition of each sampled population;
regions demarcated by green lines are Forestry Tasmania's seed
zones. Within the black line there is no putative ancestral
haplotype (Haplotype 20) and the region is dominated by Haplotype
24 (red); the broken line indicates that the suggested boundary
defining this region is bisecting seed zones; the round dotted line
in the south demarcates Forestier Peninsula which is dominated by
Haplotype 30.
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harvesting (retaining contiguous and/or free standing patches) in
the majority of old growth wet
Eucalyptus forests on State
Forest land (Forestry Tasmania 2009; Neyland
et al.
2009). Regardless of silvicultural system, coupes are artificially
re-sown, usually using seed collected on site before harvesting.
However, in cases where this is not possible, Forestry Tasmania has
developed a seed zone system based on geography and environmental
attributes (e.g. altitude, rainfall and geology) to guide off-site
seed transfer (Forestry Tasmania 2010). When seed is not available
from within the same seed zone, the emphasis is on transferring
seed from another seed zone that has similar environmental
attributes, to obtain seed as closely genetically adapted to the
target environment as possible (Forestry Tasmania 2010).
We have been studying the patterns of molecular genetic variation
in
E. obliqua to determine whether there are further
considerations which need to be taken into account to better
conserve local gene pools and patterns of diversity that exist in
native forest tree gene pools across Tasmania. Reciprocal field
trials with
E. obliqua have identified local
differentiation in adaptive traits occurring at a fine scale (e.g.
within coupe or seed zone) (Wilkinson 2008; Strich 2006). However,
despite the phenotypic diversity observed in
E. obliqua
(Nicolle 2006), our recent molecular study based on putatively
neutral nuclear microsatellites (which reflect combined seed and
pollen dispersal) showed very little broad-scale differentiation
across Tasmania (Bloomfield
et al. 2011b). This result
suggests that the seed zone system is conservative for
E.
obliqua and environmental matching is likely to be the key
consideration for seed transfer. However, there was a limited
degree of spatial structure evident in the chloroplast DNA (cpDNA),
which may reflect factors such as historic migration routes or
refugia. This genetic structuring can be overlaid on the seed
zone system to better maintain the natural pattern of genetic
variation in
E. obliqua. CpDNA often shows more
spatial structuring than nuclear DNA because it is haploid (does
not undergo recombination), maternally inherited and dispersed only
by seed. We increased the number of populations in our cpDNA survey
so that we could refine the delineation of the spatial structure
observed in the cpDNA in
E. obliqua, so this new layer of
information could be integrated into the Forestry Tasmania seed
transfer guidelines.
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Figure 3. Contour map of chloroplast DNA
haplotype richness in E. obliqua across Tasmania.
Haplotype richness was calculated using FSTAT (Goudet 2001).
Individual values indicate haplotype richness per population
(rarefied to a sample size of four); a value of 1.0 indicates that
a population is fixed for a single haplotype. Red colouring
on the map indicates regions of relatively high haplotype richness
(i.e., more haplotypes per region), while blue indicates regions of
lower haplotype richness.
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An additional 34 populations of E. obliqua were sampled
from throughout Tasmania and their cpDNA microsatellite data
combined with that of Bloomfield et al. (2011b). In
total, 5 – 10 individuals from 65 populations have now been
analysed using cpDNA microsatellites, producing 19 unique cpDNA
haplotypes in Tasmanian E. obliqua (Table 1). The
statistical parsimony tree (Figure 1) displays the relationships
among the 19 Tasmanian cpDNA haplotypes identified. While nuclear
DNA microsatellites showed little spatial structure, presumably
because pollen flow is extensive in this species (Bloomfield et
al. 2011b), some structure was evident in the cpDNA (Figure
2). We found an area in central eastern Tasmania, also
encompassing the Tasman Peninsula, which lacks the putative
ancestral haplotype (blue haplotype, haplotype 20; see outlined
area in Figure 2). The suggested boundary drawn in Figure 2 to
enclose this area includes many of the eastern Tasmanian seed zones
but may bisect up to three seed zones (broken line in Figure 2).
Furthermore, the Forestier Peninsula (see round-dotted line in
Figure 2) has a high frequency of a well-derived haplotype
(Haplotype 30) which is not found anywhere else and is fixed in two
populations. Other similarly derived haplotypes (Haplotypes 52 and
53) are only found in single populations and are not fixed in their
respective populations.
The areas of greatest haplotype richness for Tasmanian E.
obliqua are in the south-east, north and north-east, where
(except for the Launceston area) glacial refugia have been
postulated (Figure 3). Furthermore, an area characterised by
distinct haplotypes was found in the central-east. While trials
have not revealed any adverse fitness effects of transferring seed
across seed zones of similar environment (e.g. Forestier Peninsula
and Lune River - Wilkinson 2008; Strich 2006), the natural patterns
of genetic variation in E. obliqua would be better
preserved with an additional guideline to prioritise seed zone
transfers between similar environments within the same major
chloroplast DNA regions as indicated in Figure 2 (as defined by
primarily haplotypes 24 and secondarily haplotype 30).
Acknowledgements
We wish to thank David McElwee for assistance with field sampling,
Sasha Wise for assistance with laboratory work and Mark Neyland and
Lachie Clark for helpful discussion.
References
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Hobart, Tasmania, 21-25 November 2011.
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Biobuzz issue fifteen, December 2011
