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Justin Bloomfield (right) and supervisor Dr
Dorothy Steane select mature stringy barks for the study.
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Justin Bloomfield
University of Tasmania
In February, Justin
Bloomfield (left) was awarded Honours in Biotechnology at the
University of Tasmania for
his work on genetic diversity in Eucalyptus obliqua in
Tasmania. Justin is presently “filling some gaps”
in the sampling, and when the complete data set has been reanalysed
the results will be prepared for publication. Below is a
brief summary of Justin’s work.
Eucalyptus obliqua (Figure 1), commonly known in Tasmania
as “stringy bark” - and on mainland Australia as
“messmate” - is widespread in south-eastern
Australia. In Tasmania it is actively harvested from native
production forests by a variety of means. Most harvesting of
E. obliqua is done using "partial" methods: aggregated
retention in wet forests and typically a combination of seed tree
retention, potential sawlog retention and/or advanced growth
retention in dry forests. Less than half of the harvesting of
E. obliqua involves a clearfell, burn and sow silviculture
regime. To maintain species patterns and to conserve local
gene pools whilst continuing timber harvesting, Forestry
Tasmania has developed and adopted a protocol for regenerating
areas that have been clear-felled, using aerial sowing of seed
after the coupes have been burnt. Tasmania has been divided
into “seed zones” on the basis of environmental
variables such as geology and rainfall.
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Figure 1. A fine specimen of a mature stringy
bark at the Lune River "plain" population.
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The regeneration guidelines require that, wherever possible, seed
should be collected on-site prior to harvesting and used to resow
the site. When there is insufficient seed on site, transfer
of seed should be within a seed zone and, if no seed is available
from a particular zone, there are guidelines for the transfer of
seed between similar seed zones. When the seed zones were
defined and last reviewed, information on
E. obliqua
regarding genetic diversity within seed zones and relationships of
populations across seed zones was not available. Forestry
Tasmania was interested to know whether genetic information on
native
E. obliqua populations would have implications for
changing the guidelines in order to maintain the spatial integrity
of the genetic variation in the species.
A study done in the 1990s by Graham Wilkinson (Wilkinson 2008; see
article
from Biobuzz 5) found genetic variation in quantitative traits
between
E. obliqua trees from very different environments
in close proximity to one another. For example, Wilkinson
collected seed from two sites at Lune River in Tasmania’s
south. One population was on a well-drained slope in wet
sclerophyll forest; the other, just 180 – 620 m away, was on
an exposed, wet plain at the bottom of the slope. Another two
sites were selected at Forestier Peninsula, on Tasmania’s
south east coast. Wilkinson planted all seed lots in a common
garden at each of the four sites. Over 45 months, he measured
a range of quantitative traits and found that there were
significant phenotypic differences between the progeny from the
four localities. For one part of his study, Justin resampled,
as closely as possible, Wilkinson’s parent trees (some
adjustments were made because of harvesting in some areas).
Justin wanted to test whether the variation in quantitative traits
that Wilkinson observed between populations in close proximity to
one another was the direct result of natural selection and
adaptation in the face of ongoing gene flow between localities, or
whether random genetic drift due to genetic isolation of
populations was involved.
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Figure 2. Tasmanian E. obliqua
distribution map overlayed with the locations of the populations
sampled in this study. Lune River, Forestier and Mount Lofty
had two populations sampled at each location. The distribution map
indicates whether E. obliqua is present (black dots) or
absent (no dots) within a 10 km square grid (Williams and Potts
1996).
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In his study, Justin sampled 300 trees from 14 populations
across the species’ range in Tasmania (see Figure 2).
The populations included Wilkinson’s four sites at Lune River
and Forestier Peninsula. Justin fingerprinted all the trees
using seven microsatellite loci. Microsatellites are short
regions of repetitive DNA - eg, ACACACACAC – that tend to be
highly variable because, during replication, DNA synthesising
enzymes tend to get a bit muddled with all the repetition and
sometimes accidentally add in or cut out one or more repeats,
thereby producing different sized microsatellites in different
lineages. Microsatellite markers are generally assumed to be
selectively neutral, so when we analyse these data we are examining
the underlying neutral genetic diversity rather than diversity that
results from selection.
From the fingerprint data, Justin was able to work out
population-level diversity statistics. He found that all the
populations had similar and reasonably high levels of genetic
diversity (expected heterozygosity, H
e = 0.79; observed
heterozygosity, H
o = 0.78). There was very little
differentiation
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Figure 3. Relationship between geographic
distance and Nei’s (1972) genetic distance among 14 Tasmanian
E. obliqua populations. r2 indicates
strength of relationship between the two distance measures.
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between populations, suggesting that there is gene flow (eg,
through pollen and/or seed movement) between the populations that
were sampled. With respect to Wilkinson’s populations,
there was no significant differentiation in microsatellite profiles
between the paired sites. This means that the quantitative
variation observed by Wilkinson is likely to be a result of
environmental selection and adaptation in the face of ongoing gene
flow between phenotypically different populations. There are
no obvious genetic isolation mechanisms operating between the
adjacent populations. This is in stark contrast to the
results of
Foster et
al. (2007) who found that dwarf populations of
E.
globulus growing adjacent to tall populations of the same
species are highly differentiated in their microsatellite profiles
and genetically isolated by differences in flowering time.
On a larger scale, over the whole of Tasmania, there was a subtle -
but significant (
r2 = 0.41) - pattern of
differentiation of populations that formed a classic
“isolation by distance” trend (Figure 3). This
means that the further apart two populations are, the less related
they tend to be (and
vice versa). This sort of trend
is often seen in species with large continuous distributions.
There was also higher “allelic richness” in the east
than in the west (Figure 4).
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Figure 4. Allelic richness across seven
microsatellite loci in 14 Tasmanian E. obliqua
populations. Yellow circles indicate higher than average allelic
richness in a population; red triangles represent below average
allelic richness in a population. Average allelic richness across
all populations is 10.3.
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Allelic richness is a sensitive measure of genetic
diversity, so in E. obliqua there tends to be more genetic
diversity in the eastern regions than further towards the
west. Hence, inland areas of Tasmania may have been colonised
from the east (eg, during the last glacial maximum inland areas
were too cold to support forest trees) and areas on the west coast
may have experienced some loss of genetic diversity due to
isolation or because they were colonised last. Eastern
Tasmania is, thus, an important zone for the conservation of
genetic diversity in the species.
Returning to the issue of seed zones and seed transfer guidelines,
can we comment on whether Forestry Tasmania has got them
right? From our data so far it would appear that in E.
obliqua some of the phenotypic variation that we observe
between populations is a result of environmental selection and
adaptation in the face of gene flow. We observed that there
is more genetic diversity in eastern populations and we are hoping
to confirm this with more evidence from increased sampling.
Seed transfer guidelines recommend minimising the distance and
environmental gradients across which seed is transferred; at this
stage we can not recommend any improvements to this
strategy.
Biobuzz issue eight, March 2009
