In Borralho, N., et al. (2004). Eucalyptus in a Changing World. Proc. of IUFRO Conf., Aveiro 11-15 October 2004
EXPLORATION OF THE EUCALYPTUS GLOBULUS GENE POOL
Brad M. Potts1,2, René E. Vaillancourt1,2, Greg Jordan1,2, Greg Dutkowski1,2, João da Costa
e Silva1,3, Gay McKinnon2, Dorothy Steane1,2, Peter Volker1,4, Gustavo Lopez1,5, Luis
Apiolaza1,4, Yongjun Li1,6, Cristina Marques7 and Nuno Borralho8
1CRC for Sustainable Production Forestry,
2School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Tasmania, Australia
3Universidade Técnica de Lisboa, Instituto Superior de Agronomia, Departamento de Engenharia
Florestal, Centro de Estudos Florestais, Tapada da Ajuda, 1349-017 Lisboa, Portugal
4Forestry Tasmania, GPO Box 207, Hobart, Tasmania 7001, Australia
5present address: Centro de Investigación y Tecnología de ENCE, Ctra. Madrid-Huelva km. 630 - Ap. 223
- 21080 Huelva, España
6present address: Statistical Animal Genetics Group, Institute of Animal Science, Swiss Federal Institute of
Technology (ETH) ETH Zentrum (UNS D 5) CH 8092 Zurich Switzerland
7 RAIZ-Direcçao de Investigacao Florestal, ITQB II Av. Republica, Apartado 127
2781-901 Oeiras, Portugal
8RAIZ, Instituto da Floresta e Papel, Quinta de S. Francisco, Ap 15, 3801-501 Eixo, Portugal
The first Europeans to discover Eucalyptus
globulus were French explorers in 1792. Its seed
was rapidly spread throughout the world in the
19th century and this was the species by which
much of the world first knew the genus.
However, it was in the industrial forests of the
20th century that this species, once considered
the ‘Prince of Eucalypts’, achieved greatest
prominence due to its fast growth and superior
pulp qualities. Formal breeding first commenced
in 1966 in Portugal and in the late 1980’s large
base population trials from open-pollinated seed
collections from native stands were established
in many countries. These trials have provided
unprecedented insights into the quantitative
genetic control of numerous traits of economic
and ecological importance and how this variation
is spatially distributed in the native range of the
species. However with large, fully pedigreed
breeding populations becoming available for
quantitative analysis and the rapidly expanding
knowledge of DNA sequence variation, we are
now at the threshold of a new understanding of
this important eucalypt gene pool. Indications of
the significance of non-additive genetic effects
are becoming available. The E. globulus
chloroplast genome has now been sequenced
and several genome maps have been published.
Studies of the variation in nuclear microsatellites
and the lignin biosynthesis gene CCR confirm
the complex, spatially structured nature of the
native gene pool. Strong spatial structuring of
the chloroplast genome has provided a tool for
tracking seed migration and the geographic
origin of exotic landraces. Highly divergent
lineages of chloroplast DNA have been
discovered and studies of the hypervariable JLA+
region argue that some components of the E.
globulus gene pool have been assimilated from
other species following hybridisation.
Eucalyptus globulus Labill. (Tasmanian blue
gum) is one of the most important pulpwood
plantation species in the world (Eldridge et al.
1993; Potts 2004). It is a forest tree with a native
range on the island of Tasmania, the Bass Strait
Islands and adjacent coastal regions of Victoria
on continental Australia (Jordan et al. 1993;
Dutkowski and Potts 1999; Fig. 1). It is one of
four closely related taxa, which are variously
given specific taxonomic status (Eucalyptus
globulus, E. bicostata, E. pseudoglobulus and E.
maidenii - Brooker 2000) or treated as
subspecies of E. globulus (Kirkpatrick 1974;
Pryor and Johnson 1971). These taxa are herein
treated at the specific level (Brooker 2000). The
cores of these four taxa are morphologically and
geographically distinct, but linked by
morphologically and geographically intermediate
(intergrade) populations (Kirkpatrick 1974,
1975a; Jordan et al. 1993; Jones et al. 2002a;
Eucalyptus globulus was discovered on the
island of Tasmania (Recherche Bay; Fig. 2) in
1792 by French explorers and was one of the
first eucalypt species to be formally described
(Labillardière 1799). The primeval eucalypt
forests of Tasmania were amongst the tallest
forests in the world and E. globulus trees up to
101 m in height were recorded (Lewin 1906;
Hickey et al. 2000). The proximity of native E.
globulus to the coast ensured rapid use and
knowledge of its timber by the early explorers
and colonists (Potts and Reid 2003). The high-
density timber was durable and resistant to the
destructive Teredo sea-worm and it became
prized for marine construction, including
shipbuilding (Lawson 1949). By the late 1800’s,
trees 60-90 m high were regularly harvested
from southeastern Tasmania and shipped
throughout the world for wharf piles (100-120
feet in length and 20 inches square) (Lewin
1906). By 1905, four million feet of wharf piles
had been supplied for British Admiralty
contracts, acknowledged as ‘….the longest and
biggest and the most durable that had ever
reached British shores’ (Lewin 1906). The timber
was also in great demand for railway sleepers,
street paving blocks and mine supports.
However, while re-growth of this species was
common, by 1925 the large mature timber trees
of E. globulus were scarce (Irby 1925) and
clearing for agricultural purposes in the late 19th
Century had substantially reduced the natural
distribution of E. globulus in Victoria (e.g.
Gippsland) and on King Island.
Following its discovery, Eucalyptus globulus
seed was rapidly spread throughout the world. It
was the first eucalypt to be extensively planted
and known outside of Australia (Jacobs 1981;
Doughty 2000). By the end of the 19th Century it
had been introduced to France, Chile, South
Africa, Portugal, Italy, India, Spain, USA,
Uruguay, Algeria, Tunisia, Argentina, Peru,
Ecuador, Zimbabwe, China, Ethiopia and Bolivia
(Table 1). In most countries, it is likely that many
independent seed introductions occurred
throughout the 19th century, either directly from
native Australian sources or from other exotic
sources (Zacharin 1978). Much Australian seed
was distributed by Ferdinand von Mueller, the
director of the Royal Botanical Gardens in
Melbourne, who considered E. globulus the
“Prince of Eucalypts” and championed its
introduction worldwide (Doughty 2000 p. 51; Hay
2002). However, France is believed to have
been a key secondary distribution point
(Zacharin 1978), and the species has been
cultivated near major Mediterranean ports such
as Toulon since 1813 (Doughty 2000), for
example, seed for the 1863 planting of E.
globulus near Santander in Spain was taken
from the Iles de Hyeres, near Toulon (Orme
1977). The Frenchman, Prosper Ramel, played
a major role in establishing E. globulus
plantations in southern Europe and North Africa
Figure 1. Distribution of core and intergrade
populations of E. globulus, E. bicostata, E.
pseudoglobulus and E. maidenii
Populations are classified based on
morphological analysis of their capsule
morphology (Jordan et al. 1993). Capsule
size, shape and number per umbel are the
main taxonomic features separating the four
species. Eucalyptus maidenii has up to seven
capsules per umbel and the smallest
capsules. E. globulus has solitary fruit and the
largest capsules. Both E. bicostata and E.
pseudoglobulus are three fruited, but E.
pseudoglobulus has smaller capsules and
longer pedicels than E. bicostata. There is
more or less continuous variation in capsule
morphology between the cores. The
populations sampled in the 1987/88 CSIRO
collections were spread throughout the
distribution of core E. globulus and E.
globulus intergrades (modified from Jordan et
Table 1. The spread of E. globulus as an exotic during the 19th Century
The date of first recorded introduction (1st record) into each country, the estimated area (ha)
planted to E. globulus and the number of major breeding programs
record Country Comments Area (ha)date
native Australia Mainly established since 1995 349,7962004 2
France Probably from the 1800 Baudin expedition which visited
Tasmania (Zacharin 1978). By 1810 trees of this species are
recorded at Château Malmaison (Penfold and Willis 1961, see
also Duyker 2003 p. 233)
1823 Chile 27 plants of unknown origin intended for Peru were planted
between Valparaiso and Arauco, more genetic material came
from France in 1838 (Andrade and Vecchi 1920) and 1865
9 plants from Australian seed raised in Mauritius were planted in
the Governor’s garden at Cape Town. Once the most ubiquitous
exotic tree, pest and disease problems, particularly Gonipterus
scutellatus, resulted in planting virtually ceasing by the 1940’s.
Most of the 1,016 ha estate at that time is believed to have
descended from the original 9 trees (Poynton 1979)
1829 Portugal Villa Nova de Gaia near Oporto Zacharin 1978), main plantations
probably derived from Australian introductions from mid to late
1829* Italy Possibly even earlier as plants growing in the Camalduli Gardens
near Naples and described in 1829 as E. gigantea by Dehnhardt
were later identified by von Mueller as E. globulus (Zacharin
1843 India Nilgiri Hills, followed by importation of large quantities of seed for
many years (Zacharin 1978) NA 1
Record of a single ornamental planting at Nelson Zacharin 1978,
but probably earlier introductions. Plantings ceased by 1960’s
due to insect pests
1860* Spain Possibly as early as 1847 (Zacharin 1978) 500,0002002 1
1853* USA Suisan Valley near San Fransisco, California (Helms 1988) NA
1853 Uruguay Initial seed from Cape Town, South Africa (Brussa 1994; Cardozo
et al. 2003) 200,0002004 3
1854 Algeria Seed from France through Prosper Ramel 30,0001977
1854 Tunisia Seed from France through Prosper Ramel 30,0001977
1857 Argentina Plantings on the farms of Don Leonarde Pereyre Iraola (Zacharin
1978) 17,0002004 1
1860 Peru In the Sierras NA
1865 Ecuador Introduced for reforestation of elevate windswept plateaux 31,8631994
1872 Indonesia Experimental plantings on Mt Sindoro grew well, as did later
plantings on Mt Perahu (1700-2300m) (Zacharin 1978) NA
1890 Zimbabwe NA
1890 China Possibly even earlier as a large 'blue gum' was noted in the
grounds of the French legation in Kunming in 1894 (Morrison
1895). 1894-96 introductions to South China from Italy probably
included E. globulus (Zacharin 1978)
1895 Ethiopia Near Addis Ababa, via France (Zacharin 1978), mainly for fuel
wood and charcoal (Pohjonen and Pukkala 1990) 100,0001990 1
1900 Bolivia Seed from Argentina NA
NA = not available * not from Doughty (2000)
The spread of E. globulus around the world was
aided by the unpalatability of its juvenile foliage
to cattle, sheep and goats (Jacobs 1981).
Eucalyptus globulus was also believed to reduce
the occurrence of diseases such as malaria by
helping to dry out swampy soils and through its
leaves producing beneficial volatile chemicals
(Doughty 2000). It became known as the ‘fever
tree’ and in the latter half of the 19th century
millions of trees were planted to combat malaria
in many countries (e.g. Tre Fontaine Monastery
near Rome, Italy) (Stanford 1970; Zacharin
1978). Simultaneously, cultivation of E. globulus
for timber, fuel and oil production expanded
rapidly in many parts of the world. Its spread into
California, for example, occurred during the gold
rush where it was widely promoted by
Government agencies and private investors as a
source of construction timber and fuel (Santos
1997; Doughty 2000). However, by the
beginning of the 20th century warping and
splitting problems in the exotic-grown timber
greatly reduced interest in its use for
construction. Nevertheless, younger trees were
used for firewood, charcoal and mine props. The
latter was one of the main reasons for the early
plantings in Chile (near Lota in the VIII Region,
By the mid to late 1800’s, E. globulus plantings
were well established in southern Europe and
Northern Africa, Chile, California and India
(Andrade and Vecchi 1920; Penfold and Willis
1961; Jacobs 1981). The modern development
of Ethiopia and creation of its capital Addis
Ababa has been directly attributed to the
successful introduction of E. globulus in 1894-95
(Penfold and Willis 1961; Pohjonen and Pukkala
1990). This is one of the great successes of rural
afforestation and E. globulus is now integral to
the life of most Ethiopians around Addis Ababa
as a source of fuel wood, charcoal, poles for
house construction and numerous other
products. However, the industrial plantations of
the late 20th century, established to feed
voracious pulp and paper markets, brought E.
globulus to its greatest prominence. No sector of
world forestry has expanded as rapidly as this
industrial use of eucalypts (Turnbull 1999) and
the eucalypt kraft pulp industry is dominated by
only two species. Eucalyptus globulus is the
premier species (Cotterill et al. 1999;
Grattapaglia 2003; Villena 2003) and favoured in
temperate regions, whereas E. grandis and its
hybrids are used in the subtropics and tropics.
While exact figures are difficult to obtain, it is
estimated that 800,000 hectares had been
planted with E. globulus by 1973 (Poynton
1979), 1.7 million ha in 1995 (Tibbits et al.
1997), and planting rates in the last decade
would suggest that it may now exceed 2.5
million ha. Countries where E. globulus is grown
include Australia, Chile, China, Columbia,
Ethiopia, India, Peru, Portugal, Spain, USA and
Uruguay (Eldridge et al. 1993). The Iberian
Peninsula has the main concentration of
plantations. It’s the main eucalypt species
cultivated in Portugal, with the plantation estate
estimated at c. 60,000 ha by 1961 (Penfold and
Willis 1961), 430,000 ha by 1988 (Eldridge et al.
1993), 550,000 ha by 1997 (Tibbits et al. 1997)
and about 700,000 in 2002 (Toval Hernandez
2002). Spain has a similar estate, estimated at
390,000 ha in 1988 (Eldridge et al. 1993) and
500,000 ha in 2002 (Toval Hernandez 2002).
The E. globulus estate in Chile was estimated at
44,561 ha in 1960 (Penfold and Willis 1961) and
232,000 ha by 2003 (Raga 2001). Australia has
an estimated 349,796 ha of E. globulus
plantation, most of which has been established
since 1995 (National Forest Inventory 2004).
Despite the early spread and utilization of E.
globulus around the world, it took more than 150
years before formal domestication (provenance
testing and breeding) started. By this stage,
landraces adapted to their exotic environments
had developed on most continents (Eldridge et
al. 1993), no doubt under a combination of
natural (e.g. adaptation to harsher frosts -
Almeida et al. 1995 and drought - Toro et al.
1998) and artificial (e.g. improved form - Lopez
et al. 2001a) selection. Some of these landraces
are thought to have originated from a narrow
genetic base (Eldridge et al. 1993), which for
example may have contributed to the poor
performance of E. globulus in South Africa
(Table 1; Poynton 1979; Gardner et al. 2003).
Through the 1960’s and 70’s, small experimental
trials of Australian provenances of E. globulus
were undertaken in many countries for species
selection, increasing interest in the species for
industrial pulpwood plantations (e.g. Orme 1977;
Eldridge et al. 1993). Formal breeding first
commenced in 1966 in Portugal using
phenotypic selections from local, landrace
plantations (Dillner et al. 1971; CELBI 1979a,
1979b). By 1973, an extensive genecological
study of natural variation in the Eucalyptus
globulus complex had been completed
(Kirkpatrick 1975a; Kirkpatrick 1975b), followed
in 1975-76 by the first major native stand seed
collection for provenance testing for forestry
(‘Orme collection’ - Orme 1977). The Orme
collection encompassed provenances from all
four species in the complex, and was planted in
more than 20 trials in Australia, Colombia,
Portugal, Spain, Uruguay and USA between
1977 and 1985 (Orme 1988; Eldridge et al.
1993; Almeida et al. 1995; Barbour and Butcher
1996). By the late 1980’s, growth and wood
property results from bulk provenance and family
trials established from this collection in Australia
and overseas had clearly focused temperate
zone breeders on E. globulus and its intergrade
populations (results are reviewed in Eldridge et
al. 1993). Local landrace provenances of E.
globulus included in these trials did not
necessarily grow better than at least some of the
native stand provenances, and there was no
clear evidence of major differences in growth
rate between the native Australia provenances.
With increasing worldwide interest in breeding E.
globulus for pulpwood plantations, the largest
ever native-stand seed collection of E. globulus
and intergrade populations (in the Otways and
Strzelecki Ranges) was undertaken in 1987 and
1988 by the Australian Tree Seed Centre
(ATSC) of CSIRO in collaboration with
Australian and overseas forestry companies
(616 parent trees from 49 collecting localities -
Gardiner and Crawford 1987, 1988; Jordan et al.
1993; Dutkowski and Potts 1999). While studies
of the Orme collection continued through the
1990’s (Volker et al. 1990; Almeida et al. 1995;
Kube et al. 1995), particularly for wood
properties (Miranda and Almeida 2001, Miranda
et al. 2001b, a, Miranda and Pereira 2001,
Miranda and Pereira 2002), the main focus of
breeding programs rapidly turned to exploiting
variation in this new collection. This collection
was planted as family lots in over 50 progeny
trials worldwide, including Australia (Jarvis et al.
1995), Chile (Sanhueza and Griffin 2001; Infante
and Prado 1989, 1991, Ipinza et al. 1994), China
(Zang et al. 1995), Ethiopia (Gizachew 2002),
Portugal (Araújo et al. 1996) and Spain (Vega
Alonso et al. 1994; Soria and Borralho 1998;
Toro et al. 1998) and formed the base (e.g.
Jarvis et al. 1995) or a key fusion population
(e.g. Griffin 2001) for many breeding programs.
Several other native stand seed collections have
been undertaken since. However, these have
been smaller and targeted specific areas. There
are now active breeding programs for this
species in at least 7 countries (Table 1), many of
which are 2 or more generations from the native
or landrace population. While most countries
with well developed landraces have based their
original breeding population on local landrace
selections (e.g. India, Venkatesan et al. 1984;
Portugal, Dillner et al. 1971), many are infusing
into it new material from more recent Australian
collections (e.g. Griffin 2001).
THE GENE POOL
THE QUANTITATIVE GENETICS VIEW
The base population trials established from
native stand seed collections have provided
invaluable insights into the genetic control of
numerous traits of economic and adaptive value
and how the genetic variation affecting these
quantitative traits is structured in nature. The
large CSIRO seed collections of 1987 and 1988
provide the best perspective on the geographic
patterns of quantitative genetic variation in E.
globulus. Many trials only contain seedlots from
one of these collections. However, our studies
have focused one five trials established by
APPM Forest Products (now Gunns Ltd) in
northern Tasmania that combine both
collections. These trials were established in
1989 across a range of planting environments,
using randomized incomplete block designs (5
replicates, 21-28 incomplete blocks per
replicate, 2-tree plots). They contain between
450 and 596 open-pollinated families (Jordan et
al. 1994; Dutkowski and Potts 1999).
Spatial structure of the gene pool: Our studies of
these five trials show highly significant, often
independent and spatially structured,
quantitative genetic differentiation between the
49 sampling localities in virtually all traits
examined (Table 2). Maps of the broad-scale
distribution of genetic diversity in key economic
and biological traits are now available (see
references in Table 2). The overall pattern of
genetic variation amongst localities, in what
could be viewed as mainly adaptive traits, has
been summarized by classification of the E.
globulus gene pool into a hierarchy of 13 races
and 20 subraces (Fig. 2; Dutkowski and Potts
1999). This classification includes
morphologically intermediate populations (i.e.
the E. globulus intergrades - Fig. 1) and
accounts for a significant component of the
genetic variation in these trials (Fig. 3). While the
genetic variation in growth and survival exhibited
weak spatial structuring, there were clear
regional patterns in bark thickness, wood
density, flowering precocity and leaf morphology
(Table 2; Dutkowski and Potts 1999). There
were few simple correlations with climatic
variables, although multivariate analyses
indicated a major latitudinal cline differentiating
King Island and intergrade populations on
continental Australia and western Tasmania
from core E. globulus populations in eastern
Tasmania. This cline is paralleled by a
genetically based trend for flowering to occur
earlier in the growing season in eastern
Tasmania and on the Furneaux Group of islands
(Gore and Potts 1995; Apiolaza et al. 2001).
However, there was marked continuous variation
within these major groups. There is an east-west
cline in bark thickness and drought tolerance in
the Otway Ranges coincident with a decline in
rainfall. A major north-south cline within the
continuous populations of core E. globulus on
the east coast of Tasmania suggests adaptation
to some environmental change, but without a
simple association with macro-climatic variables.
Bark thickness increases in northern populations
(Dutkowski and Potts 1999), as does drought
tolerance (Dutkowski 1995a; Toro et al. 1998).
There is also an increase in susceptibility to
marsupial browsers (O'Reilly-Wapstra et al.
2001) in northern populations, associated with a
decrease in defensive chemistry (O’Reilly-
Wapstra et al. this volume).
An early discovery from these trials was the
deviant nature of the shrub-like population of E.
globulus sampled from exposed coastal cliffs at
Wilsons Promontory, Victoria. This population
exhibits rapid transition to adult foliage type,
precocious flowering and maintains the dwarf
habit in field trials (Jordan et al. 1999; Jordan et
al. 2000). It became clear that previous grouping
of seedlots from the Bass Strait island races,
King Island together with Furneaux (Orme 1988)
was inappropriate as these races were clearly
genetically different in many adaptive (e.g.
drought tolerance Dutkowski 1995b; Toro et al.
1998, susceptibility to herbivores Jordan et al.
2002) and economic traits (e.g. wood density -
Dutkowski and Potts 1999). There is also nearly
a 100 day difference in peak flowering time
between these two races (Gore and Potts 1995;
Apiolaza et al. 2001), resulting in virtual
reproductive isolation when they are grown
together. The core E. globulus populations on
the Furneaux islands were clearly genetically
differentiated from adjacent populations in
northeastern Tasmania, having shorter juvenile
leaves, thinner bark and more precocious
flowering and vegetative phase change
(Dutkowski and Potts 1999).
Table 2. Published studies of quantitative genetic variation in the five Tasmanian trials of a range-
wide collection of OP seedlots of Eucalyptus globulus by CSIRO in 1987 and 1988, and average
(and range) of single site estimates of heritability of within race variation (h2op) for each trait type.
The h2op values are averaged across different ages, sites, or different ways of measuring the same
trait (e.g. growth includes dbh, height and volume measures across ages 1 to 8 years) and the
number of such variables averaged for each trait is indicated (N). Except where indicated1, h2op
values have been recalculated and values may differ slightly from those published. The mixed
linear model used to estimate variance components treated race and replicate as fixed effects and
family within race, incomplete block, and plot (where appropriate) as random terms. The heritability
calculation follows Jordan et al. (1999), and assumes a coefficient of relatedness for open-
pollinated families of 0.4. It should be noted that our parameter estimates also refer to the
heritability of within-race or within subrace differences, and the phenotypic variance does not
include the incomplete block term
Trait N h2
op (range) Publications
Cellulose content 1 0.84±0.27 Apiolaza et al. unpubl.
Flowering time 4 0.68 (0.60-0.81) 1 Gore and Potts 1995, Apiolaza et al. 2001
Vegetative phase change 2 0.54 (0.46 - 0.63) Dutkowski and Potts 1999, Jordan et al. 1999, Jordan et
Frost resistance 1 0.52 (0.017)1 Tibbits et al. unpubl.1 (seedling T50)
Wood density (Pilodyn) 5
0.46 (0.21 - 0.59)
McDonald et al. 1997, Dutkowski and Potts 1999
Muneri and Raymond 2001; Apiolaza et al. unpubl.
Predicted pulp yield 2 0.46 (0.42-0.49) 1 Muneri and Raymond 2001; Apiolaza et al. unpubl.
Flowering precocity 2 3 0.44 (0.37 - 0.50) Chambers et al. 1997, Dutkowski and Potts 1999, Jordan
et al. 1999
Bark thickness 5 0.42 (0.22 – 0.71) Dutkowski and Potts 1999
Lignotuber development 2 0.35 (0.27-0.43) Whittock et al. 2003 1
Juvenile leaf morphology 5 0.32 (0.13-0.46) Potts and Jordan 1994a, Dutkowski and Potts 1999
22 0.28 (0.17 - 0.39)
Potts and Jordan 1994b, Borralho et al. 1995, Borralho
and Potts 1996, McDonald et al. 1997, Dutkowski and
Survival (8yr) 2 5 0.27 (0.14 - 0.41) Chambers et al. 1996, Dutkowski and Potts 1999
Microfibril angle 1 0.27±0.24 Apiolaza et al. unpubl.
Herbivory 4 0.23 (0.00-0.46)
Jones and Potts 2000 1, Jordan et al. 2002, O'Reilly-
Wapstra et al. 2001 1
Fibre length 1 0.16±0.17 Apiolaza et al. unpubl.
Coppicing 1 0.07
Whittock et al. 2003 1
1 Not re-analysed, used published estimates. 2 Binomial model
Indirect assessment of wood density at age 5
based on Pilodyn penetration added to the
knowledge of economically important traits
(McDonald et al. 1997; Dutkowski and Potts
1999). Rapid growth was the main aim of
selection during the 1980’s and, while no major
regional variation was detected, the King Island
race had performed consistently well resulting in
interest for seed collection (Volker and Orme
1988; Orme 1988; Eldridge et al. 1993).
However, with the clear definition of a pulpwood-
breeding objective in the early 1990’s (Borralho
et al. 1993), wood density and pulp yield also
became important traits for selection. With the
significant regional differences in wood density
evident, seed collections during the 1990’s for
breeding and deployment were focused on
specific components of the gene pool,
particularly the fast growing, high density
intergrade races such as the Strzelecki race.
The previously favoured King Island race had
the lowest wood density (Dutkowski and Potts
1999) and poorest drought tolerance (Toro et al.
1998) of all races sampled. The economic value
of the various components of the diverse E.
globulus gene pool may change again with more
information on other wood properties relevant to
present and future breeding objectives (fiber
dimensions - Miranda et al. 2001b; extractives,
cellulose content and pulp yield - Miranda and
Pereira 2002, Muneri and Raymond 2000;
microfibril angle Muneri and Raymond 2000;
tension wood Washusen et al. 2001; Washusen
and Ilic 2001; sawing properties Greaves et al.
While all traits we have examined show
significant genetic variation among localities, the
largest component of the genetic diversity in
quantitative traits within the E. globulus gene
pool lies within sampling localities, much of
which is between families (Fig. 3). Localities
were defined to ensure all the trees sampled
occurred within 10km of each other (Potts and
Jordan 1994b), but trees were at least 100m
apart (Gardiner and Crawford 1987). The
variation between these open-pollinated families
could represent true additive genetic differences
between the trees sampled or be due to other
factors, including variation in the parental
outcrossing rates and hence variation in the
expression of inbreeding depression (Hardner
and Potts 1995; Hardner et al. 1996). In the
former case, our studies have clearly indicated
that fine-scale spatial structuring of genetic
variation is superimposed on the broad-scale
racial patterns of genetic variation discussed
previously. This structure has no doubt arisen
through a combination of limited seed dispersal
(family groups <50m; Hardner et al. 1998; Skabo
et al. 1998) and/or adaptive clines in response to
environmental gradients (Jordan et al. 2000).
b) Molecular affinities (microsatellites) c) Quantitative genetic affinities
OP genetic parameters: Most estimates of
narrow-sense heritabilities and genetic
correlations required to understand the response
of E. globulus populations to selection are from
open-pollinated (OP) progeny trials (e.g. Volker
et al. 1990; Woolaston et al. 1991). Eucalyptus
globulus has a mixed mating system (Hardner et
al. 1996) and hence we have standardized our
calculations of heritabilities (e.g. Lopez et al.
2002) and breeding values (Dutkowski et al.
2001) from open-pollinated progenies by
assuming a coefficient of relatedness of 0.4
(termed h2op) rather than 0.25 used for complete
half-sibs. It should be noted that our heritabilities
and genetic correlations refer to the additive
genetic (co-)variation within genetic groups (i.e.
races, sub-races or localities), although
between-race (co)variances may be accounted
for in statistical models used for genetic
parameter estimation. The within-race
heritabilities derived from the five Tasmanian
base population trials are summarised in Table
2. Other estimates for growth and wood
properties are reviewed in Lopez et al. (2002)
and Raymond (2002). Survival, growth and
herbivore damage were the least heritable traits
whereas flowering time, vegetative phase
change and wood properties such as cellulose
content and wood density were the most highly
heritable traits in these trials (Table 2). Our
average h2op of 0.25 for diameter across ages 4
and 8 in the five trials is consistent with the
average of 0.21 reported from numerous trials
by Lopez et al. (2002), although our average
Figure 2. a) Racial classification of Eucalyptus
globulus based on genetic differences in
quantitative traits; and b) molecular and c)
quantitative genetic affinities of the races
Races are separated by solid lines and labelled
in roman type and subraces are separated by
broken lines and labelled in italic type. The
stippled area is the natural distribution of E.
globulus and intergrade populations (see Fig.
1). The figure (left) has been modified from
Dutkowski and Potts (1999) based on new
information from Lopez et al. 2001b which
changed the boundary between the Southern
and South-eastern races. The classification
and boundaries are being refined as more
information becomes available and the latest
version is available at
ex.html. The UPGMA dendograms are based
on b) Nei’s genetic distances derived from an
analysis of 8 microsatellite loci using nearly 400
native trees and c) Mahalanobis’ distances
based on 38 quantitative traits measured on
families in the 5 Tasmanian base population
trials (see Dutkowski and Potts 1999).
heritability for Pilodyn penetration (0.46) is
slightly higher than normally reported (Lopez et
al. 2002; Costa e Silva et al. 2004a).
The expression of quantitative genetic variation
amongst these open-pollinated families is
surprisingly stable. Age-to-age correlations for
growth were high at both the family within race
(rg, genetic correlation) and race (rrace,
correlation between subrace least squares
means) levels. Between dbh at ages 4 and 8
years, the average rg was 0.93 (n=5) and the
average rrace was 0.80 (n=5). Two-year old
growth measurements were taken at only one
trial and were significantly correlated with 8-year
measurements at the family within race
(rg=0.71), but not the race level (rrace=0.57).
There was also a strong correlation between the
different measures of growth at a given age (e.g.
dbh4 vs. height4: averages rg=0.89 and
rrace=0.80, n=5). For a given trait, the expression
of genetic variation on the five sites was also
highly stable, at least within races. Within races,
the across-site correlations for the various
growth traits (averages rg=0.71 and rrace= 0.48;
n=40), survival (averages rg=0.85 and rrace=0.41,
n=10), flowering precocity (averages rg=0.68 and
rrace=0.84, n=4), Pilodyn penetration (averages rg
=0.91 and rrace=0.67, n=10), and bark thickness
(averages rg=0.88 and rrace=0.89, n=10) were all
positive but only 89% of the across-site
correlations at the race level were positive. For
growth and survival, rrace was clearly less in
magnitude than rg, suggesting that genotype x
environment interaction is stronger at the racial
level. This trend is clearly evident in the
magnitude of the genotype x environment
variance components in the partition of the
variation in 4-year dbh and Pilodyn penetration
shown in Fig. 4. Virtually all main effects and
interaction terms were statistically significant.
However, with the exception of the race x site
interaction components for diameter, the
interaction variance components were small
compared to the site stable components of
genetic variance (i.e. race and family within race
(Environmental and Genetic)
Site x Family (1%)
Site x Race (2%)
Incomplete blocks (5%) Genetic
(a) Four year Diameter
Site by Family (1%) Family (13%) Site by Race (3%)
Incomplete blocks (7%)
(b) Average Pilodyn Penetration (5 years)
Even beyond Tasmania and Australia, recent
analyses (João Costa e Silva et al., in
preparation) suggest that the expression of
growth differences between these open-
pollinated families is still relatively stable (Table
3). In this case, across-site genetic correlations
were estimated for DBH between families within
subraces (rg), and between subraces (rsubrace)
rather than using the broader race as the genetic
group. As expected, the across-site correlations
decreased slightly as growth was compared
between sites within regions in Australia
(weighted averages rg=0.85 and rsubrace=0.85, not
shown), different regions within Australia
(weighted averages rg=0.76 and rsubrace=0.73, not
shown) and from different countries (weighted
averages rg=0.70 and rsubrace=0.51, Table 3).
Figure 3. Hierarchical partition of
quantitative genetic variation in E.
globulus. Average % total
phenotypic variation within replicates
due to variation between families
within localities, between localities
within races, and between races for
different types of traits (see Table 2).
Sawfly damage represents two of
the herbivory traits in Table 2. The
additive genetic variation within
localities is expected to be 2.5 times
the family variance component
(Potts and Jordan 1994b)
Figure 4. Percentages of variation across
all five Tasmanian base population trials
due to genetic, environmental and other
causes in (a) diameter, and (b) Pilodyn
penetration. The traits were standardised
so that the residual variance for each site
was 1. Pilodyn penetration was the average
of two shots per tree. The terms indicated
were fitted as random effects in the model
(replicate was treated as fixed) using
ASREML and their significance tested
using a one-tailed likelihood ratio test. All
interaction variance components were
highly significantly (P<0.001) greater than
zero except for the site by family interaction
Subrace performance was less stable across
countries than the performance of families from
the same subrace, suggesting that genetic
differences at the subrace level with impact on
performance are more responsive to
Table 3. Average values for estimated across-site correlations in dbh of E. globulus from the 87/88
CSIRO collections. Ages of dbh measurements ranged from 4 to 6 years in Australia (15 sites), 3 to
4 years in Chile (3 sites), 3 to 5 years in Portugal (3 sites) and 4 to 5 years in Spain (3 sites).
Across-site correlations were estimated at the subrace (rsubrace) and family within subrace (rg)
levels, and the numbers of site pairs used in the calculations are given in parenthesis. Although all
sites had connections for subraces, there were pairs of sites with no connections for families,
owing to the use of different family samples within subraces. The statistical analyses used
bivariate mixed linear models which fitted unstructured correlation matrices for subrace and
family within subrace effects (note that, in the absence of family connections, only rsubrace was
estimated and a diagonal matrix replaced the unstructured correlation form for family effects,
which is analogous to assuming zero across-site covariances), as well as accounting for
heterogeneous variances for these terms, design features (such as incomplete blocks, plots, etc)
and residual effects (João Costa e Silva et al., in preparation)
Family within subrace - rg Subrace - rsubrace
Australia Chile Portugal Spain Australia Chile Portugal Spain
Australia 0.78 (93) 0.74 (27) 0.71 (31) 0.63 (27) 0.76 (105) 0.49 (45) 0.56 (45) 0.50 (45)
Chile 0.87 (3) 0.85 (6) 0.69 (9) 0.76 (3) 0.41 (9) 0.47 (9)
Portugal 0.73 (1) 0.63 (6) 0.60 (3) 0.53 (9)
Spain 0.66 (3) 0.39 (3)
Note: Using the numbers of site pairs as weighting factors: weighted average rg within countries = 0.78; weighted
average rg across countries = 0.70; weighted average rsubrace within countries = 0.75; weighted average rsubrace across
Reliability of OP genetic parameters: The
genetic interpretation of the patterns and levels
of quantitative genetic variation amongst the
native stand OP families is a key issue in
understanding the genetic architecture of E.
globulus. Adjustment of the coefficient of
relatedness to better estimate levels of additive
genetic variance and breeding values from OP
progeny, as discussed above, does not account
for inbreeding depression, non-random mating
and the variable levels of outcrossing which
occur amongst OP families of E. globulus from
native stands in Australia (Hardner et al. 1996;
Jones et al. 2002b). This means that additive
and non-additive genetic effects may be
confounded in genetic parameter estimates
based on OP progeny.
We have been studying an experiment
established in 1990 specifically aimed at
comparing genetic parameters and parental
breeding values estimated from families derived
from OP’s with those obtained using pollen of
the same 26 base parents in a factorial mating
design (CP). Results at age 2 years (Potts et al.
1995; Hodge et al. 1996), now supported by
those at 6 years (Volker 2002.), indicate that for
growth traits: (i) variability within OP families is
high compared to CP families, (ii) OP
heritabilities are inflated even after adjustment
(also Hardner and Potts 1995); (iii) breeding
values estimated from OP and CP progeny are
poorly correlated (e.g. DHB age 6yrs: a rg
between the CP and OP cross-types of -0.1,
P>0.05; Volker 2002) and (iv) estimated levels of
genotype x site interaction are less for OP
progeny then CP, suggesting that OP estimates
of additive x environment interactions are
downwardly biased. Growth is one of the key
traits affected by inbreeding depression, and the
later finding is consistent with the suggestion
that the high within race/subrace genetic
correlations for growth across sites may well
reflect stable differences in inbreeding
depression amongst OP families rather than
consistent expression of additive genetic effects.
In contrast to growth traits, good correlations
between OP and CP breeding values have been
found for frost resistance (Volker et al. 1994),
Mycosphaerella leaf damage (Dungey et al.
1997), leaf size and shape (Dungey 1991),
Pilodyn penetration (Volker 2002) and the timing
of transition to adult foliage and first flowering
(Jordan et al. 1999). For these traits, variation
amongst OP progeny should reliably estimate
variation in additive genetic effects, but clearly
the same cannot be said for growth traits.
The importance of non-additive genetic effects:
Understanding the relative importance and
nature of non-additive (dominance, epistasis and
maternal) genetic effects in E. globulus is now a
key research issue. This information is required,
for example, to help decide whether to shift from
OP seed orchards to more expensive clonal and
full-sib family deployment strategies for the
species. Additive and non-additive effects can
only be separated with full pedigree control (CP),
and dominance and epistatic effects can partly
be partitioned where individuals within full-sib
families have been cloned (Costa e Silva et al.
2004a). There are few CP studies of the relative
importance of additive and non-additive genetics
effects in E. globulus (Table 4). Evidence for
significant non-additive genetic effects on growth
in E. globulus comes from i) the existence of
severe inbreeding depression (Hardner and
Potts 1995; Hardner et al. 1998), ii) existence of
positive mid-parent heterosis in inter-
provenance/inter-race crosses (Vaillancourt et
al. 1995; Hodge et al. 1996; Volker 2002; Lopez
et al. 2003; see also Hardner et al. 1998) and
(iii) significant non-additive interactions between
parents from the same population (Table 4). In
the latter case, results have been variable, but it
appears that the non-additive genetic control of
growth (e.g. dominance ratio - d2) can in cases
be comparable to the additive one (CP
heritability - h2). Indeed, the additive genetic
control of growth is clearly low and CP
heritabilities for DBH (Table 4) tend to be
approximately half the average of the many
values reported from open-pollinated progeny
trials of this species (Lopez et al., 2002).
However, in the study of Volker (2002) the non-
additive effects on 6 year DBH were less stable
across sites than the additive genetic effects.
Costa e Silva et al. (2004a) is the only study to
separate dominance and epistatic components
of variation. This study used RAIZ breeding trials
based on crosses amongst selections from the
Portuguese landrace. Both non-additive
components were small and insignificant for
DBH and Pilodyn penetration (Table 4). Similar
results were also obtained for height (Costa e
Silva et al. 2004b). However, the possibility
cannot be dismissed that epistasis may be more
important in other populations.
Table 4. Estimates of the percentage of the phenotypic variation within E. globulus populations,
which is due to additive, dominance and epistatic effects. Estimates are either (i) averaged over
individual sites or (ii) calculated across sites and include gxe terms in the estimate of the
Population Trait Additive
Across sites- Volume
0.08 ± 0.012
0.02 ± 0.013
0.05 ± 0.03
0.15 ± 0.06
Vaillancourt et al.
Hodge et al. 1996
Taranna x King
Island, 8x26 F1
(5 sites) 5 single sites -DBH 6yr
Across sites - DBH 6yr
0.08 ± 0.03
0.25 ± 0.07
DBH 4yr (5 sites)
Pilodyn 4yr (2 sites)
Costa e Silva et al.
DBH 1-3yr (5 sites)
Density (3 sites)
Li et al. unpubl
1 pooled within crosstypes 2intra-provenance crosses 3interprovenance crosses 4 combines between and within subrace
dominance estimates whereas as other estimates refer to within subrace/provenance dominance alone
Lopez et al. (2003) is the only study to
investigate the significance of maternal effects in
E. globulus. The maternal effects on seed weight
and germination rate were shown to only have a
transitory effect on growth and were insignificant
after 1 year of field growth. Additive genetic
effects on growth became expressed by age 40
months, suggesting that maternal seed or other
maternal environmental effects are unlikely to
inflate variation between OP families.
Interestingly, the fact that differences between
reciprocal inter-race F1 crosses were significant
by one year after field planting raises the
intriguing possibility that there may be some
maternal genetic effects on growth (e.g. through
maternally inherited plasmid genes or
THE MOLECULAR GENETICS VIEW
Our key molecular information on the structure
of the E. globulus gene pool is from studies
involving three different marker systems. Two
studies have used nuclear microsatellites. These
loci can effectively be considered neutral to
selection and affinities between populations are
more likely to be influenced by time since
isolation, bottlenecks and recent pollen and seed
mediated gene flow. One study examined the
affinities of the commercially important Strzelecki
race (Jeeralang population) to core E. globulus,
E. pseudoglobulus and E. bicostata (Jones et al.
2002a). This Victorian E. globulus intergrade
race is located in a geographically intermediate
position between the three species (Fig. 1). Its
capsule morphology is variable and ranges from
large single capsules resembling E. globulus to
smaller, three-fruited forms resembling E.
bicostata or E. pseudoglobulus. The population
is intermediate in microsatellite frequencies
between core Tasmanian E. globulus and the
other species, but has very close affinities to
core E. globulus on the coastal plain in south
Gippsland (Southern Gippsland race) (Fig. 5).
The second microsatellite study involved 8
nuclear microsatellite loci assayed for nearly 400
native trees from 11 races of E. globulus
collected from many of the same areas as
sampled in the 1987/1988 CSIRO collections
(Fig. 2, Steane et al. unpubl.). This study
showed two independent clines in gene
frequency associated with latitude and longitude.
The latitudinal cline was the most differentiated.
It appeared to be an extension of the cline
between species (Fig. 5) and separated the
Victorian races from those in eastern Tasmania,
with the Western Tasmania and King Island
races intermediate. The longitudinal cline
separated western and eastern populations in
Victoria, Bass Strait and Tasmania. The E.
globulus races, as defined on genetic
differences in morphology and putative adaptive
quantitative traits, fall into three main molecular
groups (Fig. 2b). The first comprises the races in
western (Otway Ranges) and eastern (Strzelecki
and South Gippsland) Victoria. The South
Gippsland race within this group comprises
many remnant farmland populations. It is an
outlier in the quantitative genetic classification
(Fig. 2b), most likely due to high inbreeding
depression in growth traits (Borralho and Potts
1996). It also classifies taxonomically as core E.
globulus (Fig. 1), which does not match its
underlying molecular genetic affinities (Figs. 2b).
The second group comprises the continuous
core E. globulus races in eastern Tasmania and
the disjunct Furneaux race. This molecular
affinity would suggest that the large quantitative
genetic differences which occur between these
races are adaptive and have evolved relatively
recently or in the face of substantial gene flow.
The third group comprises the intermediate King
Island and Western Tasmania races, which has
slightly closer affinities to the second group. In
both the west and the east, the Bass Strait
island races had closest molecular affinities to
their nearest Tasmanian races, suggesting that
a more recent connection or gene flow has
occurred between the Bass Strait islands and
Tasmanian races than with those in Victoria.
-1.5 0 1.5
The second type of molecular marker we have
examined has provided a completely novel
perspective on the E. globulus gene pool. We
sequenced the hypervariable JLA+ region of the
chloroplast DNA (cpDNA) from 264 samples
from 157 different populations (Freeman et al.
2001; McKinnon et al. 2004a; McKinnon et al.
2004b). CpDNA is non-recombining and
maternally inherited (McKinnon et al. 2001b) and
therefore reflects patterns of variation driven by
seed dispersal alone. The cpDNA of E. globulus
is highly variable (we have identified more than
100 JLA+ haplotypes) and the gene pool contains
several highly divergent lineages, the main ones
are the Central (C), Southern (S), and Eastern
Tasmanian (ET) lineages (Fig. 6). The variation
in cpDNA is highly spatially structured
geographically, at both the lineage and
haplotype level, providing a ready tool for
determining the geographic origin of
unpedigreed germplasm such as the various E.
globulus landraces (Fig. 6). The C lineage is
widespread and found in many other species in
the section Maidenaria. A sub-lineage, Cc, is
widespread in Tasmania, occurs in the Otway
Ranges, but has not been found in populations
in eastern Victoria. The sub-lineage, Cg, is the
dominant cpDNA in eastern Victoria but does not
occur in Tasmania. Both the Cc and Cg lineages
occur on the Furneaux Group of islands, but
these Cc haloptypes are different to those in
northern Tasmania. The lack of an eastern
continuity in haplotypes strongly argues for an
eastern barrier to seed migration into and out of
eastern Tasmania (Freeman et al. 2001). In
contrast, the sharing of the same Cc haplotypes
between the Otways, King Island and Western
Tasmania supports a more recent migration
route. While the Western Tasmania, King Island
and Otways races are separated by large
disjunctions, this connection was most likely
associated with the formation of a land bridge
between Tasmania and continental Australia
during the Last Glacial (McKinnon et al. 2004a).
The S and ET cpDNA lineages are less
widespread and also have been found in other
species of the section Maidenaria growing in
southeastern and eastern Tasmania respectively
(McKinnon et al. 2001a). Patterns of cpDNA
variation within Maidenaria are more related to
geography than species boundaries, suggesting
extensive introgression between species
(McKinnon et al. 2001a). In the case of E.
globulus, the distribution of the S lineage is
thought to coincide with a glacial refuge in which
hybridisation and ‘chloroplast capture’ occurred.
We now have evidence of sharing of specific
haplotypes on a very localized scale between E.
globulus and the rare Tasmanian endemic E.
cordata, which is believed to be due to the
widespread E. globulus assimilating the cpDNA
of this rare species through hybridization
(McKinnon et al. 2004b). Shared haplotypes are
highest in E. globulus sampled within 2 km of
known E. cordata populations and drops to zero
at a distance of 25 km from the nearest known
E. cordata population. This cpDNA evidence
leads us to believe that E. globulus may be a
‘compilospecies’ that, through evolutionary time,
has assimilated genes of other species into its
gene pool through hybridisation.
Figure 5. Ordination of Nei’s (1972) genetic
distances between populations of E. globulus
and closely related species, based on 8
microsatellite loci. The figure shows that the
species cores are clearly differentiated. The E.
globulus intergrade population in the Strezlecki
Ranges (Jerralang) and the populations
morphologically resembling core E. globulus on
the coastal plain in Gippsland, Victoria are
intermediate and overlaping in the figure (Jones
2 B 1 Cc
1 Cg 1 Cg
3 Cc 1 s
King Island Furneaux Group
2 B 1 Cc
1 Cg 1 Cg
3 Cc 1 s
King Island Furneaux Group
2 B 1 Cc
1 Cg 1 Cg
3 Cc 1 s
King Island Furneaux Group
2 B 1 Cc
1 Cg 1 Cg
3 Cc 1 s
King Island Furneaux Group
Figure 6. The marked spatial structure in variation in chloroplast DNA in Eucalyptus globulus.
(a) The distribution of major chloroplast DNA lineages in the native population (Freeman et al. 2001).
Arrows show inferred seed-dispersal routes between continental Australia and Tasmania based on shared
haplotypes; the dotted line shows an apparent block to seed dispersal between the Furneaux Group and
Tasmania based on the distribution of specific haplotypes. ( b) The distribution of the same chloroplast
DNA lineages in plantations in Portugal established from the landrace. DNA was obtained from 26 samples
from diverse Portuguese localities by Victor Carocha (RAIZ) and their cpDNA sequence compared to a
large database of over 269 samples of E. globulus trees from known native localities in Australia. The
Portuguese landrace contained 10 different chloroplast haplotypes; six have only been found in
southeastern Tasmania, and one has only been found in eastern Victoria and Flinders Island. The
remaining three haplotypes have as yet only been found in the Portuguese samples. These results
suggest that the Portuguese landrace is likely to have different geographic origins and be genetically
While there is no obvious evidence of such
hybridisation in the morphology of the E.
globulus trees we sampled, it is logical that such
hybridisation may have also contributed
significantly to nuclear gene diversity in the E.
globulus races in eastern Tasmania. This
hypothesis is consistent with the finding that
Eastern Tasmania not only has the highest
cpDNA diversity, but also the highest nuclear
microsatellite diversity (Steane et al. unpubl.).
We used a third marker system to study whether
we could detect evidence for hybridisation
affecting genetic diversity in functional nuclear
genes. We studied a lignin biosynthesis gene
cinnamoyl coA reductase (CCR; McKinnon et al.
in prep), which mapping studies have shown co-
locates with QTL affecting cellulose and pulp
yield production in E. globulus (Thamarus et al.
2004). It is a single-copy gene and exhibits
considerable diversity in the species (Poke et al.
2003). PCR-RFLP analysis of CCR from 208 E.
globulus samples combined with sequencing of
specific haplotypes showed two major lineages
of CCR are present in E. globulus. One lineage
dominates southern and western Tasmanian E.
globulus and is shared with other Maidenaria
species. The other lineage is prominent in
Victorian and northern Tasmanian E. globulus,
and surprisingly shows homology to CCR from
Eucalyptus saligna (section Latoangulatae).
Consistent with the other marker systems, CCR
shows a major latitudinal cline in the frequency
of key haplotypes and greatest diversity in
Tasmania. The relative frequencies of different
CCR haplotypes divide the range of E. globulus
into four geographic regions: (1) Victoria, (2) the
Furneaux Group and northeastern Tasmania, (3)
King Island and western Tasmania and (4)
southeastern Tasmania. The eastern and
western Victorian races are not differentiated
and there is a significant north-south cline in
haplotype frequencies in the continuous
populations of core E. globulus in eastern
Tasmania. Drift in glacial refugia, selection
and/or historical hybridisation may have been
instrumental in generating this cline. A potential
role for hybridisation is suggested by the finding
that southeastern E. globulus with the same
cpDNA lineage as the endemic E. cordata (S
lineage) also have significantly higher levels of a
CCR haplotype that is closely related to the
dominant CCR haplotype in E. cordata than
those E. globulus with C haplotypes.
A more detailed knowledge of the E. globulus
gene pool is emerging from integrated studies of
quantitative genetic variation in traits of potential
adaptive significance with studies of DNA
variation in the nuclear and chloroplast
genomes, which are probably mostly neutral but
may affect the phenotype in some cases. Each
perspective reveals similarities and differences
in the variation pattern as would be expected
from combining genetic information affected by
time since isolation, population size and gene
flow with that driven by selection and resulting in
both divergence and convergence in phenotype.
There is increasing evidence that the E. globulus
gene pool is not closed and that E. globulus has,
through its evolutionary history, assimilated
genes from co-occurring species into its gene
pool through hybridisation. Differential rates of
gene introgression during the hybridization
process (e.g. E. grandis x globulus backcrossing
- Myburg et al. 2004) could provide a
mechanism for rapid recovery of parental
phenotypes during backcrossing which would
explain the relative constancy of taxonomic
traits. Great genetic diversity at both the broad
geographic and local scales is available for
selection in virtually all traits examined, although
their degree of additive genetic control varies
markedly. There are now unique opportunities to
use the molecular genetic framework being
developed to better understand the quantitative
genetic behaviour of breeding populations.
Identification of the key genes responsible for
the large quantitative genetic differences
observed at all spatial levels is now a key
research issue. Similarly we are just starting to
understand the importance of non-additive
genetic effects such as epistasis. If differential
co-adaptation has occurred between the highly
differentiated races, the effects of epistasis on
growth may become greater as breeding
programs proceed to advanced generations
using inter-race hybrids. The highly diverse
cpDNA and its strong spatial structuring provides
unprecedented opportunities for tracing the
geographic origin of maternal lineages and the
recent sequencing of the E. globulus chloroplast
genome (Steane et al. unpubl) opens
opportunities to improve the resolution of this
marker system. Whether the diverse cpDNA
lineages within the gene pool are adaptive is
unclear. However, there is clearly the possibility
of maternal effects through these haplotypes
affecting plant growth directly or through
cytonuclear interactions (Lopez et al. 2003).
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