Regional allozyme divergence in Sugar Gum, Eucalyptus cladocalyx
by
Matthew Rawlings
CPBR Summer Scholarship Program Report 2000/01
The following project was undertaken at the CSIRO Division of Forestry and Forest Products as part of the Centre for Plant Biodiversity Research’s Summer Scholarship program 2000-2001.
Eucalyptus cladocalyx F.Muell. has recently shown considerable potential in dryland forestry trials in the Murray-Darling Basin and wheatbelt of Western Australia. Patterns of genetic variation on eight populations, representing its natural distribution, were assessed using allozyme markers. Three cultivated stands from Victoria, including var. ‘nana’ a variant of horticultural origin, were also included in the study to ascertain their natural origins. Results indicated relatively low levels of overall genetic diversity and large regional genetic differentiation among populations. Negligible gene flow between regions was indicated and high levels of inbreeding detected. Diversity estimates were higher in the cultivated stands compared with the natural populations. Allele profiles of the two cultivated Victorian stands were consistent with having origins in the southern Flinders Ranges while var. ‘nana’ originates from the Eyre Peninsula. Low levels of genetic diversity and large differentiation among populations have important implications for the conservation and domestication of the species.
Introduction
Eucalyptus cladocalyx is a small to medium sized tree endemic to South Australia where its natural distribution occurs in three major regions: Flinders Ranges, Eyre Peninsula and Kangaroo Island. Among these populations there is considerable variation in habit. Trees from the Flinders Ranges tend to be straight, often obtaining heights of up to 35m (Fig. 2), while trees from on the Eyre Peninsula are typically have a short and crooked habit (Fig. 3). Trees on Kangaroo Island tend to be intermediate in these extremes of habit.
Eucalyptus
cladocalyx is a widely cultivated species across southern Australia, with
plantings stretching from western Victoria across to Western Australia. It is
typically used in amenity plantings, as a windbreak, and also for wood production.
Results from recent research conducted at CSIRO’s Australian Tree Seed
Centre indicate the potential of the species in forestry to be considerable,
particularly in the Murray Darling Basin and wheatbelt of Western Australia.
Historical records indicate that from 1878 onwards the South Australian Woods
and Forests began collecting and dispatching seed from trees considered to be
of ‘good form’ from the Wirrabara State Forest. Whether these activities
established the plantations in Victoria at Lismore (est.1890’s) and Wail
(est.1912) is uncertain.
Eucalyptus
cladocalyx var. nana is a dwarf variant of horticultural origin.
It is widely grown as an ornamental, used in street plantings, or as a windbreak.
The origin of the name ‘var. nana’ is unclear, with the earliest
published reference occurring in the Yates’ farm annual of 1937. Conflicting
references have been made as to the origin of var. nana with some suggesting
the Eyre Peninsula while others the Flinders Ranges. The aim of this study was
to characterise genetic diversity in natural populations of E. cladocalyx
and ascertain the origins of the cultivated stands from Victoria noted above
and of var. ‘nana’.
Population Samples
Eight populations from the species’ natural range were sampled (see Map below).
Seed from cultivated stands at Lismore and Wail in Victoria and a cultivated stand of var. ‘nana’ from near Horsham, Victoria, were sampled.
Allozymes were used because of their wide use in population genetics studies over the past 30 years. They remain a quick, efficient and cost effective means of assessing genetic diversity at population level. Another advantage is that the data generated can be directly compared with similar studies. Populations were represented by fifty progeny from ten mother trees (i.e., five progeny from each of ten mother trees per population were assayed).
Findings
Relatively low levels of genetic diversity and moderate to large deficiencies in heterozygotes were detected in natural populations of E. cladocalyx. In summary the allozyme data for natural populations revealed:
By contrast, diversity estimates given for the cultivated stands have slightly higher levels of genetic diversity. Allele profiles of the two cultivated Victorian stands were consistent with having origins in the southern Flinders Ranges. The var. ‘nana’ cultivated stand had the diversity estimate based on mean expected heterozygosity. Cultivated stands also had deficiencies in heterozygotes and high levels of inbreeding, although these were not as high as those found in the natural populations.
Analyses of population clustering patterns illustrated that populations on the Eyre Peninsula have closer genetic similarities with one another than those in either the Flinders Ranges or Kangaroo Island populations. These analyses also suggested that var. ‘nana’ has closest affinities with populations from the Eyre Peninsula.
Implications
From a utilisation perspective, the high levels of interpopulation differentiation suggest that large differences in performance could be expected in forestry trials. There is also the potential for investigating heterosis involving interpopulation crosses. The relatively high levels of inbreeding in E. cladocalyx indicate that seed production stands warrant careful planning to ensure that closely related trees are not in close contact. Seed collections from natural stands should take place following heavy widespread flowering to minimise the probability of collecting inbred seed. From a conservation perspective, the three disjunct regions of the species have large genetic differences suggesting negligible gene flow. Any future threat to the persistence of populations in these regions would have a major and irreversible impact on overall genetic diversity in E. cladocalyx.
Acknowledgements
The Centre for Plant Biodiversity Research, CSIRO Division of Forestry and Forest Products (Tree Improvement and Genetic Resources program, and The Australian Tree Seed Centre) for providing me with the Summer Scholarship and the opportunity to work on this project. Special thanks to Maurice McDonald for his supervision, sizeable contribution and encouragement throughout the project, along with Penny Butcher and Charlie Bell for their assistance. Stephen Midgley, Tim Vercoe and all the staff at The Australian Tree Seed Centre for their support during my stay.
Note:
This report is a summary of a manuscript in preparation titled ‘High levels of genetic divergence and inbreeding in Eucalyptus cladocalyx (Sugar Gum)’ by M.W. McDonald, M. Rawlings, P.A. Butcher and J.C. Bell. For further information on the study contact Maurice McDonald, CSIRO, Forestry and Forest Products, Australian Tree Seed Centre, PO Box E4008, Kingston, ACT, 2064. Ph. 02 6281 8211; email: maurice.mcdonald@csiro.au