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In recent years domestication research has been assisted by genetic studies focusing on species of commercial interest from temperate and tropical Australia (see reviews in Moran, 1992; Butcher et al., 1999; Moran et al., 2000). The patterns of variation revealed by surveys using molecular markers (allozymes and DNA markers) can help define more efficient sampling strategies for domestication programs and for identifying key populations for conservation. For example, a survey of allozyme variation in A. melanoxylon (Blackwood) revealed two genetically distinct regions that should be treated as distinct entities in breeding and conservation programs (Playford et al., 1993). Surveys of genetic variation have also been reported for A. auriculiformis (Northern Black Wattle) (Wickneswari & Norwati, 1993), A. mearnsii (Black Wattle) (Searle et al., 2000), A. acuminata (Jam) (Maslin et al., 1999; Byrne et al., 2001) and A. tumida (Pindan Wattle) (McDonald et al. in prep.). More detailed genetic studies have been conducted on A. mangium (Mangium): very low levels of genetic variation were detected using allozymes (Moran et al., 1989a); by contrast, major differences in the level of genetic diversity in populations from the south of the species range (Daintree, Queensland) compared to levels in populations from New Guinea, were identified using RFLPs (Butcher et al., 1998). It was then possible to identify how much variation had been captured in breeding programs and its origin. Populations from New Guinea, which had the highest levels of genetic variation, showed superior growth in field trials. Accordingly, seed sources from this region are the focus of breeding programs and are widely used in plantations. A genetic linkage map has also been constructed for A. mangium to locate genes associated with quantitative traits which can be used for marker assisted selection (Butcher et al., 2000a, 2000b; Butcher & Moran, 2000).
In addition to elucidating the patterns of variation within species and species groups, the level of inbreeding has been determined for different populations and species of acacia. Predictions of genetic gain from recurrent selection in breeding programs are based on the assumption of random mating. If a high proportion of seed is produced from selfing, the gains from selection will be far less that expected. Outcrossing rates were first calculated for plantations of important tannin-producing species, A. mearnsii and A. decurrens (Green Wattle), using recessive morphological markers. Both species were highly outcrossing (Philp & Sherry, 1946; Moffett, 1956). Allozyme surveys of natural populations of A. auriculiformis and A. crassicarpa (Thick-podded Salwood) (Moran et al., 1989b; Khasa et al., 1993, 1994), A. melanoxylon (Muona et al., 1991), A. mearnsii (Grant et al., 1994) and A. aulacocarpa sens. lat. (McGranahan et al., 1997), showed these species were also highly outcrossing. By contrast, considerable variation has been reported in the mating system of A. anomala (Chittering Grass Wattle) (Coates, 1988) and A. mangium (Butcher et al., 1999). Outcrossing rates in A. mangium varied widely over the species geographic range, indicating that the use of a single breeding coefficient would introduce bias into estimates of gains from selection and breeding (Butcher et al., 1998). Populations of the rare species A. anomala, from two sites in southern Western Australia, had different breeding systems, one being predominantly outcrossing, the other reproducing clonally (Coates, 1988). Different conservation strategies would therefore be required for the two sites.
Studies of variation using molecular markers can assist in clarifying relationships among poorly defined taxa. For example, allozymes were used to examine patterns of genetic variation in the tropical arid-zone species, A. elachantha (under the name A. cowleana) and A. holosericea sens. lat. (Candelabra Wattle) planted in West Africa for fuelwood and to produce seeds for human consumption (Moran et al., 1992). This study revealed three distinct allozyme types within Candelabra Wattle, with different ploidy levels and largely confined to different geographic regions. A taxonomic revision of the group led to the description of a new species, A. colei (Cole's Wattle), and the reinstatement of A. neurocarpa (Maslin & Thomson, 1992). Similarly, findings from an allozyme survey of the Salwoods highlighted major divisions within A. aulacocarpa sens. lat. (McGranahan et al., 1997), consistent with a taxonomic revision of the group which identified new taxa (McDonald & Maslin, 2000).
References
Butcher, P.A. & Moran G.F. (2000), Genetic linkage mapping in Acacia mangium 2. Development of an integrated map from two outbred pedigrees using RFLP and microsatellite loci, Theor. Appl. Genet. 101: 594-605.
Butcher, P.A., Moran, G.F. & Perkins, H.D. (1998), RFLP diversity in the nuclear genome of Acacia mangium, Heredity 81: 205-213.
Butcher, P.A., Glaubitz, J.C. & Moran, G.F. (1999), Applications for microsatellite markers in the domestication and conservation of forest trees, Forest Genetic Resources Information 27: 34-42.
Butcher, P.A., Moran, G.F. & Bell, R. (2000a), Genetic linkage mapping in Acacia mangium 1. Evaluation of restriction endonucleases, inheritance of RFLP loci and their conservation across species, Theor. Appl. Genet. 100: 576-583.
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Coates, D.J. (1988), Genetic diversity in the rare Chittering Grass Wattle, Acacia anomala Court., Austral. J. Bot. 36: 273-286.
Grant, J.E, Moran, G.F, Moncur, M.W. (1994), Pollination studies and breeding system in Acacia mearnsii, in A.G.Brown (ed.), Australian Tree Species Research in China. ACIAR Proceedings No. 48, pp. 165-170. Australian Centre for International Agricultural Research, Canberra.
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Maslin, B.R., Byrne, M., Coates, D.J., Broadhurst, L., Coleman, H. & Macdonald, B. (1999), The Acacia acuminata (Jam) group: an analysis of variation to aid Sandalwood (Santalum spicatum) plantation research. Report to the Sandalwood Business Unit. Unpublished Internal Report. Western Australia Department of Conservation and Land Management, Perth.
Maslin, B.R. & Thomson, L.A.J. (1992), Re-appraisal of the taxonomy of Acacia holosericea, including the description of a new species, A. colei, and the reinstatement of A. neurocarpa, Austral. Syst. Bot. 5: 729-743.
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Moran, G.F., Muona, O. & Bell, J.C. (1989b), Breeding systems and genetic diversity in Acacia auriculiformis and A. crassicarpa, Biotropica 21: 250-256.
Muona, O., Moran, G.F. & Bell, J.C. (1991), Hierarchical patterns of correlated mating in Acacia melanoxylon, Genetics 127: 619-626.
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Playford, J., Bell, J.C. & Moran, G.F. (1993), A major disjunction in genetic diversity over the geographic range of Acacia melanoxylon R.Br., Austral. J. Bot. 41: 355-368.
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