To develop a breeding strategy for M. alternifolia it is important to know certain aspects of the biology of the species. These include both sexual and asexual reproduction and the range of variation in economically important traits along with their relationships and heritabilities. These aspects are listed.
1. Morphology. The flowers of M. alternifolia are morphologically bisexual and are insect pollinated. Outcrossing predominates with less than 10% self-pollination (Butcher et al. 1992). This low rate of self-pollination is highly advantageous for a breeding strategy based on open pollination as it reduces the likelihood of inbreeding.
2. Flowering. The abundance and periodicity of flowering is variable both within and between populations (Doran et al. 1997). Flowering usually occurs during October and November, for trees that are more than 2-3 years old. Flowering is heaviest during these months in years that have wet winters (Baker 1996, 1997). It appears that a wet winter is associated with both an increase in the number of trees flowering in a population and with the number of flowers per tree.
3. Pollination. Although the controlled pollination of M. alternifolia is possible (Moncur 1997), pollinating by hand is inherently slow and expensive. To undertake controlled crosses in a breeding strategy, the potential gains from such crosses have to be balanced against the costs, particularly when resources to a breeding program are limited. Potential gains from such crosses include the use of the progeny as a source of pedigree material for genetic studies as well as producing unrelated families for further selection.
4. Propagation. Mass propagation of this species is by seed. Cloning is possible by micropropagation (Hartney and Svensson 1992) or cuttings (Whish 1993) but the use of clones in yield trials over several years is needed to test if clones are suitable for plantation production. Cuttings from selected trees can be used in the establishment of clonal seed orchards to capture greater genetic gains.
5. Distribution. Information on the natural distribution of M. alternifolia (see above) is used when collecting seed for progeny trials. Seed is collected from the terpinen-4-ol rich oil trees located throughout the natural distribution of the species in NSW. Although seed normally matures 12-18 months after flowering (Colton and Murtagh 1990), mature seed can be collected at any time as some trees retain their seed crops for several years.
6. Genetic parameters. Genetic parameters express estimates of genetic and non-heritable variations of a population in respect to some characteristic (Allard 1960). In addition to being needed for the evaluation of different breeding strategies, these estimates together with their nature, magnitude and inter-relationships are necessary to assess improvement by selection. Heritability is a measure of how strongly a trait is influenced by genetics (Hanson 1963). When heritability is high, gain from selection will be high. Genetic parameters and expected gains from selection and breeding as estimated in several M. alternifolia progeny trials are—
(a) Heritabilities of 0.51 (Doran et al. 1997) and 0.67 (Butcher 1994) for oil concentration, 0.21 for plant height and 0.14 for stem diameter (Doran et al. 1997), 0.25 for plant dry weight and 0.27 for coppicing ability (Butcher 1994). These estimates indicate that improvement, particularly in oil concentration would follow selection for single traits.
(b) The absence of genetic correlation between oil concentration and growth traits (e.g. basal diameter, which is highly correlated with leaf yield (Doran et al. 1997)). This suggests that oil concentration and leaf biomass should be able to be improved simultaneously in a breeding programme. Butcher (1994), however, reported a negative genetic correlation of 0.42 between oil concentration and plant dry weight implying that genetic gain would have to be balanced between these two traits.
(c) Calculated gains of 17% for oil concentration and 14% for coppicing (Butcher 1994). These estimated gains were derived from one generation of breeding at a selection intensity of one tree in ten when a combined index selection was used, restricting plant dry weight to pre-selection levels.
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