Breeding strategies can range from simple and cheap to complex and expensive (Eldridge et al. 1993). Limited resources will often determine the complexity of the strategy and hence the potential gain. The right strategy for a breeding programme will maximise the genetic gain from available resources. When choosing the appropriate breeding strategy, there are four basic components to consider.
1. Method of selection (mass or recurrent). Recurrent selection maintains family identity for the base, breeding and propagation populations while mass selection does not. With fewer measurements, records or statistics, mass selection provides a cheaper source of genetic improvement, however genetic gains are usually less than for other strategies. Also, mass selection has a greater potential for inbreeding as the identity of the propagation population is unknown.
2. Method of mating (open or controlled). The full pedigree of both parents is maintained with controlled pollination to maximise genetic gain. The cost, time and skill required however can be prohibitive.
3. Type of breeding population (single, multiple, nucleus or interspecific). The main breeding population for each strategy is usually established from open pollinated seed that has been collected from a large number of unrelated trees in natural stands or plantations. This is to maximise the likelihood of genetic variation from which to make selections. A single breeding population is the cheapest to establish. This population may be thinned to become a seed orchard to supply improved seed for the next generation or is used as a source of selected material that is propagated vegetatively or as seed from controlled crossing to a separate seed orchard. Multiple populations subdivide a breeding population to meet possible future selection needs while avoiding the possibility of inbreeding. Nucleus breeding concentrates on selecting a small nucleus population from the main breeding population. From this nucleus, improved seed or clones are then produced. Interspecific breeding maintains a breeding population of each parent species which is to be crossed. Outstanding individuals of each species are then crossed to produce hybrids which are usually propagated as clones.
4. Method of mass propagation (seed or clones). Seed production is relatively inexpensive while cloning, although expensive, offers some increase in genetic gain.
CASE STUDY: IMPROVEMENT OF MELALEUCA ALTERNIFOLIA
An outline of the breeding strategy and breeding plan undertaken by CSIRO Forestry and Forest Products, Canberra and NSW Agriculture, Wollongbar for improving both the amount and quality of oil yield in M. alternifolia is given below. The main breeding objective is to increase oil yields in new plantations by 30% in five years (by 1998) with a 60% increase at the conclusion of the first generation of breeding, about year 2001.
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