The breeding seedling orchard in the multiple population breeding strategy

Genetic improvement includes selection, testing and breeding from the species down to the clonal level. Tree breeding programmes in tropical countries are often required to work with many provenances of many species for many sites. A Multiple Population Breeding Strategy (MPBS) was proposed in response to this need and the breeding seedling orchard (BSO) devised to combine the conventional hierarchy of sequential testing, selection and seed production populations in a single planting. In the MPBS the breeding population is divided into subpopulations which are kept separate so as to produce trees with different gene complexes. These can then be used as replicate populations (to avoid inbreeding effects in the operational seed), diversified populations (to exploit genotype-environment interaction), heterotic populations (to exploit heterosis) or structured populations (to place more emphasis on the elite elements). The BSO lies between a Seedling Seed Orchard (SSO) and a progeny test (PT). The functions of the SSO or PT are inevitably compromised in the BSO; the nearer it lies to the SSO the higher the selection intensity and seed production whereas the nearer it is to the PT the better the genetic information and the selection precision. Objectives should be clearly stated before the BSO is designed if these conflicts are to be resolved. Breeding intensity within the BSO can be simple (mass selection), intermediate (half-sib pedigree control) or intensive (full-sib pedigree control). In determining the genetic constitution of the BSO the crucial issues are: sampling the base population effectively, avoiding inbreeding and selecting the genetic checks. The elements of environmental design, plot size, shape and spacing, replication and sub-blocking, and siting can all be crucial in achieving the objectives of the BSO. Genetic gain is dependent on the effectiveness of selection which in turn is dependent on its intensity, precision and the number of criteria. The conclusion is that this strategy is flexible, provides the potential to respond to new materials and new demands and allows the breeder freedom to be adventurous without taking unacceptable risks. At the same time it accommodates the need to work with many populations of many species at different levels; and it conserves variation.

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