Population management in new plant breeding approaches

Abstract

This chapter briefly reviews some examples of the application of population and quantitative genetics to the development of new breeding methods. Plant breeding methods are all centered on the basic concepts of exploiting natural or artificially induced genetic variation in crop plants through selection, agronomic trials, and repeated cycles of this controlled evolutionary scheme. Methods vary in terms of the breeding system of the crop species (e.g. outbreeding, inbreeding, asexual), breeding objectives and the plant breeder’s primary training or research area. Over the past three decades a significant shift to population genetic and evolutionary thinking has modified many breeding approaches in terms of methods for synthesizing initial gene pools, recurrent selection procedures for improving source populations, and the desired genetic heterogeneity in the end product cultivars. This shift has been induced by several factors: greater recognition of the role of variation in populations due to increase in evolutionary training, greater availability of genetic resources, ecological and environmental issues, and changes in agriculture toward long-term biocontrol and multi-species strategies. Theoretical ideas have also played a role in understanding these new methods although most basic principles are too general or elementary to be cited as the direct basis for developing any particular breeding methods. For example, recurrent selection involves some general background in polygenic inheritance models and in the basic theory of selection response. However, even with much empirical work from numerous crop breeding projects, only general guidance, not specific rules, can be obtained from them. In managing genetically heterogeneous host populations for developing durable disease resistance, indeed several interesting discoveries have been made on the genetic basis of resistance, cost of virulence, pathogenicity in mixed populations, epidemiology of different fungi, and so on.

In the management of forestry and rangeland communities, selection criteria include numerous population and community ecological variables related to persistence (recruitment), niche evolution, coexistence, community response to invasive species, and herbivory. Likewise, the development of new crops from wild biota often requires a basic understanding of the variation patterns, adaptations, reproductive biology, and role of natural enemies. Assembly of their genetic resources and initial domestication steps require perhaps a few major genes controlling such traits as seed retention at maturity, earliness, or apomixis; a great deal of population management using quantitative genetics then yields improvements in yield or crop adaptability to a region and an agroecosystem. There is clearly a need to involve some population biology in the training of plant breeders.