1. Introduction to Plant Breeding
by Charlie Brummer; Institution of Plant Breeding, Genetics and Genomics, University of Georgia
Plant breeding, at least in some sense, dates back to the origins of agriculture. Although much early human aided selection was likely inadvertent, the earliest agriculturalists were responsible for domesticating virtually all the major crops we grow today. Two excellent resources for information on the domestication and evolution of crop plants are Zohary and Hopf (2000) and Smartt and Simmonds (1995). Following domestication, the biggest effort in breeding (via unconscious or conscious selection) was the adaptation of crops to new environments. Obviously, this is still occurring today, for example, in selecting maize for adaptation to high plant densities. Once adapted, plant breeders can focus on improved yield, quality, biotic and abiotic stress tolerances, etc.
Plant breeders are interested in phenotype, the physical appearance of a particular plant. Grain yield is a phenotype, and if you as a breeder are selecting for grain yield, then that’s want you want to maximize. The phenotype is determined by a combination of genotype and environment, with an element of random chance (that is, how genes get turned on or off, how proteins get made and folded, etc. all includes a stochastic element simply because getting the right thing to the right place at the right time in a cell can be variable just due to chance). The genotype is defined as the entire genetic constitution (or more specifically, the genetic constitution at a particular locus or set of loci) of an individual plant, and the environment specifies the conditions–weather, soil, competition of neighbors and weeds, pests, etc.–under which the genotype grows throughout its life.
The complete set of phenotypes that a particular genotype could produce under all possible environmental conditions is called its norm of reaction. The change in the phenotype of a particular genotype in response to the environmental conditions is termed phenotypic plasticity. Individual genotypes may not all respond similarly to different environmental conditions–that is, the norms of reaction among individuals may be different–a phenomenon known as genotype by environment (g x e) interaction. Norms of reaction and phenotypic plasticity are widely discussed in the ecological and evolutionary literature (see Schmalhausen (1986, originally published in 1949) and Schlichting and Pigliucci (1998)), and these concepts clearly have a close relationship with GxE, which is of interest to plant breeders. The variation of a given genotype across a field can be easily visualized by looking at a site-specific management GIS map of yield, for example.
Although as scientists we might be interested in what genes control a phenotype, and even in trying to manipulate those genes more efficiently, the end result–grain in the bin, sod on the lawn, or hay in the barn–is the phenotype, and that’s what we want. Different genotypes can lead to similar phenotypes, or as is often said: “There are many roads to Toledo,” and as long as we get there, the way we did is not so important. That said, knowing how the genes interact with the environment to lead to a particular phenotype may help improve the phenotype more efficiently by augmenting our selection methods.
The goal of plant breeding is to develop and use methods that effectively select for the best phenotypes leading to the development of improved cultivars. Generically, plant breeding includes two major facets: (1) the selection of superior plants in a manner that is both quick and accurate, and (2) the sexual recombination of the selected plants in order to generate new genotypes (and hopefully phenotypes).
One of the early commentators on plant breeding and plant breeders is, not surprisingly, Charles Darwin. In Chapter 1 of On The Origin of Species (1859), he makes a number of telling comments on breeders, both plant and animal. In particular, he mentions that the aspiring breeder must have a good eye, paying close attention to detail in order to identify the superior variants that will lead to the gradual improvements by selection. He further notes that in order for breeders to be successful in their selection, phenotypic variation must be present, and the more the better. Finally, in order to identify those favorable variants, Darwin recommends observing large populations of plants.
Darwin, C. 1859. The Origin of Species Prometheus Books, New York.
Schlichting, C.D., and M. Pigliucci. 1998. Phenotypic evolution: A reaction norm perspective Sinauer Associates, Sunderland, MA.
Schmalhausen, I.I. 1986. Factors of Evolution: The theory of stabilizing selection University of Chicago, Chicago.
Smartt, J., and N.W. Simmonds. 1995. Evolution of Crop Plants. 2nd ed. Wiley, New York.
Zohary, D., and M. Hopf. 2000. Domestication of plants in the Old World. 3rd ed. Oxford Univ. Press, Oxford.