Diversifying Selection in Plant Breeding

Open access, freely available online " Some qualities nature carefully fi xes and transmits, but some, and those the fi ner, she exhales with the breath of the individual as too costly to perpetuate. But I notice also that they may become fi xed and permanent in any stock, by painting and repainting them on every individual, until at last nature adopts them and bakes them into her porcelain " —Ralph Waldo Emerson T he history of domesticated plant form and function evolves along a two-tiered track that doubles back on itself, offering panoramic vistas of natural forces intertwined with the creative force of human endeavor (Figure 1). For approximately 10,000 years, human beings have modifi ed the traits of plants and animals, giving rise to hundreds of thousands of domesticated breeds that today form the foundation of the world's food supply. Modern breeds are descendents of the wild species from which they were derived. The process of domestication dramatically changed the performance and genetic architecture of the ancestral species through the process of hybridization and selection as originally described by Charles Darwin (1859). Despite the low yields and poor eating quality of most wild ancestors and primitive crop varieties, these ancient sources of genetic variation continue to provide the basic building blocks from which all modern varieties are constructed. Breeders have discovered that genes hidden in these low-yielding ancestors can enhance the performance of some of the world's most productive crop varieties. In this essay, I will provide some historical context for the paper by Gur and Zamir in this issue of PLoS Biology (Gur and Zamir 2004). I will discuss how " smart breeding " recycles " old genes " to develop highly productive, stress-resistant modern varieties and why this approach is particularly attractive to increase food security in regions of the world with high concentrations of genetic diversity. The job of the plant breeder is to create an improved variety. This may be accomplished simply by selecting a superior individual from among a range of existing possibilities, or it may require that a breeder know how to effi ciently swap or replace parts, recombine components, and rebuild a biological system that will be capable of growing vigorously and productively in the context of an agricultural environment. How the breeding is done and what goals are achieved is largely a matter of biological feasibility, consumer demand, and production …