The vast majority of the human population and most civilizations have depended on the productivity of plant-based agriculture for sustenance, vitality, and quality of life during the preceding five to fifteen millenia. During this time, and especially since the rediscovery of Gregor Mendel's principles of heredity in the late nineteenth century, the genetic content (genome) of crop plants has become a more important resource for crop management and production. The dynamic genomes of crop plants contain tens of thousands of genes that interact with themselves and the environment to determine the many traits affecting crop productivity. Gradually, humans have learned that the genomes of crop species and their relatives contain a range of genetic variation for many traits; how the genomes are transmitted from parent to progeny; a few of the myriad relationships among genomes, genes, the environment, and traits; some methods and mechanisms for maintaining or modifying the genomes; and how to select, capture, propagate and deliver the desirable genetic variation in forms suited to the agricultural systems and their societies. Much remains to be learned and understood.
Plant breeding is the science ultimately concerned with the holistic and systematic creation of cultivars, cultivated varieties of plant species better suited to the needs and pleasures of human societies. In many ways, plant breeding is analogous to a large river system such as the Mississippi: it has a primary source (the gene pool of the plant species), a main river (the elite gene pool of plant breeding methods), tributaries (new technology and scientific disciplines), the ability to adapt to the prevailing conditions and forces of nature, and it is replenished by recycled water (germplasm, all genotypes of a species). Plant breeders devise and deploy methods that, in accordance with their resources, the nature of the plant and production environment, and the prevailing goals of society, integrate information and material from the tributaries to produce better cultivars. The scientific tributaries have included the biological (e.g., genetics, botany, biochemistry, plant pathology, entomology), physical (e.g., mathematics, chemistry, computer), and analytical (e.g., experimental design and statistics) sciences. Some important technological tributaries include methods for storing seed or other propagules, the computer for data analysis and management, and tools or machines for conducting the many experiments and evaluating the progeny needed to create a superior cultivar. New tributaries include genomics, molecular biology, and genetic engineering. While the tributaries have varied with the nature of the crop species and the resources and goals of the societies they support, the primary roles of the plant breeder—integration, evaluation, and selection—have been constant.
Components and Challenges of Plant Breeding Programs
Plant breeding programs consist of several steps that are usually conducted as reiterative procedures:
- hire talented and cooperative scientists (e.g., plant breeder, scientists in other disciplines, and technical staff)
- understand the ecology of the plant, the target environment, the system of crop production, and the consumers
- define the target environment for crop production (e.g., Where and how are the crops grown? What is the prevailing ecology therein?)
- assemble and maintain the necessary physical resources
- identify clear goals for selection regarding the type of cultivar, the traits, and their expression
- select or create testing environments representative of the target environment
- survey and choose germplasm to serve as parents and sources of genes (the crop and other species, cultivars, accessions [individual samples of seed] from germplasm reserves, and genes)
- identify and create genetic variation among the parents and their progeny by evaluating the parents, mating the parents, and evaluating their progeny and occasionally by modifying the parents' genome or introducing genes through genetic engineering and transformation
- evaluate and select the progeny that optimize production in the target environment.
When practiced on a continuous basis, these steps have achieved impressive results for several species and target environments.
Plant breeding programs negotiate numerous challenges along the path of improvement. The reproductive biology and growth habit of the plant are primary factors that dictate breeding methods, their implementation, progress from selection, and the type of cultivar (e.g., hybrid , pure line, clonal, population , or other). Some important considerations include the mode of reproduction (sexual, vegetative, or both), flower structure (perfect or imperfect), prevailing type of pollination (e.g., autogamous [self] or allogamous [other], wind, or insect), and methods to induce flowering, make controlled matings between the selected parents, and produce an adequate supply of progeny for evaluation and distribution. Considerations of the growth habit would include the length of the juvenile period (especially with trees) and if the species has an annual or perennial habit in the target environment.
The organization of the plant's genome also affects breeding strategy and the rate of progress. The plant genome is partitioned into the nucleus, mitochondrion (mt), and plastid (pt; e.g., chloroplast). The mt and pt genomes contain relatively few genes (hundreds) and in most angiosperm species are transmitted to the progeny exclusively through the cytoplasm of the female gametes (the egg cell in the embryo sac). The maternal inheritance of those genomes may dictate which parents are used as males and females. Plant nuclear genomes contain tens of thousands of genes as parts of several independent chromosomes, are inherited biparentally through the male (sperm nuclei in the generative cell of the pollen grain) and female gametes, and often contain more than two complete sets of chromosomes (polypoidy). For example, maize (Zea mays L.) and rice (Oryza sativa L.) are diploid because the nuclei of their somatic cells contain two complete sets of chromosomes, one each from the maternal and paternal parents. In contrast, cultivated alflalfa (Medicago sativa L.) and bread wheat (Triticum aestivum L.) are autotetraploid and allohexaploid because their somatic cells contain four (from the same species) and six (from three different progenitor species) complete sets of chromosomes, respectively. Polyploidy challenges breeders because it leads to more complex inheritance patterns and may hinder identification of desirable progeny in segregating populations.
The ecology of the target environment and the plant affect the evaluation and selection of parents and progeny in myriad ways (e.g., climate, soil, organic diversity, and the subsequent stress on crop production). The relative merit of the germplasm (e.g., parent, progeny, or cultivar) may vary greatly and depend upon certain elements of the environment (i.e., geno-type and environment interaction, GxE). For example, a disease-resistant cultivar may have superior productivity when evaluated in a disease-laden environment but the same cultivar may be inferior when tested in a disease-free environment. GxE is a major challenge for every plant-breeding program because so many factors could influence the plant's growth and productivity during its life cycle. GxE is managed by testing germplasm in samples of relatively few environments and treatments intended to resemble the prevailing conditions of the target environment. Inadequate testing may result in a poor choice of genotypes, less genetic progress, and, sometimes, truly inferior cultivars.
Accomplishments of Plant Breeding Programs: Some Examples
Plant breeders have achieved some significant genetic modifications of plant species. Crop domestication , although unrecorded for most plants, provided the critical foundation for subsequent cycles of distribution, adaptation, mating, and selection. Some products of those cycles include rice and wheat of short stature and increased yield, beets (Beta vulgaris L.) with increased sucrose concentration, Brassica napus L. with edible oil, the forms of Brassica oleracea L. (e.g., cauliflower, cabbage, kale, broccoli, kohlrabi, and brussels sprouts), and high-yielding maize (corn). The achievements with rice and wheat (the Green Revolution) significantly enhanced food production for billions of persons and were partially recognized in 1970 when Norman Borlaug received a Nobel Prize for his role in developing and promoting new cultivars of wheat.
In the United States maize is an example of a crop that has been quickly and significantly modified through plant breeding. Maize is a tropical grass domesticated by central American natives, possibly from the wild relative teosinte (Zea mexicana ). It was cultivated throughout the Americas before the colonization by Europeans, who adopted and expanded maize production. In the 1920s breeding methods changed dramatically: inbred lines (parents) were developed through generations of self-pollination and selection; the inbred lines were mated in specific combinations; many combinations were tested; a few combinations exhibited exceptional vigor and productivity; and the seed produced from selected matings was grown as the crop (i.e., the F1 or hybrid generation of the mating between the inbred lines). Previously, breeding methods in the United States emphasized selection of seed and individual plants produced through random, uncontrolled matings within locally adapted varieties and such open-pollinated seed was grown as the crop. In the 1930s farmers quickly substituted hybrid cultivars for the open-pollinated cultivars because the hybrids had higher and more consistent yield. Concomitantly, management practices changed. The average grain yield of maize increased from 30 to 120 bushels per acre from the 1930s to the 1990s. About 50 percent of the increase is due to genetic changes mediated by breeding for higher yield of grain, resistance to biotic and abiotic stress, and the ability to respond to more intensive management (e.g., increased application of fertilizer and seeding rates).
Plant Breeding Programs and Germplasm Reserves. The target environment and societies' goals sometimes change in dynamic and unpredictable ways that render existing cultivars obsolete. Cultivars with improved adaptation may be bred if genetic variation (i.e., genes and combinations thereof) exists for the traits of interest. To manage this uncertainty, germplasm reserves or gene banks have been established worldwide with the primary goals of collecting and maintaining the broadest possible array of genetic variation for economically important plant species. In the United States a network of Plant Introduction Stations are financed by the federal and state governments to provide this service. The reserves are important because:
- the gene pool of existing cultivars represents only a subsample of the genetic variation for a given species
- favorable genes are certainly contained in other gene pools
- human activity has reduced the native gene pools of most crop species and their wild relatives in agricultural and natural settings
- the reserves have provided useful genes
- methods for investigating gene pools have been crude but there are good prospects for improvement
- our ability to engineer genes and complete organisms to meet the demands of crop production is woefully inadequate and will need all the help provided by nature.
see also Breeder; Cultivar; genetic Engineering; Green Revolution; hybrids and Hybridization; Polyploidy; Propagation; Reproduction, Sexual; Seeds.
Crabb, Richard. The Hybrid Corn-Makers. Chicago, IL: West Chicago Publishing Company, 1993.
Fehr, Walter R. Principles of Cultivar Development, Vol. 1: Theory and Technique. New York: Macmillan, 1991.
Sauer, Jonathan D. Historical Geography of Crop Plants: A Select Roster. Boca Raton, FL: CRC Press, 1993.
Smartt, J. and N. W. Simmonds . Evolution of Crop Plants. Essex, England: Longman Scientific & Technical, 1995.
Wallace, Henry Agard, and William L. Brown. Corn and Its Early Fathers. Ames, IA:Iowa State University Press, 1988.
Reproduction for slaves was not a practice marked by love or marriage but rather a function of their status as property. Their reproductive rights belonged to their owners, many of whom viewed breeding as a profitable business venture. Animal imagery abounded: the strongest males were referred to as "bucks" or "thoroughbreds" and put out "to stud." Females were judged in much the same manner, their worth determined by either their skills as a cook and personal servant or their breeding potential. The hardier the stock, the higher the price for the breeding males and females, as well as for their offspring on the auction block. Healthy males sold for from $900 to $2000 each, with females of reproductive age (with thirteen to twenty being prime years) selling for as much as two thousand dollars.
Many slaveholders "mated" their robust males with as many females as possible to create a stable of sturdy slave labor. Physically powerful male slaves were feared and admired by white men and women, and their prowess and virility sometimes took on mythic proportions. These males were not allowed to marry or settle down and could be loaned out like a horse for stud duties. The Federal Writers' Project interviewer of the slave Charlotte Martin recounts what she told him about her owner: "Wilkerson found it very profitable to raise and sell slaves. He selected the strongest and best male and female slaves and mated them exclusively for breeding" (Born in Slavery, vol. 3, p. 167).
Newborn slave boys were inspected for their future usefulness, as J. W. Whitfield, a slave born in New Bern, North Carolina, explained: "When a boy-child was born … they would reserve him for breeding purposes if he was healthy and robust. But if he was puny and sickly they were not bothered about him" (Born in Slavery, vol. 2, p. 139). Charlotte Martin's interviewer confirms this observation: "The huskiest babies were given the best of attention in order that they might grow into sturdy youths, for it was those who brought the highest prices at the slave markets" (Born in Slavery, vol. 3, p. 167).
For slave women, reproduction could be harsher still—they could be forced to breed with one or more males or raped by slaveholders. Attractive bondwomen faced the worst circumstances. If they were pretty or shapely, they were desired by men of all races and ages and often hated by the white women of the homestead, who suspected their own fathers, husbands, and brothers would chase after them. It was always perceived to be the female slave's fault, however, if she was raped—she either seduced her attacker or was unable rise above her base, animalistic nature.
According to Alice Wright, an Alabama-born slave, breeding was of utmost importance at the plantation where she lived and worked. "My father said they put medicine in the water (cisterns) to make the young slaves have more children…. If his old master had a good breeding woman he wouldn't sell her. He would keep her for himself" (Born in Slavery, vol. 2, part 7, p. 246). Indeed, slaveholders frequently kept female slaves for themselves, not only for breeding purposes but as mistresses. It was an open secret that slave owners slept with female slaves; their mixed-race children lived side by side with their white offspring—unless a slave child bore too close a resemblance to the slave owner, in which case he or she was quickly sold. However, in some cases, as Charlotte Martin explained to her interviewer, slaveholders overtly proffered their mixed-race children, believing their biological input made these children more intelligent and useful than black slaves. The interviewer reports: "Sometimes the master himself had sexual relations with his female slaves, for the products of miscegenation were very remunerative. These offspring were in demand as house servants" (Born in Slavery, vol. 3, p. 167).
Observers of slavery from both within the United States and beyond its borders weighed in on the issue of the breeding of slaves. George Fitzhugh, a Virginian who championed slavery, quoted approvingly an editorial from the Edinburgh Review extolling the excellent treatment of black slaves in the United States as opposed to poor whites in England: "No man in the South, we are sure, ever bred slaves for sale. They are always sold reluctantly, and generally from necessity, or as a punishment for misconduct" (Fitzhugh 1857, p. 236). Josiah Conder, a Briton who opposed slavery in the British West Indies, found slavery objectionable not on humanistic grounds but because of the "fecundity of the negro race." As he wrote in his antislavery tract, blacks were an unruly race incapable of controlling themselves reproductively: "Nature herself is the constant enemy of the slave-owner, threatening him continually with an inundation of his living capital, that shall destroy his profits, and ruin him with his own wealth" (Conder 1833, p. 24). In truth, slaveholders sold their property whenever they pleased, and if there ever were too many mouths to feed, a trip to the auction took care of the problem. For slave owners, the inhuman practice of breeding was in fact a profit-making venture.
Born in Slavery: Narratives from the Federal Writers' Project, 1936–1938. Manuscript Division, Library of Congress. Arkansas Narratives, vol. 2, part 7. Available from http://memory.loc.gov/ammem/snhtml/snhome.html.
Born in Slavery: Narratives from the Federal Writers' Project, 1936–1938. Manuscript and Prints and Photographs Division, Library of Congress. Florida Narratives, vol. 3, part 7. Available from http://memory.loc.gov/ammem/snhtml/snhome.html.
Conder, Josiah. Wages or the Whip: An Essay on the Comparative Cost and Productiveness of Free and Slave Labour. London: Hatchard, 1833.
Fitzhugh, George. Cannibals All! or, Slaves without Masters. Richmond, VA: V. Morris, 1857. (Repr., ed. C. Vann Woodward, Cambridge, MA: Belknap Press, 1960.)
breeding, in agriculture and animal husbandry, propagation of plants and animals by sexual reproduction; usually based on selection of parents with desirable traits to produce improved progeny. In conventional breeding, progeny inherit genes for both desirable and undesirable traits from both parents. Breeders conserve desired characteristics and suppress undesirable ones by repeatedly selecting meritorious individuals from each generation to be the parents of the next. This process leads to a population expressing a combination of inherited traits that distinguishes it from the rest of the species. In plants, such a population is described as a variety or cultivar; in livestock, it is called a breed. Purebreds result from one or more generations of inbreeding, or mating of close relatives, such as brother to sister or offspring to parent (backcrossing).
Inbreeding produces families or lines with increasing degrees of genetic uniformity, or homozygosity, in successive generations. In highly homozygous families, dominant genes are uniformly transmitted and expressed; recessive genes are also more likely to be expressed, and to produce undesirable traits, including loss of general vigor and fertility. In some plants, such as wheat, that are naturally self-fertilizing and homozygous, deleterious traits are readily eliminated by natural selection; there is no loss of vigor.
In naturally cross-pollinated or open-pollinated plants, and in animals, loss of vigor in inbred lines can be restored by outbreeding to unrelated or distantly related lines; a first-generation hybrid is more vigorous than either of its purebred parents. Animal breeders exploit the phenomenon of hybrid vigor, or heterosis, in producing crossbred cattle, sheep, swine, and other domestic animals. Much of the corn (Zea mays mays) grown in the United States and other agriculturally developed countries is the hybrid of two different inbred lines, or the double-cross hybrid of four inbred lines.
Selective breeding developed with the domestication of useful species during the Neolithic period: the oldest known remains of cultivated crops and domestic animals show signs of purposeful improvement. For centuries, selective breeding proceeded empirically. Beginning in the 18th cent. various breed associations formed to register purebred herds and flocks and keep track of pedigrees. Plant breeders collected seeds and documented their genealogies. The basic principles of heredity, originally published by the Austrian biologist Gregor Mendel (see Mendel, Gregor Johann) in 1866, were rediscovered in 1900.
With subsequent discoveries in genetics, and progress in artificial insemination and other breeding techniques, plant and animal breeding have become increasingly scientific. More recent advances in biotechnology and genetic engineering allow breeders to transfer specific genes and gene complexes among plants and animals, bypassing the limitations of conventional sexual reproduction. Knowledge of genomes and the techniques of genetics also enhance conventional breeding: In marker-assisted breeding, genetic markers are used to identify the desired characteristics in a plant while it is a seed or seedling, reducing the time needed to find the most promising individuals with those traits. Seeds and seedlings selected using marker-assisted breeding must still be grown and evaluated and then subjected to field trials in a variety of growing regions to determine their ultimate value.
breed·ing / ˈbrēding/ • n. the mating and production of offspring by animals: palolo worms use the moon to time their breeding. ∎ the activity of controlling the mating and production of offspring of animals: the breeding of rats and mice for experiments. ∎ training and education, esp. in proper social behavior: a girl of good breeding. ∎ the good manners regarded as characteristic of the aristocracy and conferred by heredity: a lady of breeding.