A plant breeder tends to divide crop plant species into two groups: self-pollinating and cross-pollinating. A self-pollinating plant is capable of fertilizing itself, while a cross-pollinating one cannot. Breeding methods for self-pollinating plants are radically different from those for cross-pollinating ones. A self-pollinating plant tends to be highly homozygous because all of its genes came from the same parent. The plant's mother was also its father! As you might expect, these plants have undergone a significant amount of inbreeding. As the species has evolved, it has
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adapted to and thrives on inbreeding. On the other hand, a cross-pollinating plant is likely to be highly heterozygous, because its mother and father were different plants. Cross-pollinating species tend to require a high level of heterozy-gosity in order to be productive.
Self-pollinated crop plants include wheat, rice, oats, barley, peas, beans, tomatoes, and peppers. Some fruit trees, such as apricots, nectarines, peaches, and citrus fruits are also primarily self-pollinated. The most primitive form of breeding in this group of plants is called pure-line selection. This method simply involves collecting seed from each of several plants, growing all the seed from each plant in a row, and selecting the most desirable row. All the plants in a row will be related to each other, since they came from the same plant. The seeds from the best row can then be propagated as a new "pure-line" variety.
A breeder with an understanding of reproductive biology can make crosses between self-pollinating plants. In fact, today, most plant breeders create hybrid populations by crossing desirable parents. Then, as the hybrid plants self-pollinate, highly diverse groups of plants will be created. These offspring of the self-pollinated hybrids provide the basis for selecting pure-line varieties. Crosses between normal wheat varieties and dwarf plants have been created to develop dwarf varieties. More of the energy in these plants is directed toward producing seeds rather than stalks, resulting in higher yields.
Norman Borlaug, who is known as the "Father of the Green Revolution," was awarded the Nobel Prize in 1970 for developing new strains of wheat in Mexico. The new high-yielding strains, which were produced from crosses with a dwarf variety from Japan, were widely planted.
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Before the project began in 1944, Mexico had imported wheat for many years, but the new varieties were so successful that, within 20 years, Mexico had quadrupled its wheat production and had enough to export to other countries. India and Pakistan experienced similar gains. It has been argued that, by increasing grain yields in developing countries, Borlaug has saved more lives than anyone in history (Fig. 14.6).
Although the Green Revolution has been a dramatic success, gains have not been unqualified. Irrigation and inorganic fertilizer is needed to support the rapid growth of these plants. Because many inorganic fertilizers are produced from fossil fuels, the cost of farming increases with fuel prices. On the other hand, the high-input system of agriculture has allowed food production to increase without a corresponding increase in the amount of land used for farming. This means that in areas where population growth is strong, high-yield farming prevents deforestation of wild areas. The key to solving the food supply problem, of course, is to control population growth on a global scale.
Cross-pollinated species include corn, rye, alfalfa, and clover, as well as most fruits, nuts, and vegetables. The simplest form of selection in cross-pollinated crops is mass selection. With this method, many plants from a population are selected, and seeds from these plants are then used to create the next generation. Again, the seeds from the best plants are chosen and propagated, and so on, for many generations.
Stern-Jansky-Bidlack: I 14. Plant Breeding and I Text I I © The McGraw-Hill
Introductory Plant Biology, Propagation Companies, 2003
This slowly molds the genetic makeup of the population to fit the breeder's preferences. For example, suppose variations in seed size are due to genetic differences and the breeder always collects from plants with the largest seeds. After several generations of selection, the breeder has genetically altered the population so that a larger proportion of plants carry the genes for large seed size.
Outcrossing (crossing between genetically different plants) in cross-pollinated crops often results in hybrid vigor, or heterosis. These plants are large, vigorous, fertile, and high yielding. Conversely, cross-pollinated plants exhibit inbreeding depression in the form of small size, poor vigor, low reproductive capacity, and a high proportion of abnormal plants. Breeders, however, do not avoid inbreeding of cross-pollinated species. In fact, a major breeding method involves forced self-pollination for several generations to create inbred lines in which deleterious alleles have been eliminated. Then, selected inbred lines are crossed to produce hybrid seed. Using this method, the most dramatic success story to date involves corn (maize), in which crosses between unrelated inbred lines often produce hybrids that dramatically outyield their parents. In 1908, in one of the earliest hybrid corn studies, plant breeder G.H. Shull crossed two inbred lines of corn, each of which produced 20 bushels per acre. The hybrid offspring yielded 80 bushels per acre, a quadruple increase in yield. Most of the corn in the United States is grown from hybrid seed (Fig. 14.7).
Some early American and European varieties, called heirloom varieties, are grown as open-pollinated populations of plants. Each variety is a mixture of genotypes, and all plants are allowed to pollinate each other, or open-pollinate, during seed production. These varieties are not as uniform as modern varieties, but their genetic variability allows them to produce a crop under many different environments. For example, a dry year and an insect pest might severely compromise all the plants in a hybrid variety if the variety is not genetically capable of surviving these conditions. An open-pollinated variety is likely to have some plants that are adapted to drought and can resist insect damage. The Seed Savers Exchange in Decorah, Iowa, has been created to preserve and distribute heirloom varieties. Heirloom varieties are important for farm market producers and growers of organic crops. They also contribute genetic diversity to breeding programs.
Progress in plant breeding is absolutely dependent on genetic variability. It is impossible to improve a population if there is no genetic variability for the trait of interest. For example, a breeder who needs to develop rust fungus-resistant wheat must begin with a population containing at least one plant with some degree of resistance. Plant breeders are, therefore, concerned with the germplasm resources of crop plants. A crop plant's germplasm is the sum total of its genes. It is important to have access to crop plant germplasm containing f|H|N
Figure 14.7A Crosses between inbred lines (top) often produce high-yielding hybrids (bottom). Hybrid vigor is called heterosis.
genes for traits that are important for current breeding efforts. However, we must also realize that in the future, new traits may become important. For example, in a decade or two, we may need to develop plants with resistance to a new pathogen.
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The plant varieties used in agriculture today have resulted from centuries of selection for specific traits. They are often genetically uniform and, therefore, may not be good sources of new genetic variability for further advances in breeding. In addition, their homogeneity makes them vulnerable to outbreaks of pests. The Irish potato famine in the mid-1800s occurred because people in that region relied on a few varieties of potatoes for most of their food supply. When a fungal pathogen spread through the fields, all the plants succumbed, and the people lost their entire potato crop. As recently as 1970, 15% of the United States corn crop was wiped out by a fungal pathogen. Genetic uniformity in the corn varieties grown at that time resulted in this significant loss.
In order to meet current and future needs for plant genetic diversity, gene banks have been established worldwide. The International Plant Genetic Resources Institute is responsible for coordinating international efforts to collect and conserve crop plant germplasm. The Institute also assists regional and national gene banks. The collaborative efforts of state and federal government agencies, along with private industries, comprise the National Plant Germplasm System in the United States Scientists regularly conduct collecting expeditions in regions where wild relatives of crop plants are found (Fig. 14.8). They bring plant samples back to the gene banks, where they are catalogued, propagated, and screened for desirable traits. Some seeds or other propagules are put into long-term storage for future needs, while others are given to plant breeders for immediate use.
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