植物育种2.doc

Plant Breeding: Historical Perspective 25 CHAPTER 1Plant Breeding:Historical PerspectivePlant breeding can be defined as the application of techniques for exploiting the genetic potential of plants. Success in plant breeding depends on the ability to identify promising parents, to combine desirable attributes through hybridization, and to recognize and select effectively among segregating populations. It has taken humans many centuries to discover these basic concepts and to develop systems of plant breeding. Plant breeding as a science is new and is still under development, with plant biotechnology and molecular biology recently added as new tools. Even before any of the basic concepts of plant breeding were understood, before anything was known about genetics and the gene, 1 and before sex and hybridization in plants were recognized, considerable progress in plant improvement occurred. This progress can, by definition, be said to be plant breeding, and by transforming wild plants into economically desirable crops, early humans achieved the first major accomplishment of civilization, namely, agriculture. It was plant domestication through plant breeding that allowed agriculture to develop. This chapter provides the historical background that led to the enormous advances in plant breeding about A.D. 1900. That history reveals a painstaking process that began 8,000 to 10,000 years ago and was accomplished as an art with out the benefit of science, which was not applied to plant breeding until the discovery of the laws of genetics in 1865. How success was achieved is largely speculation.The normal reaction to a study of the historical development of plant breeding, especially a study covering the period when plant breeding was more an art form than a science, is that it will serve no useful purpose. On the contrary, its historical development is an integral part of understanding modern plant breeding. To ignore the past is like moving blindfolded into the future. History provides vision, and “where there is no vision, the people perish” (Proverbs29: 18, King James Version). History can enrich the imagination of the plant breeder, for it is with imagination that a dream can be turned into reality. 1. Words or terms that are defined or explained in the Glossary are set in boldface the first time they appear in the text and at strategic points throughout the text.THE EARLIEST STEPS IN PLANT BREEDING Wild plants usually have small, unappealing seeds that shatter at maturity to ensure seed dispersal and perpetuation of the species in the wild. Considerable improvement was needed to transform such plants into economically desirable forms with large, edible seeds or fruit that were retained on the plant after maturity. Variability undoubtedly existed, but whether humans recognized this variation and deliberately selected for it or whether progress was indirect and unconsciously conducted is open to debate. Amold (1985) suggested that all crops were developed by humans through conscious selection. Jenkins (1966) argued that very little if any conscious selection occurred. The opportunity for speculation about the early forms of plant breeding is considerable, the key point being that progress did occur. Neolithic farmers may have adopted a series of agricultural practices that embodied selection for improved plants. Harvesting the seed of wild plants for food purposes has a negligible effect, if any, on the genetic structure of the population. It is only when such seed subsequently is sown that a genetic shift can be expected (Harlan, de Wet, and Price, 1973) (Figure 1.1). Exactly when humans unraveled the great secret of managed growth and changed from collecting seed of wild plants for food to placing seeds in the soil and tending their growth cannot be determined accurately. However, such controlled production should not be viewed as a single act or discovery but rather as a process that began in several areas (Harlan, 1975). The transformation to nonshattering types, that is, those that retained their seeds at maturity, was neither a simple nor a rapid process but one that occurred over a considerable time span. Other features such as seed dormancy needed modification before plants were agriculturally acceptable. Dormancy is a biological mechanism that ensures the survival of seed until conditions occur that are favorable for establishment and growth. Seeds that were dormant might be lost to a subsequent population seeded by humans. Other seeds having specific requirements may have been influenced to germinate by cultural practices, such as varrying the seeding depth, that in turn modified light, temperature, and moisture requirements. Again, selection occurred under human influence but not as an intentional act. Perhaps it was in the area of seed size that deliberate human action exerted conscious selection and direct plant breeding. Large seeds would be favored over small seeds, and humans might select the large seeds for planting. If seed size was genetically controlled, a large-seeded population eventually would emerge (Figure 1.2). Our knowledge of when and where such early developments in plant breeding took place is based on archaeological evidence. The first efforts to grow wild plants probably occurred in Southeast Asia and culminated in the domestication of some the many plants with which humans experimented in 13,000 B.C (Solheim, 1972). By 10,000 B.C. advanced knowledge existed of such plants as rice (Oryza stiva), several legumes such as the pea (Pisum spp), either the bean or the broad bean (Phaseolus or Vicia spp) and possibly the soybean (Glycine spp). It was at this point that crops became domesticated. Possibly ten separate genera of horticultural crops were domesticated in this region. Although the beginnings of civilization in China and Southeast Asia are obscure and fragmented (Treistman, 1967; Chang, 1981), the number of successful species domesticated indicates considerable success in plant breeding. Braidwood (1960) suggested that the first successful experiment in food production inFigure 1.1 it is still possible to find plants that are undomesticated and which, if successfully domesticated, may have considerable economic value. One such crop is wild rice (Zizania palustris L.), a grasslike plant found in shallow lakes and rivers in North America and highly prized by gourmets for its distinctive, nutty flavor and chewy texture.The feature that distinguishes a wild plant from its domesticated counterpart is the ability to disperse seed at maturity. Because wild rice shatters its seed, the traditional method of harvesting is to press the panicles against the gunwales of a canoe or boat and dislodge the seed into the bow of the boat. This nondestructive method of gathering seed allows harvesting to occur several times per season. Wild rice is an annual, and because so much seed is dropped, the stand for the following year is assured.This traditional method of harvesting has been going on for centuries, but if nonshattering was genetically controlled, on progress toward domestication could be expected. If harvesting was delayed so as to favor those plants that retained their seed, and if this seed was subsequently planted, a shift toward domestication might occur.Adding to the complexity of domesticating wild rice is the fact that seed cannot be stored dry for an extended period of time. Normally wild rice exhibits seed dormancy that may last three months or longer when stored wet.Deliberate selection for nonshattering seed habit led to the discovery in 1963 of a cultivar with reduced shattering habit. Although shattering losses still occur, the rise of a more shatter-resistant cultivar has permitted controlled production and the use of harvesting equipment similar to that for cultivated rice (Oryza sativa L.)Learning to domesticate the wild rice crop is analogous to the situation of crop development by early agriculturists.Photo courtesy Ontario Ministry of Agriculture and Food.Figure 1.2 Early agriculturists may have consciously selected large seeds from among a mass of seeds and if seed size was genetically controlled, progress through plant breeding would occur.The small wheat seeds on the left are those of einkorn (Triticum monococcum), one of three kinds of wheat originally domesticated in southeastern Turkey. It is a diploid with seven chromosome pairs. The wheat on the right is common wheat (Triticum aestivum L. em. Thell.). The seed of common wheat in this photograph is 3.6 times larger by weight than the seed of einkorn wheat. It is possible that a fivefold increase in weight per grain was achieved in the evolution of wheat from a wild grass.Other wheats that were domesticated are tetraploid forms (14 chromosome pairs) called emmer (Triticum dicoccum) and Triticum timopheevi, which has no common name. As humans manipulated these wheats, combinations were formed that produce the hexaploid common wheat with 21 chromosome pairs.The degree of transformation, as evident from this photo, is substantial, and although it took centuries to achieve, the plant breeding accomplishments of early agriculturists are significant.The sequel to controlled agricultural production was to free humans from the task of obtaining their daily food supply. Those so freed were able to achieve advances in art and technology, the ingredients of a civilization.Humans were able to domesticate wild species only if natural variability existed. A limited number of genera dominate the agricultural scene, possibly because of limited variation in other species. The Gramineae must have possessed a range of genetic variability, for eight genera make up a major share of important world crops, namely, Avena (oat), Hordeum (barley), Oryza (rice), Pennisetum (millet), Secale (rye), Sorghum (sorghum), Triticum (wheat), and Zea (maize). Photo courtesy Ontario Ministry of Agriculture and Food. Eurasia took place on the forest-free areas along the Tigris and Euphrates rivers in present-day Iraq, extending into Iran and southeast Turkey on the north and south into Lebanon, Israel, and the southern Jordan highlands. This horseshoe-shaped area is referred to as the Fertile Crescent and is an area where cereals and pulses show the rich diversity so essential for selection, a component of plant breeding. Imprints on clay vessels suggest that wheat, barley, rye, oat, and millet were produced between 9000 and 7000 B. C.The third site of independent agricultural development and plant breeding was in south central Mexico between 6700 and 5000 B. C. Squash and avocados were domesticated first (MacNeish, 1964), and in time a remarkable number of species were domesticated, with maize (Zea mays) as the foremost example.The evidence of independent domestication of diverse crops in three separate areas suggests that advances in plant breeding were not isolated accidents or that such advances occurred without conscious human effort. Domestication in the Middle East and China included the self-fertilized cereals and soybeans. These species have relatively small and inconspicuous reproductive organs, and it is highly doubtful that humans practiced any hybridization. Self-fertilization leads to homozygous plants, but opportunity for selection could arise from within the heterogeneous population that had developed by natural hybridization and spontaneous mutation.Maize is a cross-fertilized crop that was in an advanced state of development and extensively grown when the Europeans came to the Americas. Adapted maize cultivars2 extended from the southern part of South America to the north shore of the St. Lawrence River, from sea level to elevations of 3,355m (11,000 ft.). Types included flint, flour, pod, and popcorn as well as red, blue, black, yellow, white, and variegated kernels. There is no doubt about the competence in plant breeding of the American Indians, as it took the development of F1 hybrid maize of modern agriculture to exceed the performance capability of Indian maize. It is understandable that European settlers grew to respect the maize crop and its developers, for maize is said to be the greatest gift from the Indians.Exactly how the American Indian accomplished the monumental task of transforming the maize plant from an unresolved but probably primitive and unproductive grass (Figure 1.3) to a major food crop remains a mystery, for the details of how it was accomplished were not passed to European settlers. Weatherwax (1954) suggested the task was accomplished without the realization of what was being done, and hence how it was achieved was not understood. The credibility of this view is enhanced by the fact that the modification occurred over 4,000 years.Some features of the maize plant may have facilitated breeding advances compared to the small grain cereals in Europe, but this should not discredit the plant breeding accomplishments of the American Indian. A large ear, for example (Figure 1.3), provided a major asset in that selection pressure was directed toward individual plants that produced 200 to 1,000 progeny. In addition, the maize plant is cross-fertilized, which assures heterozygosity, yet large separate male and female sex organs provide for manual removal of the staminate parts of some plants to control pollination. Moreover, maize is a full-season crop, and as the crop was moved to short-season areas, natural selection eliminated plants that did not reach maturity. If humans practiced selection, it was directed to individual ears, in contrast to small cereals that are mass harvested and sown in broadcast stands. The combination of natural selection coupled with deliberate and either conscious or unwitting selection resulted in a highly desirable domesticated crop. For agriculturists, there was no turning back, for once a crop was domesticated it became dependent on humans for survival (Figure 1.4).Evidence exists that the American Indians did not understand sex and hybridization 2. In addition to the term cultivar, plant breeders use a number of terms to designate plants at various stages of development in a plant breeding program. Terms include line, clone, hybrid, type, cultigen, stock, biotype, selection, strain, race, breed, brand, accession, segregate. All are defined in the Glossary.A. B.Figure 1.3 Considerable speculation exists about the ancestry of cultivated maize. Three principal theories suggest that (1) the ancestor was a wild pod-popcorn, possibly extinct but the existence of which is based on archaeological evidence found in Mexican caves; (2) cultivated maize represents a domesticated, annual teosinte that is the closest relative of corn (the seed-bearing section is shown in Photo B); and (3) annual teosinte is not an ancestor but rather the product of the hybridization of Zea diploperennis with a maize plant in the early stages of domestication. The third theory was developed by Wilkes (1977,1985), based on the discovery in 1977 in Mexico of a previously unknown wild relative of maize, a perennial teosinte, Zea diploperennis (Photo A), having the same chromosome number as corn and cross-fertile with it. Wilkess hypothesis has gained support and has compelled almost all students of the ancestry of maize to modify their views (Nault and Findley, 1981-1982). Crosses between Zea diploperennis and a primitive Mexican popcorn of Zea mays produced annual teosinte plants. Evidence is strong, therefore, that the ancestry of cultivated corn is biparental, with Zea mays and Zea diploperennis serving as coequal ancestors (Mangelsdorf, 1986).The accomplishments of the American Indian in developing modern corn cultivars from the first crosses 4,000 years ago in Jalisco, Mexico, are indeed major plant breeding milestones. The extent of breeding progress can be appreciated by comparing annual teosinte ears in Photo A with yellow dent corn ears in Photo B. Photo A courtesy National Gardening Association, Vermont. A. B.Figure 1.4 In the development of primitive crop plants, wild, weedy, and domesticated forms can be found .wild forms can survive without help of humans; weedy forms survive because agriculture provides a suitable environment despite efforts to get rid of them; and domesticated forms require care and cultivation for survival. Domestication is associated with the loss of the brittle rachis in cereals or the brittle cob in maize, the structures on which the seeds are carried.The clumps of maize plants in Photos A and B are striking evidence of the fate of modern-day maize if left on its own for survival. The massive ear of modern maize cultivars is characterized by many rows of seed adhering tenaciously to a rigid cob that is enclosed by husks and adheres to the stock by a sturdy shank. When an ear of maize reaches the soil, it is usually intact and numerous seedlings emerge that are so crowded that they either choke each other or develop into numerous spindly plants that fail to reproduce seed. Photo B shows tassels but no productive ears. Domesticated plants are dependent on humans for survival.In contrast, teosinte, ancient form of maize, is a self-sowing wild plant that disperses its seed as the rachis segments form the disarticulating and slender cob. Teosinte seeds are arranged in two vertical rows on opposite sides of the rachis (distichons), as compared to polystichons in modern maize ears.Domestication of a crop should not be confused with crop cultivation. Domestication involves genetic changes that make the plant bette