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Genetic Resources Conservation Program, Univ. of California, One Shields Ave., Davis, CA 95616
* Corresponding author (abdamania{at}ucdavis.edu).
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ABSTRACT |
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Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • FAO, Food and Agriculture Organization of the United Nations • SDS, sodium dodecyl sulfate
Genetic Resources Conservation Program, Univ. of California, One Shields Ave., Davis, CA 95616
* Corresponding author (abdamania{at}ucdavis.edu).
Received for publication August 17, 2005.
This paper has been written to look back at the early period of crop genetic resources conservation and inform readers of what has been achieved so far and what needs to be done in the future. The recognition of the value of crop genetic resources and early efforts at collecting germplasm by pioneer plant explorers, such as F.N. Meyer and N.I. Vavilov, and some of the strategies they employed are described. Historic examples of collection, evaluation, and utilization of genetic resources, notably by the late J.R. Harlan and other U.S. agronomists, are highlighted. The use of wild progenitors in improving biotic and abiotic stress tolerance has been covered. Previous and present status of genetic resources collection and storage, both ex situ in gene banks and in situ in the natural habitats of crops and their wild progenitors, is discussed. The creation of agrobiodiversity is a dynamic process and hence the work on conservation of genetic resources has to continue. With the increasing use of biotechnology in crop improvement, the value of germplasm already collected and conserved will substantially increase as researchers seek out new sources of useful genes in the future.
Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • FAO, Food and Agriculture Organization of the United Nations • SDS, sodium dodecyl sulfate
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INTRODUCTION |
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Agriculture as we know it is a relatively recent historical phenomenon that began just over 10,000 yr ago in the Fertile Crescent of West Asia and later in Central America with the domestication of plants and animals from wild species (Cavalli-Sforza, 1996). Subsistence farming took root in West Asia with wheats (Triticum spp.), barleys (Hordeum spp.), lentils (Lens spp.), and chickpeas (Cicer spp.), whereas corn (Zea mays L.), squashes (Cucurbita spp.), and beans (Vicia spp. and Phaseolus spp.) were grown by early farmers in Central America (Harlan, 1995). But it was only after the first cities were established and the rapid increase in world population as a result of urbanization that farmers began to produce more food than what was enough to feed their own families to meet the demands of the city dwellers (Diamond, 1997). Through the increase of food availability after the beginning of agriculture during the Neolithic period, the human species has incredibly multiplied its own numbers at the expense of the rest of the world's biota. Today, however, large tracts of land of the former Fertile Crescent are desert, semidesert, steppe, or heavily eroded soil or salinized terrain unable to produce enough food to support its own human population despite its early lead (Diamond, 1997).
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CROP DOMESTICATION AND EVOLUTION OF LANDRACES |
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After domestication, the habitats of wild progenitors and other wild relatives of crop plants were neglected (Damania, 1994). Farmers considered them weeds and eliminated them from their fields, although they were often found on the borders or vicinity of cultivated fields, exchanging genes with the domesticated species (Gepts, 1998). However, uncontrolled grazing and periodic slash and burn practice also led to the severe genetic erosion and destruction of their habitats in their centers of domestication (Damania, 1994).
For thousands of years, early farmers selected the best-looking plants from their fields for use as a seed source for the next season's sowings. In the course of selection, season after season, the farmer gave preference for certain traits, such as seed color, size, and taste, that are heritable from one year to the next (Bennett, 1970). This was the mechanism that gave rise to the landraces. Once this landrace diversity is replaced by modern crop varieties of any kind, their natural gene pool is lost forever, unless the landraces have been conserved in situ in the field or collected and stored ex situ in a gene bank for future use for plant breeders, together with the species of wild progenitors from which each crop was domesticated (Lev-Yadun et al., 2000).
Plant breeding based on means other than mere selection in the farmers' field did not begin until the mid-1800s. This activity gathered pace after the turn of the century and breeders were already using wild and primitive forms in breeding programs following the rediscovery of Gregor Mendel's work on genetic inheritance of the pea plant (Pisum sativum L.) (Maliani and Bianchi, 1979). After World War II, massive food aid and research programs initiated by private foundations based in the United States, such as the Rockefeller Foundation, with the aim to increase crop production in the developing countries led to the Green Revolution wheat and rice programs in the 1960s and 1970s. The research led to high-yielding homogenous and homozygous inbred wheat and rice cultivars at international centers in Mexico and the Philippines, respectively, that were developed from self-pollination from crosses (hybrids) of various promising lines. This approach increased yields of these crops dramatically, especially in South Asia, in a short period of time (Ganguly, 1999).
However, this introduced crop germplasm increased yields at the cost of local landraces that were the indigenous products of several hundred years of development and selection. During the last four decades, erosion of genetic resources has taken place at a pace rapid enough to cause the elimination of numerous old crop varieties and diverse obsolete forms in their centers of genetic diversity—one might even now say, with some justification—centers of former diversity. For example, improved Italian and other cultivars have replaced 97% of the indigenous durum wheat [T. turgidum var. durum (Desf.) Yan ex P.C. Kuo] varieties in Greece (Biesantz et al., 1990). Also, all the sorghum [Sorghum bicolor (L.) Moench] races of South Africa have disappeared after the introduction of white high-yielding varieties from Texas (Plucknett et al., 1987). Fortunately, as a result of germplasm collection missions initially performed by the Food and Agriculture Organization of the United Nations (FAO) and later by the national germplasm conservation programs, such as the USDA, most of this material is now conserved in gene banks and available to breeders for crop improvement programs (Plucknett et al., 1987).
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EARLY EXPLORATIONS FOR GENETIC RESOURCES |
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Under direction of Sir Joseph Banks, the British performed many collecting expeditions and brought back plants to botanic gardens, especially to the Royal Botanic Gardens at Kew near London. They also established the botanic garden at St. Vincent in the Windward Islands in the Caribbean. It was this garden that was responsible for the introduction of the breadfruit tree [Artocarpus altilis (Parkinson) Fosberg] to the neotropics. Captain William Bligh, of the Royal Navy, brought six varieties of the breadfruit tree to St. Vincent in 1793 from the Tahitian islands, after his earlier attempt in 1789 had failed when his crew mutinied and seized his ship, the HMS Bounty. It is said that the 1789 mutiny was partly due to the strict rationing of water to the crew in favor of the 1000 breadfruit seedlings and partly due to the Captain Bligh's harsh treatment of his crew. Following the seizure of the ship, the mutineers threw 1000 seedlings overboard in to the sea despite vociferous protests by the two horticulturists from the Royal Botanic Gardens on board (Guarino et al., 1995). Bligh was later exonerated in a military trial for the loss of his ship and its cargo, and was given another command whereby he was able to fulfill his aborted mission (Alexander, 2003).
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DANGERS OF NARROWING OF THE GENETIC BASE AND CULTIVAR UNIFORMITY |
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The French later also introduced coffee to Ceylon (the modern Sri Lanka), which was soon to become a British colony. In 1869 the coffee rust, Hemileia vastatrix Berk. & Broome, invaded Ceylon. The Ceylonese coffee plants, like their clones in Brazil, had very little genetic resistance and the losses were so devastating that the industry could not recover. The British plantation owners soon turned to growing tea (Camellia spp.) instead, after having established two botanic gardens that played a crucial role in new introductions at Peradinya and Hakgala, the latter at a higher elevation than the former. Latin America remained free of the coffee rust until 1970, when it suddenly made its appearance in southern Brazil. In 1994, the price of coffee went up considerably due to reduction in supplies as a result of the partial failure of the crop in Brazil brought about by attacks of the coffee rust disease. Coffee breeders were forced to turn to wild-growing coffee plants for resistance genes. Recently resistant hybrids have been obtained by crossing cultivated coffee C. arabica Benth. with a wild form C. robusta L. Linden (=C. canephora Pierre ex A. Froehner) that is still found in Uganda and on the Indian Ocean island of Mauritius (Damania, 1985).
There has been a clear trend toward morphological and genetic uniformity in the modern varieties during the last 150 yr, resulting in the narrowing of the genetic base and exposure to attack from new pathogens against which the crop has no immunity. In other words, a narrow genetic base of a crop variety may lead to an increased risk of losses caused by new strains of parasites, insect pests, and diseases (biotic stress) (Peterson and Higley, 2000), or due to unusual environmental phenomenon such as temperature extremes or the greenhouse effect (abiotic stress) (Jenks and Hasegawa, 2005).
For example, in two successive years, 1845 and 1846, the late blight disease caused by a fungus [Phytophthora infestans (Mont.) de Bary] cut Ireland's potato (Solanum tuberosum L.) production by half, resulting in a famine that cost hundreds of thousands of lives and triggered mass emigration, mainly to North America (Miller and Wagner, 1994). The potato is a native of South America where it has its center of diversity but is grown throughout the world. In the two years mentioned above, the potato crop was devastated throughout Europe, but the population of Ireland was especially vulnerable. There was nothing else to eat, since the Irish cultivated potatoes exclusively as England controlled supplies of other foods. Between 1846 and 1851 at least 1 million Irish died of starvation or associated diseases and another 1.5 million left the country (Miller and Wagner, 1994). The potato crop was vulnerable because only a couple of varieties with a narrow genetic base were planted in the entire country. Clonal propagation of potatoes allowed the cultivation to continue up to that time with a narrow genetic base relying on a single cultivar derived from closely related parentage (Hawkes, 1994).
Consequently, wild genetic resources of potato, frantically collected mostly from the Andes in Peru (Hawkes, 1958), have been used to produce resistance not only to the late blight but also to other diseases, such as bacterial wilt, scab, leaf roll, and a number of other pathogens. Late blight is still around and periodically attacks potato crops in the United States with new, more aggressive strains, constituting a threat to potato production all over the country and the world. But this time we have diversity in wild relatives of the potato to combat this disease and limit crop losses to acceptable levels.
To mark the centennial year of Charles Darwin, a symposium at the Chicago meeting of the American Association for the Advancement of Science (AAAS) took place 28–31 Dec. 1959. The program of this meeting included a Section on Agriculture that was to (i) review the plant and animal germplasm resources useful to U.S. industries, (ii) measure the progress made in the genetic improvement of these resources, and (iii) to chart a course for the future development for the conservation of plant and animal genetic resources to provide food and fiber security for the peoples of the world (Hodgson, 1961).
A landmark conference sponsored by the International Biological Program and FAO was held in Rome 18–26 Sept. 1967. Resulting from papers presented at this meeting by the foremost experts in the field, a book has formed the basis of modern genetic resources conservation efforts, and is sometimes referred to by students as the "Frankel and Bennett Bible" (Frankel and Bennett, 1970). The alarm bells regarding disappearance of the world's vast and often undervalued genetic resource were sounded and a call for action given (Zedan, 1995).
In the former Soviet Union, due to a period of consecutive mild winters during the late 1960s, high-yielding winter wheat ‘Bezostaya’ (T. aestivum L.) spread rapidly outside its original area of adaptation, so that by 1972 it occupied about 15 million hectares. But, in that year harsh winters returned with a vengeance and damage to the crop was extremely high, forcing that country to resort to imports to make up the deficit and avoid great human hardship. These catastrophic events and the general vulnerability found in our major crops globally due to their narrow genetic base led to a series of technical meetings at the FAO, which reviewed the situation and made important recommendations (Frankel and Bennett, 1970).
In 1972, the Consultative Group on International Agricultural Research (CGIAR) convened an expert working group at Beltsville, Maryland, which urged the creation of the International Agricultural Research Centers (IARCs) for crop improvement in developing countries, and the collection and conservation of the germplasm of the world's most important food and feed crops. They recommended that a global network of crop genetic resources conservation centers (gene banks) be established. The creation of the International Board for Plant Genetic Resources (IBPGR) in 1974, with headquarters at FAO, Rome, Italy, was an outgrowth of the CGIAR's involvement in this effort (Frankel and Hawkes, 1975). This began a great international effort to collect and conserve world's crop genetic resources before they became extinct (Zedan, 1995).
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GENETIC RESOURCES COLLECTION AND GREAT PLANT EXPLORERS OF THE LAST CENTURY |
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Vavilov's work (Vavilov, 1992) has formed the basis of much of the study of plant genetic resources that is performed today. It is mainly due to him that plant breeders became aware of the fact that variation in cultivated plants is geographically unevenly spread and that the bulk of the genetic diversity in our important agricultural and economic crops is geographically confined to relatively few nuclear areas he termed the Centers of Origin. However, Zohary (1970) pointed out that the concept of Centers of Origin should not be confused with Centers of Diversity. While Centers of Diversity are a biological fact, the term Centers of Origin is only a Vavilovian interpretation. Vavilov meticulously organized and took part in over 100 plant collecting missions. His co-workers, including S.M. Bukasov and M.G. Popov, conducted other missions. His major expeditions outside the Soviet Union included those to Iran (1916), Central and South America, the United States (1921, 1930, 1932, respectively), Afghanistan (1924), and the Mediterranean, including Lebanon, Syria, and Ethiopia (1926–1927), and Central Asia (1929) (Vavilov, 1992). These meticulously planned and executed missions for crop plants that would improve agriculture in the Soviet Union were based on Vavilov's own concepts in the sphere of evolutionary genetics (i.e., the Law of Homologous Series in the Inheritance of Variability, Vavilov, 1951, and the theory of the Centers of Origin of Cultivated Plants, Vavilov, 1992). As a result of his firm and unwavering confrontation with the authorities regarding his views on genetics, Vavilov was arrested in August 1940. He died in a cell in the Saratov prison in January 1943. Today, many consider Vavilov to be the father of crop plant exploration and collection. Others that came after him used his genetically diverse collections to breed new varieties of crop plants (Hawkes, 1983).
In 1906 Aaron Aaronsohn discovered wild progenitor of durum wheat, T. dicoccoides (Korn. G. Sweinf.), close to the town of Rosh Pinah in northern Israel at an altitude of 1213 feet (Aaronsohn, 1910). Following this discovery, the USDA sent Orator F. Cook in 1910 to Syria and Palestine (Cook, 1913) to collect the same wild wheat which he called T. hermonis Cook since Aaronsohn had discovered it on the slopes of Mt. Hermon. A large number of ears were collected by Cook and sent back to the United States (Cook, 1913). Later, during World War II, the USDA sent Harvard ethnobotanist Richard E. Schultes to the Amazon River basin in South America to search for rubber [Hevea brasiliensis (Willd. ex A. Juss.) Mull. Arg.] germplasm. The attack on Pearl Harbor in 1941 and the loss of the Philippines had cut U.S. access to the products of rubber plantations of the Far East. Synthetic rubber tires on heavy airplanes cracked on impact with the runway on landing and only tires with natural rubber compounds would survive. Although the main source of rubber is H. brasiliensis, there are eight other plant species that could also be used and it was these that Schultes was after in the Amazon region. I describe what happened in his own words during one of his expeditions in 1947: "About 5 AM there was a jolt and a loud sound of crashing and splitting wood. The barge ran into a leaning tree along the river's edge and the already badly damaged cabin was smashed. With my flashlight, I saw that the tree was in young fruit, with a recently fertilized ovary that is, so I broke off a few branches. When dawn came, I examined the plant—it was Micranda Bak. var. minor G.J Lewis that I was especially anxious to collect!" This incident is quoted to emphasize the point that plants that may be illusive for several days can be chanced on quite suddenly and dramatically (Schultes and Raffauf, 2004).
Recently, tomato (Lycopersicon esculentum Mill.), which is grown over large areas in California, has become a classic example of a cultivated plant that has been markedly improved by the introduction of genes from obsolete cultivars and wild relatives. Resistance to scourges of tomatoes, such as Fusarium and Verticillium wilts, and to nematodes, was bred from these sources by Charles M. Rick in the Department of Vegetable Crops at the University of California, Davis. Rick also collected a wild tomato, L. cheesmanii Riley, in the Galapagos Islands in 1971. This species, which was found growing only 5 m from the sea, turned out to be an extremely good source of salt tolerance in tomatoes. Another wild tomato [L. pennellii (Correll) D'Arcy], found growing among rocks and cacti in Peru, was a potential source of drought resistance for California varieties (Rick, 1973).
Another interesting exploration and utilization success story that could be mentioned here concerns the late Paulden F. Knowles. Knowles, then a scientist working on oilseed crops in the Department of Agronomy at UC Davis, traveled to Upper Egypt in June 1958 to collect safflower (Carthamus tinctorius L.) and other oilseed crop germplasm. While devoting a major portion of his exploration time and effort toward safflower, Knowles, like all efficient explorers, also collected genetic resources of other crops useful to California and the United States, such as flax (Linum spp.), sunflower (Helianthus spp.), castor (Ricinus communis L.), lettuces, sesame (Sesamum indicum L.), peanut (Arachis spp.), soybean [Glycine max (L.) Merr.], forages, cereals, and food legumes either from the fields or farmers' storehouses. Knowles was made aware of the oilseed-producing lettuces of Egypt when he happened to visit the Plant Breeding Station (PBS) at Giza (near the Great Pyramids just outside Cairo) and met Mustapha Serry who was in charge of the Oil Crops Section of the PBS at that time. During discussions, Serry mentioned a primitive form of lettuce (Lactuca sativa L.) being grown in Upper Egypt for its seeds from which edible oil was extracted. Sensing an opportunity for collection of a germplasm potentially useful for crop improvement in California and the United States, Knowles requested and received a sample of the seeds of this plant from the PBS nursery. Serry also informed him that seeds of this particular sample contained 30% edible oil and the plants did not show any disease symptoms in Egypt. Knowles noted this piece of information and wasted no time in traveling to Upper Egypt the following week (Knowles, unpublished report, 1959).
In Upper Egypt, Knowles collected several more seed samples of a primitive lettuce form, Lactuca serriola L. (or prickly lettuce), from farmers' stores or traders' warehouses since all of the harvest had already taken place. Knowles also collected nine safflower and one castor seed sample at Kena. This germplasm from Egypt was brought to the United States and subsequently evaluated for disease resistance at University of California–Davis. The prickly lettuce sample collected from Kena, Upper Egypt, possessed strong resistance to lettuce mosaic virus. The resistance was attributed to a recessive allele called mo. Subsequently, crosses with California lettuce cultivars were made. Thus, the gene responsible for resistance to lettuce mosaic was transferred to four lettuce cultivars grown in the western United States (Vanguard 75, Winterset, Autumn Gold, and Salinas 88), resulting in considerable savings to the U.S. farmers (Knowles, 1959). This sample (PI 251245) and others collected by Knowles have been multiplied and are now conserved in the USDA Lettuce Genetic Resources Collection at the Vegetable Production Research Unit, Salinas, CA, and also duplicated for safety at the Western Regional Plant Introduction Station, Pullman, WA (Qualset and Shands, 2005).
During the mid-1970s, 96% of green pea produced in the United States came from only two varieties and nearly 69% of sweet potato [Ipomoea batatas (L.) Lam.] was based on a single variety. However, the corn blight disaster had alerted plant breeders, and efforts were made to broaden the genetic base of crops and diversify varieties grown (Tatum, 1971). But, with the arrival of genetically modified crops such as corn, cotton (Gossypium spp.), and soybean, the need to conserve our genetic resources is greater than ever before. Because, if the recent biotechnologies including genetically modified organisms fail, farmers could revert to their original varieties obtained from conservation centers.
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STRATEGIES FOR SYSTEMATIC COLLECTION OF GENETIC RESOURCES |
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The problem facing most germplasm collectors today is how to sample genetic resource material to include an adequate amount of useful variability and still manage to keep the number of collected samples to a practical size. This is an urgent problem especially in cases where the plant population to be collected may not be there if the collector returns to the site a dozen or so years later. For instance, I retraced Vavilov's route in Syria in 1991 with Soviet scientists from St. Petersburg who had brought along detailed information including photographs taken by Vavilov in 1926 (Vavilov, 1997). Approximately 65 yr later, we could neither find the villages Vavilov had mentioned, nor could the local villagers recognize the places where the pictures were taken. It is imperative that sampling strategies to collect the most useful germplasm occurring among different geographical areas as well as within those areas be determined. The information on patterns of distribution, extent within cultivated and wild populations, and ways to maintain variability once captured is vital. Collections of the same species made from widely separated geographical areas usually differ from one another in morphological characteristics, but certain characters occur much more frequently in some areas than in others. Various sampling strategies for genetic resources collection have been listed and discussed by Porceddu and Damania (1991).
In recent years biological conservation efforts have typically emphasized habitat protection as a crucial means of maintaining the fast-eroding genetic diversity in the face of global warming and subsequent climatic change. There are two basic methods for conservation of crop plant genetic resources, in situ (or on site) in nature without any human major disturbances, and ex situ (off site) in gene banks at subzero temperatures and low seed moisture content. During the last three decades, man-made disturbances have sharply eroded the original habitats of most field crop species and their wild relatives in the centers of diversity. Until recently, off-site conservation in gene banks was exclusively used for the preservation of crop plant genetic resources. However, some biologists are of the opinion that further evolution of the germplasm ceases once it is preserved in gene banks. To counter this line of thought, in situ methods have been proposed where evolutionary processes continue to operate in nature, allowing the germplasm to adapt itself to the changing environment. This method of conservation is gaining acceptance as it not only preserves the wild progenitors of our major crops on site but also conserves their habitat through frequent monitoring and limited grazing (Damania, 1994). The biological diversity of the wild progenitors is to be found in some areas in the Near East within the Fertile Crescent.
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GENE BANKS AND EX SITU CONSERVATION OF GENETIC RESOURCES |
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In the United States, collection and evaluation of germplasm has been performed since the 1900s, but the first repository of crop seeds was not established until 1947 when the Regional Plant Introduction Station in Ames, IA, began operation. In 1958, the first national facility for storage of germplasm at low temperatures in cold rooms was constructed at Fort Collins, CO. These facilities were recently upgraded (Qualset and Shands, 2005).
An IBPGR survey in 1975 revealed that there were only eight long-term genetic resources conservation centers globally (almost all of them in the industrialized countries). But only 7 yr later, the total had jumped to 33, and today there are over 1000 major germplasm collections in gene banks all over the world (Qualset and Shands, 2005).
Crop genetic resources grown from true seed are stored in three main types of gene banks. In long-term gene banks, whose aim is to store material for 50 to 100 yr, samples are kept at –10 to –20°C in airtight containers. Usually before such storage, the seed samples are dried to a moisture content of 5 to 7%. In medium-term facilities, as most working collections are stored, temperatures of 0 to 5°C are maintained and seeds may last up to 10 to 15 yr. The short-term collections or breeder's collection are usually kept in paper envelopes or cloth bags or tin cans at 5 to 15°C without any seed drying. Samples are constantly withdrawn for evaluation and distribution from the latter two collections, but those under long-term storage are rarely disturbed unless seeds are unavailable from the other two types of storage (Hanson, 1985).
Genetic resources of vegetatively propagated crops are difficult to maintain and require growing out continuously. In recent years, the collections of such crops have been maintained as tissue cultures. For example, it is now possible to store potato plants in test tubes. Cold temperatures and the use of certain culture media can slow growth in vitro. At 6 to 10°C, potato plantlets can survive for up to 2 yr. This method saves time and money and reduces risk of loss since the material need not be planted in the field every year (Volk et al., 2005).
Botanic gardens that conserved genetic resources once played a crucial role in introduction of germplasm of new crops from far off lands. They have lost their importance as centers of conservation in modern times as state-run crop improvement programs, international agencies, and private companies have taken over the work of exploration and collection (Plucknett et al., 1987).
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IN SITU CONSERVATION OF GENETIC RESOURCES |
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The Fertile Crescent, an arc that traverses Jordan, Israel, Palestine, Lebanon, Syria, parts of southern Turkey, and northwestern Iraq, is one of the world's main centers of crop biodiversity for very important crops such as wheat, barley, oat (Avena sativa L.), lentil, faba bean (Vicia faba L.), vetch, and several fruit crops such as olive (Olea europaea L.), fig (Ficus spp.), pistachio (Pistacia vera L.), almond [Prunus dulcis (Mill.) D.A. Webb], and plum (Prunus spp.). Here, naturally occurring populations of the wild progenitors and relatives of these crops can still be found. For a comprehensive list of important crop species with origins in these areas, see Damania (1998).
During their long history of propagation in crop fields in the centers of origin and primary diversity, landrace populations of major crop species were in close association with their wild and weedy relatives. They occasionally exchanged genes and enriched genetic diversity in respective species and evolved together, during which time they were most certainly exposed to a multitude of biotic and abiotic stress selection pressures. Recent studies have demonstrated extensive genetic diversity in the original populations of the wild gene pool in the natural habitat. Such extensive diversity cannot be preserved by the standard ex situ collection procedures (i.e., it is generally accepted that during field collection usually only about 50–150 plants per site are sampled and many genotypes may be left out). On the other hand, under in situ conservation, a much larger and continuously evolving genetic diversity is preserved. Indirect evidence from studies on landrace and wild cereal populations indicate that in situ methods should be effective for the conservation of genetic diversity in both cultivated and wild species.
To overcome the limitations of the ex situ collections, preservation of populations of crop wild relatives in their natural habitat is important for long-term benefits of national programs and the international community as a whole. The germplasm so conserved is adequate not only for fulfilling current research needs but also those for the future, such as responding to changed climatic conditions due to global warming, changing rainfall patterns, acid rain, and habitat destruction.
In situ conservation projects must be politically viable and share broad national development goals, such as increased farm income, besides conservation. Political viability depends on acceptance of the project by interest groups other than genetic resources scientists and nature conservationists (i.e., by farmers, consumers, and government officials). Altieri and Merrick (1987) have proposed that farmers themselves can preserve their traditional varieties in farming systems conservation projects. In fact, Ceccarelli et al. (2001) have found that participatory crop improvement programs between breeders and farmers not only manage to raise yields through germplasm enhancement, but also preserve the farmers' locally-adapted varieties. It is almost impossible for subsistence farming systems in developing countries, saddled as they are with economic problems and rapidly increasing population pressures, to set aside tracts of land just because they contain crop genetic resources that are as yet unknown or for possible future use. In the United States, the apple orchard and fields at Sturbridge Village, MA (ca. 1837), or the gardens at Williamsburg, VA, provide suitable examples of living historical farms that combine tourism and research (Dutton, 1979). The task of conservation of wild relatives of crop plants in situ can be difficult to enable, but the stakes are high.
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EVALUATION OF GENETIC RESOURCES |
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Systematic evaluation of large collections requires a multidisciplinary team. Interaction between members of an evaluation team should be at optimal level for results that enable greater utilization of germplasm. In the past, an evaluation team was limited to breeders, geneticists, agronomists, and pathologists, but in recent years teams have included entomologists, crop physiologists, and biochemists, among others. The diversity of subjects involved, and in some cases the highly technical nature of tests to be performed, emphasizes the need for participation of a variety of research institutions. Moreover, as stressed before, evaluation needs to be conducted under different environments to unravel the true potential of a genotype. Very few national programs have the resources to satisfy all requirements themselves and hence international collaboration in evaluation is not only desirable but also essential.
At present, greater priority is being given to the task of characterizing, evaluating, and documenting data on the current inventory of accessions stored in gene banks. Characterization is defined as recording information on those characters that are highly heritable, can be easily seen by the eye, and are expressed in all environments. It should provide a standardized record of readily assessable characters, and together with passport data (information that goes with samples during seed exchange), it could identify an accession in the gene bank. Evaluation, on the other hand, is the assessment of the value of a sample for its usefulness in crop improvement. The information generated would not only promote greater utilization of the germplasm, but also make optimal use of storage space by identifying duplicates and eliminating redundant germplasm. It is now widely agreed that the gene bank curator must be regarded as being responsible for characterization of germplasm, whereas the more in-depth evaluation should be the task of a germplasm scientist based in a crop improvement program. A short-list of germplasm accessions can then be recommended to the plant breeder for utilization in his crossing blocks. In fact, it is categorically stated that genetic resources have been utilized without elaborate characterization but never without evaluation mostly by, or in close interaction with, plant breeders (Erskine and Williams, 1980).
The process of germplasm evaluation involves the following steps:
Germplasm that survives detailed evaluation and testing is often suitable as potential crossing material for one or two specific traits. Hence, it could be useful as a donor of these traits rather than as lines for release as commercial cultivars.
At the present time, several evaluation projects have been undertaken in response to the current selection criteria of the breeders. This approach may not be optimal because the probability of a sample being examined for a given attribute is solely dependent on current needs. However, a comprehensive approach can provide information suitable for a wider use. It is not implied here that it is possible to always ascertain breeding aims of the future. For example, during early 1900s, short stature (dwarfness) was not considered as a desirable trait as it was almost certainly linked at that time to small grains and hence low yields (Damania, 1997; Maliani and Bianchi, 1979).
Utilization of Genetic Resources— Some Examples
For the purpose of utilization, systematic analysis and description of samples are useful for both distinguishing between populations and identifying duplicates, as well as providing information on the extent of variation within a given germplasm collection (Damania, 1996). Inadequate passport data very often inhibit effective utilization of collected germplasm. It must be emphasized to collectors and gene bank managers that passport data supply extremely valuable—in many cases, the only available—information on the ecological adaptation of an accession, and hence no effort should be spared to fill this important gap in documentation of germplasm. Systematic evaluation is an expensive and time-consuming process, as any worker in this discipline would agree. Therefore, it is imperative to carefully choose the traits that one wishes to evaluate. Priority-wise, the inclusion of traits in an evaluation program will result in optimal use of physical facilities, manpower, and financial resources.
The best way to retain a comprehensive gene pool for future use in plant improvement is to collect and maintain the seeds of old landraces, rare forms, and wild relatives of crop plants from diverse native sources. With extensive cooperation between workers in diverse disciplines and in many countries we can prevent permanent loss of remaining crop diversity and extinction of the wild relatives. In specific terms, plant explorations and collections have a dual role of (i) making available for utilization the greatest possible amount of genetic variability in cultivated and wild crop species, and (ii) showing us the range of variability that a species is capable of and its ecological as well as geographical range of distribution.
American missionaries taught school and often helped plant collectors; they made collections from markets and sent the seed/plant material back to the United States. An interesting collection, evaluation, and utilization story involves USDA plant introductions PI 125367 and PI 134116. While I was working at ICARDA in Aleppo, Syria, my colleagues and I were looking for accessions of wheat that would tolerate very low rainfall as well as soil salinity, the latter being an increasing problem in many countries of West Asia. In five consecutive years, we evaluated nearly 12,000 accessions from the collection donated to ICARDA by the USDA in 1982. While evaluating a set of 5000 of these samples on the shores of saline Lake Jabboul (mean annual rain only 150 mm), 55 km from Aleppo in the 1986–1987 season, we found that only four accessions were able to produce seed and did not dry up and die out like the rest. Out of these four accessions, we noted that two came from Afghanistan. A computerized database was queried and we discovered that these two accessions were collected somewhere in Afghanistan in 1939 by Harlan and Koelz. I immediately wrote to Jack R. Harlan (who had by then retired to New Orleans, LA) to ask if he remembered making these collections and to give us more details if he did. Harlan wrote back that he had gone to Afghanistan much later than 1939 but his older brother, Wilbur Harlan, had taught school in Kabul during that period. I was able to locate Wilbur Harlan (also retired) with the address supplied by Jack, and he said that he and his colleague Koelz as a routine collected those two samples for the USDA from market in Kabul, Afghanistan (W. Harlan, personal communication, 1987). Obviously, farmers had brought the seeds to the market from neighboring wheat-growing areas (according to USDA data, one of the two landrace samples was called "Kandahari," implying that it may have been transported to Kabul from the Kandahar province). The samples had remained in the USDA collection for almost 50 yr without anyone being aware of their high tolerance to combined stress of soil salinity, drought, and high temperatures, until we grew them out and exposed them to those stresses in Syria (Jana et al., 1990). Crosses were made with ICARDA's improved lines and today the genes from these two wheat accessions are to be found in nurseries that ICARDA regularly distributes to the national programs in West Asia and North Africa.
Among other things, Jack Harlan advised if I was looking for high temperature tolerance in wheats I should be collecting in areas close to the Iran–Afghanistan borders (J. Harlan, personal communication, 1987). Since those areas were not safe at that time I could not explore this lead. But Erna Bennett had covered some mountainous areas in Afghanistan in the mid-1960s and she had observed that from one valley to the next, bread wheat (T. aestivum L.) populations displayed differences in awning, pubescence, straw thickness, and other traits that are associated with differences of aspect, altitude, soil moisture regimes, cultural practices, and social and geographic isolation (Bennett, 1970).
Another story of dramatic utilization in the United States that also involves Jack R. Harlan is the following: Harlan explored parts of Turkey in 1948 and, on that mission, he collected a wheat sample from a field in Fakiyan Semdinli which subsequently entered the USDA plant introduction system as PI 178383. On evaluation in the United States, the wheat looked "terrible" in Harlan's own words, had no apparent quality attributes, lodged under irrigation, and possessed no winter hardiness. But according to Harlan (1975), 15 yr later when stripe rust (Puccinia striiformis Westend. f. sp. tritici Eriks. (P.s. tritici) suddenly attacked wheat in the Pacific Northwest, PI 178383 was screened once again (among other wheat germplasm accessions), and was found to have resistance to four races of stripe rust, 35 races of common bunt {Tilletia tritici (Bjerk.) Winter, =T. caries (DC.) Tulasne and T. laevis Kühn [=T. foetida (Wallr.) Liro]}, 10 races of dwarf bunt (Tilletia controversa Kühn), flag smut [Urocystis agropyri (Preuss) A.A. Fisch. Waldh.], and snow mold [Fusarium nivale (Fr.) Ces., =Microdochium nivale (Fries) Samuels & Hallett]. Today, genes from PI 178383 appear in the ancestry of virtually all the wheat grown in the Pacific Northwest. Oddly enough, it later turned out that PI 178383 did not originate in Turkey at all and this story does not end here. Nearly 40 yr later, in 1986, USDA collector Calvin Sperling went to Turkey to trace back the site where Harlan had made his important collection in 1948 and sampled more wheats with similar desirable traits in that area. Sperling located the village of Fakiyan Semdinli, but while talking to the farmers, he learned that they had originally migrated to Turkey from Iraq's Kurdistan. But, it is uncertain if the traditional crop varieties have survived. Unfortunately, that area of Iraq which forms part of the Fertile Crescent has been the scene of major upheavals and human tragedy in recent years and continues to be out-of-bounds to plant collectors.
The Sugarcane mosaic virus has brought the cane (Saccharum officinarum L.) sugar industry to the brink of disaster in many countries. In 1926, attacks of this virus in the state of Louisiana brought production from a high of 181,440 Mg down to 42,638 Mg of refined sugar. Introducing resistance genes from a wild relative, Saccharum spontaneum L., solved the problem. This cross was first made in Java, Indonesia, in 1921 following a similar attack there (Comstock and Lentini, 2005). The battle to improve disease resistance and improve yields knows no boundaries.
In 1974 a single wild alfalfa plant (Medicago hemicycla Grossheim) was found to have resistance to all four biotypes of Stemphylium botryosum Wallr. or leafspot disease that was causing huge losses to alfalfa in California. Soon after, the genes for resistance were transferred to a cultivated type of alfalfa by a team at University of California–Davis Department of Agronomy led by Professor Ernest H. Stanford (Borges et al., 1976).
Ethiopia is considered to be a center of diversity for tetraploid wheats. Several years ago, improved durum wheat lines developed in Mexico were introduced to Ethiopian farmers but were found to be susceptible to local races of stem rust [Puccinia graminis (Pers.) f. sp. tritici]. Since resistance can be found where the virulence is strongest, each season durum breeders at the International Maize and Wheat Improvement Center (CIMMYT), Mexico, crossed Ethiopian tetraploid wheat landraces resistant to stem rust with their improved germplasm. The best lines resistant to stem rust with acceptable yields were selected for further testing in Mexico and Ethiopia and some lines resistant to most races of stem rust were developed (Damania, 1991).
Resistant varieties of wheat have also been developed to counter pests such as wheat stem sawfly (Cephus pymaeus L.). Instead of depending on resistance genes, breeders have turned to solid stems in which the lumen is filled with pith. ‘Rescue’, released in Canada in 1947, successfully utilized solid stem genes to control larvae of the sawfly. A high percentage of wheat landraces collected from Morocco in 1987 were found to possess solid stems, and these have already been used in crosses at ICARDA. Two local durum varieties from Morocco, BD 0287 and BD 1154, were also found to be moderately resistant to Mayetiola destructor (Say) or the hessian fly (Damania, 1990).
However, it has been recently pointed out that occasional incidences of serious epidemics in traditional agriculture are not related to lack of diversity in landraces per se, but to the uniform susceptibility to a particular new disease (Lenné, 2000). For instance, black leaf streak disease (Mycospaerella fijiensis Morelet) of banana (Musa acuminata Colla) devastated both local and commercial varieties worldwide as it spread in the 1960s (Brown, 1998). Resistance was finally found in New Guinea, where black leaf streak is endemic, and where there is extremely high genetic variation in M. fijiensis populations. Therefore, it is recognized by genetic resources researchers that it is not only important to collect germplasm of the target species, but also of the pathogens. Hence, crop plant collectors of legume species routinely now also collect the root nodules of mycorrhizal fungi.
Use of Wild Species in Crop Improvement —The Example of Wheat
Three reasons account for the insufficient exploitation of wild-types in wheat breeding: (i) past collections of wild progenitors have been fragmentary and scanty; the material available does not represent the true spectrum of variability among the species; (ii) work on wild forms has primarily concentrated on evolutionary and taxonomic studies, and (iii) the within-population variability of wild species has not been adequately examined.
It is difficult to pFredict which wild species can contribute most to crop improvement. A logical approach is to concentrate on the wild progenitor, since it readily produces fertile hybrids without problems with chromosome pairing. Triticum turgidum subsp. dicoccoides (Korn ex Asch. & Graebn.) Thell., the wild progenitor of durum wheat, is a useful source of genes for earliness, high protein content (Avivi, 1978), and disease resistance (Grama et al., 1983). These traits are of immense importance for improving durum productivity and stability in unfavorable crop growing environments of Central and West Asia and North Africa.
Tahir (1983) evaluated samples of T. turgidum subsp. dicoccoides for various agronomic traits. The samples were collected from high altitudes in Syria and Iraq, where the wheat crop always suffers from the effects of frost, drought, and stripe rust. The material was grown at two sites in Syria: (i) at Sarghaya, a cold high elevation site (1450 masl) with brown soil and good rainfall, and (ii) at Tel Hadya, a moderate rainfall site (284 masl) with red soil and a Mediterranean type climate tempered with a continental effect of the Syrian desert. Considerable variability was found in plant height, heading time, spike length, 1000-kernel weight, stripe rust, and frost resistance.
Seventy-five crosses were made between T. durum Desf. and T. turgidum subsp. dicoccoides accessions from the above experiment. The parents and their progenies were artificially inoculated with indigenous stripe rust isolates and resistant lines were selected for further evaluation. There was considerable variability in the progenies for the agronomic characters recorded. Despite low temperatures of –1°C to –12°C at Tel Hadya and Sarghaya, respectively, all but two lines did not suffer frost damage. Lines SY 20184 and IQ 55132 suffered slight frost damage comparable with the control winter wheat ‘Bezostaya’ from Russia. In addition, lines SY 20010, SY 20017, SY 20021, and SY 20089 were highly resistant to stripe rust (Tahir, 1983).
Triticum turgidum subsp. durum x T. turgidum subsp. dicoccoides crosses have shown that selection for high protein content as well as yield can be readily transferred to the cultivated form (Srivastava et al., 1988). When one follows the evolution from the wild species to the modern wheat cultivars, it can be seen that wild species such as T. turgidum subsp. dicoccoides have the highest protein content; obsolete wheat forms such as T. dicoccum Schrank (syn. T. turgidum subsp. dicoccum) are intermediate; and modern durum cultivars have the lowest protein percentage. Selections from the progenies of crosses can maintain a high protein percentage (Damania et al., 1992). Similarly, 1000-kernel weight of some durum varieties is much higher than that of T. turgidum subsp. dicoccoides. Nevertheless, high 1000-kernel weight is retained in the selected progenies. The introgression of high protein content from poor-yielding materials (such as T. turgidum subsp. dicoccoides) and its recombination with the high yield potential of improved varieties is therefore possible.
However, use of wild gene resources by hybridization with cultivated forms takes more time and effort than conventional breeding with cultivated material alone because of undesirable genetic linkages. In a simple T. durum x T. turgidum subsp. dicoccoides cross, characters from the wild species (such as brittle rachis, glume hairiness, profuse unsynchronized tillering, hybrid necrosis, grass clumping, and loose crown) persist in subsequent generations. One can make rapid progress when top-crossing this material with durum wheat because both possess genome AABB.
Protein content and quality, as evaluated for pasta production, are not significant criteria for preference by wheat farmers in West Asia and North Africa. Selection for strong gluten content of durums for pasta production by sodium dodecyl sulfate (SDS) sedimentation test and mixograph performance is relatively new, as most Italian pasta was produced domestically (in Italy) until recently. Damania et al. (1988) selected 10 hybrid lines of T. durum x T. turgidum subsp. dicoccoides and analyzed them for protein (gluten) content of a kind that is favorable to pasta cooking qualities. Electrophoretic studies of gliadin composition revealed the presence of band Rm 45, a genetic marker normally associated with good pasta cooking quality in cultivated durum wheats. This marker is found in 40% of the hybrid genotypes. However, tests revealed that in the majority of hybrids where band Rm 45 was present, low SDS-sedimentation volume gave them poor cooking quality. Triticum turgidum subsp. dicoccoides may be therefore more useful as a gene resource for high protein content than for good cooking properties.
The genus Aegilops (goatgrass) is one of the wild parents of cultivated bread wheat (Kimber, 1993). In recent years, some researchers have studied variability in Aegilops species, whereas in the past more emphasis was placed on tetraploid wild emmer, Triticum turgidum subsp. dicoccoides (Körn. ex Asch. & Graebn.) Thell. For example, Damania and Pecetti (1990) characterized many Aegilops species and identified lines with potential in wheat improvement for stressful environments.
Knott and Dvorak (1976) reviewed the use of alien germplasm as a source of stress resistance. Most work on incorporating characters from alien species, including Aegilops, into durum wheat has dealt with disease resistances because of their simple inheritance mechanism. For instance, genes for resistance to stem rust have been successfully transferred from wild Triticeae, such as Agropyron elongatum Host ex P. Beauv. [syn. Elytrigia elongata (Host) Nevski], to durum wheat (Rao, 1978). Ceoloni et al. (1988) reported the usefulness of diploid Aegilops longissima Schwein., Muschl. & Eig as a source of resistance to powdery mildew (Erysiphe graminis f. sp. tritici E. Marchal).
Researchers at CIMMYT have produced about 150 synthetic hybrids using different genotypes of Aegilops squarrosa L. (syn. Ae. triuncialis var. triuncialis) (D genome donor to bread wheat). Materials have been identified from within this collection with: (i) good resistance to a variety of diseases such as Karnal bunt (Tilletia indica Mitra), spot blotch [Helminthosporium sativum Pammel. (King & Bake)], and scab (Fusarium spp.); (ii) improved tolerance to salinity; and (iii) high levels of polymorphisms useful for restriction fragment length polymorphisms mapping of genetic linkages. Aegilops squarrosa has also been used as a source of genes for resistance to greenbug (Schizaphis graminum Rodani) (Mujeeb-Kazi, 1993).
Most researchers now feel that additional studies on Aegilops spp. are needed to provide information on the evolutionary pathways, distribution of characters and their frequencies, as well as associations among characters. The most effective ways to enable greater use of these species in durum wheat improvement involve (i) the selection from the products of crosses between wild and cultivated forms for earliness and drought, frost, and disease resistances; and (ii) improvement of heterogeneous populations of wild forms by modifying the frequencies in favor of desirable genes, followed by single plant selection.
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ABSTRACT
INTRODUCTION