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Proc. Natl. Acad. Sci. USA Vol. 96, pp. 5937-5943, May 1999 Colloquium Paper This paper was presented at the National Academy of Sciences colloquium "Plants and Population: Is There Time?" held December 5-6, 1998, at the Arnold and Mabel Beckman Center in Irvine, CA. Plant genetic resources: What can they contribute toward increased crop productivity? DAVID HOISINGTON*, MIREILLE KHAIRALLAH, TIMOTHY REEVES, JEAN-MARCEL RIBAUT, BENT SKOVMAND, SUKETOSH} TABA, AND MARILYN WARBURTON International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apartado. Postal 6-641, 06600 Mexico City, Mexico ABSTRACT To feed a world population growing by up to 160 people per minute, with >90% of them in developing countries, will require an astonishing increase in food pro- duction. Forecasts call for wheat to become the most impor- tant cereal in the world, with maize close behind; together, these crops will account for ~80% of developing countries' cereal import requirements. Access to a range of genetic diversity is critical to the success of breeding programs. The global effort to assemble, document, and utilize these re- sources is enormous, and the genetic diversity in the collec- tions is critical to the world's fight against hunger. The introgression of genes that reduced plant height and increased disease and viral resistance in wheat provided the foundation for the "Green Revolution" and demonstrated the tremendous impact that genetic resources can have on production. Wheat hybrids and synthetics may provide the yield increases needed in the future. A wild relative of maize, Tripsacum, represents an untapped genetic resource for abiotic and biotic stress resistance and for apomixis, a trait that could provide devel- oping world farmers access to hybrid technology. Ownership of genetic resources and genes must be resolved to ensure global access to these critical resources. The application of molecular and genetic engineering technologies enhances the use of genetic resources. The effective and complementary use of all of our technological tools and resources will be required for meeting the challenge posed by the world's expanding demand for food. Today, on the eve of a new millennium, we are approaching a critical era in the evolution of our planet and species we are in a race between growing population and food production. This era was cast in Paul Ehrlich's The Population Bomb (1), perhaps prematurely, as a time when population would out- pace the earth's resources, including its capacity to produce food. The threat of the Malthusian crisis forecast by Ehrlich appears to have diminished as we have witnessed a slowdown in the rate of population growth. But the challenge of feeding a world population growing by up to 160 people every minute (>90% of them in developing countries) remains daunting. It is forecast that, by 2050, world population will increase from the current level of ~6 billion to >8 billion people. Feeding this population will require an astonishing increase in food production. In fact, it has been estimated that the world will need to produce as much food during the next 50 years as was produced since the beginning of agriculture 10,000 years ago (2~! Today, it appears more likely that a population/food crisis may be born, not from an exponentially increasing world population (though in some of the world's poorest regions, population growth remains exceedingly high), but from an ill-founded sense of complacency about food production. PNAS is available online at www.pnas.org. Our staggering requirement for food must be viewed in the context of statistics that indicate that the area available for food production has, essentially, remained constant since 1960 (3~. Despite some new land being brought into cultivation, soil erosion and urbanization have offset these gains. In addition, less resources (both human and financial) are being devoted to overcoming major production constraints. Financial support for agricultural research has decreased for the last several years and is expected to continue its slow decline as most developed nations continue to focus on domestic issues rather than addressing the multitude of problems facing the world's de- veloping nations. How will we feed the world in the coming years? For the foreseeable future, conventional agriculture will be our pri- mary response, with cereal grains playing a pivotal role. The International Food Policy Research Institute has predicted that, by the year 2020, almost 96% of the world's rice con- sumption, two-thirds of the world's wheat consumption, and almost 60% of the world's maize consumption will be in developing countries. Forecasts call for wheat to surpass rice in its apical role in feeding the poor of those nations. It will likely become the most important cereal in the world, with maize close behind; together, these crops will account for ~80% of the cereal import requirements of developing coun- tries. Many economists stress, however, that increased pro- duction in developing countries will be essential for achieving food security. Maize and wheat are each expected to have an annual global demand of ~775 million tons eacht and will be of critical consequence in the race between crop production and population growth . This paper focuses on the potential of genetic resources, particularly those of maize and wheat, to help meet the continually expanding demand for these major grains. It will indicate how such resources have contributed in the past, and how they may advance our efforts in the future. The Role of the Consultive Group on International Agricultural Research (CGIAR) in Preserving Genetic Resources Simply stated, plant breeding depends on the correct combi- nation of specific alleles at the 50-60,000 genetic loci present in a plant's genome. The knowledge of where these alleles are best found and the combination and evaluation of these into a Abbreviations: CGIAR, Consultative Group on International Agri- cultural Research; CIMMYT, Centro Internacional de Mejoramiento de Maiz y Trigo; FAO, Food and Agriculture Organization; IPR, intellectual property rights. *To whom reprint requests should be addressed at: International Maize and Wheat Improvement Center (CIMMYT), Aptdo 370, P.O. Box 60326, Houston, TX 77205. e-mail: D.Hoisington@CGIAR.org. TRosegrant, M. W., Sambilla, M. A., Gerpacio, R. V. & Ringler, C., Illinois World Food and Sustainable Agriculture Program Confer- ence, May 27, 1997, Urbana-Champaign, IL. 5937

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5938 Colloquium Paper: Hoisington et al. single species can be considered the "art" of breeding. Obvi- ously, access to a wide range of genetic diversity is critical to the success of any breeding program. The work of the 16 centers that collectively form CGIAR represents the largest concerted effort toward collecting, pre- serving, and utilizing global agricultural resources. Together, the centers hold nearly 600,000 samples of the estimated 6 million accessions stored globally (Table 1~. The remaining germplasm are stored in other international, regional, and national gene banks, many of which collaborate closely with the CGIAR centers. If one considers that the Food and Agriculture Organization (FAO) has estimated that the total number of unique accessions globally are on the order of 1-2 million, the CGIAR centers account for an estimated 30-60% of the world's unique holdings under long-term conservation. Given that the CGIAR has focused its efforts on the crops of highest significance in world agriculture, this proportion could be even greater. The materials in the CGIAR gene banks include traditional varieties and landraces, nondomesticated species, advanced cultivars, breeding lines, and genetic stocks. The effort re- quired to assemble, document, and maintain these collections is enormous but well justified as the genetic diversity present in the gene banks represents a critical component in the world's fight against hunger. The International Center for Maize and Wheat Improve- ment, better known by its Spanish name of Centro Interna- cional de Mejoramiento de Maiz y Trigo (CIMMYT), has a global mandate for improving the productivity and sustain- ability of maize and wheat in developing countries. The collection, documentation, and evaluation of the genetic re- sources of maize and wheat are a critical part of meeting this mandate. CIMMYT's newly established Genetic Resource Center contains ~120,000 accessions of wheat and 18,000 accessions of Latin American maize (of the 25,000-35,000 accessions in partner gene banks in Latin America). This represents the largest collection of these two important cere- als. It is, perhaps, the only effort that is actively pursuing the documentation and evaluation of its collection on a routine basis. Before discussing several issues related to the use of these genetic resources, it is important to mention a few examples of their contributions to crop improvement. Although these examples have been drawn mostly from maize and wheat, there Table 1. Summary of CGIAR's germplasm holdings (4) Center Total holdings 70,940 136,637 13,911 2,448 109,029 110,478 39,756 13,470 1,051 80,646 17,440 595,806 Major species Cassava, Phaseolus, rice Maize, wheat Potato, sweet potato Agroforestry species Lentil, chickpea Chickpea, sorghum, groundnut Yam, rice, maize, cassava Forage legumes and grasses Banana, plantain Rice Rice CIAT CIMMYT CIP ICRAF ICARDA ICRISAT IITA ILRI IPGRI IRRI WARDA Total IITA, International Institute for Tropical Agriculture; CIAT, Cen- tro Internacional de Agricultura Tropical; CIP, Centro Internacional de Papa; ICRAF, International Centre for Research in Agroforestry; ICARDA, International Center for Agriculture in the Dry Areas; ICRISAT, International Crops Research Institute for the Semi-Arid Tropics; ILRI, International Livestock Research Institute; IPGRI, International Plant Genetic Resources Institute; IRRI, International Rice Research Institute; WARDA, West African Rice Development Association. Proc. Natl. Acad. Sci. USA 96 (19994 are a large number of similar examples for many other major food crops handled by the CGIAR Centers. Contributions of Wheat Genetic Resources Wheat is truly global, being one of the few crops grown over most of the world. It belongs to the genus Triticum, which oriainated almost 10,000 years ago in the historic Fertile Crescent, an area in the Middle East. Triticum arose from the cross (supposedly in nature) of two diploid wild grasses to produce tetraploid wheat, which today includes the many cultivated durum (pasta or macaroni) wheats ( OCR for page 5937
Colloquium Paper: Hoisington et al. Rust Resistance. Some of the most devastating and universal crop diseases are caused by fungal pathogens. Among them, the rust pathogens are the most widespread and generally cause the largest crop losses per season. Many genes have been found that provide resistance to specific races of each rust pathogen. Within wheat, leaf, yellow, and stem rusts are major pathogens. Fungicides can provide a level of control; however, the chemical option is often limited for many farmers, partic- ularly in developing countries, by high costs and lack of knowledge about application. In addition, the negative envi- ronmental effects of chemical applications can be consider- able. The incorporation of host plant resistance genes into mod- ern wheat varieties has allowed yields of resistant wheat that have not been treated with fungicides to nearly equal those of the same varieties under fungicide applications (6~. Many of these varieties have incorporated single major genes that convey resistance to specific races of the rust pathogen. Of >40 known genes for leaf rust resistance, 12 originated in species other than T. aestivam and T. turgidum while 20 of the 41 known genes for stem rust resistance originated in species other than T. aestivam and T. turgidum (ref. 11; also see Table 2~. Even among the genes originating from T. aestivam, many come from landraces. Unfortunately, many of these major genes have already been "broken"; i.e., the specific race has mutated to become viru- lent against the specific resistance gene. Efforts to identify and incorporate genes that confer "durable" resistance are there- fore preferable. Several such genes have been identified and incorporated into modern wheat varieties. One of the most important is Lr34, which was originally found in the cultivar "Frontana" (131. Lr34 has been incorporated into >50% of the wheat varieties grown in the world today; together with several modifier genes, it has resulted in stable resistance to leaf rust Table 2. Important genes in wheat that were found in related species (12) Trait Locus Source Disease resistance Leaf rust Stem rust Stripe rust Powdery mildew Wheat streak mosaic virus Karnal bunt Pest resistance Hessian fly Cereal cyst nematode Quality traits Grain protein High protein Low molecular weight glutenins Lr9 Lrl8 Lrl9 Lr23 Lr24 Lr25 Lr29 Lr32 Sr2 Sr22 Sr36 Yrl S Pml2 Pm21 Pm25 Wsml Aegilops umbellulata Triticum timopheevi Thinopyrum T. turg7tdum Ag. elongatum Secale cereale Ag. elongatum T. tauschii T. turgidum Tritzcum monococcum Triticum timopheev~i Triticum dicoccoides Aegilops speltoides Haynaldia villosa 1. monococcum Ag elongatum Qqantitative T. turgidum trait loci H21 H23, H24 H27 Cre3 (Ccn-D1) Quantitative trait loci S. cereale T. tausch~i Aeg~lops ventricosa T. tauschii T. turg~dum T. dicoccoides T. turgidum Proc. Natl. Acad. Sci. USA 96 (1999' 5939 in the 1980s and 1990s. Most of these durable sources of resistance have not come from alien sources but from cultivars and landraces that evolved in the past to contain broad levels of resistance to pathogens. The cultivar "Hope," bred in the United States earlier in the century (14), was later used by Borlaug as a source of stem rust resistance in the Rockefeller/Mexico wheat program. The Hope resistance was based on the Sr2 gene that, when com- bined with other unidentified genes, produced a more durable resistance. The Sr2 originally came from a tetraploid wheat variety known as emmer and has since been incorporated into many wheat varieties worldwide, providing excellent levels of resistance. Recently, a linked molecular marker was developed (15) that may allow more rapid identification and manipula- tion of this important gene. It is fair to say that the incorpo- ration of the Sr2 and Lr34 genes from genetic resources into cultivated wheat varieties represent milestones in the grain's genetic advancement. Most likely, the gains of the Green Revolution could not have been made without them. Veery Wheats. Genetic resources have contributed more than single genes to crop improvement efforts; entire chro- mosomal segments also have been introduced with noteworthy results. Perhaps the most important of these is the lB/lR translocation that was identified as a simple transfer between rye and wheat in the former Soviet Union cultivar "Kaukaz." The lB/lR translocation, which carries a number of genes from rye, confers resistance to various diseases (fungal and viral pathogens) and adaptation to marginal environments (16~. This translocation has been deemed so important that it has been incorporated into >60 wheat varieties, including the prominent Veery lines, that occupy >50% of all developing country wheat area, almost 40 million hectares (174. Yield Potential. Yields of the major cereal crops (rice, wheat, and maize) have increased steadily over the past years, al- though the rate of these yield increases appears to have slowed.! To meet cereal production demand in the next decade, we must continue to increase yields; even more daunting, we must increase them at an ever-increasing rate. How will such growth be supported, particularly when the rates of increase over the past few years appear to have declined? For rice and wheat, the use of hybrids may be one possibility, although it remains to be demonstrated what level of heterosis (hybrid vigor) can be achieved in either crop. Heterosis levels currently detected in wheat are ~10-25~o, lower than the 25-35% levels historically found in maize, one of the first hybrid cereal crops. The reasons for wheat's lower heterosis levels have not been determined, but one possibility is the lower level of diversity generally found in self-pollinated crops such as rice and wheat. Many groups continue to search for alternatives to the existing germplasm. In wheat, it is possible to reproduce the hybridization event that created hexaploid wheats from a cross of tetraploid with diploid wheat. These so-called "synthetics" represent a source of novel genetic variation (18~. Research at CIMMYT has led to the develop- ment of >600 new synthetic wheats, crosses between various durum wheats and T. tauschii accessions. Many of these crosses have produced rapid improvements in important characteris- tics, including disease resistance, abiotic stress tolerance, and yield. Although much work remains to be done, the use of molecular genetic techniques now allows us to identify the gene segments most likely responsible for improved perfor- mance and, thus, to focus on more directed crosses in the future. A recent example, now under investigation at CIM- MYT, is the role of the Lrl9-containing segment. This gene (or segment) originally came from Agropyron elongatum and was tRosegrant7 M. W.7 Sambilla7 M. A.7 GerpaciO7 R. V. & Ringler7 C.7 Illinois World Food and Sustainable Agriculture Program Confer- ence7 May 277 19977 Urbana-Champaign7 IL.

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5940 Colloquium Paper: Hoisington et al. first incorporated into the wheat variety "Agatha" (11~. Yield trial data indicates that varieties containing the Lrl 9 gene yield at least 10% more than counterparts without Lrl 9 (Ravi Singh, personal communication). The gene was originally transferred for its possible role in conferring leaf rust resistance, but its potential to increase yields may become a more important factor for breeders, thus demonstrating the often unantici- pated potential of these alien transfers. Use of Genetic Resources in Maize Improvement Unlike wheat, the use of genetic resources in maize improve- ment has not been well documented at the global level and may not be as great. Although ~50,000 accessions of maize exist in germplasm banks around the world (19, 20), most of these have never been adequately evaluated for useful traits. Reasons cited for low utilization include lack of evaluation data, documentation, and information; poor coordination of na- tional policies; and poor linkages between gene banks and breeders (21~. The untapped potential of these genetic re- sources is indicated to some extent by the progress that U.S. breeders achieved through a combination of plant improve- ment and pedigree breeding. Using double and three-way crosses, varieties were produced that helped double U.S. yields between 1930 and 1966; by 1995, single crosses and the use of better hybrid materials by breeders helped triple 1930 yields. Meanwhile, there have been frequent warnings about the genetic vulnerability of maize and the potential of exotic germplasm to reduce the threat (22-24~. It has been estimated that, in the U.S., OCR for page 5937
Colloquium Paper: Hoisington et al. (IPR) to agricultural products, even plant varieties. This is in great contrast to the 1960s and 1970s, when such protection was considered a detriment to global progress in plant im- provement. With initiatives such as the 1991 strengthening of the Union for the Protection of New Varieties of Plants Convention and the 1993 Multilateral Trade Negotiating Rounds in the General Agreement on Tariffs and Trade, IPR was widened to include inventions and breeding technology. Looking to better protect genetic resources, FAO estab- lished the International Undertaking on Plant Genetic Re- sources in 1983. At its outset, the Undertaking subscribed to the rule of free and unrestricted interchange of germplasm and recognized all plant genetic resources as the "heritage for mankind." The Undertaking was modified in 1989 and 1991 to include resolutions regarding compensation and ownership of genetic resources. In 1992, the Convention on Biological Diversity officially recognized the sovereign rights of individual nations over biological diversity and the resources within their territories. The Treaty has been ratified by most developing countries and many developed countries but has yet to be ratified by the U.S. A result of the Convention has been a modification of the agreement between FAO and the CGIAR centers. In 1994, each CGIAR Center signed an agreement with FAO putting their genetic resources "in trust" under the auspices of the FAO. These agreements describe the roles and responsibilities of the centers as trustees and include articles that state: (i) The Center shall hold the designated germplasm in trust for the benefit of the international community, in particular, devel- oping countries; (ii) neither the Center, or any recipient, will seek IPR protection over the designated germplasm or related information; and (iii) the Center will undertake to make samples of the designated germplasm and related information available directly to users for the purpose of scientific research, plant breeding, or genetic resource conservation without re- striction. Hawtin and Reeves (37) provide an excellent review of the historic aspects and current status of IPR in the CGIAR centers. The authors point out that several important issues must still be resolved, such as the meaning of the phrase "germplasm and related information" and the definition of when a variety is sufficiently different from the original in-trust germplasm from which it was derived (the issue of derived varieties). These are under study, and it is expected that, following the next round of negotiations, some, if not all, of these issues will be clarified. What effects these agreements will have on the use of genetic resources is unclear. The interpretation of related information, in particular, will have implications for IPR options. If the interpretation restricts IPR over materials such as genes obtained from genetic resources held in trust, these important resources will continue to be incorporated in conventional ways, but the application of biotechnology may be more limited. Molecular Approaches to Utilization of Genetic Resources There is a multitude of examples illustrating the use of genetic resources to improve modern plant varieties, but one can readily posit that biotechnology could substantially enhance this use. Molecular genetics now allows the routine genetic analysis of nearly any characteristic of interest. As these traits are better understood, the underlying genes can be isolated and used to identify corresponding genes in a wide range of genetic resources. Molecular genetics has already had a tremendous impacts on plant breeding. Progress in the development of new PCR- based marker systems (amplified fragment length polymor- phisms and microsatellites) has opened the vast majority of plant genomes to investigation. Linkage maps for many species have been developed and often have been combined with Proc. Natl. Acad. Sci. USA 96 (1999' 5941 suitable phenotypic data to identify genomic regions contain- ing the genes coding for specific traits of interest. Many of these regions have been further characterized (e.g., by map- based cloning) and even manipulated (via genetic engineering and marker-assisted selection) in breeding programs (38~. Undoubtedly, these efforts will continue and will even be enhanced by newer molecular techniques such as DNA arrays (39) and automation. DNA markers also have been used extensively to charac- terize germplasm, a process popularly known as fingerprinting, to evaluate the genetic relationships among accessions (genetic diversity) and provide important information in the areas of ecology, population genetics, and evolution. Within wheat, several studies have reported the application of molecular markers within Asiatic wheat landraces (40~. Molecular marker technology has advanced our understanding of genetic resources more than any other type of genetic data. A major hurdle, however, must be overcome soon: how to store, access, and analyze the vast amounts of data that will be produced by using molecular markers. Databases, such as the International Crop Information System, are under develop- ment to store pedigree and performance information for most of the crop species within the CGIAR. Efforts are also underway to expand the capability of the system to include molecular genetic and diversity information. This data will allow scientists to more exactly determine "relatedness" be- tween accessions and even trace specific genomic segments through pedigrees. The potential uses of this information capability for gene identification and verification are enor- mous and point to the importance of this area. Fortunately, computing hardware and software systems are keeping pace with advances in molecular technology so that appropriately large-scale and real-time applications should be feasible. Bioinformation has been called the next revolution in the world; it is certainly of major import to our utilization of genetic resources. Genetic Resources: What Are the Potential Impacts? Agriculture before the 18th century completely depended on landraces for new varieties. During the industrial revolution, the entire nature and practice of agriculture was transformed forever. The discovery of genetics by Mendel in the mid-19th century and the subsequent work of other plant geneticists provided the knowledge base that made dramatic increases in agricultural productivity possible. Undeniably, these discov- eries have led to vast improvements in agriculture, but they also have led to a decline in the genetic diversity of the crops in many farmers' fields. Landraces and traditional varieties have been replaced by less diverse modern cultivars and hybrids. What lies ahead? A tremendous challenge now awaits us an increased demand for food production spurred on by an increasing world population, a decreasing natural resources base, and declining investments in agricultural research. How will this challenge be met? Genetic resources will be fundamental to our efforts to improve agricultural productivity. These resources, fortunately stored in gene banks around the world, evolved an assortment of alleles needed for resistance and tolerance to the diseases, pests, and harsh environments found in their natural habitats. Consumer characteristics enter the picture when one considers that farmers over the years have selected their preferred varieties based on yield, color, texture, and taste. Many of these combinations cannot be easily duplicated artificially, even with the help of modern molecular techniques. What should be alarming is the fact that many of these valuable genetic resources are essentially "sitting on the shelf" in what have been dismissively termed "gene morgues." The conservation of a resource only becomes important if the resource has or

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5942 Colloquium Paper: Hoisington et al. Proc. Natl. Acad. Sci. USA 96 (1999' farmer (often the net consumer) produce additional food in his or her own fields may be the surest and quickest way to increase food security in these countries Can genetic resources contribute to maintaining the rate of yield increase for the major crop species (or even increase it) without requiring significantly higher inputs (human and fi- nancial)? Molecular genetic studies indicate that self- pollinating species generally have lower levels of molecular diversity (as measured by the level of molecular polymor- phism). Controversy still exists as to the exact relationship between molecular diversity and yield potential, although studies with maize indicate that at least among related mate- rials, molecular diversity is positively and significantly corre- lated with yield (43, 44J. Based on these studies, measures of molecular diversity could be used to identify new germplasm sources that, when crossed with existing varieties, would result in enhanced yields. Work at CIMMYT using synthetic wheat clearly indicates that this strategy is extremely promising. The array of resources at our disposal, together with new biotechnology techniques, provides us with a healthy measure of optimism for meeting the world's future food requirements. One technique or resource alone will not suffice; rather, the effective and complementary use of all of our technological tools and materials will be required to meet this enormous challenge. We should not, however, let our growing capabilities lull us into complacency. We cannot afford to wait on this issue- we must move quickly and effectively. The world must commit itself now to feeding its future generations to ensure that, indeed, there is enough time for us to produce plants that can provide food for all humankind. 1. Ehrlich, P. R. (1975) The Population Bomb (Ameron, Mattituck NY). 2. James, C. (1997) ISAAC Briefs No. 4. (International Service for the Acquisition of Agro-biotech Applications, Ithaca, NY), pp. 31. 3. Evans, L. T. (1998) Feeding the 10 Billion: Plants and Population Growth (Cambridge Univ. Press, London). 4. Langridge, P. & Chalmers, K. (1998) in Proceedings of the 9th International Wheat Genetics Symposium, Saskatoon, Saskatche- wan, Canada, ed. Slinkard, A. E. (Univ. of Saskatchewan Press, Saskatoon, Canada), Vol. 1, pp. 107-117. 5. Chapman, C. G. D. (1986) Plant Genet. Resources Newslett. 65, 2-5. 6. Smale, M., Aquino, P., Crossa, J., del Toro, E., Dubin, J., Fischer, T., Fox, P., Khairallah, M., Mujeeb-Kazi, A., Night- ingale, K. J., et al. (1996) Understanding Global Trends in the Use of Wheat Diversity and International Flows of Wheat Genetic Resources: Economics Working Paper 96-02 (CIMMYT, Mex- ico City). 7. Kihara, H. (1983) in Proceedings of the 6th International Wheat Genetic Symposium, ed. Sakamoto, S. (Plant Germplasm Insti- tute, University of Kyoto, Kyoto, Japan). 8. Krull, C. F. & Borlaug, N. E. (1970) in Genetic Resources in Plants: Their Exploration and Conservation, eds. Frankel, O. H. & Bennett, E. (Blackwell Scientific, Oxford). 9. Borlaug, N. E. (1988) J. R. Swedish Acad. Agric. Forestry 21, Suppl., 15-55. 10. Gale, M. D. & Youssefian, S. (1986) in Progress in Plant Breeding, ed. Russell, G. E. (Butterworth, London). McIntosh, R. A., Hart, G. E., Devos, K. M., Gale, M. D. & Rogers, W. J. (1998) in Proceedings of the 9th International Wheat Genetics Symposium Saskatoon, Saskatchewan, Canada, ed. Slinkard, A. E. (Univ. of Saskatchewan Press, Saskatoon, Can- ada), Vol. 5, p. 236. 12. System-wide Genetic Resources Program (1996) Report of the Internally Commissioned External Review of the CGIAR Genebank Operations (System-wide Genetic Resources Group, Rome). Dyck, P. L. & Samborski, D. J. (1982) Can. J. Plant Pathol. 7, 351-354. 14. McFadden, E. S. (1930, J. Am. Soc. Agron. 22, 1020-1031. 15. Johnston, S. J., Sharp, P. J., McIntosh, R. A., Guillen-Andrade, H., Singh, R. P. & Khairallah, M. (1998) Proceedings of the 9th International Wheat Genetics Symposium, Saskatoon, Saskatche 11.

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