Host Plant Resistance and Conservation of Genetic Diversity
"Convince me that you have a seed... and I am prepared to expect wonders."
- H. D. Thoreau, essayist, naturalist and philosopher
"Time is short; we must hurry!"
- N. Valivov, pioneer genetic conservationist in the 1930s
The Urgency of Global Genetic Conservation
The world's biota are under siege. A human population nearing 6 billion places unprecedented pressures on the biosphere.. Approximately 95% of the terrestrial surface is now occupied by human settlements or ecosystems managed for food and materials production. As a result, natural ecosystems are destroyed and fragmented, species are destroyed or doomed, and global genetic diversity is diminished. The scale of this destruction directly threatens human civilization, which is dependent in numerous ways on biological diversity (Ehrlich and Wilson, 1992). The enormity of the crisis has stimulated international efforts to conserve biological diversity and to ensure sustainable use of its components (Earth Summit and Diversity Treaty).
Conserving Agricultural Genetic Diversity
Before the extent of this crisis was recognized, genetic diversity in agricultural crops began to erode as political and social changes of the industrial era transformed agriculture. Genetic conservation began early this century when foresighted agriculturists saw that the loss of primitive varieties or "landraces" meant loss of genetic variation essential for sustained crop improvement. Pioneers like Soviet geneticist Nikolai Valivov and USA scientist Harry Harlan encouraged and conducted systematic collection of the seeds of indigenous crop varieties and developed an understanding of geographic distribution of genetic variation in crops. The extent of this variation is impressive to those of us accustomed to the uniform appearance of crop plants in use today. For example, indigenous cultivars of maize from Mexico can vary in their time to maturity from 60 days to 16 months, the plants can range from 40 to 700 cm in height, and ear lengths range from 4 to 40 cm! Variation in fruit form in tomato (Lycopersicon esculentum) (Fig. 1) and leaf type in Brassica oleracea acephala from the Mediterranean (Fig. 2) are striking morphological manifestations of genetic diversity. Morphological variation is just one indication of variation in countless physiological, chemical, and developmental traits.
Figure 1: Fruit of Lycopersicon spp. and cultivars of L.esculentum (cultivated tomato). Some accessions from the C. M. Rick Tomato Genetics Resource Center (U. C. Davis) and the USDA NPGS Lycopersicon collections.
Figure 2: Some of the variation in plant form among Brassica oleracea acephala accessions.
In the early 1970s, international concern escalated to alarm over the continuing erosion of crop genetic resources despite efforts to forestall it. The devastating epidemic of southern corn leaf blight (Helminthosporium maydis) in the USA in 1970 occurred because 70% of the corn in grown the USA had been developed using a single source of cytoplasm that was susceptible to the pathogen. This and other evidence that genetic uniformity was dangerous triggered a more concerted effort to conserve crop genetic diversity. The The National Plant Germplasm System of the USDA (NPGS) was restructured, and the International Agricultural Research Centers (IARCs) were united under the direction of the Consultative Group on International Agricultural Research (CGIAR) and the International Board of Plant Genetic Resources (IBPGR).
Today germplasm in these collections and those of other private and public institutions around the world contain over 3,600,000 accessions (genetically distinct germplasm samples) representing at least 100 crop species and their wild relatives. Among the over 200 national collections, the largest is the NPGS with over 500,000 accessions. Another 500,000 accessions are held in collections of the IARCs. These collections consist of propagative tissues, usually seeds, maintained in cold storage and periodically propagated to preserve their viability and genetic identity. This technique is known as ex situ conservation. The collections are curated to provide plant breeders and other scientists ready and free access to facilitate crop improvement. The ex situ collections are essentially the only genetic resource used for crop improvement.
Pest-Resistant Crops and Genetic Diversity
Traits conferring host plant resistance (HPR) to insects and pathogens are among the most important for crop improvement. There is probably no crop into which pathogen resistance has not been widely introduced using genes obtained from the international ex situ germplasmcollections. Insect resistance has also been introduced into several hundred crop varieties during the last 20 years (Smith, 1989) and its importance is increasing as insecticides lose efficacy due to pest adaptation or are removed from use to protect the environment and human health (Eigenbrode and Trumble, 1994). Often, successful crop production is impossible without resistance to insects and pathogens that otherwise cannot be economically controlled. In many cases, multiple genes are required for sustained resistance to counter pest adaptation. Thus, maintaining agricultural productivity to meet world food needs depends on access by agricultural scientists to plentiful sources of HPR genes.
Existing ex situ collections may not be adequate for HPR breeding in the future, however. Pest resistance genes are rare and predominantly found in unimproved varieties or wild accessions. For example, resistance to insects is found in only 0.01 to 2% of rice accessions (Heinrichs, 1986) and much of this occurs in exotic land races. In potato, high levels of resistance to the green peach aphid (Myzus persicae) has been identified in about 6% of examined accessions of wild Solanum species, but in 0% of over 360 accessions of S. tuberosum and other cultivated Solanum species (Flanders et al., 1992). The pattern is similar for resistance to other insect pests of potatoes. In cultivated tomato (Lycopersicon esculentum) insect resistance is rare, but it is more prevalent in wild accessions of L. esculentum var. cerasiforme (Eigenbrode et al., 1993) and common in more distantly related Lycopersicon hirsutum (Farrar and Kennedy, 1992).
Pathogen resistance too is most frequently found in unimproved or wild accessions. Rice gets resistance to two of Asia's four main rice diseases from a single wild rice accession from central India. Resistance to 12 different diseases in cultivated tomato originates in wild Lycopersicon species. Wild crop relatives have yielded pathogen resistance in barley, cassava, sweet potato, sunflower, grapes, tobacco, cacao, sugarcane and wheat. Many wheat breeding lines in the USA carry exotic genes for resistance to pathogens: powdery mildew (Erysiphe graminis); dwarf bunt (Tilletia controversa); several rust species (Puccinia spp); and leaf blotch (Septoria nodorum), and to insects: Hessian fly (Mayetiola destructor) and greenbug (Schizaphis graminum). Resistance to an important new pest, the Russian wheat aphid (Diuraphis noxia) has been found in about 50 accessions of wheat and barley (0.5% of the accessions screened) and most of these are landraces are from Afghanistan and the former Soviet Union.
But wild accessions and landraces that provide the most HPR genes are underrepresented in many germplasm collections. Only about 8% of accessions in the world crop germplasm collections are wild crop relatives and some collections have little or no wild germplasm (NRC, 1993). In most crops (with the exception of wheat, oats, potato, and tomato) it is estimated that less than 20% of the genetic diversity represented in wild species and unimproved cultivars is conserved ex situ (Reid and Miller, 1989). Some critics contend that even this estimate may be low because estimates of the total available variation are inaccurate. Time and resource pressures to obtain material has resulted in relative undersampling of geographically remote areas (reviewed by Fowler and Mooney, 1990).
Although more germplasm collecting is needed and continues, preservation of the accessions may be problematic. Resources are already strained to maintaincollections at their present size. Losses and erosion in extensive collections inevitably occur due to contamination, drift, errors in record keeping, physical facilities failures, inadvertent selection during increases, and other causes (NRC, 1993). Collections require backups at one or more locations to protect against these kinds of losses, but accessions in some crops do not have sufficient backups. In other crops, germplasm duplication is excessive among international collections, but coordinating optimal reductions is logistically difficult. Curators and managers of these collections are working hard to minimize these problems.
Another difficulty with extensive ex situ collections is that many accessions are inadequately characterized, or have undesirable traits that discourage breeders from using them. Evaluation and enhancement (improving agronomic suitability by breeding) within the context of curation requires considerable resources. To address these problems with available resources many collections are now managed using the ëcoreí concept (Frankel and Brown, 1984). A subset of accessions is chosen to maximize representation of the alleles in the entire collection. Unfortunately, a typical (10%) core will contain approximately 70% of the collection alleles (NRC 1993) and may miss rare alleles such as those conferring pest resistance. Meanwhile, devotion of resources to a core can lead to more rapid erosion of main collection along with its potential pest resistance genes. Available resources are simply not sufficient to preserve and characterize genetic variation ex situ .
In situ Genetic Conservation
One approach to preservation, if not characterization, of crop genetic diversity is establishment of in situ reserves. These are preserved habitats harboring wild populations of crop species or related species; or they are regions where cultivation of indigenous landraces is continued by "custodian" farmers in exchange for subsides. In situ genetic reserves can help conserve wider genetic variation less expensively than ex situ conservation because the costs of germplasm storage are eliminated. Additionally, while ex situ collections are genetically static or may even erode, genetic variation of in situ populations should be maintained by natural processes or indigenous cultural practices. Accumulation of deleterious mutations will also be prevented in this way. Genetic and ecological patterns in reserve populations, e.g., pest resistance or microclimate tolerance, can be identified in situ and used to extract useful traits.
The advantages of in situ genetic conservation is widely recognized, but efforts in this area are just beginning. Geographic reserves have been established for a number of crops including apples, pears, pecan, palm species, forage crops, cassava, peanut, teosinte, and Allium, Avena and Triticum species. But in situ conservation involves more than just setting aside habitat reserves. To be useful, such reserves must be managed to document and maintain genetic variability, usually necessitating several ecogeographically distinct reserves for a particular species. In situ reserves must be managed along with ex situ collections to facilitate extraction of useful genes. This level of in situconservation does not exist for any crop or crop relative. The closest coordination between ex situ and in situ conservation may be the efforts of CIMMYT (Centro Internacional de Mejoramiento de Maíz y Trigo) to oversee the establishment of teosinte geographic reserves. In fact, just how to accomplish effective in situ genetic conservation is still an area of study. The theory and methods are under development generating a complex scientific literature (Loeschcke et al., 1994 provide excellent reviews; also see recent issues of: Biological Conservation, Conservation Biology, Conservation and the just-established electronic journal Conservation Ecology.
Agricultural Genetic Conservation and Global Biological Diversity
How much genetic diversity must be conserved for its agricultural potential? The application of molecular biology to crop breeding theoretically makes accessible the world gene pool (3 x 1010 loci ?, each with multiple alleles). The spectacular success of insect resistance derived from genes of the soil bacterium Bacillus thuringiensis illustrates that any genome potentially can contribute to HPR and other invaluable crop traits. So agricultural genetic conservation and conservation of the global genetic diversity are becoming less distinct. Many of the theoretical considerations and methods are similar as well. In situ conservation approaches will become more important as global pressures reduce the size of natural habitat reserves to their theoretical minimum sizes necessary to maintain species viability. Ex situ reserves of some wild species will play a part in preservation and restoration of some habitats. Often, cooperative efforts between ecological and crop germplasm conservators will be appropriate. For example, some wildland habitats can serve as in situ genetic reserves for crop relatives.
The relationship between agriculture and global conservation is complex. Although agricultural productivity depends on global genetic diversity, and may provide some technology that can help preserve it, at the same time agriculture is one of the most environmentally destructive human activities. First, because of its scale ñ approximately 50% of the terrestrial surface is under cultivation. Second, because of its intensity, most agroecosystems largely supplant natural communities. Ironically, vascular plants, which carry the majority of genes with potential for crop improvement, are most severely affected by agricultural expansion. And agriculture is also harmful to genetic diversity because of pesticides effects on nontarget organisms.
Thus, although it is estimated that agricultural production may need to double to feed projected human populations during the next 25 years, increasing production can no longer be accomplished by increasing the area under cultivation. And the ancillary negative effects of agriculture on global ecology must be minimized. To accomplish these goals, genetic improvement of crop yields and tolerance to abiotic and biotic stresses (HPR) will be critical. But there is a potentially dangerous interdependence: as global biological diversity erodes, so does the genetic potential needed to conserve it.
Accomplishing Genetic Conservation
The imperatives for conserving needed genetic diversity outlined herein include expanding ex situ germplasm collections of wild crop relatives and unimproved varieties; improving accessibility to breeders of genes in unimproved accessions through better characterization and enhancement; expanding use of in situ reserves; and reducing agricultureís negative effects on global biodiversity. Accomplishing these goals will require substantial resources; just adequately conserving the existing ex situ collections will require tripling of present funding (Iwanaga, 1993). New techniques will help improve the cost-effectiveness of ex situ preservation. For example, cryogenic storage can increase the time germplasm can be stored without regeneration. Newer molecular techniques for the characterization of germplasm are faster and cheaper. In addition to the technological challenges, social, political and economic factors complicate genetic conservation. National and private proprietary issues can impede cooperation. Most diversity resides in undercapitalized countries that cannot afford to conserve it. Extraction and of this diversity by developed countries can easily become a form of economic exploitation. Unchecked market forces will result in unjust distribution of genetic resources and overexploitation. Strong international agreements are needed.
For more links relevant to conservation of global plant genetic resources go to: Plant Genetic Resources Information of the UN Food and Agriculture Organization (FAO), Selected Sites Relevant to Plant Genetic Resources.
The International Convention on Biological Diversity (ICBD), signed by many heads of state after the Earth Summit in Rio de Janeiro acknowledged, at least in principle, the interelatedness of conservation and sustainable production, the need to channel international public and private resources towards conservation of biological diversity, the requirement to protect the interests of the less developed countries in which so much genetic diversity resides while providing incentives to attract investment in conservation from the more developed countries, and the requirement to somehow curb human use of global resources while assuring a good quality of life for everyone. How are these efforts progressing? Follow-up conferences such as International Conference on Population and Development, Cairo, 1994 have focused critical agendas such as curbing human population growth, rather than on resources for diversity conservation (Earth Summit Watch page). Recently, crop genetic conservation has gained a sharper focus. The IARCs have been placed under the auspices of the Food and Agriculture Organization (FAO) of the United Nations which could help coordinate conservation efforts globally. The upcoming FAO-sponsored Fourth International Conference on Plant Genetic Resources (agenda is on-line) will bring together world experts in germplasm to assess the state of genetic diversity, to review the newest technical and legal tools to facilitate conservation, and to determine the capacity of human resources and institutional structures to meet conservation needs. We can be hopeful that these efforts will help martial the necessary resources and international resolve to save our precious genetic resources.
A Final Thought
I have focused on the utilitarian value of genetic diversity, as has most of the international effort and debate. This is probably inevitable; the pursuit of resources drives our economic and political world. Certainly the value of genetic diversity is immense, and host plant resistance is just one example. But some hold that a merely utilitarian view of biodiversity can never succeed in conserving it ñ that our orientation towards the use of nature is the root cause of the crisis. The refusal by the USA to sign ICBD indicates how a focus on the utility of conservation can hamper cooperation. This point of view has been articulated eloquently by many American thinkers (Thoreau, Leopold, Carson, Abbey, and others). Recently, Ehrenfeld (1981) asks us to consider conservation not for its utility, but because it is simply the right thing to do.
Perhaps achieving a sustainable world will require humanity to somehow acknowledge together the sanctity as well as the utility of the wonders Thoreau was prepared to find in the seed.
- Ehrenfeld, D. The arrogance of humanism (Oxford University Press, Oxford, 1981).
- Ehrlich, P. R., Wilson, E. O. 1991. Biodiversity studies: science and policy. Science 253: 758-253.
- Eigenbrode, S. D., J. T. Trumble & R. A. Jones, 1993. Resistance to beet armyworm (Spodoptera exigua [Hubner]), hemipterans, and Liriomyza spp. in Lycopersicon. Journal of the American Society of Horticultural Science 118: 525-530.
- Eigenbrode, S. D. & J. T. Trumble, 1994. Plant resistance to insects in integrated pest management in vegetables. Journal of Agricultural Entomology 11: 201-224.
- Farrar, R. R. & G. G. Kennedy. Sources of insect and mite resistance in tomato in Lycopersicon s. in Genetic improvement of tomato (ed. Kalloo, G.) 121-142. (Springer-Verlag, Berlin, 1992).
- Flanders, K. L., J. G. Hawkes, E. B. Radcliffe & F. I. Lauer, 1992. Insect resistance in potatoes: sources, evolutionary relationships, morphological and chemical defenses, and ecogeographical associations. Euphytica 61: 83-111.
- Fowler, C. & P. Mooney. Shattering: food, politics, and the loss of genetic diversity 1-278 (University of Arizona Press, Tucson, AZ, 1990).
- Frankel, O. H. & A. H. D. Brown. Plant genetic resources today: a critical appraisal. in Crop Genetic Resources: Conservation and Evaluation (eds. Holden, J.H.W. & Williams, J.T.) 249-257 (George Allen and Unwin, London, 1984).
- Heinrichs, E. A., 1986. Perspectives and directions for the continued development of insect-resistant rice varieties. Agriculture, Ecosystems and Environment 18: 9-36.
- Iwanaga, M. Enhancing links between germplasm conservation and use in a changing world. in International Crop Science I 407-413 (Crop Science Society of America, Madison, WI, 1993).
- Loeschcke, V., J. Tomiuk & S. K. Jain. Conservation Genetics 1-440 (Birkhauser Verlag, Basel, Switzerland, 1994).
- National Research Council. Managing Global Genetic Resources: Agricultural Crop Issues and Policies (National Academy Press, Washington DC, 1993).
- Reid, W. V. & K. R. Miller. Keeping Options Alive: the Scientific Basis for Conserving Biodiversity (World Resources Institute, Washington, D. C., 1989).
- Smith, C. M. Plant Resistance to Insects: a Fundamental Approach (John Wiley & Sons, New York, N.Y., 1989).
A few other related web sites
- Plants for a Future. A private foundation with information about plant conservation and use.
- The Economic Botany Collections of Royal Botanical Gardens at Kew. One of the oldest botanical collections.
- Warwick Horticultural Research International (Warwick HRI). The Genetic Resources Unit of HRI maintains a vegetable gene bank (VGB) built in 1980 at the National Vegetable Research Station at Wellesbourne, England.
- Collection of Botany-Related URLs contains web and net sites describing botanical gardens etc. international, over 80 sites.
- International Treaty on Plant Genetic Resources for Food and Agriculture