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Duvick, D.N. 1990. Genetic enhancement and plant breeding. p. 90-96. In: J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press, Portland, OR.

Genetic Enhancement and Plant Breeding

Donald N. Duvick


  1. INTRODUCTION
  2. GENETIC ENHANCEMENT, A DISCIPLINE
  3. USES OF GENETIC ENHANCEMENT
  4. WHO DOES GENETIC ENHANCEMENT?
  5. IS GENETIC ENHANCEMENT A DYING ART?
  6. A NEW CONCEPT OF GENETIC ENHANCEMENT
  7. A NEW FUNDING BASE FOR GENETIC ENHANCEMENT?
  8. USES OF BIOTECHNOLOGY FOR END-PRODUCT (CULTIVAR) DEVELOPMENT
  9. GENETIC ENHANCEMENT AND DEVELOPMENT OF NEW CROPS
  10. SUMMARY AND CONCLUSIONS
  11. REFERENCES

INTRODUCTION

Plant breeding, a respected discipline, has been practiced as a distinct profession for about a century. Genetic enhancement is a new term, of uncertain meaning, certainly not describing a distinct profession. What precisely is genetic enhancement? How does it differ from plant breeding? Why should we discuss it at all? The concept of genetic enhancement relates to a fairly new concept: genetic vulnerability. And genetic vulnerability is a direct by-product of successful plant breeding. Let me explain.

GENETIC ENHANCEMENT, A DISCIPLINE

Plant breeding as an art probably pre-dates civilization; it certainly has shown results (cultivated plant varieties) for the past 10,000 years. As a science and a full-time profession, however, plant breeding is largely a 20th century discipline. Its successes, as we all have heard many times over, have given rise to great expanses of a few highly favored cultivars, which in turn have attracted epidemic spread of diseases and pests, uniquely adapted to their ever restricted range of hosts. From experiences with such epidemics, the concept of genetic vulnerability has been developed, along with its corollary, genetic diversity.

Promotion of genetic diversity requires introduction of new, unrelated breeding materials into basic and advanced breeding pools. Resulting new cultivars thus can display a greater diversity of genotypes, and present less opportunity, as grown on the farm, for epidemic disaster.

But to bring greater diversity into the breeding pools means going outside the range of elite adapted local materials. It requires bringing in unadapted and, thus, unproductive cultivars—or even weedy or wild species—that have only a few useful traits, usually tightly linked with a panoply of other undesirable, yield-reducing traits. Such exotic material, when crossed directly with elite adapted lines, uniformly gives unsuccessful breeding results. Useful cultivars rarely or never can be developed from such wide crosses.

But skillful breeders can overcome this problem. They do not try, immediately, to use the unadapted material for production of elite new cultivars. They settle, instead, for selection of intermediates: lines or populations that, while maintaining the good new traits from the exotics, also contain some of the germplasm of the adapted elite cultivars. The broadening alien germplasm thus is at least partly naturalized, and most importantly, the new product, genetically enhanced breeding material, now is somewhat attractive to those plant breeders who are charged primarily with the task of reliably and repeatedly turning out top quality new cultivars.

Thus, the breeder who has performed the important function of partially acclimatizing alien germplasm, while conserving its essential genetic contributions, has performed the operation of "genetic enhancement." Genetic enhancement is needed to prevent genetic vulnerability brought about by the successes of modern plant breeding.

Why is genetic enhancement (I think the plant breeders' term "prebreeding" may be more or less synonymous) thought of as a new requirement in plant breeding? Since all crop cultivars came originally from wild species, genetic enhancement—selection toward useful cultivar types—surely was needed for the original modification of the wild species. What is the difference between then and now?

The difference is that 10,000 years ago—or even 1,000 years ago—there were no obvious divisions between highly selected, elite cultivars and purposely introduced, grossly unadapted exotics. Farmer selectors worked with whatever materials they found locally, and imperceptibly, generation after generation, they selected towards the types they needed. We, of course, don't know how or if such selection was planned; we are reasonably sure, however, that the early farmer-selectors did not purposely hybridize highly select, specialized cultivars to unadapted exotics or species, with the specific aim of bringing in new traits. Thus, our farmer-selector progenitors didn't need to do "prebreeding" or genetic enhancement. From our present sophisticated point of view, that was all they did do. They continually and gradually enhanced genetic materials at hand, slowly making them more amenable to cultivation for human use.

Until sophisticated hybridization and selection schemes were devised, there were few or no sharp breaks between adapted breeding materials and newly introduced, unadapted breeding materials. Sharp breaks, if they existed, were not planned. It is true that when immigrants brought a favored cultivar from the homeland to a new place—as to a new continent with different day-length or different climate—there was a break: the transported cultivar was unadapted to its new home until gradual selection (within the heterogeneous cultivar, or among progeny resulting from its accidental hybridization to indigenous cultivars) gave rise to new, genetically enhanced (better adapted) progeny. But such enhancement was neither planned, nor frequent.

USES OF GENETIC ENHANCEMENT

There are at least three distinct uses of genetic enhancement. The first is to prevent genetic uniformity and consequent genetic vulnerability. Only recently has pre-breeding—genetic enhancement—become a necessary, frequent, and planned part of all plant breeding activities, an essential part of germplasm diversification strategies.

Genetic enhancement has a second important purpose, that of raising yield levels to new heights. This goal is more often hoped for than achieved, but it is true that most break-through cultivars have highly diverse parentage. The semi-dwarf wheats, the high yield dwarf rices, the first U.S. hybrid sorghums, and even the first U.S. corn belt dent maize cultivars are examples. In each case, extensive prebreeding preceded development of the breakthrough, high-yield cultivars. The prebreeding was used to adapt diverse kinds of germplasm to new genetic backgrounds and new geographic locales.

A third use of genetic enhancement also needs mention. Genetic enhancement is used to bring in new quality traits not found in local cultivars. New levels of protein percentage in wheat or unusual starch properties in maize are examples.

WHO DOES GENETIC ENHANCEMENT?

Genetic enhancement is now necessary and useful-but it is not yet well-recognized as being so. This is evident because there is no group of scientists or professionals who call themselves "genetic enhancers", or "prebreeders"; there is no "American Society of Generic Enhancers," no "National Council of Commercial Pre-Breeders." Nor is there a recognized body of prebreeding theory, nor textbooks on the subject. Nor can I call to mind people whose chief reputation in plant biology—basic or applied—rests on their accomplishments in prebreeding. Neither fame nor fortune—nor even generous funding—comes to those who do only prebreeding (with one or two exceptions, that prove the rule).

Yet the job gets done-exotic germplasm is incorporated into adapted, elite stocks, and new cultivars often do contain significant segments of exotic germplasm, giving new kinds of disease and insect resistance, new levels or kinds of stress tolerance, or greater yield capability.

By and large, the job of generic enhancement is done by a special subset of plant breeders and geneticists in the public sector, scientists who either know they must pre-breed in order to make further progress in variety development of their crop, or know they like the challenge of finding, incorporating, and adapting useful germplasm from unlikely places. But in most cases, the main basis of their professional reputation, and of their ability to attract research funds, has been their successful production of finished cultivars. These scientists have supported their avocation, prebreeding, with their vocation, cultivar production.

An important deviation from this rule, that prebreeders also are cultivar developers, may be arising in the international research centers such as the International Rice Research Institute (IRRI), the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) or the International Maize and Wheat Improvement Center (CIMMYT). Such centers were formed with the express purpose of making cultivars and advanced breeding materials for use in developing countries. Today, as national breeding programs are maturing in most developing countries, the centers are rethinking their missions. Some people believe the centers should reduce or even cease cultivar development, in favor of development solely of genetically enhanced breeding materials, to be made available to all plant breeders in developing countries. Further, the centers often are sites for large germplasm depositories, and some believe the centers thus uniquely are suited to characterize and choose potentially useful materials in their collections of landraces and/or wild related species, and then to pre-breed—genetically enhance—them for use by others for cultivar development.

These centers have the same problem, potentially, as breeders in U.S. land grant universities and other tax-supported institutions: in the absence of widespread recognition of genetic enhancement as a profession with easily identifiable products whose value can be calculated, centers that concentrate on genetic enhancement might lose their popular recognition and thus eventually the public funds that support them. This points up further the need for better definition and recognition of the products of genetic enhancement, and of its proper place in plant breeding and variety development. Cultivar development, of course, has had no lack of professional recognition; in fact, as a profession, it has become a successful commercial activity. Many plant breeders now work in private industry, they are supported by the products of their activity-improved cultivars and hybrids-and their numbers increase, annually.

But, plant breeders working for private industry never have done much prebreeding. Private breeders usually depend on the public sector to incorporate needed exotic genes, to remold exotic strains into adapted breeding materials. Genetically enhanced stocks, made by public breeders for their own use in cultivar production, have been freely available to private breeders as well, to use as they wish; privately employed breeders, pressed for short-term results, have depended heavily on publicly employed breeders for genetically enhanced stocks.

IS GENETIC ENHANCEMENT A DYING ART?

And herein arises a problem. Trends are for public breeders to abandon cultivar production and move to more basic biological studies. Beginning young plant breeding scientists in the public sector are going in this more basic direction. Breeders of the older generation are not redirecting their activities, but they are retiring. The gradual change resulting from these two trends is breaking the connection in the chain of development from basic biology through genetic enhancement and on to cultivar production. Publicly employed plant biologists by and large are entering the plant breeding research field at levels well before genetic enhancement-in the more precisely defined (and better funded) science of molecular biology, for example. A gap thus may be developing, in the progression from basic plant biology to final cultivar production.

Some public breeders, seeing this problem, are trying to build up support specifically for genetic enhancement, but because genetic enhancement is not a well-recognized discipline—and perhaps because some people feel the term "genetic enhancement" is only a plant breeder's euphemism for final cultivar production—its support is neither large nor widespread. Private breeders, in general, have not taken up the torch of pre-breeding—or if so, are not talking about it. We may be building towards a shortage of genetically-enhanced breeding materials, a shortage that will become evident a few years further on.

A NEW CONCEPT OF GENETIC ENHANCEMENT

Perhaps I should differentiate among kinds of generic enhancement. Genetic enhancement, up to now, has utilized standard hybridization, segregation, and whole plant selection techniques. Backcrossing, cyclic population improvement, and pedigree selection among selfed progeny are examples of methods employed. But with the advent of molecular genetics and cell biology, a new kind of biotechnology-assisted genetic enhancement (or prebreeding) is possible. Heretofore unavailable genes for insect resistance may be transferred from alien species into elite genotypes (e.g., Bt genes from bacteria to maize); or, cellular fusions may be used to construct new systems of cytoplasmic male sterility (e.g., in Brassica species); or, restriction fragment length polymorphisms (RFLPs) may be used to build up tables of RFLP linkages with agronomically useful traits (e.g., as in wheat, to allow faster, planned introgression of alien germplasm). Such novel genetic potentials have raised the possibility of entirely new product lines-new kinds of plant cultivars, never before available.

The prospect of being able to market novel breeding stocks or novel varieties, produced via genetic engineering or other new cell and/or DNA technologies, has attracted the attention of entrepreneurially inclined scientists and business people. New companies, or new divisions in existing companies, have been formed to exploit the new commercial opportunities in plant biology.

Commercial applications of biotechnology in plant prebreeding activities generally have been along one of two lines:

1. Some companies have developed fully integrated research and development programs, designed to utilize a combination of molecular biology and standard plant breeding techniques for prebreeding and final cultivar production. Their intention is to turn out, and market, finished cultivars with novel traits or with greatly improved existing traits.

2. Other companies (or sometimes the same companies) have opted to utilize molecular genetics (for example, genetic transformation) or cell biology (for example, selection in tissue culture) to modify—to genetically enhance—existing elite germplasm stocks which then can be marketed as source materials for development of new cultivars containing the novel, engineered traits. The "engineered genetic enhancement" may be done on contract, or it may be done with expectation that the new enhanced genotypes will be marketed (via sale or licensing) to end-users such as seed companies or integrated food-processors. In either case, genetic enhancement is done as a commercial end in itself; the end-users, the purchasers of the enhanced materials, will do the final step of cultivar development.

A NEW FUNDING BASE FOR GENETIC ENHANCEMENT?

But these biotechnology-using genetic enhancers are different from the earlier generation of genetic enhancers. Not only do they use different technology (and, often, different source materials, such as bacteria), they also have a different source of funding. They do not depend on public funds; they intend to be supported by profits from sale or licensing of their enhanced germplasm. They intend to be paid by the seed companies that use their products. The seed companies in turn are expected to raise the price of their seed as sold to the farmer, to cover the extra costs of cultivar development and thus keep profit margins at safe levels. So in the end, the farmer would be expected to pay for genetic enhancement, whereas up to now the public at large has borne the cost. In some cases, genetically-enhanced materials, produced via new biotechnologies, might be used to reduce a seed company's costs of breeding or production, e.g., if new kinds of male sterile systems for hybrid seed production were to be developed. In such cases price increases for seed would not be required. But in general, genetic engineering is envisaged as adding a desired and affordable extra value—extra profitability—to crops grown by the farmer. The farmer would-according to this plan-be able to afford an extra charge for the seed because of the extra value in the seed.

The new payment scheme for generic enhancement thus opens the door for private enterprise and in theory will bring in funds needed for development of genetic enhancement. The scheme as yet exists only in theory, however. Up to this time, biotechnology-based genetic enhancement businesses have been supported mainly by venture capital funds, not by sale of genetically-enhanced product. Prices, and terms of sale, license or partnership only now are being worked out. And such arrangements necessarily must be made with no knowledge or experience of how much added value really will accrue from the genetically-enhanced materials, or when such value might be ready for sale in products, or whether farmers would want it, or be willing—and able—to pay for it. Thus, the new kind of genetic enhancement-genetic engineering enhancement-still is an unproven accomplishment, financially as well as technically.

If it succeeds, it will add a new dimension to the economics of farming, for farmers will be paying a larger share of the development cost of their seed inputs, and the general public (through general tax funds) will pay less. And a new category of food-and-agriculture-related workers in private enterprise will be formed: genetic engineering prebreeders.

But a question arises: if this new scheme (of using private enterprise for genetic enhancement via biotechnology) succeeds, will traditional genetic enhancement (without aid of biotechnology) still remain as a separate, publicly-supported discipline? Or will those who are practicing genetic enhancement-for-hire with the tools of biotechnology add the tools of standard genetics and plant breeding to their repertoire and proceed to do "all-purpose genetic enhancement," with the goal simply of developing and selling useful genetically-enhanced breeding stocks, produced with whatever technology works best? Such a development might erase the currently popular but really artificial distinction between biotechnology and older technologies, used in aid of genetic enhancement.

Genetic enhancement's financial future is uncertain, as is its technological future. Much will depend on whether some of its products obtained via any technology can be produced in relatively short term, with obvious and quantifiable value, and at a price that buyers can afford. If so, commercial funds and talent will be attracted; if not, commercial efforts at prebreeding as an end in itself will be abandoned. Then public funding, and scientists at public institutions, will need to continue to carry the load, unless the seed industry steps up rates and amounts of in-house activity in genetic enhancement, with or without aid of biotechnology.

Even if genetic enhancement does become a commercially successful enterprise, publicly-funded efforts will continue to be needed for those basic, preliminary kinds of genetic enhancement (early prebreeding) that require long-term effort with uncertain or even unpredictable results. (Examples include interspecific and intergeneric hybridization in wheat and wheat relatives; conversion of short-day, tropical maize landraces to long-day temperate adaptation; genetically engineered changes in amino acid ratios of soybean storage proteins.) Such long-term, chancy work will not attract private enterprise. And, many seed crops are not dealt with at all by private enterprise breeding, due to their small scale or otherwise low profit margin potential. Thus, for well-rounded genetic enhancement programs in any crop, public research in genetic enhancement always will be needed.

USES OF BIOTECHNOLOGY FOR END-PRODUCT (CULTIVAR) DEVELOPMENT

Biotechnology-the sum of the technologies deriving from molecular and cell biology-will have utility in plant breeding beyond the prebreeding stages. Genetic transformation—nonsexual insertion of alien genes and/or gene regulating systems—is envisaged as eventually being applied, quickly and easily, to finished cultivars, to endow them with a series of useful new traits. However, experience is beginning to show that genetic background affects expression of genes and gene systems that are transferred in vitro, as is also the case when they are transferred via regular sexual mechanisms. Long periods of trial and error are needed to bring transformed lines into desired levels of expression. Thus, for some years to come, genetic transformation perhaps will be used only as a prebreeding technique, one in which alien genes are put into breeding pools of elite germplasm, following which standard plant breeding methods are used to extract good new varieties containing the alien genes.

Cell culture techniques—particularly protoplast fusion techniques—will have utility for making new combinations of nuclear and cytoplasmic genes. Immediate utility is seen for manufacturing new male sterility systems to make F1 hybrid cultivars, or to develop new kinds of herbicide resistance. With time and experience it may be possible to increase primary productive potential as well, by selecting and fixing optimum mitochondrial and chloroplast genotypes, balanced with proper nuclear genotypes. Such success seems far in the future, however, since at present little is known about optimization of nuclear, mitochondrial, and chloroplastic combinations. DNA technology will help plant breeding in a more traditional way, when information is further developed on maps of restriction fragment length polymorphisms (RFLPs) and their linkages with useful traits in crop plants. In vitro tests, using RFLP markers, will allow near perfect accuracy in selecting for such useful linked traits. RFLP technology also will be used for precisely characterizing cultivar lineages and relationships, for positive identification of miss-identified or mislabeled cultivars, for characterizing gene frequencies of populations, and even to test hypotheses of landrace evolution, and global movement of crop genotypes.

Perhaps the greatest contribution of molecular biology to plant breeding will be a deeper and more precise understanding of gene action and inheritance, of cellular physiology and biochemistry, and perhaps of whole plant physiology. With such deepened understanding, breeding and selection via standard sexual methods will be planned and executed with greater vision and economy. Such knowledge and execution will be, in part, a consequence of efforts to make genetic transform-nation a practical reality. The fall-out of knowledge from genetic engineering efforts, wisely utilized by observant biologists, will be of great benefit to plant breeding, and to plant culture in general.

GENETIC ENHANCEMENT AND DEVELOPMENT OF NEW CROPS

In addition to broadening the genetic base of established crops, genetic enhancement-and plant breeding in the sense of final cultivar development-can be used in two other ways: to develop new crops from heretofore uncultivated species, and to change old crops into new crops.

Breeding of the first type (creating new crops where none existed before) usually is interpreted also to include development of advanced cultivars from landraces. Usually the advanced cultivars are made for industrial countries and the landraces come from developing countries. Soybeans and amaranth thus are spoken of as "new crops" in the United States, even though they have been cultivated for centuries as landraces in China, and in Central and South America, respectively

Challenges are great and are about the same, for changing either wild species or landraces into advanced cultivars, able to withstand monocropping and intensive culture at high production levels. Selection for productivity inevitably requires narrowing the germplasm base, in the course of selecting plant architecture and physiology able to withstand crowding and the stress of high yields, and amenable to machine planting, weeding and harvesting. Planting the narrowly selected genotypes in large areas gives excellent access to specially adapted disease and insect pests, as already noted, and so the need for tolerance or resistance always is next to be added to the list of breeding priorities for new crops. Thus, genetic enhancement first narrows and then later must broaden the genetic base, in the course of new crop development.

And as soon as users of the new product get used to larger quantities of it, they will want to standardize and then to change the quality of the product-as, in oil percentage, grain storability and hardness, or flavor and texture. The rule is simple: the more successful the crop, economically, the greater the demands on (and complaints about) the plant breeders.

Breeding of the second type-changing old crops into new ones-has been considered as a broad-scale possibility only since concepts of genetic engineering were developed. For example, ability to produce human insulin in bacteria gave rise to thoughts of producing insulin, or other useful primary gene products, in crop plants. Plants were envisaged as now able to turn sunshine, air and water (and soil nutrients) into high value specialty chemicals, instead of commodity chemicals like corn starch or soybean oil.

Most of these ideas still are only ideas, but workers are proceeding to put some of the least complicated ones into action. Modification, via genetic engineering, of the ratios of fatty acid components of rapeseed oil is contemplated, for example. Standard breeding to modify chemical components has been used in Canada already, to make rapeseed into a new crop: "canola." Canola oil, unlike standard rapeseed oil, is essentially free of erucic acid and glucosinolates and therefore is safe for human consumption when used as a cooking oil. Another possibility derives from recent success in cloning the R gene (first identified by Mendel) in peas which provides the opportunity to identify and transfer, in vitro, genes giving new starch branching properties and thus, potentially, useful new kinds of specialty starches in old starch-producing crops such as maize or wheat. A word of caution is needed for those looking to replace major portions of commodity crop production areas with specialty-product new versions of the old crops. High value specialty products, by definition, require only small amounts of product to saturate the market. They won't replace very much of the production area now used for production of commodity crops. On the other hand, they will make intensive use of crop breeding and crop management skills. They are "brain-power intensive" crops, and as such, must command higher preunit prices—premiums—or they won't be grown.

SUMMARY AND CONCLUSIONS

Genetic enhancement in the sense of prebreeding is a new term, not well-known, especially to public funding agencies. Genetic enhancement for crop plants has become necessary in recent years to broaden the relatively narrow genetic base of modern crop cultivars. Such broadening is needed to supply new kinds of pest resistance, to bring in new levels of productivity and stability of performance, and to provide useful new qualities to food and feed products.

Up to now, genetic enhancement has been performed chiefly by plant breeders in the public domain, including those in the international research centers. But enthusiasm for certain kinds of genetic enhancement via biotechnology has stimulated attempts to commercialize such enhancement. A trend thus may have started, to shift the cost of genetic enhancement from the general taxpayer to the farmer who buys value-added seed. This trend toward commercialization already is well-established for final cultivar development. Commercial plant breeding is well-established for the major field crops, especially for those that are grown as F1 hybrids. However, the more basic kinds of genetic enhancement, with or without aid of biotechnology, will continue to be done by publicly supported geneticists and plant breeders.

Biotechnology will provide essential and innovative support to standard plant breeding in the years to come, bringing in new generic systems, new techniques for selection and identification of genotypes, new ways of making hybrid crops, and, most importantly, deeper understanding of plant gene action, biochemistry and physiology.

Plant breeding utilizing genetic enhancement, and sometimes assisted by biotechnology, will be used in the future to develop new intensive-culture crops from wild or weedy species, or from landraces. Plant breeding utilizing genetic enhancement, and strongly assisted by biotechnology, also will be used someday to allow old crops to produce new products, such as specialty chemicals.

In conclusion, plant breeding, defined as cultivar development based on genetically-enhanced breeding materials and assisted by deep understanding of genetic processes at the molecular level, has a vital role to play in tomorrow's agriculture, especially in tomorrow's/s new crop agriculture.

REFERENCES


Last update February 12, 1997 by aw