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a 611 Joanne Lane DeKalb, IL 60115
b Dep. of Crop Sciences, Univ. of Illinois, 1102 South Goodwin Avenue, Urbana, IL 61801
* Corresponding author (atroyer{at}uiuc.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: ASTA, American Seed Trade Association • EDVs, essentially derived varieties • GD, genetic diversity • IVs, independent varieties • RFLP, restriction fragment length polymorphism • RM, relative maturity • SNPs, single nucleotide polymorphism markers • SSRs, simple sequence repeats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Proposals are being considered by the American Seed Trade Association to broaden proprietary rights by restricting use of materials and methods freely available for plant breeding research (Hunter, 1991; Parks, 1993). Two key terms in the proposals are minimum distance, how different does germplasm have to be to be considered not the same, and dependency (relatedness), how much of the germplasm of a new inbred originated from one of its parents. Both of these terms relate directly to amount of genetic distance (diversity). The tentative ASTA guideline threshold for dependency is, and always has been, 75%; the guideline threshold will be reviewed subsequent to a multiple year experiment in progress (J.S. Parks, 2001, personal communication).
Garza et al. (1962) developed a method to identify corn hybrids with the same pedigree. They compared selfed and crossed pairs of the same and different double cross corn pedigrees for seven traits on an individual plant basis and for nine traits on a plot basis. No significant differences were detected between hybrids with the same hybrid pedigree, but significant differences were detected between different hybrid pedigrees. Their method was based on the Mendelian principle that an F1 hybrid either sibbed or selfed produces the same F2 gene frequency.
Troyer et al. (1983)(1988) used the Garza method to determine genetic diversity (GD) among corn hybrids. Comparisons of the hybrid-by-hybrid cross with the mean performance of the two hybrids were made to determine if the two hybrids were identical (0.0 GD) or if the two hybrids were unrelated (1.0 GD). Performance of the hybrid-by-hybrid cross is quantified by the inverse relationship of GD to inbreeding depression. Data were collected for grain yield, growing degree units to pollen shed, grain harvest moisture, and plant height, with grain yield giving the best spread of estimates for GD. Paszkiewicz et al. (1986), Smith et al. (1990), and Williams and Hallauer (2000) confirmed use of the Troyer et al. (1988) method (using grain yield) to determine GD among hybrids.
Smith and Smith (1989a) studied the use of morphological traits, isoenzymes, chromatography, and electrophoresis of zein seed storage proteins (reversed-phase high-performance liquid chromatography or RP-HPLC); RFLP profiles; pedigree records; F1 grain yield; and GD to test for minimum distance between inbreds of corn. They concluded the following: morphological data of inbreds cannot be used to determine distinctness because it correlates poorly with pedigree records, with F1 grain yield, and with heterosis. Inbreds can have similar morphologies but be of different genetic constitution. Biochemical (isoenzymes and zeins) data could not provide unique identification of some inbreds related by pedigree at 88 or 89%. RFLPs were more efficient than isozymic or HPLC methods; RFLPs discriminated among closely related (75–95% by pedigree) inbreds. Smith et al. (1991) showed scatter plot association of F1 grain yield vs. RFLP distance was r2 = 0.87 (P < 0.01), and scatter plot association of heterosis (F1-F2 grain yield) vs. RFLP distance was r2 = 0.76 (P < 0.01).
Smith and Smith (1989b) proposed using 40 to 120 well dispersed RFLP probes to profile existing and derived inbreds where a derived inbred is an independent variety (IV) if 25% or more of its genotype is different from existing inbreds. Conversely, if a derived inbred is 75% or more similar to the original, the new inbred is an EDV. If the parent inbreds are related, the 75% suggested rule would apply only to the RFLP profile that differed between the two related parent inbreds because only the differing part will segregate and be affected by selection.
Melchinger et al. (1991) assessed similarity for RFLPs among 32 related and unrelated U.S. Corn Belt inbreds using two restriction enzymes and 83 DNA probes. Eighty-two probes detected polymorphisms with at least one enzyme. On average, 4.3 variants were found per probe–enzyme combination across all 32 inbreds. With few exceptions, Modified Roger's Distance (MRD) between related lines agreed well with expectations on the basis of coancestry coefficients determined from pedigree data. Results suggested that RFLP data can be used for assigning inbreds into heterotic groups and quantifying genetic similarity between related lines, but it appeared that a greater number of probe–enzyme combinations are required to obtain reliable estimates of genetic distance.
Mumm and Dudley (1994) analyzed 46 RFLP probe–enzyme combinations on 148 U.S. corn inbreds to calculate genetic distance and to cluster lines into related groups. Although the clustering agreed very well with pedigree records, they suggested more markers should be used.
Senior et al. (1998) report simple sequence repeats (SSRs) are a viable, cost effective alternative to RFLPs and isozymes for genotyping corn inbreds and determining genetic similarities. Bongard-Pierce et al. (2000) and Rafalski et al. (2001) report single nucleotide polymorphism markers (SNPs) will be available soon for genotyping corn inbreds. They state molecular marker technologies such as SSRs and SNPs have advanced beyond isozymes and RFLPs and will enable much more extensive genome coverage in shorter periods of time with less money. This higher throughput genotyping will enable much more precise germplasm characterization.
Rasmusson and Phillips (1997) reported incremental genetic gains were made for several traits in a narrow (87.5% related by pedigree) barley (Hordeum vulgare L.) gene pool. They question that the variation on which selection is based in elite gene pools is provided almost exclusively from the original parents. They suggest that many mechanisms exist to generate variation de novo such as gene amplification and transposable elements. They hypothesize that epistasis (nonallelic gene interaction) is more important than commonly viewed and that it arises from de novo generated diversity as well as the original diversity.
Williams and Hallauer (2000) compared GD values with RFLP values (50 DNA probes) for 36 hybrids of varying diversity made up with 17 inbreds from Reid Yellow Dent and from Lancaster Sure Crop cultivars. Grain yield had significant differences among hybrid comparisons and was used for GD. The correlation between GD and RFLP methods for all hybrids was r = 0.80 (P < 0.01) and for logical (Reid x Lancaster) hybrids it was r = 0.91 (P < 0.01). They suggested RFLP analysis for initial screening and GD for additional needed information from field tests.
The question remains as how best to quantify relationship between related inbreds. Degree of relationship will be used for determining dependency and minimum distance. Our objectives were to assess hybrid agronomic performance of some popular, closely related inbreds on the basis of molecular methods, and to offer thoughts on developing inbreds and on germplasm ownership.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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DEKALB hybrid DK524 and hybrid B73 x Mo17 have B73 as a parent in common. The other parent of DK524 is a proprietary inbred that is 50% Mo17 germplasm. They were compared by head-to-head comparisons from several experiments in the U.S. Corn Belt to test the inference that they have the same pedigree (Smith, 1988) and to compare Mo17 with its related, proprietary inbred. Large maturity difference between hybrids reduced number of test locations where the hybrids were together.
Two genetically diverse pairs of extremely popular, closely related inbreds were each compared in two hybrid combinations by head-to-head comparison from many experiments over multiple years in the U.S. Corn Belt. One pair was B73 compared with Holden (Foundation Seeds) inbred LH119 that is derived from B732H93. Indiana inbred H93 is derived from Iowa inbred B375GE440 Ht source. Inbreds B73 and LH119 have 88.6% (62/70) RFLP similarity. Holden inbred LH51 was the tester parent in hybrids. The other pair was Mo17 compared with LH51 that is derived from Mo175C103Ht source. Inbreds Mo17 and LH51 have 88.2% (60/68) RFLP similarity; B73 was the tester parent in hybrids. These four inbreds (B73, LH119, LH51, and Mo17) have each been grown in hybrids on tens of millions of hectares in the U.S. Corn Belt (Zuber and Darrah, 1980; Don Eggerling, 2000, personal communication). These inbreds and their hybrids were chosen because they were very popular and widely grown, are well known to students of corn breeding and because extensive data were available.
Only those RFLP markers that are polymorphic between compared inbreds are used to determine similarity. Monomorphic markers are not considered informative because the parts of the genome assayed by these markers are considered identical.
Hybrid performance trials involving closely related inbreds were grown in two-row plots in two to four replicate tests for 6 yr in eight to 12 U.S. Corn Belt states. Plots were 5.3 m long including a 70-cm alley with 76 cm between rows. Data were collected for grain yield adjusted to 155 g kg-1 moisture, grain harvest moisture, spring vigor score (green mass of a row profile before the corn is 25 cm tall), early stand count (seeds planted/seedlings present), growing degree units to flower (10°C minimum, 30°C maximum), plant and ear height, stalk lodging at harvest (stalks broken below the ear), root lodging at harvest (plants leaning 30° or more), stay-green score (amount of green leaf area in late fall), final stand (by count near flowering time), and RM (relative maturity) by grain harvest moisture.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Closely related inbreds, Mo17 and LH51, are compared in hybrids with the same inbred tester (Table 2). The newer, slightly later, taller hybrid, B73 x LH51, had 5% higher grain yield, 25% better stay green, 20% fewer root-lodged plants, and 38% fewer dropped ears than B73 x Mo17. LH51 was significantly improved over Mo17 in hybrids with B73. LH51's backcross parentage results in 88.2% (60/68) RFLP similarity with Mo17. Smith and Smith's (1989b) proposal would make LH51 an EDV. Smith (1988) reported that inbreds LH51 and Mo17 differed by only one locus (ACP1-3 vs. ACP1-2, respectively) on the basis of a set of 21 isozymes.
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Inbreds B14, B73, and Mo17 were popular public inbred lines. They were products of a previous era when Agricultural Experiment Stations, supported by public funds, devoted significant effort to developing superior inbreds. Virtually all present popular inbreds are developed only by commercial companies. Only a few publicly supported programs continue to develop public inbreds, and their efforts are very small in comparison to the overall hybrid seed industry.
Evidently introgression of a small amount of DNA can have significant effects on agronomic performance. The quality (positive effect) of introgressed DNA improving useful agronomic traits is more important than quantity of DNA similar to the IV parent. Data in Tables 1, 2, and 3 indicate that unique, superior inbreds (LH51, LH119 and inbred male of DK524) were developed even though pedigree and molecular markers would consider them EDVs at 75% similarity. These historical, elite, popular inbreds that significantly increased farmers' profits would certainly have been less popular, and perhaps would not have been used at all, if the progenitors were protected by private breeders, and other breeders were handicapped by Smith and Smith's (1989b) proposal of 75% DNA similarity to an IV parent.
Developing Inbreds
Over the past century, variable, segregating, open-pollinated, corn cultivars derived from crosses of flint x dent races were replaced with more uniform corn hybrids resulting from crosses of uniform inbreds. More widely adapted hybrids replaced locally adapted open-pollinated cultivars. More widely adapted hybrids also reacted better to variable weather. These changes in corn adaptedness caused a reduction in the number of U.S. seed-corn companies from thousands in the early 1940s to less than 300 presently (Johnson, 1957; Troyer, 1999).
Open-pollinated corn cultivars initially were selfed with emphasis on local adaptation and diversity (Jenkins, 1936). Then crosses of inbreds were selfed on the basis of combining ability and bringing useful traits together—the pedigree method (Hayes and Johnson, 1939). Sprague recognized the Reid Yellow Dent x Lancaster Sure Crop heterotic pattern (Iowa State Corn Breeding Annual Report 1939, 1940). Breeding became more focused within heterotic groups. Single-cross hybrids replaced double crosses. Specific combining ability became relatively more important than before. Iowa State used population improvement and early testing in Stiff Stalk Synthetic to develop three very popular inbreds: B14, B37, and B73 (Hallauer et al., 1983). The replacement of many inbreds with three Stiff Stalk inbreds reduced genetic diversity in hybrid corn.
We agree with Rasmusson and Phillips (1997) that epistasis is a relatively more important form of gene action than commonly viewed. Epistasis is the interaction among genes at different loci. In corn hybrids, epistasis is determined by comparing two-parent, single-cross hybrids; three-parent, three-way hybrids; and four-parent, double-cross hybrids in multiple, balanced sets to provide identical gene frequencies among averages for hybrid types (see Hallauer et al., 1988 for a review). Identical gene frequencies among types of hybrids preclude simple gene model explanations for significant yield differences among hybrid types (A.R. Hallauer, 2001, personal communication). Stringfield (1950) found single crosses yielded 3.7% more than double crosses. Jugenheimer (1976) summarized three different experiments where single crosses averaged 6% higher yield than double crosses. The four-parent, double-cross formula (Jones, 1922) made hybrid corn practical, but virtually all U.S. commercial corn hybrids are now two parent, single-cross hybrids to take advantage of this epistasis (Fig. 1) .
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Breeding strategy for better corn hybrids slowly changed over a long period of time from finding the best hybrid combination among many existing inbreds to developing new, superior, widely adapted inbreds. Improved performance testing programs and improved information management methods identified widely adapted inbreds (Bradley et al., 1988; Troyer, 1996). A relatively few elite inbreds (about 20 at a given time) became popular source materials (Troyer, 1990). Heterotic groups and patterns are based on these few elite inbreds. Backcross populations of elite inbreds increased in favor over F2 populations for inbred development.
Stringfield (1950) also compared the two possible first backcrosses with the F2 for a four inbred diallel set of six single crosses (providing average identical gene frequencies for the two types of populations) and found the average of the two backcrosses (12 genotypes) to be significantly higher yielding than the average of the F2s (six genotypes). This yield epistasis is less obvious than superiority of single-cross hybrids. He states: "Inbreds which survive the selection procedure may tend to have favorable combinations of genes. A favorable combination might contribute more vigor than a random assortment individually just as good." We suggest this selection for "favorable combinations of genes," i.e., within-inbred epistasis, is important and is increasing over time as average yield increases (Fig. 1).
Duvick (1999) states yields of inbreds, and of their single crosses, have risen continually since the 1930s. Since the 1960s, parent inbreds with higher grain yields were and are consistently favored for single-cross hybrid seed production. We suggest this positive association between inbred and hybrid yield drives selection for within-inbred epistasis. Stringfield's (1950) results, showing positive yield epistasis both for selfing backcrosses to develop newer inbreds and also for crossing fewer inbreds to make simpler hybrids, support the importance of intact inbred genomes (within-inbred yield epistasis) in corn. Favorable interaction within intact inbred genomes explains the positive yield-epistasis within and between inbreds and simpler hybrids.
At present, a closely related inbred is usually used as a donor parent to improve the more elite inbred in terms of grain yield or grain yield/harvest moisture ratio. It is important to keep the good traits of the elite inbred while adding improved traits (Troyer, 2000). This results in a higher frequency of acceptable, new inbreds—it raises new-inbred, mean performance. A one-backcross population averages 75% of the elite genome, two-backcross populations average 87.5%, and three-backcross populations average 93.75%. More backcrosses obtain more of the elite genome (genome conservation) in the resulting improved inbred. More backcrosses reduce segregation and recombination of genes resulting in more recovery of original linkage groups of the recurrent elite inbred. This conserves favorable, unique combinations of genes. Genome conservation breeding helps maintain and improve within-inbred epistasis (Troyer, 1994, 2000). Such breeding would be hindered by Smith and Smith's (1989b) proposal of 75% DNA similarity to an IV parent.
Germplasm Ownership ConceptsPlant breeding instruction and plant breeding experience teach us to use the best materials and the best breeding methods (Darwin, 1859, 1868; Troyer 1990, 2000; Hallauer, 1992; Allard, 1999). Restricting materials and restricting breeding methods will reduce the rate of grain yield increase. Availability of all germplasm for breeding is to the advantage of everyone. A level playing field for corn breeders provides easy access to all elite inbreds in popular hybrids to allow backcrossing for genome conservation breeding.
The dependency principle (IVs versus EDVs) for determining inbred proprietary rights was developed in Europe with the expectation that selection had occurred in F2 populations where maximum segregation and recombination are expected. The dependency principle is not appropriate for genome conservation breeding where backcross or multiple backcross populations are used for selection. In genome conservation breeding, an inverse relationship generally exists between the quantity and the quality (overall positive benefit) of germplasm added to an improved inbred. More valuable gifts come in smaller packages—less is more. For example, Selig et al. (1999) studied BC1 and BC2 inbreds derived from Brazilian x U.S. Corn Belt populations by RFLP analysis and generally found inbreds with the highest testcross grain yields had the smallest amount of introgressed Brazilian germplasm. More dependency (greater proportion of the recurrent parent) indicates higher quality germplasm was added from the nonrecurrent parent. Otherwise the new, improved inbred would not be improved. More dependency on the recurrent parent of an improved inbred indicates the breeder used more skill in choosing the nonrecurrent parent and in devising screening methods to select the improved inbred than someone with less knowledge about the background and behavior of corn. This skill should be rewarded—not penalized.
How does threshold level for determining EDVs (dependency) affect corn breeding? A lower threshold for EDV causes more EDVs and fewer IVs. More EDVs cause more lawsuits and more royalty payments, and thus more lawyers and more accountants. Spending for germplasm is limited. The money for more lawyers and more accountants results in fewer corn breeders. Duvick (1984) calculated that increasingly greater numbers of corn breeders are required to maintain a constant rate of grain yield improvement. Thus, more EDVs would cause a lower rate of grain yield improvement because of fewer corn breeders. Shifting funds from income producing research (fewer corn breeders) to overhead expense (more lawyers and accountants) is an overly selfish business strategy counterproductive to the common good.
Barton (2000) presents data showing the number of intellectual property lawyers in the USA is growing faster than the amount of research. Number of intellectual property lawyers relative to research expenditures has increased 67% in the last 11 yr to 75 intellectual property lawyers per $1000 million of research spending. He states cost of lawyers alone approaches $10 000 to obtain a patent and $1.5 million per side to litigate a patent. To respond to this problem, he proposes three reforms: raising the standards for patenting to reduce the number of patents, freeing research by decreasing use of patents to bar research, and controlling invalid patents by easing attack on them.
Lewontin and de Miranda Santos (1997) view current trends in U.S. intellectual property protection as serious threats to future innovations in the agricultural sector. They note a contradiction in patenting plant varieties, patenting genes broadly used in plant breeding, and patenting the very process that allowed these varieties to be created. They question how many seed companies involved in plant breeding will survive the increased transaction costs to access the patented genetic materials. They point out the difficulty in determining the relative importance of a single trait to the overall performance of a variety. Plant varieties are products of many distinct genetic materials, usually developed over a long period of time. Some pedigrees involve 50 or more parents and are literally meters long using standard size type. Who gets credit?
The International Union for the Protection of New Varieties of Plants (UPOV) Convention provides: "The authorization of the breeder of a protected variety shall not be required (a) for the utilization of that variety as an initial source of variation for the creation of other varieties, and (b) for the marketing of any resulting variety." This principle was reaffirmed when the UPOV Convention was revised in 1978 and in 1991. Lewontin and de Miranda Santos (1997) recommend protecting plant varieties through plant breeders' rights as long as it allows breeders to use protected varieties as a source of variation for the development of new ones.
The U.S. Plant Variety Protection Act, Title II, Chapter 4,Sec 41, p.8 states: "Distinctness. - The distinctness of one variety from another may be based on one or more identifiable morphological, physiological or other characteristics (including any characteristics evidenced by processing or product characteristics, such as milling and baking characteristics in the case of wheat) with respect to which a difference in genealogy may contribute a difference." Sec. 42, p.9 states: "distinct, in the sense that the variety is clearly distinguishable from any other variety the existence of which is publicly known or a matter of common knowledge at the time of the filing of the application." Title III, Chapter 11, Sec.114, p. 20 states: "The use and reproduction of a protected variety for plant breeding or other bona fide research shall not constitute an infringement of the protection provided under this Act."
Coffman (1998) states that if present trends continue in the patenting of genes, two or three companies will have a major influence on the global food system. Small companies will be forced to license technology from private industry suppliers or again look to the public sector for advanced breeding material. Research exemptions may leave the public sector breeder in the difficult position of developing technology that he may not be able to distribute. Coffman concludes that the future of plant breeding depends on the policies that we evolve for the management of intellectual property related to crop improvement.
Our Founding Fathers viewed education and research as means for economic growth. They saw the application of science (technology) as a solution for practical problems (Hayami and Ruttan, 1985). Article 1, section 8 of the U.S. Constitution states, "The Congress shall have power to promote the progress of science and the useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries."
Inbreds presently used in our industry are EDVs that have had the rights and privileges of IVs. These EDVs got us where we are today. Paying IV royalties indefinitely would probably require an amendment to the U.S. Constitution and would logically lead to all royalty payments going to descendants of developers of widely adapted cultivars that make up the background of today's corn hybrids or to descendants of some pre-Columbian Native Americans because our U.S. patent system rewards the first inventor regardless of application date. If we wish to reward recent and future improvements in corn, it would be prudent to raise the threshold and to set a short time limit for IV royalties.
Plowman (1993) emphasizes stable funding and access to the best possible genetic resources (germplasm) as factors contributing to plant breeding success. He expects these factors to remain important in the future. He suggests a research exemption of patented material to use as a parent of a new variety. He argues that most of the profit from a new variety is made in the first 5 yr and that more than 5 yr are required to develop and market a newer variety.
Many related inbreds exist because of widespread use of the pedigree breeding method, and because all hybrid seed-corn companies started with the same public inbreds, and then, surviving companies used superior competitive hybrids as breeding material (Troyer, 1999). Determining inbred ownership has become very complicated, very time consuming, and very expensive. Annual litigation expense exceeds $100 million in the U.S. hybrid seed-corn industry (Bruce Bickner, 1999, personal communication).
No doubt newer molecular marker systems will be able to assay rapidly more informative markers on the genome of two or more inbreds providing more accurate estimates of genetic similarities. However, the entire genome (genotype) in conjunction with its environment produces a phenotype. Quantitative traits (cumulative action of many genes each producing a small effect) such as yield, grain moisture at harvest, stalk breaking, root lodging, ear dropping, and agronomic traits in general interact with environments (usually weather and cultural practices) that require many test locations to determine the relative value of a genotype. We do not expect molecular markers alone to predict agronomic performance accurately. Corn breeders use performance data to choose breeding materials. Seed-corn business managers use performance data to choose hybrids to produce and market. Agronomic performance data are necessary parts of policies for germplasm ownership.
Successful corn breeding for higher grain yield has resulted in the relatively low price of commodity corn (Fig. 1). Corn price has increased only 0.5% annually while the consumer price index (CPI) increased 3.95% annually since 1960 (Darrel Good, http://www.farmdoc.uiuc.edu//marketing/supply_demanddata.html, 2000, verified August 14, 2001; Bureau of Labor Statistics; Troyer, 1991). At this rate, the value of corn in equivalence of goods doubles because its price relative to CPI has reduced by half in only 29 yr. The farmer offsets higher costs of corn production with increased corn grain yields (Fig. 1). The higher profit margin for hybrid seed corn than for other crop seed is justified by expected higher yield from newer hybrids. This larger profit margin supports more research to develop higher yielding hybrids. During the last 30 yr of less and less expensive corn, corn use has increased 50%—from 152.4 Tg (6000 million bushels) in the 1960s to about 228 Tg (9000 million bushels) in the 1990s (Anon., 2000; USDA, 2000).
Germplasm ownership encourages commercial plant breeding research. Protection is necessary to prevent a competitor from selling the same product without incurring comparable research expenses. Continued breeding gain that increases grain yield, thus reducing feed, food, and fuel costs for the public, depends on the availability of current elite inbreds for research purposes. To use a competitor's inbred in a commercial hybrid, a reasonable royalty should be paid on the basis of the degree of superiority and the cost of developing that inbred. A minimum threshold of 90% or higher for improved EDVs seems appropriate to enable genome conservation breeding. A small royalty on EDVs should be paid to the owner of the IV for only 5 yr. The owner of an IV should have the right to use its first generation EDVs as an incentive for allowing its use, but ownership should belong to the breeders of the new EDVs (improved inbreds). Rules and regulations that stimulate commercial plant breeding research and the hybrid seed-corn business, while encouraging genetic gain for grain yield, are appropriate to make a better world for farmers, for corn breeders and for the entire human populace.
"He that withholdeth corn, the people shall curse him; but blessings shall be upon the head of him that selleth it."
—Proverbs 11:26.
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ACKNOWLEDGMENTS |
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We thank Dr. Ron Ferriss of Syngenta, and Mr. Gary Arthur of Holden's Foundation Seeds for sharing and for allowing use of their RFLP molecular marker data for closely related inbreds.
We thank Dr. Arnel Hallauer, Dr. John Dudley, Dr. Charles Stuber, and anonymous reviewers for invaluable suggestions on the manuscript.
Received for publication March 15, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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