The specific objectives of the research relate to approach element 2.1.2 (Plant Breeding and Genetics). Approximately 75% of research effort will relate to STP code 2.1.2.6 (Germplasm Enhancement and Plant Breeding) and 25% of research effort will relate to STP code 2.1.2.1 (Plant Cellular and Molecular Biology). The research project is focused on the origin, maintenance, and utilization of genetic variation for important agronomic and grain quality traits in corn. The majority of the research is focused on utilization of genetic variation via recurrent selection. Recurrent selection is a general class of selection methods used to enhance corn germplasm. Maintenance of genetic variation is being studied using quantitative genetic mating designs and models and molecular markers. The origin of genetic variation has been difficult to study, but research is in progress to investigate the role of transposable elements in generating genetic variation.

The specific objectives of the research are:

1) Improve adapted corn germplasm for important agronomic traits and grain quality traits by using state-of-the-art recurrent selection, statistical, and quantitative genetic protocols.

2) Develop and empirically evaluate quantitative genetic theory to develop improved genetic models for short- and long-term selection responses.

3) Apply state-of-the-art molecular marker techniques to study the genetic architecture of quantitative traits and to facilitate selection for important agronomic traits and grain quality traits in corn.

The cooperative Federal-State corn breeding project at Ames, Iowa has the facilities and equipment for conducting all phases of the research. Adequate land space is available at the Iowa State University (ISU) Agronomy Research Center near Ames, Iowa for research plots and pollinating nurseries. Additional sites for research plots are available at ISU research farms near Crawfordsville and Chariton, Iowa. Land for research plots is also rented from farmers near Fairfield and Carroll, Iowa. Cold storage facilities are available at the Agronomy Research Center and on the ISU campus for long-term cold storage of seed. Laboratory facilities are available at the Agronomy Research Center and on the ISU campus for drying, shelling, and processing of seed. Both land and physical facilities are good to excellent.

Mechanical equipment adapted for plots is available for planting and harvesting nurseries and research plots. This equipment permits timely planting and harvesting of research plots. Harvesters are equipped with computers for electronically reading and storing plot weights and grain moisture content of each plot. Electronic equipment is also available for recording and storing traits that are not collected on the harvesters. Under typical operating conditions we can plant 800 plots per hour and harvest 100 plots per hour. The major constraint is usually weather.

Excellent equipment is also available for processing seed. Drying and shelling equipment have been acquired to handle large volumes of nursery seed. Electronic seed counters are available to package plots for nurseries and research plots. Excellent computing hardware and software are available for generating experimental designs and analyzing data. The bulk of the computing is done on personal computers and UNIX workstations.

Some laboratory equipment is available for conducting various grain quality assays. After the vacant position is filled, money will be available for purchasing additional equipment if needed.

The following table outlines the major equipment available for conducting field research:

NEED FOR RESEARCH -

1. Description of problem to be solved.

Corn is one of the most important feed grain crops in the United States. The 1 January 1995 U.S. Crop Report (National Agricultural Statistics Service) estimated that 10.1 billion bushels of corn was harvested from 72.92 million acres for an average yield of 139 bushels per acre. Assuming an average price of $2.10 per bushel, the estimated value of the 1994 U.S. corn crop is $21.2 billion. Hence, corn is a very important crop species that contributes to the economic well being of the U.S. Because other areas of the world are rapidly adopting the production of double- and single-cross hybrids, producers in the U.S. face keener competition in producing and marketing their corn in the world markets. A major and perhaps, the only factor in maintaining the competitiveness of U.S. producers is the continued genetic improvement of corn germplasm. Genetic improvement is one of the most economical, efficient, and environmentally sound methods of increasing production and quality of the U.S. corn crop.

Ninety-five percent of the corn produced in the U.S. is commodity corn (mostly yellow dent) and 5% is specialty corn. Of the commodity corn produced, 55% is fed to livestock, 29% is exported, 12% is used for wet milling, and 4% is used for dry milling (Hallauer, 1994). Many of the current speciality corn types is outlined in Hallauer (1994). Individually and collectively, speciality corns occupy a small market niche in the U.S. You could argue that the market niche is small because so little genetic and plant breeding research has been done with speciality corns, but the argument could also be made that little research has been done because the market niche is so small. In either case, it is clear that consumers are demanding higher quality and more nutritious food; importers want higher nutritional and physical quality in the product we deliver; livestock producers want nutritionally balanced feed grains for their animals; and environmental concerns demand that more of our disposable products be produced from commodities in abundant supply. Breeding and genetic research into corn for alternative and speciality corn markets could have tremendous impact on consumers, producers, the environment, and the U.S. economy.

Estimates of the contribution of genetics to the increase in observed corn yields since 1930 have ranged from 51 to 94% with an average of 75% (Russell, 1993). That is, 75% of the increase in grain yield observed by producers is due to plant breeding and genetics. The remainder can be attributed to improved management and cultural practices. Since 1980, however, there is growing evidence that genetics is contributing more and cultural and management practices less to the increases in corn yields. Genetic improvement has been realized primarily from methods of selection used to develop lines and hybrids that have greater response to nitrogen fertilizer and higher plant densities, improved standability, and improved tolerance to biotic and abiotic stresses. Modern corn hybrids will out perform their predecessors under both low and high input management (fertilizer and pesticides), under both no-tillage and conventional tillage, under both low plant densities and high plant densities, and under both low heat and drought stress and high heat and drought stress. None of this would have been possible without a commitment in the public sector to developing improved germplasm and breeding methodology.

Future increases in corn yields will undoubtedly rely solely on genetic improvement. There is little evidence to indicate that we have reached a "genetic plateau" in corn yield. In fact, as producers shift to lower input agriculture, we will become increasingly dependent upon genetics to maintain and increase current yield and quality levels. Waggoner (1994) points out that improving both the quality and quantity of our crops per unit area not only spares land for nature, but lessens the overall environmental impact of agricultural pesticides and fertilizers. He also points to the need for maintenance research to maintain current genetic gains in a production environment of changing abiotic and biotic stresses. Genetic improvement must be done for our future in a world where we can expect to be feeding 10 billion people by the year 2025.

Elite hybrids are developed in the private sector primarily by using the pedigree selection method (Hallauer, 1990). One consequence of this has been a narrowing of the germplasm base available to producers (Smith, 1988). One of the major contributions of USDA-ARS breeding programs is the release of improved breeding populations and parental germplasm that can be integrated into private breeding programs to broaden the genetic base of elite germplasm. A second major contribution is the information derived from basic quantitative genetic research into the inheritance of agronomic and grain quality traits that is used to more efficiently and effectively apply selection methodology.

2. Relevance to ARS needs, missions, objectives, programs, and priorities.

A major objective of the ARS Program Plan is to develop the means to maintain and increase crop productivity and maintain and improve the quality of crop plants (Objective 2, Anonymous, 1991). One research emphasis area is the acquisition, evaluation, and enhancement of plant genetic resources and development of improved varieties using various approaches (traditional, molecular, genetic, and combinations) to overcome productivity barriers in major crops. Another major research emphasis area is the enhancement of plant germplasm by manipulating the plant genome at the molecular level. The proposed objectives and research relate directly to these research emphasis areas.

An objective of ARS Plant Production research is to improve the production efficiency of plants and the quality of plant products. The selection and modification of plant germplasm is one approach to fulfill this objective (Approach 2.1.2: Plant Genetics and Breeding). Development of plant germplasm that is productive and performs consistently over environments requires long-term research programs. Basic research on the relative efficiency and effectiveness of different selection methods; types of gene action operative in plant germplasm; techniques for developing cultivars that are stable in production, tolerant to major pests, and high in quality; and identification of genes that contribute to these processes is needed to maintain systematic genetic advance of plant germplasm. Research programs that have continuity, stability, and the aggressiveness to apply emerging new molecular genetic technology to corn improvement are necessary to obtain this type of basic information.

3. Current technology as related to stated objectives.

Genetic improvements that have been made since Shull (1908, 1909, 1910) outlined the inbred-hybrid concept of corn breeding were primarily based on the pedigree method of breeding. Corn breeders initially sampled open-pollinated or landrace varieties in the 1920s and 1930s and many important inbred lines were developed to produce double-crosses (Hallauer, 1990). Resampling of open-pollinated varieties was met with limited success and breeders began crossing elite lines to produce F₂ populations or crossing lines that complemented each other for specific traits. Pedigree selection was then initiated in F₂ or backcross populations. This method of selection has led to the development of improved lines that have been used extensively in producing corn hybrids. These lines have generally been referred to as second-cycle lines.

Although pedigree selection has lead to the genetic improvement of corn lines and hybrids (Russell, 1993), it has also resulted in a narrowing of the genetic base available to farmers (Goodman, 1990). Hallauer (1990) citing other studies, reported that in 1936 97.7% of the inbred lines developed by the U.S. Department of Agriculture and State Agricultural Experiment Stations were derived from open-pollinated varieties and only 2.3% were second-cycle lines. By 1960, 50% of the inbred lines developed were second-cycle lines and since 1960 the majority of the inbred lines released in the U.S. Corn Belt have been second-cycle lines. The narrowing of the genetic base that has resulted from the use of pedigree selection is evidenced in recent surveys of the U.S. corn germplasm base. Zuber and Darrah (1980) reported that the three most widely used inbred lines in 1979 were B73, Mo17, and A632. Darrah and Zuber (1986) reported that by 1984, the usage of these lines had decreased substantially, presumably due to an expansion in private breeding programs and the success of these programs in developing proprietary inbred lines. Smith (1988) used isozyme and chromatographic data to study the genetic diversity of corn hybrids grown by farmers in the U.S. corn belt. He found that 60% of the privately released hybrids studied either had B73, Mo17, or A632 as direct parents or as major contributors of germplasm. The public inbred Oh43 was also found to have made substantial contributions. Therefore, even though Darrah and Zuber (1986) reported a substantial reduction in the usage of these lines, their private sector replacements represent either identical or closely related germplasm and the inbred lines B73, Mo17, A632, and Oh43 continue to be major contributors of corn germplasm.

Broadening of the U.S. corn germplasm base can only be achieved by using germplasm sources that are unrelated to the lines currently being used to produce corn hybrids. Examples of alternative germplasm sources are narrow base synthetics (4-8 lines), broad base synthetics (9 or more lines), open-pollinated varieties, and exotic or unadapted germplasm. The incorporation of these germplasm sources into private breeding programs has been slow or nonexistent because their breeding objectives are short term. In addition, the performance of many alternative germplasm sources is poor relative to U.S. Corn Belt standards. Therefore, it is often not economically feasible to utilize these alternative germplasm sources in private breeding programs.

Alternative germplasm sources will not be used as sources of inbred lines until they have been improved to acceptable agronomic levels. Recurrent selection is a breeding method with medium- to long-term objectives that has been demonstrated to be effective for improving corn germplasm (Sprague and Eberhart, 1977; Hallauer and Miranda, 1988; Hallauer et al., 1988) and germplasm of other crop species (Hallauer, 1981; 1985; 1992). However, these studies do not provide precise comparisons among the different methods of recurrent selection. Because different methods of selection were used in different source populations, information is not available for the relative effectiveness of the different methods of selection and how they can efficiently contribute to germplasm enhancement. Another factor that confounds the comparisons among past recurrent selection studies is the use of different effective population sizes. Because small effective population sizes (e.g., 10 to 20) were often used, genetic drift and inbreeding affected the estimates of response to selection (Smith, 1983).

Recurrent selection methods will not be acceptable to applied corn breeders unless it can be demonstrated that the methods can contribute to the development of new lines and hybrids. Hence, it is necessary to determine the most efficient and effective methods of recurrent selection that will contribute to both the short-term and long-term goals of breeding programs. Integration of recurrent selection methods with the classical corn breeding methods (pedigree and backcross) will ensure systematic genetic improvement of germplasm sources for line and hybrid development.

Quantitative genetics has undoubtedly made substantial contributions to the genetic improvement of corn (Lamkey and Lee, 1993). Quantitative genetics in conjunction with statistics have been important in the development of systematic progeny testing schemes and of breeding methodology, and, perhaps most important, has provided analytic tools for comparing responses to selection for various breeding methods and progeny testing schemes. Despite these contributions, many aspects of quantitative genetic theory are naive in light of modern genetic principles. Lewontin (1977) pointed out that all quantitative geneticists know is that phenotypes are manifestations of genotypes expressed in environments. And genotypes result from the actions of genes organized into chromosomes that behave regularly during gametogenesis. Beyond these generalities, little is still known 20 years later, about the biology and architecture of quantitative traits. The number of loci controlling a trait; the number of alleles segregating per locus; the allelic frequencies; the effects of allelic substitutions; the linkage relationships among loci; the epistatic interactions between loci; and the expression and regulation of genes are still poorly understood. These factors are the basic building blocks of quantitative genetics and all quantitative genetic models must make assumptions about these factors. Molecular biology has the potential to answer many relevant questions. The challenge is to incorporate emerging information from molecular biology into models or to demonstrate that this information is irrelevant because of the robustness of the theory (Lewontin, 1977).

Molecular markers (isozymes and restriction fragment length polymorphisms) look promising as an aid to the improvement of corn germplasm and the development of improved lines and hybrids. Molecular markers have been used successfully in corn to group inbred lines into heterotic groups (Lee et al., 1989; Godshalk et al., 1990; Melchinger, 1993; Mumm and Dudley, 1994) and preliminary reports on identifying and locating loci affecting quantitative traits have been favorable (Stuber et al., 1987; Edwards et al., 1987; Stuber et al., 1992; Veldboom and Lee, 1994). Lande and Thompson (1990), Lande (1992), and Gimelfarb and Lande (1994) have derived and modeled selection indices that maximize the rate of genetic improvement for quantitative traits for various schemes of marker assisted selection. Their data suggest that marker assisted selection may lead to more efficient genetic improvement under some circumstances, but additional research is needed. Despite these advances, there is still no published research that has critically evaluated the efficacy and efficiency of applying molecular markers to corn improvement.

4. Potenetial benefits expected from attaining objectives.

Attaining the objectives of this research would lead to an expansion of the improved germplasm sources available to corn breeders; provide information on the relative effectiveness of different recurrent selection methods; provide basic information on the types of gene action controlling quantitative traits in corn; and provide empirical information on how long-term recurrent selection experiments modify the genetic variance components of corn populations which could lead to more efficient and effective selection methods.

The objectives of the research are long-term, however, recurrent selection when properly integrated with an applied breeding program, can also fulfill short-term breeding objectives. The most important point is to maintain continuity in the research program and to build on results that have been previously obtained. By this approach, a body of basic information can be developed on recurrent selection methods and related research objectives that would be of benefit not only to corn breeders, but to breeders of other crop species as well. Preliminary information suggest that the potential benefits can be and have been enormous.

5. Expected users of research.

The results of this research will be used primarily by scientists. The information derived from this research will be basic in nature and can be used by other scientists to corroborate their own research findings. Applied corn breeders, whose main objective is the development of improved lines and hybrids, will also benefit from these results by using the information to fine tune their breeding procedures. Growers will ultimately benefit from the research because improved source populations will be developed from which elite inbred lines can be developed for use in hybrids. Consumers will also benefit from this research as it contributes to more stable and high quality corn production.

6. Anticipated products of the research.

In addition to basic information that results from research on the selection and modification of plant germplasm, there are spinoffs in the form of technology transfer that are of direct use to the end users of this information. Genetically improved populations will result that can be used directly as populations per se or as sources of improved inbred lines. Genetically improved inbred lines will also be developed in cooperation with the Iowa State University corn breeder. The genetically improved populations and inbred lines contain unique genes and gene combinations not currently present in the corn germplasm pool and, therefore, contribute to a diversification of the corn germplasm base. All results of the research will be published in appropriate scientific journals.

1. Recurrent selection and related studies.

a. Long-term selection programs

Recurrent selection is a cyclical procedure that involves three phases conducted in sequence: 1) development of progenies (half-sib, full-sibs, S₁ lines, S₂ lines); 2) evaluation of these progenies in replicated experiments so that the top fraction can be selected on the basis of the traits of interest; and 3) intermating of the selected progenies to form a new random mating population in which to initiate the next cycle of selection. The completion of all three phases constitutes one cycle of selection. Each cycle of selection takes from 2 to 4 years depending on the type of progenies evaluated and the availability of a winter nursery. The primary objectives of recurrent selection are to increase the mean performance for the traits of interest and to maintain genetic variation for continued improvement of the population.

A North Carolina Design II (factorial) mating design was developed in the BSCB1(R)C0 and BSCB1(R)C11 populations to determine if selection had changed the additive and dominance variances. A series of half- and full-sib progenies were developed within each population by crossing four males ( S₀ plants) to each of four females (S₁ progenies). The S₁ progenies were developed by selfing random S₀ plants from each population. Each male was crossed to several plants (an average of 11) within a S₁ progeny and the resulting seed was bulked to obtain a representative sample of the gametic array of the original S₀ female plant. Thus, each set of four by four matings produced progenies from a sampling of eight random plants within each population. To achieve a reasonable sample of individuals, 14 sets of four by four matings were constructed within each population, yielding a total sampling of 112 random S₀ plants from each population. Therefore a total of 224 full-sib progenies from each population were produced for field evaluation.

The 448 entries were evaluated in 14 sets of a replications-in-sets randomized incomplete block experiment. Each set consisted of 16 full-sib progenies from each of the populations completely randomized within each of the two replications. The experiment was grown at Ames, Crawfordsville, and Fairfield, Iowa in 1994, are being grown at Ames and Fairfield, Iowa in 1995, and will be grown at Ames, Iowa in 1996. This experiment will provide us with data on the effect of long-term recurrent selection on genetic variances

b. BS11 selection methods study.

An initial evaluation of the response to selection was conducted by evaluating all cycles of each selection method as populations per se, in crosses with the base population BS11C0, in crosses with the inbred B79, as as S₁ populations per se. The study was evaluated at four locations in 1992, 1993, and 1994. The preliminary results will be described in the literature review. On the basis of the results of these studies, selection in BS11(5-S1)C5, BS11(10-S1)C5, BS11(20-S1)C5, and BS11(30-S1)C5 has been resumed.

Because of the differences in response observed among the four effective population size studies and the expected level of inbreeding in each population, an experiment has been designed to study how the additive genetic variance has changed with selection. In 1993, 120 S₁ lines were developed in BS11C0, BS11(5-S1)C5, BS11(10-S1)C5, BS11(20-S1)C5, and BS11(30-S1)C5 by selfing 120 random S₀ plants. In 1994, these S₁ lines were planted as females in an isolation block to be pollinated by BS11C0 as male. The resulting half-sib progenies will be on test at three locations in 1995 and 1996.

A detailed experiment is being designed to study genetic effects and rates of response in the effective population size study. The study is being designed so that the genetic models of Smith (1983) and Hanson (1987) can be fit to the data.

c. Narrow

Extensive work is underway utilizing narrow base synthetics. The objective of this study is to see if recurrent selection implemented in narrow base synthetics will produce a higher frequency of elite inbreds than recurrent selection implemented in broad-base synthetics. The goal is to provide evidence that recurrent selection can be incorporated more effectively into commerical breeding programs. Unlike all of our other recurrent selection programs, these populations will also be open to the introduction of new germplasm. The programs involves a total of four recurrent selection programs conducted in two different populations.

2. Molecular marker analysis of recurrent selection.

Molecular markers are a powerful tool for analyzing and interpreting plant breeding experiments. A vast amount of information has been obtained from selection experiments in corn that has contributed to the design of more efficient and effective breeding programs. Variation in response within and among selection programs indicates that there are many unknowns involved in understanding selection response. Although current methods of phenotypic evaluation of selection has produced useful information, they are also limited in their resolution due to limitations in the quantitative genetic models, and practical size and expense of conducting field experiments. The extensive restriction fragment length polymorphism (RFLP) maps that have been developed for corn offer an excellent tool for studying genome evolution of populations that have undergone recurrent selection. The long-term objective is to understand the creation and maintenance of genetic variation, the organization and structure of the genome, and the genetic responses to selection in corn populations. Achievement of these objectives will lead to the design of improved breeding programs that will not only improve performance at the farmer level, but also increase genetic diversity.

Attainment of the short-term objectives will provide information towards the achievement of the long-term objective that is either difficult or impossible to obtain using conventional phenotypic analyses. Simultaneous consideration of the molecular marker data with the extensive conventional phenotypic data on the populations will provide unprecedented insight into the long-term objectives. Estimation of effective population size will aid in understanding selection response patterns and could provide a means of avoiding genetic "bottlenecks" during selection and recombination. These data will also provide insight into inadequacies of current knowledge and theory regarding selection response. New questions and novel ways of understanding selection response may be suggested by analyzing data from these populations. The impact on U.S. agriculture will be direct, because the performance and improvement of all agricultural species is fundamentally related to an understanding of genetic variance and selection response in populations.

3. Random genetic drift study.

Random genetic drift, which is chance deviations in allelic frequencies due to sampling, is an important component in the design of selection and breeding programs. Under some scenarios, the theory of random genetic drift is well known and under other scenarios it is poorly understood because of mathematical complexities. Random genetic drift is directly related to effective population size and an effective size of one is equivalent to selfing. Despite decades of quantitative genetic research in corn, very little detail is still known about random genetic drift and inbreeding depression. Average rates of inbreeding depression have been extensively quantified in corn (Hallauer and Miranda, 1988), but little is known about the variance of the rate of inbreeding depression and inbreeding among sublines. The objectives of this project are: 1) to study the effects of genetic drift on the mean and variance of genotypic values in the BS13 population, 2) to test assumptions about the effective rate of inbreeding and divergence of allelic frequencies by using molecular markers, and 3) to use this information to assess the potential effects of genetic drift on the observed response to S₂-progeny recurrent selection in the BS13 population.

Two-hundred random inbred lines are being developed from BS13(S)C0. The S₁ lines were developed by selfing 200 random plants in BS13(S)C0. The 200 S₁ lines were grown ear-to-row and the first three plants of each row were selfed. One random ear was chosen from each of the 200 S₁ lines to form the S₂ generation. This process will be continued until the S₅ generations. In addition the S₁ lines will be crossed in isolation as females by using the BS13(S)C0 population as the male to form a group of half-sib families. Five of the six generations (S1, S₂, S₃, S₄, S₅, and half-sib) will be phenotypically evaluated. This aspect of the study is still under development by using quantitative genetic theory to model what combinations of generations will give the most precise parameter estimates. Phenotypes will be recorded on all 200 progenies from each of the five generations chosen to be included in the study. The study is being designed around proposals made by Lynch (1988).

In addition to phenotypic information, genotypic information from molecular markers will also be collected on the five inbred generations. The molecular marker information will allow us to estimate the true rate of inbreeding at the genome level in the population. This information can then be compared to the phenotypic data to give us unprecedented insight into the inbreeding process in corn.

4. Health and safety issues

Laboratory Hazards. Our seed processing lab is a dry lab and there are no chemical procedures being conducted. Routine hazardous materials such as paint and other household chemicals are labeled in accordance with current federal guidelines. The laboratory is inspected each year for compliance with safety regulations.

Occupational Safety & Health. Field work presents many safety issues and we maintain compliance with all current federal and state guidelines and laws. All personnel who operate machinery are trained once a year in machinery safety. Personnel also receive training in use of dust and hearing protection. We are in full compliance with the US EPA worker protection standards.

Use of funds

The project has an annual funding base of $434,783. The breakdown as percentages of the total base funding is shown below.

We have demonstrated that transposable elements (TE) are pervasive in breeding populations and their frequencies have changed in populations undergoing selection. The frequency of plants containing active TE increased from 19 to 91% after 13 cycles of half-sib and inbred family selection and decreased from 19 to 0% after 11 cycles of reciprocal recurrent selection in the BSSS corn population. The increase in TE frequency was likely due to random genetic drift (small sample sizes) coupled with a possible selective advantage associated with the region of the genome linked to the TE and that the decrease in TE frequency was due to random genetic drift. This work established that TE exist in high frequencies in breeding populations and that TE frequencies change with selection.

The improvement in grain yield in the BSSS corn population observed after seven cycles of half-sib recurrent selection and the lack of improvement observed in grain yield after six cycles of inbred family recurrent selection was unexpected theoretically. These results suggested either a failure of the theory or a violation of some of the basic assumptions required by the theory. The detailed quantitative genetic information available on this population indicates that the population should respond to inbred family selection. Inbreds released from BSSS populations improved by half-sib selection have dominated the commercial corn industry and over 50% of the hybrids grown commercially in the U.S. contain germplasm related to these inbreds. The discovery of the lack of response to inbred family selection has led the corn industry a step closer to understanding the complexities of integrating recurrent selection with inbred development.

A recently published method for identifying parental germplasm to improve a specific hybrid correctly identified the top population 60% of the time for grain yield. The construction of the germplasm and subsequent application and interpretation of the genetic model was unique. The study was robust enough to have practical applications and critical enough to provide a scientific evaluation of the methods. Statistically there was often little difference in the ranks of the populations and this lack of statistical power limits the use and interpretation of the new method. When comparing the new method with existing methods, a simpler method performed as well as the newer method. Because genetic gains in breeding programs are limited by the germplasm chosen, this study has important implications about how breeders choose germplasm. This study provides corn breeders with critical data on which to base decisions about the type of methodology to use when choosing parents in a breeding program.

We demonstrated that 11 cycles of reciprocal recurrent selection (RRS) improved the cross between the corn populations BSSS and BSCB1 for grain yield and other agronomic traits. Genetic variance in the cross between populations decreased 25% after 11 cycles of selection, but the difference was not statistically significant. Future genetic progress should be continued at current rates. The superiority of testcross selection methods in corn was demonstrated. The rate of genetic progress clearly demonstrated that RRS is superior to other selection methods and the rate of genetic gain exceeds that realized by U.S. farmers. The improved BSSS and BSCB1 populations (cycle 13 is now available) were rapidly adopted into breeding programs world-wide. Pedigrees of inbred lines from a 1993 compilation of North American corn germplasm indicated that improved versions of BSSS and BSCB1 have been incorporated into numerous synthetic populations and used as sources for the development of improved inbreds. Since 1984, 7 of the 13 inbreds jointly released by the USDA-ARS and Iowa State University were derived from BSSS and BSCB1. Other published data have demonstrated that improved BSSS and BSCB1 represent the best adapted germplasm available to corn breeders.

Lee, M., E. B. Godshalk, K. R. Lamkey, and W. W. Woodman. 1989. Association of restriction fragment length polymorphisms among maize inbreds with agronomic performance of their crosses. Crop Sci. 29:1067-1071.

Godshalk, E. B., M. Lee, and K. R. Lamkey. 1990. Analysis of the relationship of restriction fragment length polymorphisms with agronomic performance of maize hybrids. Theor. Appl. Genet. 80:273-280.

Melchinger, A. E., M. Lee, K. R. Lamkey, and W. W. Woodman. 1990. Genetic diversity for restriction fragment length polymorphisms and its relationship to genetic effects estimated from generation means in four sets of maize inbreds. Crop Sci. 30:1033-1040.

Lamkey, K. R., P. A. Peterson, and A. R. Hallauer. 1991. Frequency of the transposable element Uq in Iowa Stiff Stalk Synthetic maize populations. Genetical Research 57:1-9.

Lamkey, K. R. 1992. Fifty years of recurrent selection in the Iowa stiff stalk synthetic maize population. Maydica 37:19-28.

Keeratinijakal, V. and K. R. Lamkey. 1993. Responses to reciprocal recurrent selection in BSSS and BSCB1 maize populations. Crop Sci. 33:73-77.

Keeratinijakal, V. and K. R. Lamkey. 1993. Genetic effects associated with reciprocal recurrent selection in BSSS and BSCB1 maize populations. Crop Sci. 33:78-82.

Schnicker, B. J., and K. R. Lamkey. 1993. Interpopulation genetic variance after eleven cycles of reciprocal recurrent selection in BSSS and BSCB1 maize populations. Crop Sci. 33:90-95.

Holthaus, J. F. and K. R. Lamkey. 1995. Population means and genetic variances in selected and unselected Iowa stiff stalk synthetic maize populations. Crop Sci. 35:(Nov.-Dec., Accepted May 11, 1995)

Lamkey, K. R., B. S. Schnicker, and A. E. Melchinger. 1995. Epistasis in an elite maize hybrid and choice of generation for inbred line development. Crop Sci. 35:(Sept-Oct, accepted March 21, 1995)

The response to selection using recurrent selection methods has been extensively reviewed in recent literature (Hallauer and Miranda, 1988; Hallauer et al., 1988; Hallauer, 1981; 1985; 1992), therefore, an extensive review of recurrent selection literature will not be presented. However, a review of more recent published and unpublished results will be presented, because these results point out the need for continued basic research with recurrent selection.

Inbred (S₁ or S₂) progeny recurrent selection is expected to be one of the best methods for improvement of the population per se when considering the expected change in allelic frequency (Comstock, 1964) and correspondingly the predicted gain per year (Eberhart, 1972). During the past 20 years, inbred progeny selection has been advocated as the method of choice for several reasons (Hallauer et al., 1988). Quantitative genetic studies have shown that overdominant gene action is relatively unimportant and that the majority of the genetic variation in corn populations is due to additive genetic effects (Hallauer and Miranda, 1988). Under these conditions, the coefficients of the additive genetic variance in the predicted gain equation are two to six times larger than they are for other recurrent selection methods. Therefore, other things being equal, the gains from inbred progeny selection should be correspondingly greater that they are for other recurrent selection programs. S₂-progeny selection allows for multistage selection, that is, there can be selection among and within S₁ lines for highly heritable traits. For obvious practical reasons, inbred progeny selection is easily integrated with applied breeding programs.

Despite these theoretical advantages, inbred progeny selection does not consistently provide favorable selection response. Lamkey (1992) evaluated six cycles of S₂-progeny selection in BS13(S) and reported that there was no significant improvement in population per se performance. A more extensive evaluation (Lamkey, unpublished) has confirmed these results and has demonstrated that there has not been any improvement in testcross performance of the populations as well. In contrast, Weyhrich and Lamkey (1994) reported that five cycles of S₂-progeny selection increased grain yield 4.6% per cycle, along with similar increases in testcross performance. These inconsistencies regarding the performance of S₂-progeny selection are prevalent in the literature and there are many explanations regarding the erratic performance. Among them are overdominant gene action, epistasis, and random genetic drift. The performance of S₂-progeny selection is population dependant, but unfortunately there is no empirical evidence to predict those populations for which we should use S₂-progeny selection. Clearly more empirical research is needed into genetic mechanisms of selection response.

In contrast to S₂-progeny selection, reciprocal recurrent selection (RRS) has produced outstanding gains. Keeratinijakal and Lamkey (1993a) evaluated 11 cycles of RRS in BSSS(R) and BSCB1(R) and reported genetic gains of 7% per cycle in the population cross. In addition, there were significant linear increases in resistance to root and stalk lodging and no change in grain moisture. There was no change for grain yield in the populations per se, but there were significant linear increases in resistance to root and stalk lodging. The lack of improvement in the populations per se was attributed to random genetic drift (Keeratinijakal and Lamkey, 1993b). Helms et al. (1989) reported gains of 4.2% per cycle in the population cross of BSSS(R) and BSCB1(R) after 10 cycles of selection. Moll and Hanson (1984) reported gains of 2.7% per cycle in the population cross of Jarvis and Indian Chief after 10 cycles of selection. Eyherabide and Hallauer (1993) reported gains of 6.5% per cycle in the population cross for grain yield after eight cycles of full-sib reciprocal recurrent selection. These data indirectly suggest that some form of testcross selection is needed to make significant genetic gains in corn. It is also interesting to note that since 1984 7 of the 13 inbreds released by the cooperative Federal-State corn breeding project at Ames, IA were derived from BSSS(R) and BSCB1(R). The last line released from the BS13(S) population was B84, which was released in 1978.

Weyhrich and Lamkey (1994) reported on the comparison of seven methods of recurrent selection conducted in a common base population, BS11. The results have been interesting, in light of the previous results of selection in BS13(S), BSSS(R), and BSCB1(R). After five cycles of selection, the population per se responses per cycle as a percentage of the C0 mean were 4.6% for S₂-progeny selection, 3.8% for modified ear-to-row selection, 3.7% for full-sib reciprocal selection, 2.9% for S₁-progeny selection, 2.1% for full-sib selection, 1.6% for half-sib with an inbred tester selection, and 0.7% for mass selection. Responses of the populations crossed to inbred B79 had rankings similar to the populations per se. There are few published studies available that allow comparisons of selection responses in a common base population using standardized selection protocols. Darrah (1986) reported on the Kenya corn breeding methods study involving several methods of selection. Stojin and Kannenberg (1994) also reported on a study comparing three or four selection methods in five corn populations.

Molecular and biochemical markers in corn have been primarily utilized in assessing genetic diversity (Melchinger, 1993), determining heterotic groups (Melchinger, 1993), mapping quantitative trait loci (Stuber, 1992), and facilitating backcrossing single genes into inbred lines (Melchinger, 1990). There have been only a few published studies utilizing molecular or biochemical markers to analyze recurrent selection. Stuber et al. (1980) reported significant changes in allelic frequencies at eight isozyme loci that were greater than would be expected with genetic drift acting alone. These changes were also correlated with grain yield (the trait under selection). Changes in allozyme frequencies after eight cycles of selection using two selection methods in another population were linear, although statistical tests indicated that directional selection alone could not account for the observed changes (Kahler, 1983). It was concluded that either stabilizing selection and/or random genetic drift were responsible for the observed changes in allozyme frequency.

Inbreeding and inbreeding depression have been extensively studied in corn (Benson and Hallauer, 1994; Cornelius and Dudley, 1974; Good and Hallauer, 1977; Hallauer and Sears, 1973). With the exception of the study by Cornelius and Dudley (1974), inbreeding studies in corn have estimated only the rate of inbreeding depression by using bulk populations. Cornelius and Dudley (1974) studied individual sublines instead of bulk populations and were therefore able to estimate both rates of inbreeding depression and as well as variance components associated with the inbred generations. Lynch (1988) has pointed out many flaws in previous studies on inbreeding depression and random genetic drift and has made recommendations on the optimal design of such studies. The genetic consequences of inbreeding and small population size are of importance in many areas of biology, including inbreeding depression caused by random genetic drift in selection experiments (Lynch, 1988). There is clearly a need for well designed experiments in corn on inbreeding depression and random genetic drift.

The CRIS search of research underway covered 140 research projects being conducted primarily at State Agricultural Experiment Stations and U.S. Department of Agriculture research agencies. The emphasis at state agricultural experiment stations was as follows: Arkansas-line development; Delaware-recurrent selection and line development for disease and insect resistance; Hawaii-line development and quantitative trait loci (QTL) mapping for insects and diseases; Illinois-recurrent selection, line development, and QTL mapping for kernel quality traits, disease resistance, and insect resistance; Purdue-QTL mapping for grain milling, yield, and lodging and starch genetics; Kentucky-white food corn; Louisiana-diseases and dry down; Minnesota-line development, quantitative genetics, and genetics; Nebraska-Quantitative genetics and biometry; New York-nitrogen use efficiency; North Dakota-line development and recurrent selection in early corn; Pennsylvania-line development for grain and forage corn; South Dakota-line development, drought tolerance, and stability; Tennessee-recurrent selection, exotic germplasm, and tillage systems; Texas-Line development in quality protein maize and food corn; Wisconsin-quantitative genetics, population development, breeding methods. Of these state projects, the ones at Illinois, Wisconsin, and Nebraska are most similar to the proposed research. However, there appears to be little overlap in the specific research being conducted.

The CRIS search only identified two USDA-ARS research programs using the keywords corn breeding/quality/improvement/selection methods/quantitative genetics. The program at Columbia, MO utilizes recurrent selection and molecular markers primarily for studying resistance to lodging and insects. The program at Raleigh, NC utilizes recurrent selection in adapted and unadapted germplasm and molecular markers for QTL mapping, marker assisted selection, and studying genotype x environment interaction. The research approach of these programs is similar to the proposed research, but in many cases the objectives are different and the germplasm is different. Again, there appears to be little duplication of the proposed research with the proposed research.

THE RESEARCH PROJECT HAS BEEN EXAMINED FOR POTENTIAL IMPACTS ON THE ENVIRONMENT AND HAS BEEN FOUND TO BE CATEGORICALLY EXCLUDED FROM THE ARS NATIONAL ENVIRONMENTAL POLICY ACT.

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