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Published online 26 August 2005
Published in Crop Sci 45:1891-1900 (2005)
© 2005 Crop Science Society of America
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CROP BREEDING, GENETICS & CYTOLOGY

Extent and Distribution of Genetic Variation in U.S. Maize

Historically Important Lines and Their Open-Pollinated Dent and Flint Progenitors

J. C. Hoa, S. Kresovichb and K. R. Lamkeyc,*

a Pioneer Hi-Bred International, Inc., Johnston, IA 50131
b Institute for Genomic Diversity, Cornell Univ., Ithaca, NY, 14853-2703
c Dep. of Agronomy, Iowa State Univ., Ames, IA 5011-1010

* Corresponding author (krlamkey{at}iastate.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Ancestral open-pollinated populations of U.S. maize (Zea mays L.) are a resource to broaden the genetic base of modern maize and to ensure long-term gains in maize breeding. We report the extent and distribution of simple sequence repeat (SSR) variation in a comprehensive study of Northern Flints (NFs), Southern Dents (SDs), Corn Belt Dents (CBDs), and historically important inbred lines, and address their implications for maize improvement and conservation. We found that the loss of rare alleles in open-pollinated cultivars from elite gene pools resulted in the divergence of inbred lines as a distinct group apart from their progenitor races. In contrast, the Corn Belt Dent cultivars retained high levels of genetic variation that approximated or exceeded that found in their NF and SD progenitors. Within open-pollinated varieties, the absence of any clear pattern of population differentiation indicates frequent admixture, a relatively recent time of divergence, or poor correlation between the degree of divergence at neutral marker loci and quantitative trait variation. This research suggests that the inbred lines currently used by breeders as the primary genetic resource for line improvement do not represent the genetic variation available in the temperate races from which they are derived: only 56% of the alleles found in the Corn Belt Dents were displayed in a diverse set of inbred lines. Extensive sampling targeted at a limited number of Corn Belt Dent cultivars is a promising approach toward the identification and introgression of rare alleles that were lost during early inbred development or the maintenance of open-pollinated populations.

Abbreviations: CBD, Corn Belt Dent • NF, Northern Flint • PCA, principal components analyses • PCR, polymerase chain reaction • SD, Southern Dent • SSR, simple sequence repeat • UPGMA, unweighted pair group method using arithmetic averages


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
VIRTUALLY ALL U.S. hybrid maize, the bulk of feed corn grown in other temperate regions, and over one-half of the global annual supply of maize are derived from the Corn Belt Dent racial complex. Open-pollinated populations of Corn Belt Dent were developed in the early nineteenth century by the hybridization of two highly differentiated races, the NFs and the SDs, followed by mass selection (Anderson and Brown, 1952; Wallace and Brown, 1956; Doebley et al., 1988). Germplasm from a few widely adapted and popular cultivars eventually prevailed over the approximately 800 open-pollinated cultivars grown throughout the country, many of which are now extinct (Goodman and Brown, 1988; Hallauer, 1995).

Since 1948, genetic resources used by maize breeders for inbred development have included F2 (elite x elite inbred crosses), backcross, and synthetic populations, with a rapid decline in the use of open-pollinated cultivars (Bauman, 1981; Jenkins, 1978; Fountain and Hallauer, 1996). This type of selection program has generated productive inbreds that yield better in hybrid combination than their progenitors, but has also generated strong concerns among public and private breeders that the genetic base of U.S. maize is too narrow (Timothy and Goodman, 1979; Duvick, 1981, 1984). Today, one-half of the germplasm used in hybrid cultivars can be attributed to a single Corn Belt Dent population, Reid Yellow Dent (Troyer, 1999). Though it appears that >70% of U.S. hybrid maize is derived from no more than half a dozen lines (National Research Council, 1972; Smith, 1988), molecular marker studies indicate that historically important inbred lines have retained a much broader range of genetic diversity than expected based solely on the number of races involved in their ancestry (Smith et al., 1985). Isozyme data indicate that the most successful open-pollinated sources of elite germplasm (i.e., Iowa Stiff Stalk Synthetic, Lancaster Sure Crop, and Reid Yellow Dent) each encompass relatively high levels of enzyme variation (Smith et al., 1985).

Our goal in this study was to characterize the extent and distribution of genetic variation in historically important inbred lines and open-pollinated populations of CBD, NF, and SD racial complexes. Our first objective was to determine the proportion of genetic variation that was retained through each successive narrowing of the maize gene pool during selection for traits of agronomic importance within the Corn Belt. Our second objective was to examine the genetic structure in U.S. maize and how it relates to demographic or historical parameters such as genetic drift, migration, selection, and time of divergence. Genetic relationships among Corn Belt Dent inbred lines are well understood using allozymes (Smith et al., 1985), zeins (Smith and Smith, 1987), pedigree (Smith et al., 1990), phenotype (Smith et al., 1990), restriction fragment length polymorphisms (Dubreuil and Charcosset, 1998), microsatellites (Smith et al., 1997), and various combinations of these. However, information on their open-pollinated progenitors is limited, and the 765 accessions currently available (North Central Plant Introduction Station; Ames, IA) have not been described using highly polymorphic DNA markers in a comprehensive survey. Genetic classification of populations and races more intimately associated with the early origins of U.S. maize can be applied toward minimizing risk of genetic uniformity, ensuring long-term selection gains, and partitioning a largely untapped source of temperate-adapted breeding material into well-defined or new heterotic groups basic to most U.S. maize breeding.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials
Two hundred and twenty-five accessions were evaluated in this study: 75 historically important inbred lines and a random sample of 38 NF (Northeastern American Flint and Flour), 37 SD (28 Southeastern American Southern-Derived SD, six Southeastern American SD, and three Southeastern American Eight Row), and 75 CBD accessions (Tables 1 and 2). To compare types of germplasm with equal power, the 38 NFs and 37 SDs were pooled together into one progenitor group with 75 accessions (NF/SD). The historically important inbred lines were selected to broadly represent the genetic background of hybrid corn and include the predominant genetic backgrounds that Troyer (1999) found to be important. Each open-pollinated population (accession) was represented by eight individuals. Our choice of a sample size of eight was based on the results of a pilot study, in which estimates of allele number and expected heterozygosity in seven Corn Belt Dent populations sampled at seven plants per population closely approximated estimates using a sample size of 20 (Labate et al., 2003). At least one individual from each inbred line was assayed. Seeds of these accessions and two inbred controls (Mo17 and B73) were obtained from the North Central Regional Plant Introduction Station (Ames, IA) and planted in a growth chamber at 28°C under a 14-h light/10-h dark cycle. Leaf tissue was harvested 10 to 12 d after planting and freeze-dried before extracting DNA by a CTAB miniprep method (Mitchell et al., 1997).


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  		Table 1. Levels of genetic variation averaged across 43 simple sequence repeat loci in (i) Northern Flints and Southern Dents and (ii) Corn Belt Dent accessions: allele number (A), observed (Ho) and expected (He) heterozygosities, inbreeding coefficient (f), and Hardy-Weinberg Equilibrium (HWE).

 

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  		Table 2. The 75 inbred lines included in the study, their pedigrees, heterotic grouping, and the major OPVs that are included in their genetic background. Pedigrees were obtained directly from the inbred originators when possible, but many were obtained from Gerdes et al. (1993) and Troyer (1999).

 
Simple Sequence Repeat Marker Data
Individuals were genotyped at 43 SSR loci uniformly distributed across the maize genome (Table 3). The polymerase chain reaction (PCR) primer sequences are available on the MaizeDB (http://www.maizegdb.org/; verified 20 Apr. 2005). Primers were synthesized and fluorescently labeled by MWG Biotech, Inc. (High Point, NC). Each 20-µL PCR reaction contained 1x PCR buffer, 0.2 mM dNTP, 1.5 mM MgCl2, 1 U Taq polymerase, 4 pmol of each primer, and 25 ng genomic DNA. Thermocycling conditions were 94°C for 1 min, followed by 35 cycles of 94°C for 1 min, 55°C for 2 min, 72°C for 2 min, and a final extension of 72°C for 1 h. Amplified DNA (0.5 µL) was mixed with GeneScan 500XL TAMRA size standard and loading buffer (50% formamide), denatured by heating, and electrophoresed on 5% polyacrylamide 36 cm well-to-read gels using an ABI 377 DNA sequencer as described by Casa et al. (2000). The PCR products of Mo17 and B73 were included as controls on every gel. Raw data were scored using GeneScan v. 2.1 and Genotyper v. 3.0 software (PE Applied Biosystems, Foster City, CA). To control gel-to-gel variation in sizing fragments, SSR data were normalized according to the mean molecular weights of inbred controls that were assayed on all gels. Data collected for inbred lines were verified with SSR data collected previously for 57 of the 75 lines in this data set (data not shown).


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  		Table 3. The 43 simple sequence repeat (SSR) loci genotyped in this study and their chromosomal location and bin number.

 


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  		Fig. 1. Mean levels of genetic variation in Northern Flint (NF), Southern Dent (SD), Corn Belt Dent (CBD), and inbred lines at 43 simple sequence repeat (SSR) loci: (a) allele number, (b) expected heterozygosity, and (c) inbreeding coefficient. Circle = NF/SD; diamond = CBD; x = inbred. The simple sequence repeat (SSR) loci are listed in chromosomal order and their numbers can be associated with the SSR locus in Table 3.

 
Statistical Analyses
Using GDA v. 1.0 software (Lewis and Zaykin, 2004), we estimated observed and expected heterozygosity, allele number, and the inbreeding coefficient for each marker locus, accession, and type of germplasm (NF/SD, CBD, and inbred lines). Standard errors of these estimates were calculated as shown by Weir (1996). Overall, within, and pairwise estimates of the correlation of alleles within subpopulations (FST) were calculated in a three-level analysis: the first level was the total population (assumes all 261 accessions represents one population), the second level consisted of the three germplasm pools (NF/SD, CBD, inbred lines), and the third level was the 261 individual accessions. Ninety-five percent confidence intervals were based on the distribution of FIS, FIT, and FST estimates obtained by bootstrapping over loci 1000 times (Weir, 1996). FIS measures inbreeding in individuals relative to the subpopulation to which they belong. FIT measures inbreeding in individuals relative to the total population. Alleles unique to one of the three types of germplasm were identified. Using GENEPOP software (Raymond and Rousset, 1995, 2004), we estimated allele frequencies in the NF/SD, CBD, and inbred gene pools.

To examine associations of individuals within and among accessions, we performed cluster and ordination analyses on binary SSR data and estimated allele frequencies for (i) the entire data set and (ii) a subset of the data restricted to haplotypes for the most common allele at each locus in the CBD gene pool. This restricted data set was analyzed to account for the high number of alleles per locus and the generally low frequency of each allele. By eliminating the majority of double negatives (i.e., 0–0 matches in data values) from the data set, we sought to increase the power of detecting differentiation among individuals and populations. Pairwise genetic similarities (GS) were calculated by using the method of Dice (1945) with the NTSYSpc v. 2.0 (Rohlf, 1998) software. Dendrograms based on these GS matrices were constructed by the UPGMA (unweighted pair group method using arithmetic averages) clustering algorithm. Principal components analyses (PCA) were conducted on variance–covariance matrices of binary data and correlation matrices of allele frequencies (Sneath and Sokal, 1973).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Levels of Genetic Variation
First, we determined the proportion of SSR variation retained through each successive narrowing of the maize gene pool. Descriptive statistics from the balanced comparison of the NF/SD, CBD, and inbred lines are summarized in Fig. 1. Three of the 43 loci (13, 17, and 27) showed high levels of heterozygosity (low inbreeding) in the inbred lines. Loci 13 and 17 are dinucleotide repeats, so stuttering could explain high heterozygosity at these loci, although it was not a problem at other dinucleotide repeat loci. All 43 SSR loci were polymorphic and 710 alleles were observed: 649 alleles in the NF/SD, 599 in the CBD, and 369 in the inbred lines. The mean number of alleles in the CBD accessions (14 ± 7) was similar to that observed in their NF/SD progenitors (15 ± 8). In contrast, the mean allele number of the inbred lines (9 ± 4) was only 62% of that found in the CBD. While a small percentage (13%) of the NF/SD alleles were absent from the CBD, nearly one-half (44%) of the CBD alleles were not found in the inbred lines. These unique alleles occurred at an average frequency of 0.01 in each sample of their progenitor gene pools. In contrast, loss of genetic variation across successive gene pools was negligible when measured in terms of expected heterozygosity (He). The CBD displayed 97% (0.71 ± 0.15) of the expected heterozygosity found in the NF/SD (0.73 ± 0.14). Similarly, the inbred lines showed 94% (0.67 ± 0.16) of the expected heterozygosity estimated in the CBD. Relatively high estimates of the inbreeding coefficient (f) were obtained for all three types of germplasm, even for the accessions of open-pollinated populations: f averaged 0.39 ± 0.08 in the NF/SD, 0.26 ± 0.11 in the CBD, and 0.94 ± 0.14 in the inbred lines.

Descriptive statistics also revealed high genetic heterogeneity within NF/SD and CBD open-pollinated cultivars: allele number averaged 3.25 ± 0.99 and 3.87 ± 0.59, respectively (Table 1). Expected heterozygosity within populations averaged 0.48 ± 0.13 in the NF/SD and 0.56 ± 0.06 in the CBD. However, high estimates of the within-population inbreeding coefficient were observed in both cases: f was estimated as 0.11 ± 0.08 in the NF/SD and 0.13 ± 0.06 in the CBD.

Population Structure
To explore the genetic structure of U.S. maize germplasm, we treated NF/SD and CBD accessions as subpopulations and each germplasm pool (NF/SD, CBD, inbred lines) as a base population. Significant FST estimates indicated a moderate level of genetic differentiation within the NF/SD and CBD races (Table 4). When we treated the pools of NF/SD, CBD, and inbred lines as subpopulations and grouped the three germplasm pools as a single base population, the overall FST of 0.034 was significant, but its size indicates little divergence between the NF/SD and CBD. On the basis of FST estimates, the inbred lines were only slightly more diverged from the other two groups. Pairwise comparisons of subpopulations further indicated a low level of differentiation among the CBD and among the NF/SD accessions (data not shown).


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  		Table 4. Significant* overall, pairwise, and within-subpopulation estimates of FST for 43 simple sequence repeat loci.

 
Associations among Individuals, Accessions, and Types of Germplasm
To examine the relationships of individuals within and among accessions, we performed ordination and cluster analyses. Both PCA and UPGMA clearly separated the dents from the flints, though only 8% of the total variation was explained by the first three eigenvectors of PCA (Fig. 2a, and see Supplemental Fig. 1). In general, CBD accessions were closely associated with the SD. A small number of inbred lines grouped into this CBD/SD cluster. Most lines, however, distinctly grouped apart from their open-pollinated progenitors. Within each type of germplasm, clear patterns of differentiation were not evident among the open-pollinated cultivars. Within each CBD, NF, or SD accession, individuals typically clustered. Similar results were obtained when we performed multivariate analyses on estimated allele frequencies (data not shown).



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  		Fig. 2. Principal components analysis of 106 Corn Belt Dent populations using data for (a) all 710 alleles observed at 43 simple sequence repeat (SSR) loci and (b) the most common allele at each of 43 SSR loci. Blue: Northern Flints; black: Southern Dents; red: Corn Belt Dents; green: inbreds.

 
Nevertheless, CBD accessions and inbred lines loosely grouped according to expectation based on pedigree and established heterotic patterns. Lancaster and Reid types separated into different groups and inbreds derived from them clustered together in most cases. For instance, B70, W117, C103, C123, NC258, and Mo17, which originated from Noah Hershey's Lancaster Surecrop, clustered most closely to an accession of Lancaster Surecrop (PI 280061). In another group, Reid Yellow Dent (PI 222314) and Black Yellow Dent (PI 213724), a cross of two Reid Yellow Dent strains, grouped with lines derived from Reid or Iowa Stiff Stalk Synthetic, which primarily, but not solely, consists of Reid Yellow Dent germplasm: A632, A634, A635, A641, B68, B73, B79, B84, B104, B164, CM105, H100, I205, MS71N196, N28, and NC294 (Smith et al., 1985). Apart from the open-pollinated cultivars, Oh43 and Oh40B grouped with their derivatives, A619, PA762, Va26, H95, and L317. Lines derived from Funk Yellow Dent, 38-11, PA91, A, Illinois A, and AR4, formed another group with B96, NC298, IDT, PA884P, and R177 (Gerdes et al., 1993).

Using data for the most common allele at each locus in the CBD gene pool, PCA separated individuals into two large clusters within rather than among accessions (Fig. 2b). The first three eigenvectors explained 17.5% of the total variation. Within each of these distinct clusters, individuals were further differentiated into three groups based on the type of germplasm: CBD/SD, NF, and inbred line. Again, we obtained similar results when we performed multivariate analyses on estimated allele frequencies for both data sets (complete and restricted SSR alleles; data not shown).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Levels of Genetic Variation
Potentially, a genetic bottleneck of U.S. maize occurred when widely adapted and popular open-pollinated cultivars were selected for early inbred development. Almost 400 major cultivars were grown in 1936 (Troyer, 1999), yet sources of inbred lines for double-cross hybrids were few and primarily consisted of Reid Yellow Dent, Lancaster Surecrop, and Krug (Goodman, 1995). Today, Reid Yellow Dent alone accounts for 50% of the germplasm estimated in modern hybrids (Troyer, 1999). Nevertheless, we found that single CBD cultivars exhibit high levels of genetic variation: average gene diversity (expected heterozygosity) within each accession accounted for 79% of that estimated in all 75 CBD accessions combined. Moreover, the CBD retained nearly all of the genetic variation estimated in their NF and SD ancestors. This high genetic heterogeneity within the CBD corroborates reports of extensive allelic diversity among Reid Yellow Dent lines (Smith et al., 1985) and the early statement of Wallace and Brown (1956), that "you can get anything out of Reid." In contrast, our broad set of historically important inbred lines showed a loss of rare alleles across the maize genome based on (i) the discrepancy of results when comparing genetic diversity of germplasm pools in terms of allele number and expected heterozygosity, and (ii) the identification of many alleles that occur at low frequency in our samples of progenitor gene pools but that are absent in the elite material. The few exceptions to this general loss of genetic diversity, most notably at umc2067 (Locus 42 in Fig. 1), may reflect the effect of divergent selection in elite heterotic groups, combined with a relatively small sample of each open-pollinated cultivar.

Population Structure
In agreement with taxonomic (Anderson and Brown, 1952), cytological (Laurie and Bennett, 1985), and isozyme evidence (Doebley et al., 1988), we found a high degree of divergence between flint and dent types. Archeological records of the NF can be traced back to at least 1000 A.D. throughout the eastern USA (McKay and Latta, 2002). The SD were dominant in the southeastern USA and were probably introduced from various Mexican sources about 2500 yr later (Doebley et al., 1988; Goodman and Brown, 1988). The absence of genetic differentiation we found between CBD and SD germplasm reflects the breeding history of U.S. maize as predominantly SD genetic material (Doebley et al., 1988). Placement of a small number of CBD accessions in the NF cluster may indicate the presence of large blocks of NF germplasm in early CBD cultivars. If, on average, the number of segregating genes exceeds three per crossover in the hybridization of NF and SD, then linkage blocks can be disrupted only by many generations of controlled breeding (Brown and Anderson, 1947).

Among CBD cultivars, loose associations without clear patterns of differentiation reflect frequent admixture and a relatively recent time of divergence. Rigid type selection, at least for such cultivars as Reid Yellow Dent and Lancaster Surecrop, was not pursued on open-pollinated cultivars: farmers frequently traded and mixed seed (Wallace and Brown, 1956), and publicity that accompanied corn shows ensured widespread use of certain open-pollinated cultivars across the Corn Belt and abroad (Goodman, 1995). Emphasis on local adaptation during the development and sale of open-pollinated cultivars (Troyer, 1999) potentially increased population differentiation at selected loci. However, we cannot expect a subsequent increase in interpopulation gene diversity based on SSR markers if (i) we assume the selective neutrality of SSR loci, or (ii) selection on a polygenic trait is diluted over many loci, such that even a selected locus (or tightly linked marker) can behave as if it were nearly neutral. The absence of genetic structure in open-pollinated maize germplasm is consistent with meta-analyses of empirical studies across several species, in which the degree of divergence in quantitative traits (QST) typically exceeds that for single-locus molecular markers (McKay and Latta, 2002; Merila and Crnokrak, 2001; Reed and Frankham, 2001).

Given that virtually all U.S. hybrid maize is derived from the CBD racial complex, we expected low levels of differentiation between inbred lines and CBD populations. In contrast, we found that inbred lines consistently form a distinct group that only slightly overlaps with their CBD/SD progenitors. Furthermore, the degree of divergence between inbred lines and their ancestral populations is greater when all alleles are considered compared with the most common alleles at each locus. These results again support the statement that modern inbred lines diverged from their ancestral populations primarily through the loss of low frequency alleles. However, FST estimates from pairwise comparisons of germplasm pools were significant but low. This discrepancy of results based on different measures of genetic differentiation exemplifies how relative measures of genetic variation, such as FST, can underestimate the magnitude of population differentiation when levels of heterozygosity are high within populations (Hedrick, 1999), as in the case of open-pollinated populations of maize. Similarly, the low percentage of variation explained by PCA reflects the high level of genetic variation within open-pollinated cultivars and within each type of germplasm.

Clear substructure within open-pollinated populations can be explained by positive assortative mating or incomplete admixture of NF, SD, or CBD cultivars. Particularly, if a season is unusually short, it is likely that late plants were preferentially crossed to late ones, and early plants to early ones, both in nature and in regeneration. Discrepancy in the outcomes of PCA using the complete and restricted SSR data sets most likely reflect the effect of matches in negative character states (i.e., 0–0 matches in data values) when variance–covariance matrices are based on binary data for all alleles, most of which occur at a low frequency.

Implications for U.S. Maize Breeding and Conservation
In this study, we wanted to determine if the primary genetic resources for U.S. maize improvement (i.e., inbreds, synthetics, and improved cultivars) are representative of the genetic variation available in the open-pollinated populations from which they are derived. On the basis of the extent and distribution of SSR variation in inbred lines, CBD, SD, and NF, we conclude that modern inbred lines have distinctly diverged from their ancestral races through the loss of rare alleles due to genetic drift or selection. Given their high genetic heterogeneity, the CBD instead can serve as an effective means of maintaining the genetic variation currently available in temperate maize. More specifically, this research suggests that extensive sampling within a limited number of CBD cultivars can be a promising approach toward the identification and introgression of rare alleles lost during early inbred development or the maintenance of open-pollinated populations. Studies with closed selection programs indicated that the loss of rare alleles is driven primarily by genetic drift (Labate et al., 1997). Therefore, many rare alleles may have been lost during the initial sampling process to develop new inbred lines due to genetic drift alone. In addition, population substructure and the presence of linkage blocks from NF progenitors should be carefully considered when developing sampling and breeding strategies for using CBD to conserve and improve modern maize. Finally, further genetic characterization of these accessions, using markers densely distributed within a candidate region, can be applied to predict allelic variation at quantitative trait loci or to identify loci under selection in breeding material more intimately associated with the early origins of U.S. maize.


   ACKNOWLEDGMENTS
 
Genotypic data for this study were collected by H. Sun, M. Wolfson, and C. Harjes. The authors thank R. Kutkala, C. Harjes, and X. Wang for assistance in data management and M. Goodman, E. Buckler, F. Troyer, B. Gaut, J.F. Doebley, and M. Hamblin for critical review of this manuscript. We are grateful to J.S.C. Smith and M. Goodman for providing seed and genetic backgrounds for some of the more difficult to get inbred lines. This research was supported by a grant from the USDA-NRI plant genome program (CSREES Award No. 99-35300-7757).


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This journal paper of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project No. 3755, was supported by Hatch Act and State of Iowa funds.

Received for publication October 7, 2003.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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A. R. Hallauer
History, Contribution, and Future of Quantitative Genetics in Plant Breeding: Lessons From Maize
Crop Sci., December 18, 2007; 47(Supplement_3): S-4 - S-19.
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