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Published online 1 September 2007
Published in Crop Sci 47:1914-1926 (2007)
© 2007 Crop Science Society of America
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PLANT GENETIC RESOURCES

Pedigree Background Changes in U.S. Hybrid Maize between 1980 and 2004

Stephen Smith*

Pioneer Hi-Bred International, Inc., Crop Genetics Research and Product Development, DuPont Agriculture and Nutrition, 7300 NW 62nd Ave., Box 1004, Johnston, IA 50131-1004

* Corresponding author: stephen.smith{at}pioneer.com.


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Monitoring genetic diversity helps determine whether plant breeders are successful at maintaining germplasm resources sufficient to provide a continued basis for genetic gain and to avoid vulnerabilities potentially associated with a narrowing genetic base. This study used pedigree data of public and proprietary maize (Zea mays L.) inbred lines and sets of Pioneer brand hybrids that were cultivated widely during the period 1980 to 2004 to report on changes in genetic diversity. Pedigree backgrounds for a set of inbred lines bred by different proprietary programs were more diverse (42 founders) than a set of publicly bred inbred lines that were widely used in 1980 (30 founders). Pedigrees of Pioneer hybrids traced to 82 founders. Most of the pedigree backgrounds were contributed collectively by 25 to 50% of the founders. Germplasm was associated into three groups: (i) older public lines, newer proprietary inbreds, and two sets of later maturity Pioneer hybrids; (ii) other later maturity Pioneer hybrids and earlier maturity Pioneer hybrids used in 1980 to 1981; and (iii) remaining early maturity Pioneer hybrids. Differences in proportions of founders already present in U.S. maize germplasm, notably Reid Yellow Dent, Iodent, SMPRS5, Minnesota 13, and Leaming, contributed most as usage changed from older public lines to increased usage of proprietary lines during 1980 to 2004. Changes for Pioneer hybrids involved generally small percentages for several founders, although changes for Reid and Iodent were greater. Future genetic gains are dependent on the deployment of useful genetic diversity. Regular assays of hybrid genetic diversity using molecular markers are recommended.

Abbreviations: PVP, Plant Variety Protection • RFLP, restriction fragment length polymorphism • RM, relative maturity • SSR, simple sequence repeat

Pedigree Background Changes in U.S. Hybrid Maize between 1980 and 2004

Stephen Smith*

Pioneer Hi-Bred International, Inc., Crop Genetics Research and Product Development, DuPont Agriculture and Nutrition, 7300 NW 62nd Ave., Box 1004, Johnston, IA 50131-1004

* Corresponding author: stephen.smith{at}pioneer.com.

Monitoring genetic diversity helps determine whether plant breeders are successful at maintaining germplasm resources sufficient to provide a continued basis for genetic gain and to avoid vulnerabilities potentially associated with a narrowing genetic base. This study used pedigree data of public and proprietary maize (Zea mays L.) inbred lines and sets of Pioneer brand hybrids that were cultivated widely during the period 1980 to 2004 to report on changes in genetic diversity. Pedigree backgrounds for a set of inbred lines bred by different proprietary programs were more diverse (42 founders) than a set of publicly bred inbred lines that were widely used in 1980 (30 founders). Pedigrees of Pioneer hybrids traced to 82 founders. Most of the pedigree backgrounds were contributed collectively by 25 to 50% of the founders. Germplasm was associated into three groups: (i) older public lines, newer proprietary inbreds, and two sets of later maturity Pioneer hybrids; (ii) other later maturity Pioneer hybrids and earlier maturity Pioneer hybrids used in 1980 to 1981; and (iii) remaining early maturity Pioneer hybrids. Differences in proportions of founders already present in U.S. maize germplasm, notably Reid Yellow Dent, Iodent, SMPRS5, Minnesota 13, and Leaming, contributed most as usage changed from older public lines to increased usage of proprietary lines during 1980 to 2004. Changes for Pioneer hybrids involved generally small percentages for several founders, although changes for Reid and Iodent were greater. Future genetic gains are dependent on the deployment of useful genetic diversity. Regular assays of hybrid genetic diversity using molecular markers are recommended.

Abbreviations: PVP, Plant Variety Protection • RFLP, restriction fragment length polymorphism • RM, relative maturity • SSR, simple sequence repeat


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
EFFECTIVE MANAGEMENT of genetic diversity is a critical prerequisite to allow continued improvement of agricultural productivity. For example, Duvick et al. (2004) demonstrated an average genetic gain of 77 kg ha–1 for U.S. maize (Zea mays L.). The U.S. National Academy of Sciences (1972), however, warned, "The general practice of using good hybrids as source material for the development of new lines, however, insures a gradual reduction in the total genetic base" (p. 117). "Decreasing genetic diversity is accompanied by an increased risk of economic loss caused by some new parasite, insect pest, or unusual environmental stress" (p. 114).

Whether future genetic gains can be expected depends on the continued deployment of useful genetic diversity. Duvick (1984) hypothesized that effective breeding programs generate genetic diversity in time. Evidence from pedigrees and molecular markers showed that changes in genetic diversity had occurred during each decade for a set of maize hybrids that were widely grown in the central U.S. Corn Belt (Duvick et al., 2004; Smith et al., 2004; Feng et al., 2006). For example, Smith et al. (2004) showed that contributions from some founders had risen, then fallen; some were low and disappeared, while others reappeared; and contributions from others persisted at low levels. By the 1980s, contributions from 26% of both founders and landraces had become extinct. Large qualitative and quantitative differences were revealed for simple sequence repeat (SSR) alleles when older (1930s and 1940s) hybrids were compared with recent (1990s and 2000s) hybrids (Feng et al., 2006). For example, 23% of SSR alleles were found only in the older hybrids, whereas 30% of SSR alleles were found only in the recent hybrids. Duvick et al. (2004) showed that the number of alleles reached a high during the 1960s and has since declined in these era hybrids.

More comprehensive, industrywide surveys of the use of maize public inbred lines in the USA during the period from 1956 to 1986 have been reported by Sprague (1971), Zuber (1975), Zuber and Darrah (1980), and Darrah and Zuber (1986). Darrah and Zuber (1986) showed also that surveys based solely on the use of publicly bred inbred lines were no longer sufficiently comprehensive because the use of public lines in commercial hybrids had declined to the extent that 62% of U.S. maize production was based solely on proprietary inbred lines; 92% of hybrids used at least one proprietary line. Furthermore, pedigrees of proprietary germplasm have traditionally been maintained as trade secrets. Consequently, it became necessary to use molecular marker data to assay diversity in U.S. hybrid maize (Smith and Smith, 1991; Smith et al., 1992). No more recent reports on genetic diversity for U.S. hybrid maize have been published, however, in contrast to marker-based genetic diversity reports for other major U.S. crops, including barley (Hordeum vulgare L.; Matus and Hayes, 2002; Soleimani et al., 2005), cotton (Gossypium hirsutum L.; Bowman et al., 2003), potato (Solanum tuberosum L.; Coombs et al., 2004), rice (Oryza sativa L.; Ni et al., 2002), soybean [Glycine max (L.) Merr.; Sneller, 2003], and wheat (Triticum aestivum L.; Kim and Ward, 2000).

Nonetheless, so long as sufficiently comprehensive pedigrees are available, and provided that founders can be established, including synonyms (Troyer, 1999, 2004), these data can provide a useful basis for monitoring genetic diversity (Smith et al., 2004, 2006a, 2006b). Describing pedigrees in terms of founder line usage enables comparisons using a common denomination. In this regard, Mikel and Dudley (2006) have facilitated identification of pedigrees of proprietary inbred lines that are protected either by a Plant Variety Protection (PVP) certificate or by a utility patent, and which are therefore presumably widely used in the USA. These inbred lines will constitute a new source of publicly available maize germplasm once their patents and U.S. PVP certificates expire.

This study, therefore, reports on changes in pedigree backgrounds of maize germplasm that was widely grown in the USA between 1980 and 2004. There are two experiments: first, pedigree backgrounds of Pioneer brand hybrids that were widely grown in the USA from 1980 to 2004 were compared; second, pedigree backgrounds of public inbred lines that were the most widely used during the early 1980s were compared with pedigrees of proprietary inbreds that were reported as widely used since that time by Mikel and Dudley (2006). These comparisons collectively provide the basis for a more complete perspective of how genetic diversity in the U.S. maize industry has changed during this period. These analyses also prompt discussion on the appropriate source of data to monitor germplasm diversity used in cultivation.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Three sources of germplasm were examined. First, pedigrees of Pioneer hybrids that were widely used on farms between 1980 and 2004 (Table 1 ) were analyzed. Hybrids were selected on the basis that they individually comprised at least 0.05% of the total bag sales from surveys of farmers (Smith et al., 1992) for each of the growing seasons: 1980, 1981, 1989, 1994, 1999, and 2004. Pedigree backgrounds of Pioneer hybrids were analyzed according to their maturity class: ≥104 relative maturity (RM) or ≤103 RM. Hybrids were partitioned according to maturity to allow a more detailed examination of changes in founder usage within each maturity class. Second, public lines that were widely used to produce hybrids during the early 1980s were selected by including all the "40 most widely used lines in 1979," as listed in Table 4 of Darrah and Zuber (1986). Third, inbred lines cited by Mikel and Dudley (2006) as being important to the U.S. maize industry from 1980 to 2004 on the basis that they have been granted PVP certificates or that they are patented were selected. These researchers further refined the criteria for their selection of these inbreds according to conditions that they were the "most referenced ... for phenotypic comparisons in the U.S. Patent database" or because they were the "most recombined corn inbreds within the Pedigree database," that is, inbreds listed in Mikel and Dudley (2006, Tables 3 and 4, respectively).


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  		Table 1. Pioneer maize hybrids included in the survey of founder constitution. Hybrids are listed according to the survey years in which they were widely used and relative maturity (RM) class designation.

 

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  		Table 3. Percentage of germplasm backgrounds by pedigree–year of survey means for maize inbred lines and hybrids in the present study. Blank entries denote no contribution; zero entries denote a contribution between 0 and 0.04.

 
Pedigree backgrounds of Pioneer hybrids were determined by review of detailed proprietary company pedigree records. The pedigree of each inbred parent of each hybrid was traced back to the earliest recorded sources of germplasm (founder sources) (Smith et al., 2004). Pedigrees of non-Pioneer proprietary inbred lines were obtained from information provided by Mikel and Dudley (2006), augmented with information from Gerdes et al. (1993) and from an anonymous reviewer of this study. Whenever pedigrees of non-Pioneer proprietary inbreds tracked to germplasm that was previously bred by Pioneer, then these pedigrees were also tracked to founders using the Pioneer proprietary pedigree database. Pedigrees of public inbred lines were traced to founders using the same proprietary database. Components of non-Pioneer proprietary pedigrees that could not be tracked in the Pioneer pedigree database were classified as "unknown" founders. Comparisons were made to descriptions of founders that have already been established for U.S. hybrid maize (Troyer, 1999. 2004) to avoid artificial inflation of founder diversity that could arise through a failure to identify synonyms. Summaries of founder contributions for Pioneer hybrids within each survey period were weighted according to use data provided by farmer surveys. Summaries of founder use for inbred lines were weighted according to use data provided by Darrah and Zuber (1986, Table 4, percentage of total requirement for 1979) and by Mikel and Dudley (2006, Table 3, number of hits, and Table 4, number of times used as parent). For inbreds reported by Mikel and Dudley (2006), contributions of founder sources (categorized as public, other company, or unknown and thus presumably including proprietary to that company) were also calculated. Associations among sets of germplasm on the basis of their founder constitutions were revealed by cluster analysis using founder contribution percentage as the input variable (Fig. 1 ), using procedures described by Smith et al. (2006b). A biplot (Fig. 2 ) was also used to show associations among germplasm sets and to identify founders that contributed most to the variation according to methods described by Smith et al. (2006b).


Figure 1
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  		Figure 1. Dendrogram showing associations of maize germplasm sets on the basis of pedigree founder constitution.

 

Figure 2
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  		Figure 2. Biplot showing associations among maize germplasm sets on the basis of founder constitution.

 

   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Ninety-seven founders were identified (Table 2 ). Of the inbreds listed in Mikel and Dudley (2006, Tables 3 and 4), percentages of weighted pedigrees that were classified as unknown were 2.4 and 6.5%, respectively. Percentage contributions of founders to the pedigrees of widely used Pioneer hybrids, to the public inbred lines listed by Darrah and Zuber (1986), and to the inbred lines listed by Mikel and Dudley (2006), are presented in Table 3. Comparisons for founders that contributed at least 2.5% for any single survey year for the Pioneer hybrids, for the sets of data obtained from Darrah and Zuber (1986) and from Mikel and Dudley (2006) are also presented in Fig. 3 , 4 , and 5 .


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  		Table 2. Germplasm backgrounds of founders for inbred lines of maize.

 

Figure 3
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  		Figure 3. Histogram showing contributions by pedigree for 104-d relative maturity or greater Pioneer maize hybrids for founders that individually contributed at least 2.5% in any period surveyed and in any germplasm set.

 

Figure 4
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  		Figure 4. Histogram showing contributions by pedigree for 103-d relative maturity or less Pioneer maize hybrids for founders that individually contributed at least 2.5% in any period surveyed and in any germplasm set.

 

Figure 5
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  		Figure 5. Histogram showing contributions by pedigree for public inbred maize lines cited by Darrah and Zuber (1986, Table 4) and by Mikel and Dudley (2006, Tables 3 and 4) for founders that individually contributed at least 2.5% in any period surveyed and in any germplasm set.

 
Comparison among Maturity Groups for Pioneer Hybrids
Germplasm present in the widely used Pioneer hybrids (presence in at least one maturity class at a mean of >1%) traced to 22 founders (Table 3). Comparisons of founder contributions for founders that individually contributed at least 2.5% in any single survey year are presented in Fig. 3 and 4. Founders ARGMAIZARM, CLARAGE, DOCKDRF101, FCOP, FSOP, ILLONG, LEAMING, M3204, and MIDYELDENT were present at an elevated level (100% greater) in the ≥104 RM class compared with the ≤103 RM class of germplasm. Similarly, the following founders were present at an elevated level in the ≥104 RM Pioneer germplasm: ALBERTFLINT, LANCCOMP, LLE, MINN13, NWDENT, and SMPRS5. IODENT was present at an approximately 50% higher level in the ≤103 RM germplasm.

Comparisons in Time within Each Maturity Group of Pioneer Hybrids
Changes for the most significant founder contributions within each maturity class are presented in Fig. 3 and 4. Founders for ≥104 RM hybrids are presented in Fig. 3; founders for ≤103 RM hybrids are presented in Fig. 4. For hybrids that are ≥104 RM, the biggest changes from 1980 to 2004 (Table 3, Fig. 3) were the introduction of A237, Northwestern Dent, and Prolific Composite; the loss of M3204; increased contributions from Argentine Maiz Amargo, FSOP, Iodent, LLE, Minnesota 13, and Osterland Yellow Dent; and decreased contributions from Illinois Long Ear, Improved Yellow Dent, Lancaster Composite, Lancaster Low Breakage, Leaming, Midland Yellow Dent, and Reid Yellow Dent. For hybrids that are ≤103 RM, the biggest changes from 1980 to 2004 (Table 3, Fig. 4) were introductions of Argentine Maiz Amargo, CD6, Dockendorf 101, FCOP, Goldengate, and SMPRS5; losses of Alberta Flint, Midland Yellow Dent, and SMITHTC; increases of Iodent and Lancaster Low Breakage; and decreased contributions from Clarage, Lancaster Composite, Leaming, Minnesota 13, Osterland Yellow Dent, Reid Yellow Dent, and YA3G3-1-3.

Two founders that had contributed to the ≤103 RM hybrids did not contribute after 1994; one founder last contributed in 1980 to 1981 (Table 3). For the ≥104 RM or later maturity hybrids, there were 13 founders that made no contribution after 1994 (Table 3). Nonetheless, the numbers of founders in the pedigrees of these hybrids generally increased with time, with R2 values of 0.67 and 0.69 for the ≥104 RM and ≤103 RM, respectively.

Comparisons among Each Set of Inbreds or Hybrids as Revealed by Multivariate Analysis of Founder Percentage Data
Associations among all germplasm sets on the basis of their founder contributions are shown as a dendrogram (Fig. 1) and as a biplot (Fig. 2). Germplasm sets associated into three groups (Fig. 1). One group comprised public inbreds as listed in Darrah and Zuber (1986, Table 4), the mainly proprietary inbreds reported by Mikel and Dudley (2006,Tables 3 and 4), and two Pioneer germplasm sets (widely used ≥104 RM hybrids from 1980–1981 and from 1989). A second group comprised the remaining ≥104 RM Pioneer hybrids from 1994, 1999, and 2004 plus the 1980–1981 ≤103 RM Pioneer hybrids. The third group comprised all the remaining ≤103 RM Pioneer hybrids. Results of the biplot analysis (Fig. 2) showed similar associations among germplasm sets as revealed by the dendrogram. Both Reid Yellow Dent and Iodent made strong contributions to the associations of germplasm sets relative to other founders. Iodent, SMPRS5, and MINN13 contributed most to the grouping of the post-1989 Pioneer ≤103 RM germplasm sets, placing them apart from other germplasm sets. Reid Yellow Dent and Leaming contributed most to the grouping of the public line set, the predominantly proprietary germplasm sets listed by Mikel and Dudley (2006), and the ≤103 RM Pioneer hybrids from the 1980s. Comparisons among all sets of germplasm can also be made through observation of Fig. 3, 4, and 5. The most important changes from the old public inbred lines to those that will constitute a new generation of publicly available lines include introductions of Argentine Maiz Amargo, FCOP, LANCCOMP, LLE, M3204, MIDYELDNT, and SMPRS5; the loss of OPYELVAR; increases of Iodent and LANCLOBRK; and decreases of CLARAGE, LEAMING, REID, and YA3G3-1-3 (Fig. 5).

Pedigree Sources by Company for Inbreds Listed by Mikel and Dudley (2006)
Pedigrees of proprietary inbreds categorized as the "most referenced ... for phenotypic comparisons in the U.S. Patent database" as listed in Mikel and Dudley (2006, Table 3) tracked to the following sources (weighted percentage in parentheses): Pioneer inbred lines tracked to Pioneer proprietary (96.2%) and to public lines (3.8%); Holden lines tracked to public lines (79%), to Pioneer (20.8%), and to unknown (0.1%); DeKalb lines tracked to Pioneer (48.2%) and to public lines (51.8%). For proprietary inbreds categorized as the "most recombined corn inbreds within the pedigree database" as listed in Mikel and Dudley (2006, Table 4), pedigrees tracked as follows: a Cornelius inbred tracked 100% to public lines; Pioneer inbreds tracked to public (3.6%) and to Pioneer proprietary (96.4%); Holden lines tracked to public (69.2%), Pioneer (30.6%), and unknown (0.2%); DeKalb lines tracked to public (11.3%), Pioneer (33.7%), and unknown (55%); a Syngenta line tracked 100% to public lines.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Maize represents one of the most genetically diverse species. Mean nucleotide diversity of protein coding regions of the maize genome is 10-fold greater than that of soybean (Tenaillon et al., 2001; Zhu et al., 2003). A lack of co-linearity of maize genes, even among well-adapted U.S. inbred lines (Fu and Dooner, 2002), provides an additional basis of diversity. Furthermore, maize exhibits enormous diversity within intergenic regions, which can be associated with gene expression (Clark et al., 2004). Comparisons of genetic diversity among genera, however, do not provide information on the amount of diversity that resides in breeding programs. Nor do these comparisons indicate the amount of genetic diversity that is arrayed in cultivation, either in space or in time. The amount of genetic diversity present in breeding programs has been conditioned initially by a series of bottlenecks that occurred during domestication and subsequently during the evolutionary history of that crop species (Yamasaki et al., 2005). Prospects for further narrowing of genetic diversity occur as breeders collectively focus on a small set of related cultivars, inbred lines, or hybrids as sources of germplasm for further breeding (Tarter et al., 2004). For example, Anderson (1944) showed that the pedigrees of six popular maize hybrids traced to two open-pollinated populations and thus raised concerns about the narrowing of the germplasm base (Labate et al., 2003). The capability for continued progress by plant breeding, as evidenced by genetic gain and genetic diversity in time, and for guarding against vulnerabilities associated with a lack of genetic diversity, are dependent on retaining a sufficient reservoir of genetic diversity in breeding programs (National Academy of Sciences, 1972). Pedigree and usage data for maize inbred lines and a set of widely used U.S. maize hybrids provides opportunities to examine recent trends in the genetic diversity of U.S. maize.

Pedigree data from three sources of inquiry provided the basis for examining changes in germplasm use in U.S. maize from 1980 to 2004. First, the survey of public line use (Darrah and Zuber, 1986) identified publicly bred inbreds that contributed to U.S. germplasm during the early 1980s. Second, the survey by Mikel and Dudley (2006) identified inbred lines, most of which are proprietary and which presumably provide a more comprehensive industrywide perspective of germplasm that was used on farms and in further breeding from the early 1980s to 1990s. These inbred lines also represent the next generation of publicly available U.S. maize germplasm. Third, periodic surveys of use on farms of Pioneer brand hybrids from 1980 to 2004 provided additional sources for tracking change in the use of U.S. maize germplasm. This latter set of germplasm represented from 34 to 45% of maize germplasm used on farms in the USA during this period.

Translation of pedigree data to founder line constitution (Troyer, 1999, 2004) allows the pedigrees of public lines, Pioneer brand hybrids, and the majority of non-Pioneer proprietary maize germplasm reported by Mikel and Dudley (2006) to be presented in a common denomination. Pedigrees for most of the non-Pioneer proprietary inbred lines listed by Mikel and Dudley (2006) could be tracked to founder lines that exist in the Pioneer pedigree database because most of those pedigrees included publicly bred lines or other germplasm previously developed by Pioneer Hi-Bred International. In terms of weighted percentage of use, 2.4 and 6.5% of the pedigrees of inbred lines listed by Mikel and Dudley (2006, Tables 3 and 4, respectively) could not be translated into this common database of founder use percentage by pedigree. These founder-line data thereby facilitate an examination of changes in germplasm use that have occurred within most of the U.S. maize industry during the past 25 yr.

During the period 1980 to 2004, germplasm changes within Pioneer hybrids involved several pedigree backgrounds but with relatively small changes in contribution percentages. For the ≤103 RM hybrids, six new founders were introduced, with individual pedigree contributions ranging from 1.0 to 4.4%. Minor contributions from three founders were lost (maximum individual contributions of 2.0–2.2%). Contributions from two founders increased by 3.8 and 5.4%. Contributions from seven founders decreased, although only one founder (Reid Yellow Dent) declined by >5%. Similar results were found for the ≥104 RM class, although here there was a larger increase (11%) in Iodent usage.

Tracking changes in germplasm use from 1980 to 2004 of Pioneer brand hybrids is based directly on on-farm use surveys of that germplasm. Pedigree founder changes that have occurred for hybrids that are released by the rest of the industry, however, cannot be assessed so precisely. The use of public inbred lines (Darrah and Zuber, 1986) provides one reference point of germplasm use during the early 1980s. This germplasm directly represented 21.6% of the U.S. total requirement for hybrid seed production in 1984, as calculated from Darrah and Zuber (1986, Table 2). Public- and Pioneer-developed germplasm together represented approximately 55% of the germplasm that was directly used to make hybrids during the early 1980s.

Further evidence on the use of maize germplasm in the USA during the 1980s is available from two molecular-marker-based studies of widely used hybrids that included hybrids of known pedigree (Smith and Smith, 1991; Smith et al., 1992). Evidence from a restriction fragment length polymorphism (RFLP)-based survey of U.S. maize hybrids that were widely used on farms during the 1989 season (Smith et al., 1992) showed differences between most Pioneer and most other hybrids. Only four Pioneer hybrids were associated with the majority of hybrids released by other companies. Two of these Pioneer hybrids used the public inbred line B73 as a direct parent in contrast to other Pioneer hybrids that all used proprietary parents. Also, the grouping together of several non-Pioneer hybrids implied use of common or similar inbred lines in those hybrids. Another study reporting associations of hybrids (Smith and Smith, 1991) also showed that Pioneer hybrids made with proprietary parental lines grouped apart from most other hybrids. In that study, three Pioneer hybrids that were each made using a public inbred line clustered with most hybrids produced by other companies. In addition, nine crosses of known pedigree were also included in that study. Eight of these crosses (LH74/LH51, LH132/LH51, B73/Mo17, B73/LH51, LH119/LH51, LH132/LH123, LH74/LH123, and B73/LH38) were associated with the majority of non-Pioneer hybrids. These RFLP analyses (Smith and Smith, 1991; Smith et al., 1992) therefore indicate that during the 1980s, germplasm components of widely used Pioneer hybrids differed from those of most of the widely used non-Pioneer hybrids and that the pedigrees of many non-Pioneer hybrids were generally more recently dependent on foundation or public inbred lines. Reliance on public lines included use directly of B73 and Mo17 and indirectly of lines including B14, B37, B73, and Mo17 by virtue of their use in the pedigrees of foundation company lines that were then widely used in the production of hybrids.

Comparisons of pedigree backgrounds for public lines listed by Darrah and Zuber (1986) with the inbred lines listed by Mikel and Dudley (2006) show that pedigree backgrounds of the more recently bred inbreds, which were predominantly developed in proprietary programs, are more diverse (59 vs. 29 founders). Most of these differences, however, are small in terms of percentage (<2.0%). Only four founders have larger differences. Iodent and Lancaster Low Breakage have >5% higher contributions, whereas Leaming and Reid Yellow Dent have lower contributions (approximately 8 and 10%, respectively) for the predominantly proprietary lines as reported by Mikel and Dudley (2006) compared with the older set of most widely used public lines as reported by Darrah and Zuber (1986).

Multivariate analyses of pedigree data show that the predominantly proprietary germplasm that is listed by Mikel and Dudley (2006) is more closely aligned with Pioneer germplasm of the ≥104 RM maturity class, but then only for Pioneer hybrids that were widely used on farms during the 1980s compared with later decades. One factor causing this association is the presence of inbreds developed by Pioneer among the proprietary inbreds listed in Mikel and Dudley (2006, Tables 3 and 4). Another factor is the contribution of germplasm, notably Iodent, initially developed by Pioneer (Troyer, 1999, 2004; Mikel and Dudley, 2006), to several non-Pioneer inbreds listed in Mikel and Dudley (2006, Tables 3 and 4).

Pedigree data show an evolution of germplasm backgrounds by Holden Foundation Seeds and by DeKalb to include germplasm previously developed by Pioneer. On this basis, for the inbreds listed by Mikel and Dudley (2006), Holden used publicly available lines in 69 to 79% of their pedigrees, with the remainder being sourced from Pioneer germplasm. For inbreds developed by DeKalb that were reported by Mikel and Dudley (2006), from 34 to 48% of their overall pedigree was contributed by germplasm developed by Pioneer. Troyer's (1999, p. 601) assertion that the pedigree background represented by 33 Pioneer proprietary inbred lines was "probably representative of the entire U.S. seed industry as a group (percentages differ by company) because all companies started with the same public inbred lines in the 1920s and 1930s" would appear to be a more accurate reflection of U.S. maize germplasm in the early 2000s than was the case in the early 1990s. My review of current patents (as of August 2006) indicates additional sourcing of Pioneer germplasm, including by Syngenta with 16 of 53 (30.19%) issued patents identifying Pioneer germplasm in the parentage of those inbreds. Furthermore, Troyer (2004) noted that MBS Genetics (2002) showed that 96% of 134 inbreds that had been developed using proprietary hybrids had Pioneer brand hybrids in their parentage. Mikel and Dudley (2006, p. 1193) concluded: "much of today's germplasm originates from seven progenitor lines."

Progress in plant breeding can only be made by effectively sourcing new and useful germplasm. The demonstrated focus of several organizations on breeding from any single source of well-adapted germplasm, however, raises concerns that the U.S. maize germplasm base could be narrowing. Studies to monitor maize genetic diversity in the USA were once regularly undertaken (Sprague, 1971; Zuber, 1975; Zuber and Darrah, 1980; Darrah and Zuber, 1986). The most recent published study of widely used U.S. maize hybrids (Smith et al., 1992), however, which reported on hybrids grown in 1989, is now outdated. If only a few hybrids were grown on very large acreages, then this fact alone would raise concerns about the breadth of the genetic base and potential vulnerabilities to pests, diseases, inclement weather, and genetic erosion; especially so if the use of these hybrids remained static in time. Concentrated use of specific individually named hybrids was not found for hybrids included in this study, nor was it the case for a previous study of U.S. maize (Smith et al., 1992). When RFLP data were used as the basis for comparing these hybrids, however, there were numerous examples of hybrids with marker profiles that were >90% similar and thereby suggestive of the use of common germplasm (Smith et al., 1992). Thus, genetic diversity among hybrids can only be monitored through the use of data that reflect genotype. In this study, I have attempted to update the current knowledge of maize germplasm use, using our own proprietary hybrid data as well as drawing on publications that have mined PVP and patent databases. There are obvious limitations as to what can be inferred about genetic diversity from the use of pedigree data, however, most especially when hybrids released by different organizations are the subject of inquiry. Consequently, the only appropriate means to monitor genetic diversity on an industrywide basis is through the use of molecular markers. I, therefore, concur with Mikel and Dudley (2006) that regular assays of genetic diversity of U.S. maize hybrids are necessary and that the diversity of U.S. maize germplasm needs to be broadened.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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Received for publication December 11, 2006.


   REFERENCES
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Intellectual Property Protection for Plant Varieties in the 21st Century
Crop Sci., July 1, 2008; 48(4): 1277 - 1290.
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