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Published in Crop Sci. 44:1935-1946 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP BREEDING, GENETICS & CYTOLOGY

Changes in Pedigree Backgrounds of Pioneer Brand Maize Hybrids Widely Grown from 1930 to 1999

J. Stephen C. Smitha,*, Donald N. Duvickb, Oscar S. Smitha, Mark Coopera and Lizhi Fenga

a Pioneer Hi-Bred International, Inc., Crop Genetics Research and Product Development, DuPont Agriculture and Nutrition, 7300 N.W. 62nd Ave., P.O. Box 1004, Johnston, IA 50131-1004
b 6837 N.W. Beaver Drive, P.O. Box 446, Johnston, IA 50131-0446

* Corresponding author (Stephen.Smith{at}Pioneer.com)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Surveys of pedigree usage indicate that breeding programs can generate genetic diversity in time. There are also concerns that genetic diversity is being lost. Previous studies of pedigree usage in U.S. maize (Zea mays L.) production have reviewed usage of publicly bred inbred lines. However, proprietary inbred lines were already contributing over 90% of U.S. maize production by 1985. This chronological study of germplasm usage compliments previous studies of genetic diversity in U.S. maize production by reporting the pedigree backgrounds of 68 proprietary, widely used maize hybrids released by Pioneer Hi-Bred International during the period 1930 to 1999. Objectives are to identify founder sources for the hybrids and to reveal changes in founder contributions through time. We also compare founder usage with pedigrees of widely used U.S. public inbred lines to measure the extent of their reliance on common germplasm backgrounds. Pedigrees of the hybrids collectively traced to at least 61 founders. Several founders of the hybrids had complex pedigrees. Founder backgrounds of the hybrids could be traced to at least 22 landraces with additional contributions from other populations or landraces. Public lines used for comparison traced to 14 founders. Nine founders of the public lines were common in the era hybrids; five were unique to the public lines. Differences in founder contributions were evident for the era hybrids and the public lines. Significant contributions from both the private and public sectors were evident in the pedigrees of the era hybrids. Diversity in time was evident. Hybrids tended to associate by decade of initial release. Most new founder contributions occurred in the 1940s (35%), 1960s (36%), and 1980s (20%). Breeding networks have allowed germplasm that was once exotic to the central Corn Belt to contribute improved productivity in that region.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
INCREASED GRAIN YIELD POTENTIAL of maize is due to the successive development of better adapted varieties (Duvick, 1977, 1984a, 1984b, 1992; Castleberry et al., 1984; Derieux et al., 1987; Russell, 1991; Eyherabide et al., 1994; Baenziger et al., 1999; Duvick et al., 2004). Estimates of increased productivity due to genetic gain in U.S. maize production are about 77 kg ha–1 (Duvick et al., 2004).

One vital reason to survey genetic diversity is to monitor changes in the breadth of the germplasm base. It is important to address concerns that germplasm is lost and to understand better the processes that may lead to changes in genetic diversity over time (Vellve, 1993; Clunies-Ross, 1995; Elias et al., 2000). A narrowing of germplasm diversity in breeding and production could lead to reduced genetic gain for yield potential, increased susceptibilities to inclement weather, or to attacks from pests and diseases coupled with the threat of further genetic erosion (Natl. Res. Council 1972; Brown-Guedira et al., 2000; Sun et al., 2001). Duvick (1984a) showed, from surveys of breeders, that genetic diversity is released and arrayed on farms in a chronological dimension as "diversity in time." Stuthman (2002) noted the importance of intelligent use of genetic diversity as a prerequisite to achieve sustainability in agriculture.

It is challenging to characterize meaningfully genetic diversity (Gomez et al., 2000; Kim and Ward, 2000; Purvis and Hector, 2000). Several types of data have been used, including morphology (Gomez et al., 2000; Ortiz et al., 2002), pedigree (Delannay et al., 1983; Cox et al., 1985), and molecular markers (Donini et al., 2000; Kim and Ward, 2000). Reliance on pedigree data can lead to an overestimation of the actual level of diversity (Soleimani et al., 2002), but such criticism is less relevant when trends are being examined. Estimates of relatedness from pedigree data can differ from measures of genetic similarity that are based on comparisons of molecular-marker data (Bernardo et al., 2000). However, such examples are mostly the exception (Bernardo et al., 1997; Bernardo and Kahler, 2001). Similar evidence for changes in genetic diversity from marker and pedigree data has been reported by Parker et al. (2002). Pedigrees have been used to describe genetic diversity for cultivars of many crop species (Soleimani et al., 2002).

Usage of publicly bred inbred lines in U.S. maize from 1956 to 1985 has been summarized by Darrah and Zuber (1986). The most widely used public inbreds changed for the years 1956, 1964, 1970, 1975, and 1979. But, for the last year surveyed (1984), the inbred B73 maintained its prior status as the predominantly used, public inbred line. More recently, Lu and Bernardo (2001) reported that eight publicly bred maize inbreds, (B14, B37, B73, B84, Mo17, C103, Oh43, and H99),"largely represent(ed) the genetic background of current elite inbreds in the U.S. seed industry." Similarly, Gethi et al. (2002) stated that "older generation (public) inbred lines...are still widely used in inbred line development..." However, all of these public inbreds are now old. The earliest release was for C103 (pre-1948) and the most recent is B84, which was released in 1978 (Henderson, 1983). More recent studies have shown significantly increased participation by commercially funded breeders in the development of hybrids used in U.S. maize production. Darrah and Zuber (1986) showed that one or more privately bred inbreds were in 92% of total U.S. maize production. Frey (1996) showed that 94% of U.S. field maize breeders are employed in the private sector. Troyer (1999) indicated usage of a germplasm base in U.S. maize production that extends beyond widely used publicly bred lines. Whether previous surveys based on usage of publicly bred lines continue to represent accurate portrayals of germplasm usage in U.S. maize production depends on the extent to which those inbreds are currently used in production or in breeding.

In this study, we examine a sampling of germplasm diversity that has been commercially developed during the period 1930 to 1999 for use in U.S. maize production within the central Corn Belt. While the hybrids we are able to examine represent the products of only a single commercial maize breeding company (Pioneer Hi-Bred International, Inc.), they do represent a significant market share in the central U.S. Corn Belt. We utilize pedigrees of maize hybrids that are, or were in their era, widely grown in the central U.S. Corn Belt, (Duvick, 1977, 1984a, 1984b, 1992). Objectives are to test the hypothesis that breeders generate diversity in time. We also test a hypothetical concern that U.S. maize diversity is narrowing. And we evaluate whether previous studies of U.S. maize diversity that used only pedigree information for public inbred lines provided a complete portrayal of germplasm usage.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Pedigrees of 68 Pioneer brand hybrids were analyzed (Table 1). Each hybrid is, or was, an important contributor to maize production in the central U.S. Corn Belt. The set of hybrids includes those reported on by Duvick to measure genetic gain (1977, 1984a, 1984b, 1992) and is expanded to include hybrids that were widely used in the central U.S. Corn Belt during the 1990s. Hybrids are listed according to year and decade of initial release. Type of hybrid in respect of parental constitution (single-cross, modified single-cross, triple-cross, or double-cross) is also noted.


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  		Table 1. Maize hybrids listed according to their initial year of commercial release and type of cross (DX = double cross, TX = triple cross, MSX = modified single cross, SX = single cross).

 
Pedigree backgrounds were determined by review of detailed proprietary company pedigree records. Each inbred parent of each hybrid was traced back to the earliest recorded sources of germplasm. Each of these earliest recorded sources is classified as a founder or ancestral genotype following Zhou et al. (2000). Founder genotypes are reported in capitalized abbreviations (e.g., ALBRTFLINT) with descriptive information (Table 2). Pedigree contributions were also summarized according to landrace (e.g., ‘Clarage’) from which a founder was derived (Table 3). In some instances, a founder was itself directly derived from a landrace. In other cases, a founder (e.g., BSSS) was itself comprised of several inbreds that collectively traced back in their pedigrees to several landraces. In those instances, the pedigree contribution attributed to a landrace from that founder was noted separately (e.g., 6.3% by pedigree of BSSS could be traced to landrace Clarage). One further clarification is necessary. The landrace ‘Reid Yellow Dent’ is comprised of numerous strains (e.g., WFRYD or ‘Wilson Farm Reid Yellow Dent’, ‘Iodent’, or ‘Osterland’ are classified as different strains of Reid Yellow Dent). Individual pedigree contributions for Iodent and for Osterland are provided in addition to the summation of all Reid Yellow Dent contributions that is denoted as "All Reid Yellow Dent Total."


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  		Table 2. Germplasm backgrounds and geographic origins of founders for era hybrids.

 

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  		Table 3. Germplasm backgrounds and geographic origins of founders with respect to derivation from maize landraces (where traceable).

 
Coefficients of parentage (CP) measuring pedigree contributions of founders to each inbred, and thereby to each hybrid, were calculated on the basis of the assumption that following each parental cross, each parent then contributed equally to each progeny (i.e., there was no mutation, selection, or genetic drift), following Malécot (1948), Gizlice et al. (1994), and Cui et al. (2000). Differential parental contributions to progeny that would be expected from recurrent use of a parent were therefore taken into account. Pedigree contributions of landraces were similarly computed. Mean decade pedigree contributions were computed on the basis of the assumption that each hybrid released during a respective decade was used on an equivalent hectarage.

Methods used to compare changes in pedigrees among decades were as follows: The mean percentage contributions of founder genotypes and of landraces within each decade were computed. Pedigree usages were noted according to three levels (<1%, 1–4.9%, and >5%) of contribution. Changes in pedigree usage among decades were high-lighted if contributions were new, disappeared, or changed by more than 100% from the previous decade. Numbers of new founder contributions and their cumulative and mean percentage contributions were calculated for each decade. Associations among hybrids on the basis of pedigree data were also used to reveal pedigree changes among decades. Pedigree distances (1 – CP) were computed for each pair of hybrids and associations among hybrids were revealed following multivariate analysis of pedigree distance data. Comparisons of CP among hybrids used within each decade and between adjacent decades were also used to show chronological changes in pedigree relatedness.

Comparisons of pedigrees underlying widely used publicly bred inbred lines and the inbred lines used in this set of hybrids were also made. The following sets of public lines were used: Darrah and Zuber (1986)(see Table 11) reported percentage of total use for public inbred lines across each of six survey years. We added contributions of inbreds W117 and CM105 for the year 1984 because these lines had a reported increase of usage in that year from the previous survey year (see Table 4 of Darrah and Zuber [1986]). Founder percentages for each year were then adjusted to reflect relative contribution of the public lines reported by Darrah and Zuber (1986) in their Table 11. Lu and Bernardo (2001) reported the following eight inbred lines (B14, B37, B73, B84, Mo17, C103, Oh43, and H99) to "largely represent the genetic background of current elite inbreds in the U.S. seed industry." Percentages of founder germplasm are provided for this set of inbreds, but without weighting for relative usage. Comparisons of pedigree usage between these public inbred lines and the set of hybrids were high-lighted according to the levels of usage previously described.


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  		Table 4. Percentage germplasm backgrounds by pedigree–decade means for hybrids in the present study. Blank entries denote no contribution; zero entries denote a contribution between 0 and 0.04.

 

   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
At least 60 founder or ancestral types (Table 2) were involved in the pedigrees of these era hybrids (Table 1). Six founders could be further traced in their pedigrees to other founders listed in Table 2. These are (i) A237, bred from ‘Pride of the North’ x Reid Yellow Dent; (ii) AFLF, originated from a cross of ‘Argentinean Flint’ x ‘Ladyfinger’ popcorn; (iii) BLACKSDCO came from a cross of Reid Yellow Dent x Iodent; (iv) BSSS (Iowa Stiff Stalk Synthetic) has a complex pedigree background constituted of (pedigree contributions in parentheses), Reid Yellow Dent (43.8%), Iodent (15.6%), Osterland (6.3%), ‘Goldmine’ (3.1%), ‘Leaming’ (18.8%), ‘Illinois High Yield’ (6.3%) and Clarage (6.3%); (v) ‘Golden Glow’, bred from Pride of the North x ‘Toole’s North Star'; and (vi) ‘Krug’, which came from a cross of Goldmine x Reid Yellow Dent. Pedigree contributions from these six founders are therefore reported as originating from their founding sources (Table 2). Four other founders each had very complex pedigree backgrounds. BRCOMP was formed by intercrossing six blight-resistant, double-cross hybrids. Female composite 1 (FCOMP1) was constituted as a synthetic population from four single-cross hybrids and one double-cross hybrid. Approximate pedigree contributions to FCOMP1 were ‘Lancaster Sure Crop’ (25%), Krug (20%), SROT2 (15%), BLACKSDCO (7.5%), BOONECOWH (7.5%), FUNKS176A (5%), Iodent (5%), and ILLOEAR (1.25%). Female composite 2 (FCOMP2) was formed as a synthetic from 68 inbreds. Primary pedigree contributors to FCOMP2 were ‘Sibley’s Pride of the North', Krug, Lancaster Sure Crop, Osterland, ‘Midland’, FUNKSYDENT, ‘Jarvis Prolific’, BOONECOWH, WFRYD, ILLHY, and ‘Pride of Saline’. Pioneer far south open-pollinated (FSOUTH) was constituted from 26 southern U.S. double-cross hybrids plus contributions from ‘Tuxpan’ and ‘Mexican June’. Pedigree contributions from these founders are provided in respect of their individual contribution rather than that of their individual components. Pedigree backgrounds for the era hybrids could be traced to at least 22 landrace varieties (Table 3). Additional, small pedigree contributions came from other populations or landraces, such as Jarvis Prolific, Pride of Saline, and Sibley's Pride of the North (components of FCOMP2), and from Mexican varieties Tuxpan and Mexican June (components of FSOUTH).

Six founders (BSSS, IODENT, KRUG, MIDLAND, REID, and WFRYD) had mean pedigree contributions per decade of ≥5.0% (Table 4). Thirteen founders had mean decade contributions of from 1.0 to 4.99%: ARGMAIZARM, FCOMP2, ILLHY, ILLOEAR, ILLTWOEAR, LANCCOMP, LANCLOBRK, LANCSURCROP, LLE, MINN13, OSTERYDNT, SPROL, and TROYERREID. The majority of founders (41 or 68%) individually contributed less than a mean of 1.0% per decade (mean calculated over all eight decades). Within each decade, the number of founders contributing more than 5.0% (in parentheses) were 1930s (8), 1940s (6), 1950s (4), 1960s (6), 1970s (5), 1980s (4), and 1990s (5). The numbers of founders contributing from 1.0 to 4.99% in individual decades were 1930s (8), 1940s (9), 1950s (11), 1960s (19), 1970s (18), 1980s (16), and 1990s (16). Pedigree contributions from new founders occurred during each decade. Numbers of new founders and their total percent contributions for each decade were 1940s (6, 35.4%), 1950s (2, 7.2%), 1960s (15, 36.0%), 1970s (7, 4.9%), 1980s (10, 20.0%), and 1990s (11, 3.3%).

Six landraces (Table 5) (Iodent, Lancaster Sure Crop, Leaming, Midland, Osterland, and Reid Yellow Dent) contributed a mean per decade of 5.0% or more over all eight decades. The combined mean decade contribution of all sources of Reid Yellow Dent (Reid Yellow Dent, Iodent, and Osterland) was 44.8%. Landraces that contributed a mean of from 1 to 4.9% per decade were ‘Argentinean Maiz Amargo’, Clarage, Goldmine, Illinois High Yield, ‘Lindstrom Long Ear’, Pride of the North, and Toole's North Star. Numbers of landraces with pedigree contributions of 5% or more have ranged from four (1990s) to six (1950s, 1960s, and 2000s). Numbers of landraces contributing from 1.0 to 4.9% by pedigree per individual decades were zero in the 1930s (all contributing landraces provided 5.0% or more by pedigree contribution in the 1930s), one in the 1940s, three in the 1950s and then ranged from four to six in other decades.


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  		Table 5. Percentage germplasm backgrounds by pedigree; decade means for maize hybrids included in the present study, according to landrace contribution. Blank entries denote no contribution, zero entries denote a contribution between 0 and 0.04.

 
The most significant changes in pedigrees across decades can be summarized as follows (changes of more than 100% across adjacent decades for backgrounds contributing more than 1.0% per decade).

1930s to 1940s (Decade Contributions in Parentheses)

1940s to 1950s

1950s to 1960s

1960s to 1970s

1970s to 1980s

1980s to 1990s

Hybrids associated into six groups at a distance level of approximately 0.8 (Fig. 1) . Two hybrids (328 and 329) were exceptions because of a CP of 0.99. Hybrids 300 and 344 were the next most similar by pedigree with a CP of coancestry of 0.57. Reading Fig. 1 from top to bottom: Group A (hybrids 300–3475) were comprised of two subgroups; hybrids released from the 1930s to the 1950s and hybrids released during the 1980s and 1990s. Group B hybrids (3162–351) were primarily released during the 1980s and 1990s. Groups C (3220–3541) and D (3206–3431) were released during the 1960s and 1970s. Group E hybrids (301B–3618) were released during the 1940s to 1960s. The remaining hybrids (352–340) formed a loose association (Group F) of varieties that were released during the 1930s to1950s. Mean CPs among each pair of hybrids within each decade, from the 1930s to the 1990s were: 0.04, 0.09, 0.07, 0.09, 0.13, 0.18, and 0.19.



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  		Fig. 1. Association of era hybrids following multivariate analysis of pedigree distance (1-CP) data. Hybrids are denoted according to their decade of initial release. Groupings are referred to in the text.

 
Fourteen founders formed the pedigrees of public lines that were used for comparison (Table 6). Nine common founders (ARGMAIZARM, BSSS, GOLDENGATE, FUNKYSDENT, KRUG, LANCCOMP, LANCLOBRK, MINN13, WFRYD) were in the pedigrees of the public lines and the era hybrids. Five public line founders were absent from pedigrees of the era hybrids (‘643’, ‘A171’ ‘CUSCO’, ‘Illinois Synthetic 60C’, and ‘W185’). Four public line specific founders had contributions above 1%, (643, A171, CUSCO, and Illinois Synthetic 60C). In contrast, nine era specific founders (FCOMP2, ILLHY, ILLTWOEAR, IODENT, LANCSURCROP, MIDLAND, OSTERYDNT, SPROL, and TROYERREID) had maximum decade contributions above 5%. Two era-specific founders (ILLLONG and LLE) contributed between 1 and 4.9%. Significant comparisons between hybrids and public inbred lines for individual decades were as follows.


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  		Table 6. Founder backgrounds in public maize inbreds adjusted for relative usage from Darrah and Zuber (1986) and from Lu and Bernardo (2001).

 
1950s

1960s

1970s

1980s

Another comparison of founder backgrounds for public inbreds and these hybrids is prompted by Lu and Bernardo (2001) who cited eight public inbreds as largely representing the germplasm base of U.S. hybrid maize. These inbreds trace to eight founders (Table 6). Contributions from six founders are in common between the public inbred lines cited by Lu and Bernardo (2001) and the era hybrids reported in this study. These public lines have contributions from CUZCO and from Illinois Synthetic 60C that are not represented in any of the era hybrids. In addition, there are very different proportions of usage for all but one of the founders (MINN13) that are common to the pedigrees of these public lines and the era hybrids (Table 6). Pedigree contributions by the most widely used U.S. public lines (Lu and Bernardo, 2001) to era hybrids of the 1990s were as follows: B14, range 3 to 18%, mean 10.9%; B37, range 10 to 29%, mean 17.9%; B73, range 1 to 28%, mean 15.2%; B84, range 4 to 20%, mean 12.7; Mo17, range 1 to 5%, mean 2.9%; C103, range 0 to 8%, mean 2.9%; H99, range 0 to 4%, mean 0.3%, and Oh43, range 0 to 10%, mean 4.4%.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Pedigree data have been used to investigate genetic diversity and to determine genetic relationships in many crop species including barley (Hordeum vulgare L.) (Martin et al., 1991; van Hintum and Haalman, 1994), rice (Oryza sativa L.) (Dilday, 1990), wheat (Triticum aestivum L.) (Cox et al., 1985; Murphy et al., 1986), and soybean [Glycine max (L.) Merr.] (Delannay et al., 1983; Carter et al., 1993; Cui et al., 2000). Pedigree data have been used to investigate the breadth of diversity in U.S. maize in various studies (Sprague, 1971; Zuber, 1975; Zuber and Darrah, 1980; Duvick, 1984a; Darrah and Zuber, 1986). Pedigree data can be problematic for determining genetic relatedness (Lorenzen et al., 1995; Lorenzen and Shoemaker, 1996; Cheres and Knapp, 1998; Kisha et al., 1998; Wang and Bernardo, 2000). Pedigree data can be in error or unavailable. No account can be taken of selection, mutation, and mistaken or uncontrolled pollination. Degrees of pedigree relatedness are based on a possibly false assumption that founder lines are unrelated and "bottlenecks" that constrict flow of genetic diversity may not be apparent from observation of pedigree data. Nonetheless, studies of relationships among inbred lines of maize using pedigree data generally concur with results using molecular marker data including restriction fragment length polymorphisms (Smith et al., 1990; Bernardo, 1993; Bernardo et al., 1997), simple sequence repeats (Smith et al., 1997; Bernardo and Kahler, 2001; Lu and Bernardo, 2001), and heterosis data (Troyer et al., 1983, 1988; Paszkiewicz et al., 1986; Smith et al., 1987, 1990). Maize breeders regularly use pedigree data when developing breeding and genetic evaluation strategies. Pedigree data can, therefore, constitute an informative basis for inquiring into the breadth of diversity. The hybrids reported on in this study have formed the basis for several published studies that measure genetic gain for yield in U.S. maize production during the 20th century (Duvick, 1977, 1984a, 1984b, 1992; Duvick et al., 2004). Since these hybrids also have detailed pedigrees that are known to us, they provide a means to investigate the breadth and dynamics of pedigree-based genetic diversity in maize for the U.S. Corn Belt, an important region of global agricultural productivity.

The genetic base of the era hybrids traces back to at least 60 founders. Some founders (A237, AFLF, Krug, and Golden Glow) could be traced back further, and thus, the number of named founders is actually 61. However, consideration of the number of founders can lead to bias in estimating diversity. On the one hand, bias could be toward an over-estimation of genetic diversity because several founders are related (for example, the various strains of Reid Yellow Dent). Conversely, several founders (BRCOMP, FCOMP1, FCOMP2, and FSOUTH) trace back to additional and very diverse sources, many individual components of which are either difficult or impossible to document completely. Furthermore, strains of Reid Yellow Dent collectively encompass a broad array of genetic diversity as evidenced, for example, that several commercial hybrids have been and are being made by crossing inbreds developed from BSSS (being comprised by pedigree of 44% Reid Yellow Dent, 16% Iodent Reid, and 6% Osterland Reid) with inbreds that have been developed from Iodent (also regarded as a strain of Reid Yellow Dent). And so, collectively, these features may counteract some of the bias toward over-estimating diversity. Meaningful comparisons of pedigree backgrounds for the era hybrids with those for the most widely used public inbred lines (Table 11, Darrah and Zuber, 1986) are only possible for periods in which contemporaneous data are available (i.e., for the 1950s through the 1980s). Considering founders that were present at a level of 5% or higher in either the public inbreds or these hybrids, during the 1950s, only one founder (MINN13) was in common usage. Significant differences were evident during each decade of comparison between the founder lines that constituted the basis of the public lines and the era hybrids. Several founders were unique to either the public lines or to the era hybrids and differences in usage were apparent for most of the founders that were common (Tables 4 and 6). Therefore, the public lines with the most widespread usage in surveys conducted during the 1950s, 1960s, 1970s, and 1980s and those reported by Lu and Bernardo (2001) as most widely used, generally represent different pedigree backgrounds compared with the contemporaneous, widely used era hybrids reported here.

This study complements previous studies that have focused on usage of publicly bred inbred lines. Significant contributions in breeding from both the public and private sectors have provided the genetic gain reported by Duvick (1977)(1984a, 1984b, 1992). Most significant contributions from public-sector breeding to the era hybrids were in hybrids released during the 1930s and 1940s and lines developed from BSSS. The public line Wf9 was used in several of the 1950s era hybrids. More recently (for some of these era hybrids released in the 1960s and 1970s), the public lines C103 and Oh43 had a pedigree contribution of approximately 25%. The development by public-sector breeders at Iowa State University of BSSS was a pivotal event in the history of U.S. hybrid maize production. Inbreds B37, B73, B84, and B14 made significant contributions to many of these era hybrids. Another important development for U.S. maize production was the development of elite Iodent germplasm as evidenced by many of the widely-used hybrids of the central U.S. Corn Belt that are reported here. Reid Yellow Dent was the predominantly used germplasm in the U.S. central Corn Belt before the advent of hybrid maize. Pedigrees of these era hybrids show a continued reliance on several strains of Reid Yellow Dent both before and following the development of BSSS. For example, a performance potential that was previously latent in Reid Yellow Dent has been realized as evidenced by the combining ability of lines developed from BSSS (largely Reid) when crossed to lines that are predominantly Iodent (a strain of Reid).

The era hybrids reveal diversity in time. Changes in the pedigree backgrounds of the era hybrids occurred between each decade. New founders were introduced in each decade. An important factor in achieving genetic gain in the central U.S. Corn Belt, as exemplified by these hybrids, has been through importing new genetic diversity that was previously adapted to other regions. These era hybrids also demonstrate additional pedigree contributions from many other landraces. Consequently, the contribution of Reid Yellow Dent, which peaked in the 1940s, has declined and stabilized since the 1960s. Breeding networks have thereby enabled germplasm that was previously developed in one location of the USA and abroad to further contribute to increased levels of productivity in another region. Some founders disappeared in one decade to re-emerge in a later decade. Most infusions of new pedigrees occurred in 20-yr cycles during the 1940s, 1960s, and 1980s. Pedigree contributions from landraces that were not already represented in the 1930s occurred in the 1940s, 1950s, 1960s, and 1980s.

Associations of hybrids on the basis of pedigrees also demonstrate pedigree diversity in time according to the tendencies of most hybrids to cluster according to decades. For example, Group E comprised hybrids released during the 1940s to 1960s; Groups C and D contained hybrids released during the 1960s and 1970s; and Group B comprised hybrids released during the 1980s and 1990s. A degree of pedigree constancy over time was also evidenced by associations of a smaller number of hybrids; for example, Group A comprised hybrids released during five decades. Diversity in time is also revealed by mean CP between hybrids of adjacent decades which indicate that the rate of diversity change according to pedigrees has slowed since the 1970s. Pedigree relatedness among hybrids released during the same decade increased during the 1970s and the 1980s, and has then remained stable during the 1990s.

Maize breeding for the central U.S. Corn Belt during the 1990s (at least as represented by the era hybrids) reflects activities in previous decades (1950s and 1970s). These decades appear to be periods when germplasm that is already reasonably well-adapted to the region is further assimilated and refined. In contrast, more extensive pedigree changes occurred in the 1940s, 1960s, and 1980s. Less new pedigrees were introduced in the 1980s (20%) compared with levels (35–36%) in the two earlier decades of most change. Nonetheless, there is no evidence that the rate of yield gains has leveled out in the era hybrids (Duvick et al., 2004).

Received for publication November 5, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome