Crop Science 41:68-71 (2001)
© 2001 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Breeding Potential of Intra- and Interheterotic Group Crosses in Maize
Rex Bernardo
Dep. of Agronomy and Plant Genetics, University of Minnesota, 411 Borlaug Hall, 1991 Buford Circle, St. Paul, MN 55108-6026
Corresponding author (berna022{at}umn.edu)
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ABSTRACT
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New maize (Zea mays L.) inbreds are usually developed within a heterotic group. However, breeders sometimes use commercial hybrids as a nonconventional (i.e., interheterotic group) source of new inbreds. The effects of disrupting heterotic patterns in maize, by selfing from commercial hybrids, are not well understood. My objective was to compare intra- and interheterotic group crosses as sources of new inbreds. The inbreds B73 and H93 represented the Iowa Stiff Stalk Synthetic (BSSS) heterotic group, whereas Mo17 and B99 represented the non-BSSS heterotic group. I estimated testcross means and genetic variances in two intragroup F2 populations: (B73 x H93)F2 testcrossed to Mo17 x B99; and (Mo17 x B99)F2 testcrossed to B73 x H93. Likewise, I estimated testcross means and genetic variances in two intergroup F2 populations: (B73 x Mo17)F2 testcrossed to H93 x B99; and (H93 x B99)F2 testcrossed to B73 x Mo17. Testcrosses of 150 individual plants from each F2 population were evaluated at three Indiana locations in 1999. Testcross means for grain yield were 1.0 Mg ha-1 higher in the intra- than in the intergroup F2 populations. Grain moisture and stalk lodging testcross means were lower in the intra- than in the intergroup F2 populations. Testcross genetic variances (VTC) for grain yield were similar among the four populations. Favorable epistatic combinations may have been broken up in the (B73 x Mo17)F2 population. The results underscore that the success of developing new inbreds from an intergroup population largely depends on finding a suitable tester for the resulting intergroup inbreds.
Abbreviations: BSSS, Iowa Stiff Stalk Synthetic • h2, heritability • VTC, testcross genetic variance
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INTRODUCTION
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A HETEROTIC GROUP comprises a set of genotypes that perform well when crossed with genotypes from a different heterotic group (Hallauer et al., 1988). New maize inbreds are usually selfed from a cross between two inbreds from the same heterotic group (i.e., BSSS). These inbreds are then evaluated for their yield performance when testcrossed with an inbred from an opposite heterotic group (i.e., non-BSSS). Single-cross cultivars therefore comprise a cross between two elite inbreds from complementary heterotic groups.
Advanced cycle pedigree breeding is the most common method for developing maize inbreds (Bauman, 1981). This approach, which is best described as inbred recycling, involves crossing newly developed inbreds to form new base populations. The inbreds crossed are often related to each other. Consequently, many, if not most, of the current elite maize inbreds in a heterotic group are derived from only a few progenitor inbreds (Darrah and Zuber, 1985; Smith et al., 1999). Most of the current BSSS inbreds, for example, have been derived from B14, B37, B73, or B84 (MBS, Inc., 1999). Advanced cycle breeding has raised concerns regarding the narrowing of maize germplasm within heterotic groups (Troyer, 1999).
Maize breeders have sometimes used commercial hybrids as a source of new inbreds (MBS, Inc., 1999). Perhaps the greatest difficulty in this approach is finding a suitable tester for the resulting intergroup inbreds (Valle-Razo and Stucker, 1996). Could intergroup populations be a means of increasing the genetic diversity among inbreds? If new inbreds are developed from an intergroup population, could other inbreds also developed from an intergroup population be used as testers? The relative usefulness of intra- and intergroup crosses as base populations in maize breeding has not been evaluated. The objective of this study was to compare intra- (i.e., BSSS x BSSS and non-BSSS x non-BSSS) and interheterotic group crosses (i.e., BSSS x non-BSSS) as sources of new inbreds.
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MATERIALS AND METHODS
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Germplasm
The inbreds B73 and H93 represented the BSSS heterotic group, whereas Mo17 and B99 represented the non-BSSS heterotic group. The B73 x Mo17 cross was a widely grown cultivar in the central U.S. Corn Belt until the early 1980s. The inbred H93 was developed at Purdue University from a [(B37 x GE440)B37]BC4 population. The inbred B99 was developed at Iowa State University from a population of `Iowa Corn Borer Synthetic no. 1' that had undergone 10 cycles of reciprocal recurrent selection for combining ability with the BSSS population (Hallauer et al., 1995).
All four inbreds were used either as the parents of an F2 population or as the parents of the corresponding single cross tester. Each F2 population was random mated twice (i.e., F2–Syn–2) to dissipate the effects of linkage disequilibium on the estimates of VTC. Throughout this article, each F2–Syn–2 population is referred to as an F2 population for brevity. The two intragroup F2 populations were (B73 x H93)F2 testcrossed to Mo17 x B99, and (Mo17 x B99)F2 testcrossed to B73 x H93. The two intergroup F2 populations were (B73 x Mo17)F2 testcrossed to H93 x B99, and (H93 x B99)F2 testcrossed to B73 x Mo17. The use of F1 crosses as testers allowed each of the four (F2 population x tester) combinations to have the same set of alleles; the four (F2 population x tester) combinations therefore differed only in the partitioning of alleles either in the F2 population or in the tester.
The inbreds were crossed to form the F1 in a winter nursery at Guayanilla, Puerto Rico, in 1997. The F1 plants were selfed to form the F2 at the Purdue University Agronomy Research Center, W. Lafayette, IN, in the summer of 1997. Approximately 200 to 250 plants from each F2 population were random mated by full sibbing in a winter nursery at Juana Diaz, Puerto Rico, from October 1997 to January 1998. The second generation of random mating was performed in the same nursery from January to April 1998. Random F2 plants were testcrossed to their respective single-cross tester at W. Lafayette in the summer of 1998. Testcross seed from 150 plants in each F2 population were obtained.
Field Evaluation
In 1999, the testcrosses were evaluated in an augmented randomized complete block design (Federer, 1961) at three Indiana locations: Pinney-Purdue Agricultural Center near Wanatah, Throckmorton-Purdue Agricultural Center near Romney, and Southeast-Purdue Agricultural Center near North Vernon. An experiment at a fourth location, W. Lafayette, was lost because of a late-season storm. Three commercial check hybrids (`DK604,' LH198 x LH185, and `P33G26') were replicated 15 times at each location, whereas the testcrosses were replicated once. Specifically, the 600 testcrosses were randomly divided into 15 sets, each set corresponding to a block. Each block therefore had 40 testcrosses, 10 from each F2 population, plus the three check hybrids. The testcrosses and check hybrids were grown in two-row plots, each row 6.1 m long and spaced 0.76 m apart, at a plant population density of 66 700 plants ha-1. Standard cultural management practices were used at each location. Data were recorded for grain yield (Mg ha-1 at 155 g H20 kg-1), grain moisture (g kg-1), and the percentage of stalk lodged plants in each plot.
Data Analysis
Analysis of variance, pooled across blocks and combined across locations, was performed for each trait. The F2 populations and check hybrids were considered fixed effects, whereas the testcrosses, locations, and blocks were considered random effects. The sums of squares for entries/blocks were subdivided into the sums of squares among the (B73 x H93)F2 testcrosses/blocks, (Mo17 x B99)F2 testcrosses/blocks, (B73 x Mo17)F2 testcrosses/blocks, and (H93 x B99)F2 testcrosses/blocks. A corresponding partitioning of the entries/blocks x locations sums of squares was done. Variance components were estimated by equating the observed mean squares to their expectations (Schultz, 1955) and solving for the desired component. The significance (P = 0.05) of VTC in each population was evaluated with an F test. Heritability (h2) on a testcross–mean basis was estimated as the ratio between VTC and the phenotypic variance of a testcross mean.
Differences among the testcross means of the four F2 populations were compared by t tests (P = 0.05). Bootstrapping was used to obtain 95% confidence intervals on VTC and h2 in each F2 population, as well as on the pairwise differences in VTC and in h2 among the F2 populations. Statistical significance was declared if the 95% confidence intervals did not include zero. Estimate of VTC and h2 within each of the 15 sets were used in bootstrapping.
To account for differences in both the mean and VTC among populations, the predicted mean of the best 5% of testcrosses (
0.05) in each F2 population was calculated (Melchinger et al., 1988)
where k0.05 was the standardized selection differential for a selected proportion of 5%. The value of k0.05 was 2.06 for grain yield and –2.06 (i.e., selection for lower values) for grain moisture and stalk lodging.
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RESULTS AND DISCUSSION
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Means and Variances
The testcross means for grain yield, moisture, and stalk lodging were more favorable in the intragroup populations [(B73 x H93)F2 and (Mo17 x B99)F2] than in the intergroup populations [(B73 x Mo17)F2 and (H93 x B99)F2] (Table 1). Grain yield, averaged among populations, was about 1.0 Mg ha-1 higher (P < 0.05) in the intra- than in the intergroup F2 populations. All pairwise differences in the mean grain yield among the four F2 populations were significant, except for the difference between (B73 x H93)F2 and (B73 x Mo17)F2. The difference between the mean of the intragroup F2 populations and the mean of the intergroup F2 populations was statistically significant (P < 0.05) but small for grain moisture (mean difference of 2 g kg-1) and stalk lodging (mean difference of 0.3%).
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Table 1. Estimates of testcross means, genetic variances (VTC), heritability (h2), and mean of the best 5% of testcrosses (0.05) in intra- and interheterotic group F2 populations of maize
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All estimates of VTC and h2 were significantly different from zero for grain yield and moisture (Table 1). For stalk lodging, VTC and h2 were significant only in the (B73 x Mo17)F2 population. For grain yield, none of the pairwise differences in VTC or in h2 among the four F2 populations were significant. The superior predicted performance of the top 5% of the testcrosses (0.05), in the intra- than in the intergroup F2 populations (Table 1), was therefore simply due to the difference in the overall means of the populations rather than due to differences in VTC or h2. The lack of differences in VTC and h2 also indicated that the intragroup F2 populations studied are superior to the intergroup F2 populations regardless of the selection intensity. Although breeders have successfully developed inbreds from commercial hybrids, the results from this study suggest that keeping the BSSS and non-BSSS heterotic groups separate enhances the probability of obtaining superior single crosses. As discussed later, this conclusion is strictly valid only if an intergroup F1 cross is used as the tester for an intergroup F2 population.
The means of the DK604, LH198 x LH185, and P33G26 check hybrids were 9.30 Mg ha-1 for grain yield, 158 g kg-1 for grain moisture, and 0.7% for stalk lodging. These means were more favorable than the 0.05 values for any of the F2 populations (Table 1). This result confirmed that B73, H93, Mo17, and B99 are no longer useful as parents of base populations. An alternative is to use advanced cycle versions of these inbreds. Inbreeding depression shall result in the testcrosses of intergroup populations, but not of intragroup populations, if the advanced cycle inbreds are closely related with each other. Suppose Y and Z are advanced cycle BSSS inbreds. If Y and Z are related to each other, inbreeding depression will occur when the (Y x Mo17)F2 population is testcrossed to Z x B99. Such inbreeding depression will further decrease the usefulness of intergroup crosses.
Epistasis in B73 x Mo17
The observed means and VTC in the intra- and intergroup F2 populations were inconsistent with the expectations for a four-allele model that assumes the absence of epistasis among an arbitrary number of loci (Bernardo, 1999). In this model, the genotypic values of the four homozygotes are: 1 for (+/+); 1/2 for (+'/+'); –1/2 for (–/–); and –1 for (–'/–'). Complete dominance of the more favorable allele is assumed. One heterotic group (i.e., BSSS) has the best allele (i.e., +) at odd-numbered loci and the worst allele (i.e., –') at even-numbered loci. In contrast, the opposite heterotic group (i.e., non-BSSS) has the worst allele at odd-numbered loci and the best allele at even-numbered loci. Crosses between BSSS inbreds and non-BSSS inbreds therefore complement each other. The occurrence of the second-best allele (i.e., +') and third-best allele (i.e., –) in either of the two heterotic groups leads to three different classes of loci (Table 2). If Class I occurs in an intragroup F2 population, Classes II or III occur in the corresponding intergroup F2 population. If Class II occurs in an intragroup F2 population, then Classes I or III occur in the corresponding intergroup F2 population. If Class III occurs in an intragroup F2 population, Classes I or II occur in the corresponding intergroup F2 population.
Regardless of the class of loci that occurs, the two intragroup F2 populations are expected to have equal testcross means and equal VTC (Table 2). Likewise, the two intergroup F2 populations are expected to have equal testcross means and equal VTC. Three types of relationships exist between the intra- and intergroup testcross means and between the intra- and intergroup VTC. First, the testcross means and VTC are equal between the intra- and intergroup F2 populations (Classes I and III). Second, the testcross mean is higher but VTC is lower in the intragroup populations (Class II) than in the intergroup populations (Classes I or III). Third, the testcross mean is lower but VTC is higher in the intragroup populations (Classes I and III) than in the intergroup populations (Class II). These expectations are unaffected by changes in scale among the genotypic values [e.g., 1/4 for the (+'/+') and –1/4 for the (–/–) genotypes]. None of these expectations was consistent with the observed result of equal VTC, but higher testcross means in the intra- than in the intergroup F2 populations.
The results, however, were consistent with a model involving linked epistatic effects. Favorable epistatic combinations in repulsion phase may have been broken up by random mating the (B73 x Mo17)F2 population. Conversely, these favorable epistatic effects were conserved in the B73 x Mo17 cross. The mean testcross grain yields in (B73 x H93)F2, (Mo17 x B99)F2, and (B73 x Mo17)F2 ranged from 7.69 to 8.06 Mg ha-1, whereas the mean testcross grain yield in (H93 x B99)F2 was substantially lower at 6.09 Mg ha-1 (Table 1). The (H93 x B99)F2 population testcrossed to B73 x Mo17 was expected to have the same mean as its reciprocal population, (B73 x Mo17)F2 testcrossed to H93 x B99, if either of two conditions was met (Melchinger, 1988): (i) epistasis was absent; or (ii) epistasis was present but the parental populations were in linkage equilibrium. The F2 populations, having been random mated twice, were expected to be largely at linkage equilibrium. In contrast, linkage disequilibrium is maximized in the F1 between two inbreds.
Studies have revealed that B73 x Mo17 has favorable epistatic effects (Bernardo et al., 1989; Wolf and Hallauer, 1997), as well as repulsion phase linkage between quantitative trait loci (Graham et al., 1997) for grain yield. The possibility of larger epistatic effects in B73 x Mo17 than in the three other crosses was consistent with the lower grain yields when B73 x Mo17 was used as the tester [i.e., for the (H93 x B99)F2 population]. Suppose B73 has the AAbb genotype, whereas Mo17 has the aaBB genotype. The A and B loci are linked with a recombination frequency of r < 1/2. If favorable epistatic combinations are in repulsion phase, the allele pairs AB and ab have positive epistatic effects, whereas the allele pairs Ab and aB have negative epistatic effects. With linkage equilibrium, the net epistasis contributed by the tester (Melchinger, 1988) is zero because each of the four gametes occurs at a frequency of 1/4. But with linkage disequilibrium in B73 x Mo17, the total frequency of the AB and ab gametes is r < 1/2, whereas the total frequency of the Ab and aB gametes is (1 – r) > 1/2. The net epistasis contributed by the tester is therefore negative. Such epistasis would have caused the lower grain yield as well as an upward bias in VTC among the (H93 x B99)F2 x (B73 x Mo17) testcrosses, relative to a population in which the tester is in linkage equilibrium (Melchinger, 1988).
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CONCLUSIONS
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The possibility of epistasis in certain single crosses but not in others suggests that the relative usefulness of intra- versus intergroup populations as sources of new inbreds depends on the particular inbreds used. If B73, H93, Mo17, and B99 are representative of the germplasm used by maize breeders, then intragroup crosses are likely to be superior to intergroup crosses as sources of new inbreds. This conclusion is predicated on the use of intergroup single crosses as testers for intergroup F2 populations. It remains unclear whether this conclusion will hold true if other types of testers are used for intergroup F2 populations. For example, would the mean testcross grain yield of the (H93 x B99)F2 population be higher if, instead of using B73 x Mo17 as the tester, the random mated (B73 x Mo17)F2 population or a recombinant inbred derived from the (B73 x Mo17)F2 population is used as the tester? This question underscores that the success of developing new inbreds from an intergroup population depends on finding a suitable tester for the resulting intergroup inbreds.
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ACKNOWLEDGMENTS
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I thank Bill Foster, Phil DeVillez, Jim Beaty, Jon Leuck, Jerry Fankhauser, and Don Biehle for their assistance in growing the nurseries and yield trials.
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NOTES
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Purdue Agric. Res. Prog. J. Paper 16 282.
Received for publication May 5, 2000.
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REFERENCES
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ABSTRACT
INTRODUCTION
