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PLANT GENETIC RESOURCES

Diallel Analyses of Agronomic Traits Using Chinese and U.S. Maize Germplasm

^a Dep. of Agronomy, Univ. of Missouri-Columbia, Columbia, MO 65211, now Pioneer Hi-Bred International, Johnston, IA 50131
^b Agilent Technologies, Wilmington, DE 19808
^c USDA-ARS Plant Genetics Res. Unit, and Dep. of Agronomy, Univ. of Missouri-Columbia, Columbia, MO 65211
^d USDA-ARS Plant Genetics Res. Unit, and Dep. of Entomology, Univ. of Missouri-Columbia, Columbia, MO 65211
^e Agriculture and Agri-Food Canada, Ottawa

^* Corresponding author (DarrahL{at}missouri.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Added genetic diversity among commercial maize (Zea mays L.) hybrids may further increase yields and safeguard against vulnerability. Introducing exotic germplasm into breeding programs would increase the genetic base from which elite commercial inbreds are derived. Ten populations of maize, created from Chinese and/or U.S. inbreds or strains, were evaluated by Griffing's diallel analysis for combining ability of grain yield, stalk lodging, ear height, flowering time, and European corn borer (ECB; Ostrinia nubilalis Hübner) resistance to estimate their potential as sources of exotic germplasm for U.S. breeding programs. Grain yield general combining ability was largest for the population Mo17 Syn.(H14)C5, a synthetic improved by half-sib selection using US13 as a tester. Grain yield specific combining ability was largest in the cross Chinese Mix 2 x Mo17 Syn.(H14)C5. Chinese Mix 2 x Mo17 Syn.(H14)C5 had more stalk lodging than the B73 x Mo17 and Pioneer Brand 3394 checks. Because of the high yield potential and other moderate-to-good agronomic traits of the cross combination, Chinese Mix 2 was selected as the best population for selection. Its large specific combining ability effect with Lancaster type material, which is commonly known in breeding programs, shows potential for further improvement. No native ECB resistance in Chinese germplasm was detected (two environments in 1 yr) compared with the resistant check Pioneer Brand 3184.

Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • ECB, European corn borer • GCA, general combining ability • H24C8, MoSCSSS(H24-HRP)C8 • MoSCSSS, Missouri Second Cycle Stiff Stalk Synthetic • SAS, Statistical Analysis System • SCA, specific combining ability

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
BREEDERS have emphasized the need to broaden the maize germplasm base to ensure continued genetic gain and avert risks associated with a narrow germplasm base (Eberhart, 1971; Darrah and Zuber, 1986; Holley and Goodman, 1988; Mungoma and Pollak, 1988). Less than 1% of U.S. commercial maize is of exotic origin, and tropical exotic germplasm is only a fraction of that (Harper, 1994). Much work has been done evaluating the introgression of exotic and tropical germplasm into commonly used, elite inbreds from maize improvement programs around the world. However, the extraction of useful alleles from exotic germplasm is difficult because such materials are often unadapted and agronomically deficient (Crossa et al., 1987; Castillo-Gonzalez and Goodman, 1989).

The evaluation and use of tropical germplasm in temperate areas are frequently hampered by its photoperiod sensitivity and deleterious effects of inbreeding (Holley and Goodman, 1988; Castillo-Gonzalez and Goodman, 1989).

One approach to use tropical germplasm in temperate areas is to form populations or pools with tropical, exotic, and/or temperate germplasm; these populations can then be used as a germplasm source for temperate breeding programs. The heterozygous and heterogeneous nature of these populations hinder comparisons with currently used hybrids, making it difficult to predict precisely their value as source germplasm. However, populations with potential utility in temperate breeding programs can be selected with some certainty on the basis of combining ability (Beck et al., 1991; Vasal et al., 1993).

Maize first arrived in China about 400 to 500 yr ago (Yu and Zhu, 1996). Since then, selection pressures arising from regional preferences, topography, and climate have influenced the genetic makeup of maize in China. Significant genetic divergence is possible because of China's extremely variable topography and climate. The climatic zones of China range from temperate (early-maturity maize) to tropical (two-to-three seasons of maize annually). The peoples of different regions also had a significant effect on maize populations, selecting for various traits such as yield, food quality, color, disease resistance, and more recently, combining ability (Yu and Zhu, 1996).

Currently, maize breeders in China predominantly use inbred lines that can be clustered into three heterotic groups (Zhang et al. [on line at http://www.chinamaize.com.cn/kjjz/kylw/hylw/lw19.html, verified 16 January 2005]). The first group is domestic germplasm referred to as Dom, which includes the Sipingtou and Luda Red Cob subgroups. The second group is comprised of Lancaster germplasm, and the third group is Reid germplasm.

The predominant heterotic patterns used in China in hybrid production are Dom x Lancaster and Dom x Reid. In northern China, where spring maize is predominant, Luda Red Cob x Lancaster is used (Zhang et al. [on line at http://www.chinamaize.com.cn/kjjz/kylw/hylw/lw19.html). In general, domestic x exotic crosses perform best and therefore are the most utilized. Most of the combinations used in China are also of the dent (domestic) x flint (exotic) types (Zhang et al. [on line at http://www.chinamaize.com.cn/kjjz/kylw/hylw/lw19.html).

The diallel mating design has utility as a method to analyze crosses, or parents and crosses, for general combining ability (GCA) and specific combining ability (SCA) (Griffing, 1956), providing an assessment of their relative merits to guide selection and testing schemes. Despite the relatively short history of maize culture and modern scientific research in China (Yu and Zhu, 1996), the selection pressures imposed there suggest that Chinese germplasm could provide useful alleles to improve U.S. maize. The objectives of this study were to evaluate 10 populations of maize created from Chinese and/or U.S. germplasm for combining ability of grain yield, stalk lodging, ear height, flowering, and ECB resistance.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
We constructed a diallel set of crosses involving Chinese and U.S. public germplasm representing two major heterotic groups. Hybrids and parents were tested in separate experiments for yield and ECB borer damage in 2002 and yield only in 2003. Ten parent populations were used in the diallel, each one representing a different mixture of Chinese and/or U.S. origin germplasm (Table 1). The diallel crosses were made at the Hinkson Bottom location in Columbia, MO, in 2001. Crosses were made by bulking pollen from six to eight plants to pollinate six to eight plants of the female parent. This was repeated in the nursery for each cross, resulting in 12 to 16 pollinations per cross. Previous tests indicated reciprocal effects were not significant in crosses between the populations, so reciprocal crosses were pooled, resulting in 24 to 32 ears representing each diallel cross. Pioneer Brand 3394¹ (Pioneer Hi-Bred International, Inc., Johnston, IA) and B73 x Mo17 were included in both experiments as checks.

Plots were picked with a Gleaner K2 combine (AGCO, Duluth, GA) and grain yields were expressed as kg ha^–1 standardized to 155 g kg^–1 moisture. Stalk lodging was recorded just before harvest at all locations as the number of plants broken at or below the ear. Counts were converted to percentages of the counted stand for analysis. Ear height was recorded after flowering on 10 competitive plants plot^–1 as the distance (cm) from the ground to the node of the top ear attachment. Number of days to silk emergence was recorded as the number of days after planting to when 50% of the plants in the plot produced visible silks.

Two locations, Hinkson Bottom and Tipton, were used to evaluate resistance to first- and second-generation ECB in 2002. Each experiment included three replications and used one-row plots. For the ECB evaluations, there were 45 F₁ crosses, 10 parents, two checks that were used previously, and two additional checks, viz., Wf9 x W182E (susceptible) and Pioneer Brand 3184 (resistant). Plots were 5.2 m long and 0.9 m wide with 28 seeds planted plot^–1. For determining first-generation ECB damage, the first six plants of each plot were infested with 60 neonate ECB larvae. Infestation was repeated three-to-five days later for a total infestation of 120 larvae plant^–1. After 2 to 3 wk, five of the six infested plants were rated for leaf feeding damage by using a 1-to-9 scale on which 1 represented no damage (Guthrie et al., 1960).

At approximately mid-flowering, the last six plants in each row were infested with 60 ECB larvae in the leaf collar of the leaf below the ear to determine second-generation ECB damage. Infestation was repeated once more in 3 to 5 d, again for a total infestation of 120 larvae plant^–1. After senescence (approximately 56 d later), five infested plants per plot were split from the ground to the internode above the ear, the number of tunnels counted, and the total length of the tunnels estimated. Every hole in the stalk was counted as one tunnel and was associated with a minimum of 2.5 cm of tunneling.

Data were analyzed by Griffing's diallel analysis (Griffing, 1956) Model 1 (fixed effects), Method 2 (parents and crosses together) and Method 4 (crosses only) according to the model

where Y_ijk is the observed measurement for the ijth cross grown in the kth replication/environment combination; µ is the overall mean; g_i and g_j are the GCA effects for the ith and jth parents, respectively; s_ij is the SCA effect for the ijth cross (note that the s_ii term is necessary for the mathematical computation, but has no meaning in terms of a SCA effect for a parent); and e_ijk is the error term associated with the ijth cross evaluated in the kth replication/environment. Year-location combinations were treated as independent environments. Because of mechanical problems with the harvester, nine plots (two to four plots in three environments) were estimated for missing yield and moisture data by averaging values of the remaining replications for that location. Pooled error degrees of freedom were reduced accordingly in the combined analysis of variance. Entries were considered as fixed effects and replications and environments were considered as random effects. A combined analysis of variance was completed for each trait by the general linear model procedure in SAS (SAS Institute Inc., 1999). The entries sum of squares was then partitioned into the components of a diallel analysis both for Method 2 and Method 4, including a partition to test for differences between diallel parents and diallel crosses. Significance of the parents vs. diallel crosses partition determined which analysis, Method 2 or Method 4, was appropriate. Where the parents vs. diallel crosses term was significant, Method 4 was used to estimate GCA and SCA effects absent parental vigor effects. The diallel analysis was calculated by using a spreadsheet developed in Microsoft Excel (Microsoft, Redmond, WA). Entry means combined across environments were used to compute phenotypic correlation coefficients using SAS PROC CORR (SAS Institute Inc., 1999).

	RESULTS AND DISCUSSION

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Grain Yield
The overall grain yield mean for eight environments was 5717 kg ha^–1 with a coefficient of variation of 16.8% and an R² of 0.85. One location was discarded each year because of drought. Because of raccoon (Procyon lotor) feeding damage, a 15-ear random sample of non-damaged ears was hand-harvested (omitting end-of-row plants) from the Hinkson Bottom location in 2002 to determine grain yield and moisture percentage. Variance and yield rank consistency compared with other locations justified inclusion of the data in the overall analysis. Analysis showed significant effects for environments, replications within environments, entries, and the environments x entries interaction. Significance was found in the parents vs. diallel crosses partition of the entries variance, justifying the use of Griffing's Method 4 analysis.

Parental grain yield means ranged from 3683 kg ha^–1 for H24C8 up to 5472 kg ha^–1 for Stiff Stalk x Luda Red Cob (Table 2). H24C8 was selected for eight cycles for high rind penetrometer resistance, which has a negative correlation to yield (Abedon et al., 1999). The largest negative GCA effect was contributed by U.S. 2 x CH3, with a value of –319** kg ha^–1 (significant at P 0.01). The largest positive GCA effect of 526** kg ha^–1 was contributed by Mo17 Syn.(H14)C5. BS29 also had a highly significant positive GCA effect of 260** kg ha^–1. Parental means were less predictive of GCA results for grain yield than observed in other traits. Moreover, SCA effects for grain yield were larger than for other traits. The 0.69 ratio of GCA/SCA sums of squares substantiates the relatively more important role of SCA in determining yield levels.

View this table: [in this window] [in a new window]   Table 2. Grain yield means (kg ha–1) of crosses (upper right triangle), specific combining ability effects (sij) (lower left triangle), general combining ability effects (gi), parental means, and cross means for 10 maize genotypes evaluated in eight Missouri environments in 2002 and 2003. General and specific combining ability effects were calculated using Griffing's Method 4 analysis.

We did not expect heterotic effects in crosses between populations comparable to crosses between inbred lines because each parent population was largely heterozygous in genetic makeup. However, large SCA effects were seen for a few crosses. The largest SCA effect of 1034** kg ha^–1 was observed for the cross Chinese Mix 2 x Mo17 Syn.(H14)C5. The high-parent heterosis for this cross was 46%, which is comparable to the heterosis exhibited by F₁ hybrids. Beck et al. (1991) reported 16.0% as the largest high-parent heterosis value in a diallel analysis of 10 exotic, intermediate-maturity International Maize and Wheat Improvement Center (CIMMYT) populations. This was comparable to other diallel analyses of exotic populations (Eberhart, 1971; Vasal et al., 1993), which had high-parent heterosis values generally below 20%. The comparison of heterotic effects between our study, which had 12 crosses exhibiting heterosis values greater than 20%, and those referenced indicates our populations are likely more narrowly based than those used in these other exotic maize diallel studies. That our populations were comprised with heterotic grouping in mind accounts for their likely narrower genetic base and resulting higher heterosis values.

The cross (Mo17 x Luda Red Cob) x Mo17 Syn.(H14)C5 had the largest negative SCA effect for yield at –739** kg ha^–1. The parent population Chinese Mix 2 was composed of material that combined best for yield with Mo17 in earlier trials, whereas the cross (Mo17 x Luda Red Cob) x Mo17 Syn.(H14)C5 had a much larger proportion of common background (Table 1). The conclusion can then be drawn that the relatively high parental mean of Mo17 x Luda Red Cob, coupled with the lack of heterosis due to relatedness led to the large negative SCA effect. Likewise, the relatively low parental mean of Mo17 Syn.(H14)C5, coupled with larger heterotic gain due to less relatedness, leads to the large positive SCA effect with Chinese Mix 2.

Stalk Lodging
Stalk lodging in eight environments averaged 20.6% with a coefficient of variation of 39.2% and an R² of 0.82. The interactions of location, weather, and timing of harvest had an impact on the stalk lodging by increasing the error in the experiment. Environments, replications within environments, entries, and the entries x environments interaction for stalk lodging were significant. Significant differences were not found between parents and crosses in this case, therefore, Griffing's Method 2 analysis, which includes parents, was used when calculating GCA and SCA effects.

General combining ability effects mirrored parental means in this case, with U.S. 1 x CH11 contributing its weak stalk characteristic and H24C8 contributing its strong stalk characteristic in crosses (Table 3). U.S. 1 x CH11 had a parental mean of 46.2% (the highest mean among all entries in the experiment) and a GCA effect of 10.5%**. H24C8 had a parental mean of 6.1% (the lowest mean among all entries in the experiment) and a GCA effect of –6.8%**. Just seven of the 45 crosses produced significant SCA effects. The ratio of GCA/SCA sums of squares was 9.22, indicating only a minor role for specific effects.

View this table: [in this window] [in a new window]   Table 3. Stalk lodging percentage means of crosses (upper right triangle), specific combining ability effects (sii and sij) (diagonal and lower left triangle, respectively), general combining ability effects (gi), parental means, and cross means for 10 maize genotypes evaluated in eight Missouri environments in 2002 and 2003. General and specific combining ability effects were calculated using Griffing's Method 2 analysis.

Ear Height
Ear height averaged 119.6 cm across 10 environments with a coefficient of variation of 5.9% and an R² of 0.90. Mean squares for environments, replications within environment, entries, and the entries x environments interaction were significant. Novelty had relatively low plant and ear heights in both years as a result of low rainfall. Significant differences were found between parents and crosses, among GCA effects, and among SCA effects for ear height. The 6.01 ratio of GCA/SCA sums of squares suggests a more important role for general, or additive, effects in determining ear height.

Ear heights between the parent populations in the diallel were quite variable, leading to several significant GCA effects (Table 4). Parent BS29 had the largest mean ear height and GCA effect, contributing to a 12.2 cm increase in the ear height of its crosses. Mo17 Syn.(H14)C5 had the lowest parental mean ear height and the second largest GCA effect in the downward direction, decreasing ear height by 4.1 cm. The largest GCA effect in the downward direction was for H24C8, contributing to an ear height 8.2 cm below the mean. The largest SCA effect was in the negative direction, for the cross of Chinese Mix 1 x Chinese Mix 2, decreasing ear height by 5.7 cm. The largest positive SCA effect was 5.0 cm, produced by the cross (Stiff Stalk x Luda Red Cob) x Mo17 Syn.(H14)C5. The greatest mean ear height in the experiment was for the cross (U.S. 3 x Zi330) x BS29. The lowest ear height among the 45 diallel crosses was associated with the cross between parents H24C8 x Mo17 Syn.(H14)C5, the parents with the largest negative GCA effects. The correlation between ear height and plant height was 0.83**. In general, our plant height means were shorter than those reported by Beck et al. (1991) in their diallel analysis of 10 CIMMYT intermediate-maturity maize populations in 11 U.S. environments. The source of the exotic germplasm for our study and the possibly larger percentage of Corn Belt germplasm in our populations likely contributed this result.

View this table: [in this window] [in a new window]   Table 4. Ear height means (cm) of crosses (upper right triangle), specific combining ability effects (sij) (lower left triangle), general combining ability effects (gi), parental means, and cross means for 10 maize genotypes evaluated in 10 Missouri environments in 2002 and 2003. General and specific combining ability effects were calculated using Griffing's Method 4 analysis.

Most parents were significantly later flowering than their respective crosses, and a majority of the crosses had silks emerge earlier than the check hybrids (Table 5). The largest GCA effect was exhibited by the parent population U.S. 1 x CH11, which contributed 1.3 d earlier silk emergence in crosses. Parent populations Stiff Stalk x Luda Red Cob and U.S. 2 x CH3 both had the largest GCA effects for later silk emergence with a significant value of 0.8 d delay. These GCA effects are relatively lower than those observed in a diallel analysis of 10 intermediate-maturity exotic populations used by Beck et al. (1991), indicating that our populations are all fairly well adapted and/or contain larger proportions of Corn Belt germplasm. Importance of general vs. specific effects is indicated by the GCA/SCA sums of squares ratio of 4.92.

View this table: [in this window] [in a new window]   Table 5. Days to silk emergence means of crosses (upper right triangle), specific combining ability effects (sij) (lower left triangle), general combining ability effects (gi), parental means, and cross means for 10 maize genotypes evaluated in four Missouri environments in 2002 and 2003. General and specific combining ability effects were calculated using Griffing's Method 4 analysis.

European Corn Borer Resistance
Second-generation ECB damage in the two 2002 environments was severe with an overall mean tunnel length of 23.8 cm, with a coefficient of variation of 23.9% and an R² of 0.65. Effects for GCA were highly significant. There were no significant SCA effects and the contrast of parents vs. crosses was not significant. A ratio of 0.97 was found for the GCA/SCA sums of squares. Application of the LSD of 6.47 cm showed all entries except one, H24C8, were significantly more damaged than the resistant check, Pioneer Brand 3184. H24C8 and BS29 did have significant negative GCA effects. Otherwise, all entries except for the resistant check were either not significantly different from, or were significantly worse than, the susceptible check, Wf9 x W182E. Thus, the ECB experiment was not repeated in 2003.

Comparison of grain yield and other agronomic characteristics of our populations indicate they will not directly benefit U.S. breeding programs in the short term. However, previous work (Hallauer and Sears, 1972) has shown adapted exotic or semi-exotic material to be useful after further improvement. Our results suggest that further improvements to Chinese Mix 2 may be achieved using half-sib selection for grain yield with Mo17 Syn.(H14)C5 as a tester because of its very high SCA effect.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Grain yield data showed highly significant GCA effects and SCA effects, with 12 crosses exhibiting high-parent heterosis greater than 20%. The SCA effect and grain yield mean of one cross in particular, Chinese Mix 2 x Mo17 Syn.(H14)C5, indicated that Chinese Mix 2 has potential for future improvement in yield performance. The large grain yield GCA effect of Mo17 Syn.(H14)C5 indicates it has value as a tester in selection for yield.

The Chinese Mix 2 population has the highest potential among the material tested here to provide improved foreign germplasm to U.S. maize breeding programs from which elite inbreds can be derived. This conclusion is based on this population's moderate-to-good rank for most agronomic traits and high grain yield when crossed with Mo17 Syn.(H14)C5, a good representative population for the Lancaster-based heterotic group.

	NOTES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
¹ Mention of a trademark or proprietary product does not constitute a guarantee, warranty, or recommendation of the product by the U.S. Department of Agriculture or the University of Missouri and does not imply its approval to the exclusion of other products that may also be suitable.

Received for publication August 16, 2004.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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