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a Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108
b CIMMYT, Apdo. Postal 6-641, 06600 Mexico D.F., Mexico
c CIMMYT, P.O. Box MP163, Mt. Pleasant, Harare, Zimbabwe
d Pioneer Genetique SARL, European Research and Development Center, 24, rue du Moulin, F-68740 Nambsheim, France
* Corresponding author (halrmickelson{at}cs.com)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Abbreviations: E x Env., entry x environment interaction effects • ETO, Estación Tulio Ospina • hij, heterosis effects • h, average heterosis effects • hj, variety heterosis effects • N3, N3-2-3-3 • NPP ES, Natal Potchefstroom Pearl Elite Selection • SC, SC5522 • sij, specific heterosis effects • vj, variety effects
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Corn Belt Dent varieties Reid and Lancaster were in widespread use in the USA before the onset of the hybrid maize era and have emerged as opposing components in the dominant heterotic pattern in the USA. Darrah and Zuber (1986) reported that lines from Reid germplasm accounted for 44% of those used for seed production in 1984, followed by Iodent (26%) and Lancaster (12%). Iodent is commonly accepted as a strain of Reid.
The Zimbabwe national maize program has developed high performance germplasm adapted to their tropical midaltitude growing regions, roughly from 1000 to 1800 m above sea level (masl) and less than 23° from the equator (Dowswell et al., 1996). The Zimbabwe hybrid breeding effort, which commenced in 1932, was based on the open-pollinated cultivars Southern Cross, Salisbury White and to a lesser extent ‘Hickory King’ (Olver, 1988). Hickory King was among a group of high-yielding U.S. varieties imported by the Southern Rhodesian Department of Agriculture and distributed to farmers between 1900 and 1905 (Weinmann, 1972). Salisbury White and Southern Cross were developed in Rhodesia from variety crosses. Variation existed in Salisbury White, so during the summer seasons of 1913–1914 and 1914–1915 a "fixed" version was formed by crossing ‘Horsetooth’, ‘Boone County White’, and Hickory King. Their respective contributions to Salisbury White were 25, 25, and 50%. These three parent variety names appear on Goodman and Brown's (1988) list of the more prominent varieties in the Derived Southern Dent, Corn Belt Dent, and Southern Dent maize races. In 1960, the commercial single-cross hybrid SR 52 was released and based on inbred lines SC5522 (SC from Southern Cross) and N3-2-3-3 (N3 from Salisbury White) (Dowswell et al., 1996). At present, in eastern and most of southern Africa, lines based on the combining ability groups developed from material related to SC and N3 and another group from K64r/M162W are key components of hybrid efforts by national breeding programs (K. Pixley, 1995, unpublished data).
Over the past 35 yr, CIMMYT has developed numerous germplasm pools, populations, and open-pollinated varieties based on combinations of germplasm coming from many backgrounds (CIMMYT, 1998). Selection to improve these populations concentrated on per se performance and broad adaptation. A series of combining-ability studies were conducted to determine heterotic relationships among CIMMYT tropical, subtropical, temperate and QPM (quality protein maize) pools, and populations (Beck et al., 1990, 1991; Crossa et al., 1990; Vasal et al., 1992a,b,c). Several of the populations demonstrated good general combining ability, and various desirable heterotic combinations were identified.
A specific objective of this study was to assess heterotic relationships between the combining-ability groups Reid and Lancaster used in the temperate USA and the SC and N3 groups used in southern Africa, and their relation to a set of CIMMYT subtropical populations. The second objective and perhaps one with more utility for hybrid development was to evaluate combining ability among nine key populations used by maize hybrid breeders in subtropical, tropical-midaltitude, and temperate environments to determine their potential as exotic source germplasm to enhance grain yield.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Populations
CIMMYT Subtropical
The four chosen CIMMYT populations are subtropically adapted materials, each with components of temperate and tropical germplasm in their pedigrees. Either ETO or Tuxpeño type germplasm is present in the respective tropical components, with Population 34 containing both. A more complete description of the populations and selection procedures used during their improvement is presented by CIMMYT (1998), but a synopsis follows.
African Tropical Midaltitude
Salisbury White and Southern Cross were used because they are central to the SC and N3 combining-ability groups. Natal Potchefstroom Pearl Elite Selection (NPP ES) was included as an additional example of southern African germplasm. This variety traces to South Africa and has been used in hybrid development efforts there (Gevers and Whythe, 1987).
U.S. Corn Belt Temperate
BSSS (R) and BS 26 were chosen to represent Reid and Lancaster germplasm, respectively. BSSS (R) Cycle 11 traces to ‘Iowa Stiff Stalk Synthetic’. BSSS (R) C11 is a version of BSSS improved via a reciprocal recurrent selection scheme with Corn Borer Synthetic No. 1 (BSCB1) as the tester (Keeratinijakal and Lamkey, 1993). BS 26 is a breeding population consisting primarily of germplasm of Lancaster Sure Crop origin (Hallauer, 1986). Lines used to form BS 26 were selected based on S2 per se performance and in testcrosses with B73 x B84 (a single cross between BSSS-derived lines).
Environments
The experiment was evaluated in two environments in Mexico, two in Zimbabwe, and one in the USA. Irrigation was available to supplement rainfall in all environments and to facilitate uniform germination in Mexico. Fertilizer, herbicide, and insecticide were applied according to practices that would provide optimum growing conditions at each location.
The experiment was grown during the 1993 rainy (June–October) season and again during the 1993-1994 dry (November–April) season at CIMMYT's experiment station at Tlaltizapán, Morelos, Mexico (18°41'N lat., 940 masl). The soil is a calcareous vertisol (isothermic Udic Pellustert) with a pH of 7.6. Lime-induced chlorosis commonly occurs in maize grown on this soil, and a foliar application of ferrous sulfate was applied twice during the 1993–1994 season to reduce symptoms.
The two locations used in Zimbabwe were CIMMYT's Harare Mid-Altitude Research Station (17°48'S lat., 1506 masl) and Rattray-Arnold Research Station (17°40'S lat., 1308 masl) during the summer of 1993/1994 (November–April). Both locations have deep reddish-brown granular kaolinitic clay Rhodustalf soils.
The experiment was evaluated during the March to August 1994 growing season at the University of Florida, Gainesville (29°40'N lat., 29 masl), at a site with soil classified as Arrendondo fine sand (sandy, siliceous, hyperthermic Grossarenic Paleudult). The growing season was characterized by a rising temperature gradient from planting to harvest. Average daily maximum temperatures in March were 18°C and in July-August, 35°C. Plots were planted with an in-row subsoil no-till (strip till) planter, into killed rye (Secale cereale L.) according to procedures described by Gallaher (1977). Rye was sown in late fall and herbicide was applied to kill the rye at mid-to-late bloom.
Experimental Design
Nine parent varieties and 36 crosses were planted in a randomized complete block design at each location with three replications. At Tlaltizapán, plots included four 5-m rows spaced 0.75 m apart. An experimental unit consisted of the two central rows less one plant at the end of each row facing an open alley. Plots were overplanted and thinned to a density of 5.3 plants m-2 with one plant per hill (53 300 plants ha-1).
At the CIMMYT-Harare and Rattray-Arnold Research stations, plots included four 4.5-m and four 4.0-m rows, respectively. Row spacing, sowing procedures, and plant density were as for Tlaltizapán, with the exception that two plants per hill were established. Again, an experimental unit consisted of the central two rows, less the plants in the endhills of each row.
Nonbordered plots were used in Florida, with plots including two 3.66-m rows spaced 0.81 m apart. Plots were overplanted and thinned to 8.1 plants m-2 (80 880 plants ha-1).
Data were recorded for grain yield (Mg ha-1), days to silking (number of days from planting to when 50% of plants had extruded silks), plant height (centimeters from the soil surface to the node below the tassel), percent lodging (calculated from the number of plants visibly root and/or stalk lodged at harvest), and grain moisture (g kg-1) at harvest. Plots were hand-harvested, and the weight of ears was used to calculate grain yield at Tlaltizapán while shelled grain was used at the other environments. A grain weight to ear weight ratio of 0.8 was assumed at Tlaltizapán. Yields were adjusted to a grain moisture of 155 g kg-1. At Tlaltizapán, percent corn stunt disease was calculated from the number of plants exhibiting corn stunt disease symptoms late in the grain filling stage, i.e., before appreciable loss of green color occurred in healthy leaves. At a similar development stage, disease severity was scored for E. turcicum leaf blight and P. sorghi leaf rust at the Zimbabwe environments.
Analyses
Pattern Analysis for Assessing Entry x Environment Interaction
Analyses of variance were computed for each environment and combined across environments. Significant entry x environment interaction (E x Env.) effects were detected, so pattern analysis was used to determine the nature and magnitude of interaction. Theoretical development of this analysis was reviewed by Cooper and DeLacy (1994). The analysis package GEBEI (Watson et al., 1996) calculates proximity coordinates representing each entry and each environment using clustering and ordination procedures. For this analysis, a two-way entry mean x environment data set was created with means converted to standard units within each environment (environmental means equaled zero). Resulting coordinates were plotted simultaneously with entries as points, and environments as vectors scaled to have the same maximum magnitude as that of the entries. The origin represents an entry with average performance at all environments. The magnitude of entry effect plus the interaction effect associated with that entry is represented by the distance between the origin and the entry's point. Similar entries have small angles between their vectors, while dissimilar entries have large angles. Environments that sort entries similarly have small angles between their vectors, and relative vector length is indicative of how well performance within a specific environment is explained by the graph.
Generation Mean Analysis
Gardner and Eberhart's (1966) Analysis II was used to summarize the performance of varieties as parents on the basis of deviations in performance of the crosses from that of the parent varieties. Varieties and crosses were assumed to be fixed effects and environments random effects for this analysis. The analysis was based on fitting variety and variety cross means, Yijs, to the linear model:
 | (1) |
equals 0 when i = j, and 1 when i 
j. Heterosis effects, only present in crosses, can be further subdivided as
 | (2) |
Diversity Analysis
Phenotypic means of varieties and their crosses were used to estimate diversity among the parent varieties; procedures were adapted from Hanson and Moll (1986). Each mean represents a genotypic value and an error, Yij = Xij + eij, respectively. Marginal means are designated as Yi. = 
j Yij/n, where n equals the number of parents, and Y.. is the mean across all parents and crosses. The diversity analysis utilizes information from the diallel mating design, given that Xij = Xji. Consider the n varieties mated to the jth reference variety, the resulting n populations include (n - 1) crosses and the jth variety. Then conceptually, the deviations (Xij - X.j) place the n populations in the jth dimension on the basis of their breeding values when mated to the jth reference variety, or in n dimensions when mated to n reference varieties. Hanson (1983) identified a set of axes, using the phenotypic deviations (Yij - Y.j), so that coordinates on axis one reflect "additive-associated" gene effects. Then residual phenotypic deviations, deviations representing "dominance-associated" gene effects can be computed as
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= 
2n (Hanson and Moll, 1986). E signifies expectation, and 2
is the error variance for yij. With no dominance-associated gene effects, E[yij] = 0. It would be preferred to compute the distance between two parent varieties on the basis of the genotypic effects xij; however, only phenotypic values yij are available, so distances are estimated as
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 | (5) |
2y is the error variance for Yij. The function resolves divergence between the ith and i'th varieties on the basis of their performance for dominance-associated gene effects when mated to the n reference populations. The divergence among all pair-wise combinations of parent varieties is thus represented on an n by n coordinate system. Gower (1966) introduced principle coordinate analysis that can be used to resolve the n(n - 1)/2 distances among the n parent varieties, the n by n coordinate variability resolved to (n - 1) or fewer axes. For the purposes of this article, the representation of populations in reduced dimensions is a tool enabling visualization of inter-variety relationships. Squared distances standardized by appropriate errors, 2y, were calculated for each environment, then pooled across environments; the square-root of the pooled value was used as an across-environment distance estimate. These latter estimates were subjected to principle coordinate analysis. |
RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Entry, environment, and E x Env. effects were highly significant for plant height and days to silking (Table 2). Pattern analyses revealed that variation along Axis 1, representing mean performance across all environments, accounted for 85 and 94% of the variability in the two-way E x Env. data sets for plant height and days to silking, respectively (figures not shown). Based on the relatively small fraction of remaining variability that could result in interaction effects, further analyses of plant height and days to silking were computed across all environments. Southern Cross was the tallest and latest maturing parent followed by Salisbury White (Table 3). They resulted in the tallest and some of the latest crosses as indicated by their high values for vj. Conversely, Populations 34 and 47, BSSS (R), and BS 26 were the shortest and earliest. BSSS (R) resulted in some of the earliest crosses, 3.4 d earlier on average, determined on the basis of the calculation of 1/2(vj) + hj.
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Generation Mean Analysis
Variety and hij effects for yield were highly significant in Mexico (Table 2). Variety effects are calculated from the per se performance of parents; hij effects are the deviations in performance of crosses from the performance midpoint of their respective parents. Variety effects for parents were important predictors of cross performance in the Mexican environments in that they were associated with 78% of the sums of squares for entries. In contrast, vj effects were not significant in the Zimbabwe and U.S. environments where 85% of sums of squares for entries was associated with hij effects. Parental performance was of little value in Zimbabwe and USA to predict cross performance, with testing of crosses likely required to allow selection for yield. In brief, the importance of vj effects was greater in those environments where apparent poor adaptation had a greater impact on yield.
Variety effects for plant height and days to silking accounted for 87 and 95% of sums of squares among entries (Table 2). This suggests that even though significant hj and sij heterosis effects were observed, the expression of these traits in crosses is largely predicted by the per se performance of their parents.
Population 44 had the highest value for per se yield across environments, followed by Populations 34, 42, 47, and NPP ES (Table 4). This is also reflected by their positive values for vj. The high per se yield of CIMMYT populations is likely because of a priority given to combining desirable traits when the populations were being developed and to subsequent improvement by means of intrapopulation schemes that emphasized broad adaptation (CIMMYT, 1998).
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Diversity Analysis
Diversity analysis was used to combine the yield performance results from all crosses and parents in one analysis to define relationships among parent varieties on the basis of dominance-associated gene effects (Hanson and Moll, 1986). The procedure allowed analysis across all environments, even though E x Env. effects were significant, by combining the yield data from the five environments as if they were data for five different traits. Numeric values for h, hj, and sij heterosis effects convey the one dimensional separation among parents (Tables 4 and 5), where as diversity analysis conveys the multidimensional separation among the nine parents (Fig. 2 presents the first three coordinates of this analysis).
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Small or negative values for hj effects were observed for NPP ES, Populations 34, 42, and 44, and BS 26 (Table 4), and, consistent with this observation, these varieties grouped together in the diversity analysis near the origin (Fig. 2). Large spatial separation along Axis 1 was observed between BSSS (R) and this central group, and a somewhat smaller separation between Salisbury White and the central group along Axis 2. These separations reflect the large hj heterosis effect observed for BSSS (R) and Salisbury White (Table 4) and suggests that BSSS (R) and Salisbury White would perform well as a opposing heterotic populations with each other, or with any of the other varieties. Conversely, it suggests that the other varieties or combinations of the other varieties would perform well in an opposing heterotic pattern with BSSS (R) or Salisbury White. These other varieties represent different levels of expression for a wide array of traits, allowing a breeder opportunity to combine this expression in heterotic combinations with either BSSS (R) or Salisbury White.
Southern Cross is positioned below and behind the central group of parents with components from Axes 2 and 3 contributing to its separation from that central group (Fig. 2). The separation of BSSS and BS 26 along Axis 1, and Salisbury White and Southern Cross along Axes 2 and 3 suggests that there is little relationship between these two heterotic patterns. The largest separation between a pair of populations occurred between Population 44 and BSSS (R), also separated along Axis 1. This large separation reflects their relationship as opposing heterotic populations and suggests that they form the best heterotic combination among the varieties. BS 26 occurred near the midpoint between these two populations, suggesting that genes from Population 44 could be used to enhance the performance of BS 26 in heterotic combination with BSSS (R), that is if Population 44 is not used opposite BS 26, an approach suggested by the observed sij effects.
The relatively small separation among Populations 34, 42, 44, and 47, and NPP ES suggests that positive heterotic responses among these are minimal (Fig. 2). This is not surprising for the CIMMYT populations. They are broad-genetic-base populations that were composited with principle regard to combining desirable agronomic traits for different ecological zones (CIMMYT, 1998). Often germplasm from opposing heterotic groups was combined in the same population, for example, Population 34 contains Cuban Flint, ETO, Tuxpeño, Corn Belt Dent, and other materials. Therefore, crosses among CIMMYT's broad-genetic-base material at the population level will likely lead to partial reduction of favorable heterotic effects, resulting in only low-to-moderate levels of heterosis. Because of increased interest in hybrid maize production in the developing world, CIMMYT's maize program has more recently concentrated on breeding strategies that provide improved germplasm for both hybrid and open-pollinated variety development programs. Consequently, these populations have more recently been improved by reciprocal recurrent selection, so greater separation may be observed from testing newer versions of these populations. Some testing of lines from CIMMYT populations has occurred resulting in evidence of good interpopulation hybrids, but also evidence of lines from the same population forming good hybrids (Han et al., 1991). The latter might be expected given the broad genetic base of these populations.
Populations 34, 42, 44, and 47 were involved as parents of the five highest-yielding crosses in Mexico, suggesting that they would be good parents for hybrid breeding programs in environments comparable to Tlaltizapán (note the ranking of crosses in Table 5). Population 42 x Population 47 had the highest observed yield. Populations 34, 42, 44, and 47's resistance to corn stunt disease and lodging in this environment likely contributed to their identification as good parents. Natal Potchefstroom Pearl Elite Selection and BSSS (R) were also involved as parents of these high-yielding crosses. The high yield of crosses involving NPP ES and BSSS (R) suggests that they would benefit breeding programs in Mexico and similar environments, however, most consumers in Mexico and many subtropical environments prefer white grain types. Consequently, BSSS (R) would need to be converted to white, in addition to incorporation of higher levels of disease resistance necessary for maize production in the subtropics. Results from the study reported by Beck et al. (1991) are consistent with our results. That study involved Population 34, 42, and 47, and in Mexico, significant positive general combining ability effects for yield were observed for these three populations and Population 42 x Population 47 ranked first for yield.
Salisbury White, Southern Cross, and BSSS (R) occurred frequently as parents of the five highest-yielding crosses in Zimbabwe and the USA (note the rankings in Table 5). Also occurring as parents of these crosses were Populations 42 and 44. Although two of the three testing environments were in Africa, the numerically highest-yielding cross [Population 44 x BSSS (R)] did not involve an African parent, however, Population 44 traces, in part, to Boone County White, a variety that performed well when it was first introduced into Southern Rhodesia in the early 1900s (Weinmann, 1972). The good performance of BSSS (R), Population 44, and Population 42 suggests that they may be valuable to breeding programs in environments typified by the midaltitude breeding stations in Zimbabwe and the warm-temperate station at Gainesville, FL. Again, the use of BSSS (R) in midaltitude environments may require its conversion to white grain color and incorporation of higher levels of disease resistance.
Maize hybrid breeding, during the past several decades, has sifted through germplasm coming from many early varieties. Reid and Lancaster have come to be recognized for their substantial contribution to modern hybrids for temperate environments (Darrah and Zuber, 1986), as have Salisbury White and Southern Cross for the midaltitude environments of eastern and southern Africa (Dowswell et al., 1996). There is some indication that Boone County White-type germplasm should also be recognized for its good performance. Boone County White is one of the parents of Salisbury White (Weinmann, 1972). American Early Dent, an important variety in Egypt (Wellhausen, 1978), originated as a variety selected out of Boone County White and was released in Egypt in the 1920s. The superior performance of Population 44 in this study traces through American Early Dent to Boone County White, but Population 44 also includes a Tuxpeño-1 component (CIMMYT, 1998). Tuxpeño germplasm has substantial importance to breeding programs in the tropics (Dowswell et al., 1996). The apparent positive specific combining ability of Population 44 with BSSS (R) and BS 26 suggests that these three should be researched further.
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SUMMARY |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Received for publication June 5, 2000.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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