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Crop Science 43:844-849 (2003)
© 2003 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Heterotic Patterns among Flint Maize Populations

Pilar Soengas, Bernardo Ordás, Rosa A. Malvar, Pedro Revilla and Amando Ordás*

Misión Biológica de Galicia, CSIC, Apartado 28, 36080 Pontevedra, Spain

* Corresponding author (aordas{at}mbg.cesga.es)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Most maize (Zea mays L.) hybrids cultivated in Europe follow the heterotic pattern European flint x Corn Belt dent. However, at present, no commercial hybrids have been obtained from European flint inbreds exclusively. A flint x flint heterotic pattern could be an alternative to use in breeding programs to obtain early maturity, high-yielding hybrids, potentially useful for producing high quality flours. The objective of this work was to search for heterotic patterns within flint germplasm adapted to temperate conditions. Ten open-pollinated flint maize cultivars were crossed in a diallel design without reciprocals and evaluated in four environments in northwestern Spain. Cultivar effects explained most of the sum of squares for traits analyzed, except for yield. Crosses yielded 30% more than parental cultivars. Yield of crosses ranged from 3.94 to 5.61 Mg ha-1. Midparent heterosis values ranged from 2.5 to 64.7%. The crosses ‘Gallego’ x ‘Basto/Enano Levantino’ and Basto/Enano Levantino x ‘Longfellow’ showed high heterosis and the best averages for grain yield. On the basis of these results, these crosses could be included in a recurrent reciprocal selection breeding program to increase heterosis among the cultivars and improve their hybrids. A cluster analysis made with midparent heterosis for grain yield as a measure of distance revealed that the flint germplasm probably had similar origins and were related to the ‘Northern Flint’ race. Some differences among these cultivars were detected. ‘Norteño Largo’ was closely related to Northern Flint, while Basto/Enano Levantino was different from the rest of the cultivars. It was concluded that the development of hybrids combining good yield and the typical agronomic characteristics of flint maize is feasible.

Abbreviations: BA, Basto • EL, Enano Levantino • FI, Fino • GA, Gallego • HN, Hembrilla Norteño • LO, Longfellow • NL, Norteño Largo • NO, Norteño • RE, Relámpago Ocho Hileras • TR, Tremesino • SI, Amarillo Precoz de Simone • df, degrees of freedom • MS, mean square • UPGMA, unweighted pair-group method using arithmetic averages • h, average heterosis • hj, cultivar heterosis • sjj', specific heterosis • vj, cultivar effect


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
IT IS WELL KNOWN that the heterotic pattern Reid x Lancaster is the most common scheme used by commercial companies to produce hybrids for the temperate areas of the world. In Europe, the heterotic pattern European flint x Corn Belt Dent has also been widely used (Moreno-González, 1988; Ordás, 1991). This combination has allowed the expansion of maize in Central Europe by combining the early vigor and earliness of the European flint lines with the high grain yield of the American dent lines (Ayrault et al., 1999). Most of the flint germplasm used today in Europe, however, comes from only a few flint lines (Messmer et al., 1992a, 1992b). Most of the inbreds employed are related to EP1, F2, or F7 (Moreno-González, 1988).

Heterosis between European flint and American dent maize has been broadly studied by different authors who have outlined different alternative heterotic patterns to the commonly employed Reid x Lancaster pattern. For instance, heterosis between Yugoslavian and Corn Belt germplasm is high, and some alternative heterotic patterns are available (Misevic, 1989; 1990; Radovic and Jelovac, 1995). Also, heterosis between Spanish and Corn Belt populations is high (Ordás, 1991; Álvarez et al., 1993; Sinobas and Monteagudo, 1996). Álvarez et al. (1993) studied heterosis between two synthetic populations: EZS1, formed from flint Spanish populations, and EZS2, formed from Corn Belt maize. They proposed the heterotic pattern EZS1 x EZS2. Sinobas and Monteagudo (1996) determined the combining ability of seven Spanish varieties with the American varieties ‘Reid Yellow Dent’ and ‘Lancaster Sure Crop’ and found that Andaluz x BS13 could be an alternative heterotic pattern. Ordás (1991), after a diallel among nine populations, outlined the heterotic patterns Northern Spain x Southern Spain, Northern Spain x U.S. dent, and Southern Spain x U.S. dent. Broadly speaking, germplasm from northern Spain is pure flint while Southern Spain germplasm is mostly semident (Sánchez-Monge, 1962). The existence of these heterotic patterns indicates there is heterosis within Spanish germplasm, at least between flint and semident materials.

Until now, no commercial hybrids have been obtained exclusively from European flint inbreds. A flint x flint heterotic pattern would be useful in breeding programs to obtain early, high-yielding hybrids, potentially useful for production of high quality flours. Such hybrids would also broaden the genetic base of the commercial varieties released to farmers.

Few studies have been performed on heterotic patterns within European flint germplasm. This germplasm is earlier in maturity than Corn Belt dent, has acceptable yield, and shows some tolerance to drought. The farmers of the northwest of Spain have cultivated flint maize traditionally for feed but also for human consumption. Northern Flint types are the source of earliness for the short season maize of Canada, northern Europe, and northern Japan. South American flint types in Argentina and Brazil are presently being introgressed with dent germplasm to obtain higher yield and faster dry down (Troyer, 1999). Early flint varieties can be used in the northern U.S. Great Plains to stabilize yields by partially evading drought hazards (Troyer and Hallauer, 1968). Heterosis among European flint populations has been observed by different authors (Misevic, 1989; Ordás, 1991; Radovic and Jelovac, 1995). European inbreds showed some differences on the basis of their combining ability in crosses with U.S. inbreds belonging to different heterotic groups (Cartea et al., 1999). The objective of this research was to identify heterotic patterns that might exist within flint germplasm adapted to temperate conditions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Ten open-pollinated cultivars of flint maize were chosen for this study (Table 1) . Seven of them are Spanish cultivars and intermediate forms (Sánchez-Monge, 1962) that represent the germplasm grown in the Spanish humid (northern cultivars) and dry areas (southern cultivars), respectively, according to the grouping of Spanish germplasm by Ordás (1991). Two Argentinean cultivars and one northern flint population were also included.


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  		Table 1. Accession name, number, description, and origin of 10 flint populations of maize grown in four environments in the northwest of Spain.

 
The cultivars were crossed in a complete diallel without reciprocals in 1997 in paired rows with 15 plants per row. Five sets of paired rows were used for each cross, using each plant once, either as a male or as a female, but never as both. Each parental cultivar was multiplied to obtain seed in the same environmental conditions. About 50 ears were obtained from each cross and from each cultivar. Pollinated ears from the five pairs of rows for each entry were hand harvested and shelled in bulk.

The 10 parental cultivars and their 45 crosses were planted in four different environments in the northwest of Spain: Pontevedra (lat. 42°24'N, long. 8°38'W, 20 m above sea level) in 1998 and 1999; Barrantes (lat. 42°30'N, long. 8°46'W, 50 m above sea level) in 1998 and Pontecaldelas (lat. 42°23', long. 8°32'W, 300 m above sea level) in 1999. The soil of the four environments was middle acidic with a high content of organic matter. Fertilizers were added as follows: 160 kg N, 105 kg P2O5 and 105 kg K2O in Pontevedra and Barrantes in 1998; 185 kg N, 120 kg P2O5, 120 kg K2O in Pontevedra in 1999; 150 kg N, 100 kg P2O5, 100 kg K2O in Pontecaldelas in 1999. To control Cyperus esculentus L., 75 kg ha-1 of a herbicide formed by 10% EPTC (S-ethyl dipropylthiocarbamate), 2.5% atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine), and 0.95% dialamida (?) was applied in Pontevedra and Barrantes. Nine check cultivars were included to complete an 8 x 8 partially balanced triple lattice design: three hybrids [(A239 x A251) x A635, (A427 x A556) x A632, and A619 x A632]; three synthetic cultivars, namely EPS6(S)C3, EPS7(S)C3, and EPS10, formed with germplasm from northern and southern Spain and with U.S. Corn Belt germplasm, respectively (Ordás, 1991); and, finally, three crosses among the synthetic cultivars, which represent the heterotic patterns Northern Spain x Southern Spain, Northern Spain x U.S. dent, and Southern Spain x U.S. dent, respectively (Ordás, 1991).

Each experimental plot included two rows spaced 0.80 m apart, with 25 two-plant hills spaced 0.21 m apart. Plots were overplanted and thinned, obtaining a final density of approximately 60,000 plants ha-1 in each environment. Data observed included pollen shedding (days from planting to 50% of plants flowering), ears plant-1, and grain yield (Mg ha-1 at 140 g kg-1 of moisture). Ear length (cm), 100-kernel weight (g), kernel row number, and kernel length (cm) were estimated from a sample of 10 random ears obtained from each plot.

Individual analyses of variance were performed for each lattice design (Cochran and Cox, 1957). If the relative efficiency of the lattice design was less than 105% for a trait, data were analyzed as a randomized complete block design. Analyses were completed with PROC LATTICE (SAS, 2000). The combined analyses of lattices were made by the PROC GLM (SAS, 2000) with adjusted treatment means. Treatments mean squares were orthogonally partitioned into diallel populations (parental cultivars and crosses), check cultivars, and among groups.

Diallel populations were divided according to Analysis II suggested by Gardner and Eberhart (1966) for a fixed set of cultivars. Hallauer and Miranda (1988, section 4.3.2) give a detailed description of the model.

The model on which this analysis is based is as follows:

In this model vj is the cultivar effect, hjj' is the heterosis effect when cultivars j and j' are crossed, and k = 0 if j = j' and k = 1 otherwise. It is possible to partition hjj' further as follows:

where h is the average heterosis (the difference between the mean of crosses and the mean of cultivars), hj is cultivar heterosis (the contribution of the j cultivar to heterosis), and sjj' is the specific heterosis when cultivars j and j' are crossed.

The standard error of heterosis was calculated as the square root of 1.5 times the variance of the entry mean, following Keeratinijakal and Lamkey (1993). A cluster analysis was done to determine relationships among parental cultivars, using UPGMA (unweighted pair-group method using arithmetic averages) (Romesburg, 1984) and the midparent heterosis for yield as the measure of dissimilarity between any two populations. Midparent heterosis was computed as the difference between hybrid yield and the mean of the two parental cultivars (Falconer and Mackay, 1996, p. 255). PROC CLUSTER of SAS (SAS, 2000) was used to carry out the analyses.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
In the combined analysis of variance, kernel row number was analyzed as for a randomized complete block design, while the other traits were analyzed as lattices (data not shown). The check (A427 x A556) x A632 did not grow in Pontecaldelas and its value had to be estimated for all traits (Cochran and Cox, 1957, p. 413); the degrees of freedom for error differed for each trait because of missing data and this is reflected at the bottom of Table 2 . The environments x entries interaction was significant for all traits, except yield. Trials grown in Barrantes and Pontecaldelas were subjected to drought; the first of those locations had a high rate of natural infestation of the borers Sesamia nonagrioides (Lef.) and Ostrinia nubilalis (Hübner) (Cordero et al., 1998). Entries resistant to drought or borers would have performed better in these environments.


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  		Table 2. Mean squares from the analysis of variance of eight traits for a diallel of 10 flint populations of maize grown in four environments in the northwest of Spain.

 
Variation among the diallel populations was significant for all of the traits (Table 2). Cultivar effects (vj) were significant for all traits, except for grain yield, and explained the highest proportion of the sum of squares. Other authors have found that cultivar effects explained most of the variability for days to flowering (Castro et al., 1968; Eberhart, 1971; Genter and Eberhart, 1974; Crossa et al., 1990) and ear number (Crossa et al., 1990). Heterosis (hjj') was significant for grain yield, explaining 92% of the sum of squares. In addition to yield, only average heterosis (h) was significant among the subdivisions of heterosis and explained 69% of the sum of squares. In the diallels analyzed by Troyer and Hallauer (1968), Misevic (1989) and Eberhart (1971), heterosis explained most of the variability. On the other hand, other authors have found that most of the variability for grain yield was because of cultivar effects (Castro et al., 1968; Crossa et al., 1987; Crossa et al., 1990; Pérez-Velásquez et al., 1995). Hallauer and Miranda (1988), in a review of different studies, found that most of the variability for grain yield was due to cultivar effects and average heterosis.

No cultivar had favorable cultivar effects for all yield components (Table 3) . For example, FI had high positive cultivar effects for ears per plant (0.123), kernel row number (1.449), and kernel length (0.036 cm), and low effects for 100 kernel weight (-4.264 g) and ear length (-1.425 cm). Because of this, we could not choose cultivars based on all of the grain yield components. The contrasting cultivar effects for individual yield components, and/or the presence of a significant environments x vj interaction for all yield components except kernel length, could explain the lack of significant cultivar effects per se for yield (Table 3).


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  		Table 3. Significant cultivar effects and cultivar heterosis of different traits for a diallel of 10 flint populations of maize grown in four environments in the northwest of Spain.

 
Kernel row number, 100-kernel weight, and kernel length showed significant hj (Table 2). Basto/Enano Levantino showed high hj for 100-kernel weight (1.095 g) and kernel length (0.028 cm), and NO for kernel row number (0.357) and kernel length (0.025 cm). It would be expected that hybrids with these cultivars would perform well. Crosses with FI showed low hj for 100-kernel weight, kernel row number, and kernel length and hybrids performed poorly.

The establishment of heterotic patterns plays an important role in the selection of germplasm to develop hybrids. The analysis of diallel crosses provides preliminary data about the heterotic relationships among the parents (Hallauer and Miranda, 1988). In general, heterosis is considered an expression of the genetic divergence among cultivars (Moll et al., 1962); when heterosis or some of its components are significant for all traits, it may be concluded that there is genetic divergence among the parental cultivars. However, for grain yield, only h was significant, indicating that cultivars had similar contributions to heterosis in their crosses. Therefore, there may be similar levels of genetic divergence among them.

Crosses of the diallel yielded 30% more on the average than their midparent value. This level of heterosis is intermediate compared with the values given by other authors. Pérez-Velásquez et al. (1995) found an average heterosis of 20% among Colombian populations. Troyer and Hallauer (1968) detected heterosis of 72% among early flint populations. Oyervides-García et al. (1985) cited values of 34.8% between ‘BSSS’ and Lancaster populations. Those values are superior to the average midparent heterosis (21.1%) found by Ordás (1991) among five Spanish cultivars and four Corn Belt dent cultivars. Also, they are greater than the average midparent heterosis showed by crosses EPS6(S)C3 x EPS7(S)C3, EPS6(S)C3 x EPS10 and EPS7(S)C3 x EPS10 (15.15%, 16.33%, and 15.15%, respectively), representing the heterotic patterns Northern Spain x Southern Spain, Northern Spain x U.S. dent, and Southern Spain x U.S. dent, respectively (Ordás, 1991) in this study. Trials grown in Barrantes and Pontecaldelas were under stress conditions, thus the cultivars per se had a relatively poor performance that might favor the expression of average heterosis.

Check cultivars differed significantly from the diallel populations for grain yield (Table 2) and showed higher grain yield than diallel populations. Yield of the check cultivars ranged from 4.88 (EPS10) to 7.58 Mg ha-1 (A619 x A632), while the yield of crosses ranged from 3.94 (GA x NO) to 5.61 Mg ha-1 (GA x BA/EL) (Table 4) . Although the diallel populations yielded less than check cultivars, the flint germplasm has other characteristics of interest. A heterotic pattern flint x flint could contribute to high quality flours, earliness, or relative resistance to drought.


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  		Table 4. Means for grain yield (Mg ha-1) of diallel (above diagonal), parental varieties (on the diagonal), and check cultivars (on the right) for a diallel of 10 flint populations of maize grown in four environments in the northwest of Spain. Midparent heterosis{dagger} for grain yield is shown below the diagonal.

 
Parental cultivars yielded less than the crosses as expected. Midparent heterosis values ranged from 2.5% (GA x NO) to 64.7% (BA/EL x LO). Crosses GA x BA/EL and BA/EL x LO showed high heterosis and the best averages for grain yield. Gallego comes from the north of Spain, BA/EL from the east, and LO from North America. Gallego had a high vj for earliness (Table 3) and a high hj for 100-kernel weight (0.999 g). Basto/Enano Levantino had the best vj for number of ear rows (1.649) and the highest hj for 100-kernel weight and kernel length (Table 3). Among the 10 flint cultivars studied, GA x BA/EL and BA/EL x LO would be the best crosses to start a reciprocal recurrent selection breeding program to increase heterosis among cultivars and to improve the cross between them.

Average midparent heterosis for grain yield components was positive and significant, and varied between 1.3% for kernel length and 8.2% for 100-kernel weight (data not shown). Specific heterosis was significant for pollen shedding, 100-kernel weight, ear length, and ear rows (Table 2). The significance of specific heterosis for number of ear rows was due, probably, to the inclusion of northern flint material. In general, the highest specific heterosis effects for yield components were detected in crosses among Spanish and American cultivars, and also among the crosses of NL to the other Spanish populations. The cross of NO x FI showed high specific heterosis for number of ear rows (data not shown). Similarly, Ordás (1991) found heterosis between germplasm from humid and dry Spain. Basto x Tremesino showed a high specific heterosis for ear length, indicating that there are differences among germplasm from Eastern and Southern Spain (data not shown).

Cluster analysis done with midparent heterosis for grain yield as the measure of distance classified the 10 flint cultivars into four clusters for a distance of 0.8 (Fig. 1) . Cluster I included a cultivar from the East of Spain: BA/EL. Cluster II included cultivars from northern and southern Spain (FI, GA, NO, TR) and Argentina (RE, SI). Cluster III included LO and NL, representing the Northern Flint race. Cluster IV included GA/HN from northern Spain.



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  		Fig. 1. Cluster analysis using midparent heterosis for grain yield as the dissimilarity measure and UPGMA as the clustering method. {dagger}Abbreviations for cultivars are in Table 1.

 
There is no a clear relationship among the clusters and the geographic origin of the cultivars. Previously, Ordás (1991) had classified five Spanish races and four American populations into three clusters related to their geographic origin. Some cultivars coincided in both works, but Ordás (1991) also employed Corn Belt dent cultivars. Genetic divergence among flint cultivars might be smaller because of a possible common origin of the cultivars. Cultivars GA and FI were formed by Sánchez-Monge (1962) by joining similar samples of different geographic origin; therefore, it is not surprising that they have a common grouping. Northern Flint races could have been introduced several times in Spain beginning in the 17th century (Revilla et al., 1998). Longfellow was classified with NL, SI, and RE in previous isozymatic classification (Revilla et al., 1998). The flint germplasm analyzed probably has a common origin with the Northern Flint race. Some introductions would have hybridized with other cultivars in Spain and, as a result, led to new cultivars. Norteño Largo and LO would form a group with clear characteristics of northern flint. However, BA/EL seems to be different from the rest of the cultivars and its origin is likely different. In the isozymatic classification of Spanish races (Revilla et al., 1998), germplasm from the south and east of Spain differed from the rest, probably because of their independent origin starting from samples coming from Central America.

Although most of the crosses were similar, GA x BA/EL and BA/EL x LO are the best candidates for inclusion in a reciprocal recurrent selection breeding program to increase heterosis among cultivars and improve their hybrids.

In conclusion, flint maize is an interesting addition to the germplasm used in temperate areas. Among the diallel populations (Table 2) there was no significant vj effect for grain yield; only h, that is the difference between the mean of all crosses and the mean of all cultivars, was significant. Thus heterosis among the different populations included in this study exists. Development of hybrids combining good yield and the typical agronomic characteristics of flint maize should be feasible.


   ACKNOWLEDGMENTS
 
M. Pilar Soengas and Bernardo Ordás acknowledge a fellowship from the Xunta de Galicia and from the Ministry of Science and Technology of Spain, respectively.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Research supported by the Committee for Science and Technology of Spain (Project Cod. AGF98-0968-C02) and Excma. Diputación Provincial de Pontevedra, Spain.

Received for publication January 18, 2002.


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




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