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Published online 21 November 2006
Published in Crop Sci 46:2666-2669 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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CROP BREEDING & GENETICS

Combining Abilities and Heterosis for Adaptation in Flint Maize Populations

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

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

* Corresponding author (csgpomps{at}cesga.es)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Breeding of maize (Zea mays L.) in the Atlantic coast of Europe has focused on developing hybrids to exploit heterosis between flint and dent germplasm. Flint x flint hybrids could have some advantages because of their earliness and early vigor that are important in short growing season areas. The objective of this work was to study the adaptation of a set of 10 flint varieties crossed to flint and dent testers and in a diallel design to check the feasibility of a breeding program to obtain flint hybrids well adapted to European Atlantic conditions. In the testcrosses, general combining ability (GCA) of the flint tester EP42 for kernel moisture at harvest and days to silking was lower than those of dent testers. Improvement of flint varieties for combining ability with inbred line EP42 could be done to obtain earlier flint x flint hybrids with a better dry-down rate than flint x dent hybrids. Diallel entries were analyzed following Analysis II of Gardner and Eberhart. Variety and heterosis effects were significant for all traits. Varieties Gallego and Norteño, besides having large variety effects, also showed intermediate variety heterosis for adaptive traits and they would be the best adapted varieties to include in breeding programs. The cross ‘Gallego x Basto/Enano levantino’ combined the best yield and good specific heterosis for adaptive traits. Thus, flint hybrids to be grown in short season areas could be obtained following this heterotic pattern.

Abbreviations: GCA, general combining ability • SCA, specific combining ability


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
BREEDING OF DENT maize for high yield in North America has generally led to hybrids with later flowering, faster grain fill, and better grain dry down (Tollenaar, 1977). This maize has been introduced in northwestern Spain several times, but not very successfully because of its lack of adaptation. Farmers in northwestern Spain have traditionally grown flint maize landraces that are better adapted than dent maize, due in part to their earliness, an important consideration in short season areas (Frei, 2000). Earlier plants can flower in the longest days of the summer and mature before temperatures are too low to allow good grain dry down. Other characteristics of flint maize, like low growth temperature requirement, early vigor, cold tolerance, staygreen, and hardness of endosperm play an important role in the adaptation of maize in short season areas (Frei, 2000).

Because American dent is not well adapted and European flint is not as productive, breeding of maize in the Atlantic coast of Europe has focused on developing hybrids to exploit heterosis between the two types of maize and combine the good properties of both (Revilla et al., 1999). At present, no flint x flint hybrids are produced for the climatic conditions of northwestern Spain, possibly due to the good performance of flint x dent hybrids. However, flint x flint hybrids could have some advantages because of their earliness, early vigor, and other agronomic properties compared to dent maize (Moreno-González, 1988; Cartea et al., 1996; Malvar et al., 1997). Also, the endosperm of flint maize is more suitable to make flour, as it is used in northwestern Spain, or to make corn-flakes. A slower drying rate of flint germplasm has been suggested as a possible reason for its limited use. However, the results found by Hunter et al. (1979) indicated that there was no inherent major difference in field drying rate between flint and dent endosperm types and differences in drying rate could be due to morphology.

If differences in drying rates are due to morphology and if yield of flint maize could be improved by selection, then flint hybrids that perform well could be obtained and compete with flint x dent hybrids in the short growing seasons of northwestern Spain, the Atlantic coast of Europe, Canada, northern Japan, or Argentina. In previous work, 10 flint landraces from Spain and America were crossed in diallel design and with four different testers to study heterotic patterns for yield (Soengas et al., 2003a, 2003b). Authors found heterosis for yield among these populations that could be exploited to obtain hybrids (Soengas et al., 2003a) and identified populations that could be included in breeding programs to increase the variability present in European flint maize (Soengas et al., 2003b). However, before obtaining flint hybrids, one should also evaluate those traits that are important in northern regions and that also affect yield, to know if those hybrids have some adaptive advantage that makes them more suitable to be grown in northern areas. The objective of this work was to study the performance of a set of flint varieties crossed with different testers and in a diallel design to check the feasibility of a breeding program to obtain flint hybrids well adapted to European Atlantic conditions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Ten open-pollinated, broad-base varieties of flint maize were chosen for this study. Seven varieties were Spanish: four from the North and three from the South and East, representing humid and dry areas, respectively. Northern varieties were Gallego (GA), Gallego x Hembrilla norteño (GA/HN), Norteño (NO), and Norteño largo (NL). From dry Spain the varieties Basto x Enano levantino (BA/EL), Fino (FI), and Tremesino (TR) were chosen. Two Argentinean varieties, Relámpago ocho hileras (RE) and Amarillo precoz de Simone (SI), and another belonging to the Northern Flint race, Longfellow (LO), were included.

To study adaptive traits, varieties were crossed as males in 1997 and 1998 with a flint (EP42 line) and dent inbred testers (A632, W64A, B93) as females, and among themselves in a complete diallel without reciprocals in 1997. Testcrosses were evaluated in 1999 and 2000 at two locations in northwestern Spain: Pontevedra (42°24' N, 8°38' W, 20 m above sea level), and Pontecaldelas (42°23' N, 8°32' W, 300 m above sea level) in a randomized complete block design with three replications. Diallel entries, which include the 10 parental varieties and their F1 hybrids, were evaluated in four different environments in northwestern Spain: Pontevedra in 1998 and 1999; Barrantes (42°30' N, 8°46' W, 50 m above sea level) in 1998 and Pontecaldelas in 1999. Nine check cultivars were also included to complete an 8 by 8 triple lattice design. A more detailed description of crosses and trials is given in Soengas et al. (2003a, 2003b). Data recorded on all the plants in both experiments were early vigor using a visual scale from 1 = poor to 9 = high, days to silking (days from sowing to 50% of plants silking), and kernel moisture at harvest.

Simple correlation coefficients were calculated between yield that was previously recorded (Soengas et al., 2003a, 2003b) and adaptive traits via PROC CORR of SAS (2000). For both experiments, each location–year combination was treated as a random environment. Sum of squares due to testcrosses were divided into varieties, testers, and variety x tester interaction. The three sources of variation were assumed to be fixed effects. The mean squares for variety and testers correspond to GCA variation, whereas the mean square of the variety x tester interaction is related to specific combining ability (SCA) variation. Combined analysis of variance was performed via PROC GLM of SAS (2000). Estimates of GCA and SCA were calculated from means of crosses, according to Falconer and Mackay (1997). Least significant differences for differences between combining ability estimates of two parents were calculated following Garay et al. (1996).

In the diallel experiment, individual analyses of variance were computed for each trait following Cochran and Cox (1957), using PROC LATTICE of SAS (2000). A combined analysis of lattice designs was made with the adjusted diallel entries means, via PROC GLM of SAS (2000). The source of variation due to diallel entries was divided following Analysis II of Gardner and Eberhart (1966) for a fixed set of cultivars. The linear model on which this analysis is based is as follows:

Formula 1[1]
where Yij is the observed mean of the crosses of varieties i and j; µv is the mean of all parental varieties; vi and vj are the variety effects for i and j, respectively, calculated as the difference between a particular variety mean and the mean of all varieties; k = 0 when i = j and k = 1 when i != j; and hij is the observed heterosis of the cross of varieties i and j measured as the difference between the mean of the two parental varieties and their cross. It is possible to partition hij further as follows:

Formula 2[2]
where h is the average heterosis estimated as the mean of crosses minus the mean of parental varieties; hi and hj are the variety heterosis effects; and sij is the specific heterosis that occurs when variety i is mated to variety j. The relation of variety effects and variety heterosis with GCA of Griffing's diallel analysis (Griffing, 1956) was described by Eberhart and Gardner (1966) and can be defined as follows: GCAj = (1/2) vj + hj, The SCAij in the analysis of Griffing (Griffing, 1956) is equivalent to sij in Gardner and Eberhart analysis (1966). The Analysis II of Gardner and Eberhart (1966) was computed with a program designed at Misión Biológica de Galicia (CSIC, Spain) using the PROC IML of SAS (2000). Standard errors were calculated following Moreno-González et al. (1997).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Yield was significantly correlated with days to silking and kernel moisture at harvest (P < 0.005) in testcrosses (–0.425 and –0.731, respectively) and diallel entries (–0.673 and –0.372, respectively). The negative correlation coefficients indicated that yield increased when plants were earlier or had less kernel moisture at harvest. These results confirm that in northern regions adaptive traits are an important yield component (Frei, 2000). Although yield is the main criterion when choosing a heterotic pattern, adaptive traits could discriminate between crosses of similar yield. There was no correlation between yield and early vigor. However, growers in northwestern Spain require hybrids with a good early vigor since they reject maize with a poor performance in the first stages. Early vigor and earliness besides minimum temperatures were identified by Malvar et al. (2005) as the main factors to explain genotype x environment interaction for crosses among varieties in short growing season areas. Early vigor and earliness should be taken into account to obtain stable hybrids across environments. The performance of flint varieties for adaptation was divided into testcrosses and diallel entries.

Testcrosses
The analysis of variance of testcrosses between 10 flint varieties and four different testers showed significant differences among crosses for all traits (Table 1). The source of variation due to GCA of varieties was significant for all traits, whereas GCA of testers was significant for all traits except early vigor, probably due to the significant environment x tester interaction (Table 1). The SCA was not significant for any trait, suggesting that additive effects are more important than dominance effects (Table 1). Testers could differentiate among GCA of varieties; however, varieties should be considered a homogeneous group for their SCA.


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  		Table 1. Mean squares for 40 testcrosses among 10 flint varieties and four testers, for three traits evaluated in four environments in northwestern Spain.

 
The GCA of EP42 was significantly lower than those of A632 and W64A for days to silking (Table 2). It has been already pointed out that flint germplasm, compared with dent, represents a source of early maturity for the development of short season inbreds and hybrids (Hunter et al., 1979). The GCA of EP42 was significantly lower than those of the other lines, including A632, for kernel moisture at harvest (Table 2). However, Moreno-González et al. (1997) found that dent germplasm, represented by A632, transmitted a faster rate of kernel dry-down to the F1 progeny compared with flint germplasm, even though crosses to flint lines were earlier flowering. It is commonly believed that flints are slower in grain dry-down than dents. However, Hunter et al. (1979) found no evidence for dry-down differences in near-isogenic hybrids differing in flintness. These authors concluded that dry-down rates in the field did not depend on the endosperm type, but other factors such as husk number, tightness and length, and pericarp thickness could also affect field dry-down.


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  		Table 2. Significant general combining ability (GCA) effects for four testers evaluated in four environments in northwestern Spain.

 
Our results showed that the flint varieties studied were earlier and had less percentage of kernel moisture at harvest in crosses to flint maize compared with dent, although these results should be taken cautiously because there was only one flint line and three dent lines. Improvement of flint varieties for combining ability with the inbred line EP42 could be done to obtain earlier flint x flint hybrids with a better dry-down rate than flint x dent hybrids. These hybrids could be grown in short season areas such as the Atlantic coast of Europe.

Diallel Experiment
The source of variation for diallel entries was significant for all traits. Variety (vj) and heterosis effects (hij) were significant for all traits (Table 3). Variety effects also explained the highest proportion of the sum of squares due to the diallel entries for all traits. A previous report showed that the inheritance of early vigor involved additive and dominance effects (Revilla et al., 1999). Others authors have found that inheritances of days to flowering and kernel moisture at harvest were mainly due to variety effects (Eberhart, 1971; Genter and Eberhart, 1974; Misevic, 1989). The environments x diallel entries interaction was significant for kernel moisture at harvest. In general, interactions were the result of magnitude changes rather than rank order changes.


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  		Table 3. Mean squares from the analysis of variance of three traits for a diallel of 10 flint varieties grown in four environments in northwestern Spain.

 
Variety GA had the best vj for early vigor. Both GA and NO showed the best vj for earliness. The varieties GA, RE, and SI had the best vj for kernel moisture at harvest (Table 4). In short-season environments with cold and humid springs, early varieties with good early vigor can grow faster and reach the flowering stage faster, so ears can dry while the weather is still warm. The desired strategy is to harvest early at low kernel moisture content to reduce both grain drying costs and field losses (Cross, 1991). Low kernel moisture content makes shelling easier at harvest time and decreases harvest losses (Dijak et al., 1999). To start a breeding program to obtain flint hybrids, the best adapted varieties would be GA and NO, since they were the earliest and had good early vigor and low kernel moisture at harvest. That was an expected result, because both varieties were collected in northwestern Spain, where the trials were performed and they could be considered as good donors of adaptation traits for Atlantic coast conditions. Varieties TR and FI showed high values for days to silking and kernel moisture at harvest (Table 4). These are varieties from the southern Spain and showed a lower level of adaptation.


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  		Table 4. Significant variety effects (vj) and variety heterosis (hj) for 10 flint varieties of maize evaluated in four environments in northwestern Spain.

 
Although variety effects were the most important in the inheritance of the adaptive traits, heterosis was also significant and, therefore, dominance effects were also important. Average heterosis was significant for early vigor (Table 3). The F1 crosses had 11% more vigor than midparents. The source of variation hj was significant for days to silking and kernel moisture at harvest (Table 3). The variety NL had the best hj for days to silking. The variety LO had a significant and low hj for kernel moisture at harvest (Table 4). Varieties GA and NO, besides having large variety effects for adaptive traits, also showed intermediate hj values for days to silking and kernel moisture at harvest, although they were not significantly different from others (Table 4).

The source of variation due to sij was significant for days to silking (Table 3). The NO x RE cross showed the lowest sij for days to silking, although this value was not significantly different from most of the crosses. Crosses RE x SI, GA x NO, and BA/EL x TR showed low values of sij for days to silking, although they were not significantly different from other crosses (data not shown). These are crosses between varieties from similar geographical regions, so it seems that the low level of heterosis found between them was due to the low level of genetic divergence between these varieties.

The crosses GA x BA/EL and BA/EL x LO had been chosen as new heterotic patterns to obtain highly productive hybrids (Soengas et al., 2003a). Crosses GA x BA/EL and BA/EL x LO showed intermediate sij values for early vigor and days to silking, indicating intermediate adaptation to northwestern Spain, and negative values of sij for days to silking, indicating that earliness in these crosses is conditioned by dominance (data not shown). GA showed good values of vj and hj for all adaptive traits, whereas BA/EL and LO were intermediate (Table 4). The cross GA x BA/EL would be a better choice than BA/EL x LO. Thus, flint hybrids to be grown in short season areas could be obtained following this heterotic pattern combining good yield (Soengas et al., 2003a) and good adaptation.


   ACKNOWLEDGMENTS
 
Pilar Soengas and Bernardo Ordás acknowledge a fellowship from the Xunta de Galicia and from the Ministry of Sciences and Technology of Spain, respectively. Work was done thanks to Project Cod. AGL01-3946 of the Ministry of Sciences and Technology and Excma. Diputación Provincial de Pontevedra, Spain.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Research supported by the Committee for Science and Technology of Spain (Project Cod. AGL01-3946) and Excma. Diputación Provincial de Pontevedra, Spain.

Received for publication April 10, 2006.


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





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