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Crop Science 43:797-806 (2003)
© 2003 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Genetic Diversity, Specific Combining Ability, and Heterosis in Tropical Maize under Stress and Nonstress Environments

F. J. Betrán*,a, J. M. Ribautb, D. Beckb and D. Gonzalez de Leónc

a Corn Breeding and Genetics Program, Texas A&M University, College Station, TX 77845
b International Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, 06600 México D.F., México
c Paseo del Atardecer 360, Villas de Irapuato, 36670 Irapuato, México

* Corresponding author (javier-betran{at}tamu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Estimation of genetic diversity and distance among tropical maize (Zea mays L.) lines and the correlation between genetic distance (GD) and hybrid performance would determine breeding strategies, classify inbred lines, define heterotic groups, and predict future hybrid performance. The objectives of this study were to estimate (i) heterosis and specific combining ability (SCA) for grain yield under stress and non-stress environments; (ii) genetic diversity for restriction fragment length polymorphisms (RFLPs) within a set of tropical lines; (iii) GD and classify the lines according to their GD; and (iv) correlation between the GD and hybrid performance, heterosis, and SCA. Seventeen lowland, white tropical inbred lines were represented in a diallel study. Inbred lines and hybrids were evaluated in 12 stress and nonstress environments. The expression of heterosis was greater under drought stress and smaller under low N environments than under nonstress environments. A set of DNA markers identifying 81 loci was used to fingerprint the 17 lines. The level of genetic diversity was high, with 4.65 alleles/locus and polymorphism information content (PIC) values ranging from 0.11 to 0.82. Genomic regions with quantitative trait loci (QTL) for drought tolerance previously identified showed lower genetic diversity. Genetic distance based on RFLP marker data classified the inbred lines in accordance with their pedigree. Positive correlation was found between GD and F1 performance (F1), SCA, midparent heterosis (MPH) and high-parent heterosis (HPH). Specific combining ability had the strongest correlation with GD. Environment significantly affected the correlations between F1, SCA, MPH, and HPH, with lower values of GD revealed in the more stressed conditions.

Abbreviations: ASI, anthesis-silking interval • CML, CIMMYT maize line • DT, drought tolerance • GD, genetic distance • HPH, high-parent heterosis • IS, intermediate stress • LP, ‘La Posta Sequía’ • MPH, midparent heterosis • PCA, principal component analyses • PIC, polymorphism information content • QTL, quantitative trail locus(i) • RFLP, restriction fragment length polymorphism • SCA, specific combining ability • SGD, specific genetic distances • SS, Severe stress • TS, ‘Tuxpeño Sequía’ • WW, well watered


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE IDENTIFICATION of parental inbred lines that form superior hybrids is the most costly and time-consuming phase in maize hybrid development. Per se performance of maize inbred lines does not predict the performance of maize hybrids for grain yield (Hallauer and Miranda, 1988). Predictors of single-cross hybrid value or heterosis between parental inbred lines could therefore increase the efficiency of hybrid breeding programs. The relationship between GD and heterosis was reported before the development of genetic markers (Moll et al., 1965). Genetic linkage maps constructed by means of DNA markers are available in maize (Coe et al., 1995). Molecular DNA markers have been used to analyze the genetic relationships among maize inbred lines and to examine the relationship between DNA marker-based GD and single-cross grain yields in temperate maize (Stuber, 1989; Lee et al., 1989; Smith et al., 1990; Godshalk et al., 1990; Boppenmeier et al., 1992; Melchinger, 1993). Genetic distance has been used to predict hybrid performance and the efficiency of prediction was greater with crosses between inbred lines from the same heterotic group than in crosses between inbred lines from different heterotic groups (Melchinger, 1999). Linkage disequilibrium between DNA markers and genes involved in the expression of target traits is required for GD and hybrid performance to be correlated. The effect of the population structure on the relationship between genetic distance and heterosis was described by Charcosset and Essioux (1994).

Tropical maize is grown on approximately 45 million ha in lowland tropical environments (Pingali, 2001). Although hybrid development in tropical maize started in the 1940s, the sustainability and adoption has been variable (Vasal et al., 1999). The expanding utilization of hybrids and inbred-line-based synthetics in tropical areas has substantially increased the number of tropical inbred lines (Beck et al., 1997) developed from landraces, populations, and synthetics by pedigree breeding (Hallauer, 1990). Several racial complexes have been preferentially used as source germplasm for hybrid development including ‘Tuxpeño’, ‘Cuban’ and ‘Coastal’ tropical flints, ‘Tuson’, and ‘ETO’. Promising heterotic patterns have been detected and developed as a result of increasing characterization of maize germplasm and development of inbred lines for heterotic response (Goodman, 1985). Tropical maize, however, has a broad genetic base and shows greater genetic diversity than temperate maize. Therefore, the estimation and organization of genetic diversity in tropical maize on the basis of DNA markers would assist in determining efficient breeding strategies.

The degree of heterosis depends on the relative performance of inbred parents and the corresponding hybrids. Environment can differentially affect the performance of inbred lines and hybrids, altering the relationship between GD and heterosis. In the tropics, limited and erratic supply of water and nutrients create contrasting environments. The influence of abiotic stresses on the use of GD as a predictor of hybrid performance is poorly understood.

The objectives of this study were (i) to estimate heterosis and SCA for grain yield under stress and nonstress environments in a diallel among a set of tropical, white-grained maize inbreds; (ii) to determine the genetic diversity of this germplasm by means of RFLP markers; (iii) to calculate RFLP-based genetic distance and classify the lines according to their genetic distance; and (iv) to estimate the correlation between the genetic distance among the inbreds and hybrid performance, heterosis, and SCA under stress and nonstress environments.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Seventeen lowland white-grained tropical maize inbred lines were used in a diallel study (Table 1). The inbred lines and the selection criteria applied during their development are described by Betrán et al. (2003). These lines represent a range of tolerance/susceptibility to abiotic stresses. Nine inbred lines are first cycle advanced lines developed from populations improved for drought tolerance by recurrent selection in stressed environments (Edmeades et al., 1997). Five of the nine were from La Posta Sequía C3 (LP) and the remaining four were from Tuxpeño Sequía 6 C1 (TS). The seven CMLs (CIMMYT Maize Lines) were selected on the basis of their combining ability and agronomic performance across environments with standard agronomic management. The experimental line AC7643 (P1) is the drought-tolerant parent represented in a segregating population for the genetic dissection of target morphological traits [e.g., anthesis-silking interval (ASI)] and yield components under water-limited conditions (Ribaut et al., 1996, 1997). The 17-line set included three pairs of sister lines LP4 and LP5, TS2 and TS5, and CML273 and CML274. Seed from reciprocal crosses among the 17 inbreds was bulked to form one set of 136 F1 hybrids.


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  		Table 1. Maize inbred lines involved in the study, their pedigree and the criteria of selection in their development.

 
Evaluation and Stress Management
Lines per se and their hybrids were evaluated separately in experiments planted side by side under drought stress, low N, and optimal conditions in 12 environments (Table 2). Experimental designs were {alpha} (0,1) lattice designs (Patterson and Williams, 1976). Hybrids and lines were oversown with two seeds per hill every 20 cm (33 cm in Obregón, OB96BISHS, and 40 cm in the 1997 low-density trial at Tlatizapán, TL97AISLD) in rows 75 cm apart and thinned to the desired plant densities. All trials received standard cultural practices to control insects and weeds. Six environments were conducted under optimal fertilization and supplemental irrigation as needed to avoid moisture stress. Simultaneous and contiguous experiments were conducted during the 1996 winter season at Tlaltizapán, Morelos, México (18°41' N, 940 m above sea level) under three water regimes: (i) TL96AWW—well watered (normal conditions with approximately one irrigation every 10 d); (ii) TL96AIS—intermediate drought stress (no water applied from two weeks before silking to the end of the flowering period); and (iii) TL96ASS—severe drought stress (no water applied from four weeks before silking to the end of the flowering period).


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  		Table 2. Characteristics of the environments used to evaluate the performance of tropical inbred lines and hybrids, and average grain yield across per and across environment for lines and hybrids.

 
This location in the off season or winter is consistently free of rainfall during the vegetative and reproductive stages, allowing the control of stress level by withdrawing or delaying irrigation during flowering and grain filling (Edmeades et al., 1999). During the 1997 winter season, experiments were conducted under intermediate drought stress (IS) and two densities (TL97AISHD and TL97AISLD). The low-N (LN) evaluation, PR96BLN, was conducted at Poza Rica, Veracruz, México (21°N, 60 m above sea level) in a field with reduced natural N levels achieved by continuous cropping of maize and with no N fertilizer applied. In addition, sorghum [Sorghum bicolor (L.) Moench] intercropping between the maize rows was used to increase stress. An adjacent high-N (HN) experiment, PR96BHN, was conducted for both inbred lines and hybrids. Experimental management for the trials conducted in the same locations was the same across irrigation and N levels. Hybrids and parental inbreds were evaluated as well under the summer heat and drought stress at Cd. Obregón (OB96BISHS) in the Sonoran Desert, México (27°30' N, 30 m above sea level), and at Cotaxtla (CO97AIS), Veracruz, México (18°50' N, 60 m above sea level) during both the winter and summer seasons.

Plot ear weight was measured in the field with a manual scale in experiments CO96AWW, OB96BISHS, and CO97BWW. Grain yield was calculated on the basis of a shelling percentage of 0.80. For the rest of the experiments, ears were dried to constant grain moisture, weighed, and shelled. Grain yield was adjusted to 155 g kg–1 moisture. In experiments with severe drought and low-N stress, the plants nearest to the alley showed more vigor and bigger ears than plants within the row with competition. Only competitive plants were harvested.

RFLP Analyses
Maize genomic DNA was extracted from the 17 tropical inbred lines used in this study. DNA was purified, quantified, digested with one of two restriction enzymes (EcoRI or HindIII), separated in agarose gels (0.7%, w/v) and transferred to nylon membranes (MSI Magnagraph, Fisher Scientific) by Southern blotting. Labeled probes (digoxigenin-dUTP) were used to detect polymorphism with the antidigoxigenin-alkaline phosphatase-AMPPD chemiluminescent reaction. Details of these protocols are given in Hoisington et al. (1994). A set of 55 RFLP probes spread across the genome [from the University of Missouri-Columbia (UMC), Brookhaven National Laboratory (BNL) and Native Plants Incorporated (NPI)], were used to screen the plant material at 81 loci. Restriction fragment length polymorphisms patterns were recorded for each probe–enzyme–line combination by assigning a 1 to the presence of the band and 0 to the absence.

Statistical Analysis
Individual analyses of variance were performed for each experiment and stress level with the PROC MIXED procedure from SAS (SAS Institute Inc., 1997). Genotypes were considered fixed effects. The adjusted means [when the incomplete block design was more effective than a randomized complete block design (RCBD)] or unadjusted means (when the RCBD was more effective) were used to make subsequent calculations and to estimate SCA effects. Under low N stress, where the variation of N available in the soil can increase experimental error, grain yield was adjusted on the basis of chlorophyll content (measured as concentration in the ear leaf of five plants per plot 2 to 3 wk after flowering with a portable photometer) as a covariate.

Griffing's Method 4 Diallel analysis (Griffing, 1956) was used to estimate SCA effects for the hybrids in all environments and across environments. Midparent heterosis was calculated as

where, F1 is the mean of the F1 hybrid performance and MP = (P1 + P2)/2 in which P1 and P2 are the means of the inbred parents, respectively.

High-parent heterosis was calculated as

where HP is the mean of the best parent.

Average number of alleles per locus was calculated for all the probe-enzyme combinations. Variability for each locus was estimated PIC (Anderson et al., 1993):

where p2i is the frequency of the ith allele in a locus with I alleles.

Different classes of loci were established on the basis of the number of alleles detected per locus. Average and minimum values for PIC were computed for each class of loci. Loci presenting the lowest PIC values per given class were allocated to their corresponding bin on the Maize Database genetic map by the "Name Descriptor Lookups" option (http://www.agron.missouri.edu/query2.html; verified 14 November 2002).

Genetic distances were calculated between pairs of lines on the basis of the method developed by Nei and Li (1979)

where Nij is the number of bands common to lines i and j, and Ni and Nj are the total number of bands for lines i and j, respectively. Genetic distance may range between 0 (all bands in common) to 1 (no bands in common). Principal component analyses (PCA) and cluster analysis, using the average linkage method algorithm (Romesburg, 1984), were computed on the estimated GD.

Specific genetic distances (SGD) were calculated according to Griffing's model I of method 4 (Griffing, 1956) as proposed by Melchinger et al. (1990)

where GGD is the general genetic distance and SGD is the specific genetic distance.

Pearson correlation coefficients (r) between GD and single cross grain yield (F1), MP, HP, MPH, HPH, and SCA were calculated from means per environment and across environments. Specific genetic distance was correlated with SCA and F1. Statistical computations were performed with SAS (SAS Institute Inc., 1997).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Grain Yield, SCA, and Heterosis
Hybrid grain yields averaged 6.01 Mg ha-1 and ranged from 1.14 Mg ha-1 at TL96ASS to 9.18 Mg ha-1 at TL96BWW (Table 2). Grain yield for inbred lines averaged 2.27 Mg ha-1 and ranged from 0.15 Mg ha-1 at TL96ASS to 3.95 Mg ha-1 at PR96AWW. Genotype and genotype x environment interaction effects were significant in the analysis of variance for grain yield of both hybrids and inbred lines (Betrán et al., 2003). Substantial differences were observed for grain yield between the managed stress evaluations simultaneously planted at the same location. At Tlaltizapán during the 1996 winter season, grain yield for hybrids grown in severe (TL96ASS) and intermediate (TL96AIS) drought stress were 13 and 50% of grain yield under the well-watered regime (TL96AWW), respectively. Grain yield for the lines per se in severe (TL96ASS) and intermediate (TL96AIS) drought stress during the same season were 5 and 48% of grain yield in the well-watered regime (TL96AWW), respectively. At Poza Rica during the 1996 summer season, grain yield for hybrids in the low N environment (PR96BLN) was 33% of grain yield in the high N environment (PR96BHN), while grain yield for inbred lines per se in the low N environment (PR96BLN) was 65% of grain yield in the high N environment (PR96BHN).

Specific combining ability for grain yield was significant (Mean square = 14.74**), while its interaction with environments (SCA x E) was not significant (Mean square = 1.12). Specific combining ability effects ranged from 1.18 Mg ha-1 for the LP1 x TS2 hybrid to -3.78 for the hybrid between sister lines LP4 and LP5 (Table 3). Specific combining ability across environments was negative for hybrids involving inbred lines with the same germplasm origin (e.g., within La Posta Sequía and Tuxpeño Sequía groups) or related by pedigree (e.g., between CML273 and CML 274) and greater for hybrids involving inbred lines of different source germplasm origin (e.g., between drought tolerant inbred lines and CMLs). Midparent heterosis ranged from –37 for the hybrid between sister lines CML273 and CML274 to 290 for the hybrid between CML268 and TS5. The magnitude of MPH across environments was smaller for La Posta Sequía inbred lines, which had greater grain yield per se than Tuxpeño Sequía inbred lines, CMLs, or P1 (Table 3).


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  		Table 3. Midparent heterosis (percent; above the diagonal) and specific combining ability (SCA) for grain yield (Mg ha-1) of single crosses among 17 tropical maize inbred lines across environments.

 
The average degree of MPH and HPH per environment varied from 34 and 8 in the low N environment (PR96BLN) to 2225 and 1225 in the severe drought stress environment (TL96ASS) (Table 4). The experiments at TL96ASS and OB96BISHS had extremely high expression of heterosis because of the poor performance of inbreds under severe drought stress and the hot and dry conditions of the Sonoran Desert in the north of México. Differences in grain yield between hybrids and inbred lines (e.g., heterosis) increased with the intensity of drought stress. As a consequence, average MPH and HPH at the same location were much greater in the severe drought stress environment TL96ASS (2225 and 1225, respectively) when compared with the well-watered conditions TL96AWW (240 and 155, respectively). Grain yield of inbred lines decreased relatively more in drought stress when compared with low-N stress. Heterosis in the high-N fertilization (MPH = 157 and HPH = 126) was larger in comparison with the low N (MPH = 34 and HPH = 8), where the performance of hybrids was relatively poor. Heterosis in the drought environments was much larger compared with the one low N environment stresses. The best yielding hybrids across environments, TS4 x LP2 and CML258 x LP5, had SCA values of 0.889** and 0.948**, respectively, and MPH values of 185 and 138, respectively.


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  		Table 4. Average midparent heterosis (MPH) and high-parent heterosis (HPH), and correlation among F1 grain yield, MPH, HPH, and specific combining ability (SCA) for all the hybrids among 17 tropical maize inbred lines within and across environments.

 
Grain yield for hybrids was positively correlated with MP, HP, MPH, HPH, and SCA across environments (Table 4). The highest correlation was observed between grain yield and SCA across environments. This positive correlation was consistent for single environments being greater than 0.56 in every case. The r (F1, SCA) across environments was 0.75, double that of the r(F1,MPH) with 0.34 and r(F1,HPH) with 0.21. This is an indication that the SCA among parental lines can predict hybrid performance better than the heterosis observed, which is highly dependent on the performance of inbred lines. Furthermore, as a consequence of the differential response of inbred lines to stresses and environmental conditions relative to hybrids, the correlations that involved parent performance (MP, HP, MPH, HPH) were more erratic and inconsistent across environments than were the correlations with SCA. The correlations between inbred line and hybrid for grain yield across environments were 0.32 for MP and 0.34 for HP. Specific combining ability was positively correlated with midparent (r = 0.47) and high-parent (r = 0.31) heterosis across environments.

Genetic Diversity
From the 55 RFLP probes, 35 identified one locus, 14 identified two loci, and six identified three loci (Table 5). Among the 81 loci, a total of 377 different alleles were identified with a mean value of about 4.65 alleles per locus. The average number of alleles per locus was similar to previous studies with temperate maize that reported 4.2 (Melchinger et al., 1991), 3.7 (Livini et al., 1992), 4.0 (Messmer et al., 1992), 4.6 (Burstin et al., 1994), 5.9 (Dubreuil et al., 1996), and 4.9 (Lu and Bernardo, 2001) alleles per locus. However, this average number of alleles per locus might not be representative of the allelic diversity present in tropical germplasm because of the small number of genotypes used in this experiment, with five inbred lines within La Posta Sequía and four inbred lines within Tuxpeño Sequía. In a diversity study conducted at CIMMYT on 218 maize inbred lines using RFLP probes that identified 34 loci, the average number of alleles per locus was 9.2 (Franco et al., 2001). In this study, the majority of loci had between three and six alleles with 12 loci showing more than six alleles. The average PIC value ranged from 0.28 for the class of loci with two alleles to 0.82 for loci with nine alleles. The minimum PIC value was 0.11 for the RFLP probes BNL7.49 and umc138. Loci with fewer alleles had more alleles with a frequency greater than 50%. Restriction fragment length polymorphism probes with the lowest PIC value for each loci class were identified and located on the maize genetic map (Table 6). Loci showing the lowest PIC values could be associated with stronger selection pressure during the improvement of the source populations and inbred line development for drought tolerance (LP and TS inbred lines). Some of the markers showing less diversity in this group of inbred lines mapped in the same genomic regions. All the loci reported in the Maize DB for the 11 RFLP probes presenting the lowest PIC value are located in positions (homologous regions) that do not map randomly along the genome, but are grouped in specific chromosomal segments (bins 3.08–3.09, 5.03–5.04, 6.06–6.08, and 8.04–8.06). For example, five out of the 11 RFLP probes identified one locus at bin 8.04 or 8.06. Except for the 5.03 to 5.04 region, the three other genomic regions are associated with QTLs for grain yield or adaptive traits in previous QTL mapping studies conducted with tropical maize (Ribaut et al., 2002). This suggests that selection pressure and/or genetic drift associated with selection are especially strong on QTL regions associated with the maize plant's response in a water-limited environment, reducing the diversity present in these loci more than in neutral loci.


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  		Table 5. Characterization of eight different classes of DNA markers based on the number of alleles they identified including allele number and loci per class, the mean and the minimum polymorphism information content (PIC) value and the number of alleles per class with a frequency greater than 50%.

 

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  		Table 6. Bin location of the loci having the lowest polymorphism information content (PIC) value (underlined) within each of the different classes presented in Table 5. For each RFLP probe, the different loci based on the Maize DB maps (http://www.agron.Missouri.edu/query2.html) are also presented. The letter in parenthesis indicates the locus following the Maize DB locus nomenclature and (c) indicates that this locus has not been reported in the Maize DB, but has been identified in one of CIMMYT's linkage maps.

 
Although some of the drought tolerant inbred lines have the same origin, this set of tropical lines remains genetically diverse as expected from their origin and the different selection criteria during their development (Table 1). This is in agreement with the phenotypic variation observed in several traits such as maturity, plant height, leaf architecture, tassel size, grain yield and yield components, and grain and kernel traits measured in these inbred lines per se and in hybrid combination (F.J. Betrán, D. Beck, M. Bänziger, and G.O. Edmeades, 2002, unpublished data).

Genetic Distance among Inbred Lines
The average genetic distance among inbred lines was 0.61 with a range from 0.20 to 0.84. P1 was the most distant line with an average GD of 0.72. Most of the GDs were between 0.45 and 0.85. Sister lines (TS2 and TS5, LP4 and LP5, CML273 and CML274) were easily identified and had a GD below 0.25. The inbred lines CML247 and P1 had the greatest GD with 0.84. These two lines have been used to develop a segregating population for mapping QTLs for grain yield and anthesis silking interval, and to subsequently improve CML247 through backcross marker-assisted selection (Ribaut et al., 2002). Principal components analysis (Fig. 1) and cluster analyses (data not shown) similarly classify the inbred lines in accordance with their origin and pedigree information. As expected, inbred lines with different origins were genetically more dissimilar than inbred lines originating from the same germplasm. La Posta Sequía lines (LP#), Tuxpeño Sequía lines (TS#), and sister lines were grouped together. The correspondence between GD and the origin and pedigree of these tropical inbred lines is consistent with other studies addressing the classification of temperate inbred lines (Mumm and Dudley, 1994; Burstin et al., 1994; Dubreuil et al., 1996). Our results indicate that the use of DNA markers represents a suitable tool to classify tropical maize inbreds developed from genetically diverse germplasm groups.



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  		Fig. 1. Association of 17 tropical white maize inbred lines as revealed by principal component analysis (FAC) on genetic distances.

 
Correlation between Genetic Distance and Hybrid Performance
Genetic distance was positively correlated with F1, SCA, MPH, and HPH in all the environments when including all the hybrids (Table 7). Genetic distance was not correlated with MP or HP. The three hybrids obtained by crossing sister lines (GD < 0.25) had a large influence on the size of the correlations. Elimination of these three hybrids resulted in a significant decrease in the observed correlation (Table 7, Fig. 2a,b). Consequently, the correlation between GD and SCA across environments [r(SCA,GD)] decreased from 0.64 to 0.31, from 0.53 to 0.17 for r(GD,F1), from 0.41 to 0.14 for r(MPH,GD), and from 0.28 to 0.05 for r(HPH,GD). Similar results have been found in temperate maize, where genetic distance was correlated with hybrid performance for related lines or lines from the same heterotic group. Genetic distance was not correlated with hybrid performance for crosses between lines from different heterotic groups (Godshalk et al., 1990; Boppenmeier et al., 1992; Melchinger, 1993; Ajmone Marsan et al., 1998). Charcosset and Essioux (1994) demonstrated that differences in linkage disequilibrium among markers and QTLs between heterotic groups could explain the lack of correlation. Bernardo (1992) concluded that at least 30 to 50% of the QTLs affecting the trait (i.e., grain yield) should be linked to DNA markers to effectively predict hybrid performance with DNA marker heterozygosity. The exclusion of P1, the most distant inbred, increased the correlations between GD and F1 and SCA (Table 7). Correlation between GD and SCA increased from 0.31 to 0.45 and between GD and F1 from 0.17 to 0.38. Moll et al. (1965) also found that heterosis decreased when diversity was excessively high. This suggests that, in general, the predictability of hybrid performance seems to be better when genetic distance is smaller than a certain threshold, depending on the germplasm under consideration.


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  		Table 7. Correlation between genetic distance (GD) and specific genetic distance (SGD) with F1 grain yield (F1), midparent value (MP), high-parent value (HP), specific combining ability (SCA), midparent heterosis (MPH) and high-parent heterosis (HPH) per environment and across environments considering three cases: (i) all the single crosses (ALL), (ii) excluding crosses between sister lines (WS), and (iii) excluding crosses between sisters lines and crosses with P1 (WSP1)

 


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  		Fig. 2. Regression of SCA on GD (a), of F1 on GD (b), and SCA on GD (c) excluding crosses between sister lines (solid line) and P1 crosses (dashed line) across environments.

 
The correlations of SGD with F1 and SCA were greater than the correlation of GD with F1 and SCA (Table 7, Fig. 2c). Specific genetic distance had a positive correlation with SCA of 0.80 including hybrids among sister lines and 0.58 without them. Negative SGDs were observed for crosses between related lines, which indicates that these crosses were less heterozygous than expected by the average heterozygosity of the parents in combination with other lines in the diallel (Melchinger et al., 1990).

The comparison of correlation coefficients for those experiments in the same environment with different stress levels indicates that the correlations of GD with F1, SCA, MPH, and HPH increased when the drought-stress levels decreased. These correlations were more similar within different levels of N stress. The different environments changed the degree of correlation, but not the sign. With better yielding environments, the correlation increases. Grain yield and the proportion of SCA variance increased under nonstress conditions, perhaps indicating a better expression of effects related to heterosis. Under drought-stress conditions, the GCA effects are more important and the correlation between GD and hybrid performance decreases (Betrán et al., 1997). Burstin et al. (1994) also found that the relative proportion of SCA variance was an indicator of the degree of hybrid performance prediction by GD of parental inbreds.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Heterosis can be affected by the environment, which can have a differential effect in parental inbreds and hybrids. In a set of 17 tropical lines, heterosis was greater under drought stress and smaller under low N conditions than under nonstress conditions. The observed differences suggest the need for additional research in how abiotic stresses affect the expression of heterosis. The degree of inbreeding of the parental lines could affect their response to stresses and subsequently the amount of heterosis. Therefore, lines with different degree of inbreeding and corresponding hybrids should be tested under managed stressed and optimal environments to estimate heterosis. Specific combining ability, which is not affected by parental inbred performance, had better predictive value for F1 grain yield than heterosis. For breeding programs devoted to develop hybrids, best single-cross combinations should be selected on the basis of maximizing hybrid performance in the target environments.

The level of genetic diversity in this set of tropical maize inbreds was relatively high with 4.65 alleles/locus and PICs ranging from 0.11 to 0.82. Genomic regions with QTLs for drought tolerance previously identified showed lower genetic diversity suggesting that same alleles could have been favored during selection for drought tolerance in LP and TS lines. The selection pressure across the genome applied during the development of stress tolerant lines and populations, could be different on the basis of the presence or absence of genes involved in stress tolerance.

Genetic distance based on RFLP marker data classified the tropical inbreds in agreement with their origin and pedigree information. Similar results have been found in previous studies with temperate maize and indicate the ability of DNA markers to assess GD and classify tropical maize inbreds. As the grouping of tropical lines in heterotic groups is not as well defined as in temperate maize, or is in initial stages, DNA markers could assist to establish initial heterotic groups based on GD avoiding crosses among related inbreds. The heterosis among groups could be enhance by recycling and selecting lines within groups based on F1 performance when crossed with lines from other groups.

Positive correlations were found between GD and F1, SCA, MPH, and HPH in all the environments, but it was highly affected by including sister-line hybrids. Specific combining ability had the strongest correlation with GD, especially when considering SGD. Environment significantly affected the correlations with GD with lower values observed under more stressed conditions. In stress environments genetic variance is reduced and additive effects can be more important than nonadditive effects. Optimal nonstress environments where grain yield is maximized could be more appropriate to measure SCA effects and the predictive value of GD.


   ACKNOWLEDGMENTS
 
We thank Srs. Ezequiel Bahena, Oscar Fernandez, Fernando Gonzalez, and their assistants who conducted the field trials in a dedicated and careful manner. Thanks to Ing. Ciro Sánchez for his assistance in the execution of seed preparation and field activities. Thanks to all the personnel at the CIMMYT research stations in Tlaltizapan and Poza Rica, and at INIFAP research stations in Obregón and Cotaxtla. We also thank Greg Edmeades and Marianne Bänziger for providing guidance and advice. This research was partly funded by the United Nations Development Program (UNDP).

Received for publication July 3, 2002.


   REFERENCES
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 ABSTRACT
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 RESULTS AND DISCUSSION
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