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Published in Crop Sci. 44:274-282 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

GENOMICS, MOLECULAR GENETICS & BIOTECHNOLOGY

QTL Mapping of Resistance to Stalk Tunneling by the European Corn Borer in RILs of Maize Population B73 x De811

M. D. Krakowsky*, M. Lee, W. L. Woodman-Clikeman, M. J. Long and N. Sharopova

Dep. of Agronomy, 100 Osborn Dr., Iowa State Univ., Ames, IA 50011

* Corresponding author (mkrakowsky{at}tifton.usda.gov).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Considerable effort has been expended toward the genetic characterization of native resistance to stalk tunneling by the European corn borer [ECB; Ostrinia nubilalis (Hübner)], indicative of the importance of this pest and the difficulty in obtaining conclusive results. In this study, 191 recombinant inbred lines (RILs) of B73 maize (Zea mays L.) (susceptible to stalk tunneling by ECB) x De811 (resistant) were evaluated for stalk tunneling, anthesis, and plant height in Iowa at two locations in 1998 and one location in 1999 with the objectives of (i) determining the genetic relationship between these traits and (ii) mapping quantitative trait loci (QTL) associated with resistance to stalk tunneling. The genotypic correlation between plant height and stalk tunneling (rg = 0.1) was negligible, but the correlation between stalk tunneling and anthesis was very high (rg = –0.8) necessitating the adjustment of the means of former with the latter. Ten QTL for stalk tunneling adjusted for anthesis associated with 42% of the phenotypic variation were observed in the mean across trials, only one of which was observed in each of the individual trials. The lack of consistent QTL detection across environments is a common characteristic among studies of ECB tunneling and underscores a major problem of breeding for resistance. QTL observed in F3 lines of the same cross and in RILs of B73 x B52 are linked to three QTL each for the mean across trials herein, providing further evidence of association between these genomic regions and resistance to stalk tunneling.

Abbreviations: AIC, Aikake Information Criteria • cM, centimorgan • ECB, European corn borer • GDD, growing degree-day • LOD, log of the odds ratio • QTL, quantitative trait loci • RFLP, restriction fragment length polymorphism • RIL, recombinant inbred line • SCB, sugarcane borer • SSR, simple sequence repeat • SWCB, Southwestern corn borer


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE EUROPEAN CORN BORER is an important pest in temperate maize production. In the U.S. Cornbelt, ECB usually completes two generations in a growing season, with the first generation feeding primarily on leaf tissue (leaf-feeding generation) and the second feeding on leaf sheath and stalk tissues (stalk-tunneling generation). Damage from ECB stalk tunneling results in broken stalks, dropped ears, and, most significant in terms of yield loss, poor ear development. While germplasm with resistance to leaf feeding is readily available, resistance to stalk tunneling has been difficult to find (Mason et al., 1996).

For almost 50 yr, researchers have worked to understand the genetic basis of resistance to stalk tunneling and incorporate said resistance into germplasm with other agronomically desirable traits (e.g., yield). Several inbred lines with resistance to stalk tunneling have been developed, including B52, De811, and Mo47 (Russell et al., 1971; Hawk, 1985; Barry et al., 1995). While high heritabilities (0.63–0.78; Schön et al., 1993; Jampatong et al., 2002; Cardinal et al., 2001; Krakowsky et al., 2002) for resistance to stalk tunneling have been reported, identification of the underlying genetic components has been hindered by environmental variation, a laborious and lengthy screening process, and the polygenic nature of the trait. Recent efforts in crop biotechnology to overcome some of these problems have succeeded in incorporating monogenic resistance into maize hybrids. While the new hybrids can protect against economic losses from feeding by ECB larvae, the durability of this type of resistance is unknown. Exposure of a pest to a host that significantly reduces the pest's relative fitness can result in selection in the pest population for individuals that can overcome the resistance factor (Gould, 1986). Laboratory selection studies have been successful in selecting for resistance to Bacillus thuringiensis in a few agronomically important insect pests, foreshadowing the possibility that insect populations could become resistant in nature (McGaughey and Whalon, 1992). For this reason, as well as concern over the use of genetically modified crops for human consumption, it is necessary to continue the effort to characterize the genes involved in native resistance.

While there are disadvantages to using RILs for mapping quantitative trait loci (QTL), such as the additional time needed to develop the lines and the inability to measure dominance effects of QTL, it can be argued that these problems are balanced by the advantages, which have been described by several authors (Burr et al., 1988; Cowen, 1988; Lander and Botstein, 1989; Knapp and Bridges, 1990) and reviewed in some detail by Austin and Lee (1996). Briefly, the expected ratio of parental genotypes is 1:1, and this increased replication of homozygous parental genotypes results in increased power for testing differences between genotypic classes. Also, the reduced genetic variation within lines allows for greater precision of trait measurement and the additional recombination between linked loci should allow for better resolution of linked QTL. Only one QTL for resistance to stalk tunneling with significant dominance effects was observed in the F3 lines of B73 x De811 (Krakowsky et al., 2002), indicating that dominance is probably not important in the expression of resistance to stalk tunneling in this population and that the inability to detect dominance effects in RILs should not be of great concern herein.

The objectives of this study were to map genetic factors for resistance to stalk tunneling by ECB in RILs of B73 x De811, to compare their locations with those of F3 lines from the same population and with other populations, and to assess the genetic correlations between stalk tunneling and anthesis and plant height to determine if these traits have confounding effects on the evaluation of stalk tunneling, the possibility of which has been reported (Dicke, 1954; Coors, 1987; Schön et al., 1993). One hundred nine of the RILs used herein were derived from F3 lines used in a previous study (Krakowsky et al., 2002), enhancing comparisons between the two studies.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials
The RILs were derived from a cross of inbreds B73 and De811. One F1 plant (B73 x De811) was self-pollinated to produce F2 individuals that were advanced by the single-seed descent method to the F6 generation. Inbred B73 is widely used in temperate maize breeding programs but is highly susceptible to stalk tunneling by ECB (Guthrie et al., 1989). Inbred De811 exhibits high levels of resistance to tunneling in the stalk (Hawk, 1985).

Field Experiments
Four trials were planted in three environments: one each at the Agronomy and Agricultural Engineering Research Center (AAERC) near Ames, IA, and the Hinds Farm near Ames, IA, on 5 and 1 May 1998, respectively (AAERC 98 and Hinds Farm), and two on 20 May 1999 at the AAERC (AAERC 99A and 99B). Originally there were to be two different planting dates at the AAERC in 1999, but unfavorable weather compelled the planting of the two trials on the same date. Soil fertilization, weed control, and cultivation practices were consistent with optimum maize production for this region. The entries in each trial consisted of 200 RILs and five entries each of B73 and De811. Entries were evaluated in 3.8 m single row plots arranged in a 14 by 15 {alpha}-lattice design with two replications per trial.

Trait Evaluation
The evaluation of stalk tunneling has been described previously (Cardinal et al., 2001). Briefly, the first six plants in each plot were artificially infested with ECB larvae when 50% of the plots in a trial had reached anthesis. A plot reached anthesis when 50% of the plants in the plot were shedding pollen. Newly hatched larvae were obtained from the USDA Corn Insect Laboratory (Ames, IA). The larvae were applied at three infestation points: the primary ear leaf axil, the first leaf axil above the primary ear, and the first leaf axil below the primary ear. Larvae were applied to each plant over 3 to 4 d for a total of 300 larvae per plant. Approximately 60 d after infestation, the stalks were harvested from the field by removing the leaves, ears, and the two nodes below the tassel and cutting the stalk at soil level. The stalks were then split with a 36-cm (14-inch) electric band saw (R.L. Wilson, personal communication) and tunneling was recorded to the nearest centimeter for each of the six plants. Parallel tunnels were recorded as one.

Plots were also evaluated for growing degree-days (GDD) to anthesis and plant height to assess the correlation of these traits with stalk tunneling. GDD were calculated for each day from planting to anthesis, according to the formula [(max. °C + min. °C)/2] – 10°C, where 10°C was used for the minimum temperature and 30°C was used for the maximum temperature if the actual temperatures exceeded those limits (Cross and Zuber, 1972). Plant height was measured on the ECB infested plants in each plot in 1998 as the distance (to the nearest 5 cm) from the soil level to the first node below the tassel, and in 1999 as the length of the harvested stalks.

Analysis of Phenotypic Data
For each trait and entry, least square means (lsmeans) were calculated with complete and incomplete blocks as random effects and entries as fixed effects for each trial (Cardinal et al., 2001). Trials were also treated as random effects when calculating lsmeans across trials. Genotypic correlations (rg) were derived from the output of the MANOVA statement in PROC GLM in SAS, with entries and trials treated as random effects (SAS Institute, Inc., 1990). Adjustment of stalk tunneling for correlated traits was performed by including the correlated trait as a covariate in the model for calculation of lsmeans (Cochran and Cox, 1957). Genotype, genotype x environment, and error variances were calculated with trials, complete and incomplete blocks, entries, and the entry x trial interaction as random effects (Cardinal et al., 2001). The lsmeans calculated for the individual trials and the mean across trials were used for the QTL analysis. Broad-sense heritabilities on an entry-mean basis and their exact confidence intervals were calculated according to established procedures (Knapp et al., 1985; Fehr, 1987).

Detection of QTL
The protocols for DNA isolation, Southern hybridization, and collection of segregation data at RFLP loci have been described (Veldboom et al., 1994). One hundred and eight genomic and cDNA probes detected 113 RFLP loci. In addition, segregation data for 33 loci defined by simple sequence repeats (SSRs) were collected (Senior et al., 1996). RILs with nonparental alleles at more than 5% of the loci or heterozygotes at more than 10% of the loci were excluded from the analyses. Segregation distortion was tested by the chi-square test.

Linkage analysis was performed by MAPMAKER/EXP v. 3.0 (Lander et al., 1987). Loci were assigned to linkage groups by the program's default settings [minimum log10 of the likelihood odds ratio (LOD) score 3.0, maximum distance between loci of 50 centimorgans (cM)]. Multipoint analysis was performed by the "order" command (informativeness criteria of 160 individuals, 2 cM between loci). The 146 loci comprised a genetic map of 1551 cM with an average distance of 11.2 cM between loci.

QTL were detected by PlabQTL with cofactor selection performed as described (Utz and Melchinger, 1996, Austin et al., 2000). First the "cov select" option was used to select cofactors by stepwise regression. Outlier or influential observations, based on statistics calculated by PlabQTL, were eliminated from the data set. The LOD threshold value of 2.5 (default value) was used to declare the presence of a QTL. Previous reports suggest a LOD threshold value between 2 and 3 (Lander and Botstein, 1989) or a permutation test to calculate the LOD threshold value for a specified Type I error rate (the probability of detecting a false QTL; Churchill and Doerge, 1994). The LOD threshold value of 2.5 has been used in similar studies of QTL in maize (Lübberstedt et al., 1998; Cardinal et al., 2001; Krakowsky et al., 2002) and (i) allows for comparisons with the B73 x De811 F3 and B73 x B52 RIL populations (LOD > 2.5) and (ii) minimizes the risk of a Type II error (i.e., missing a QTL). The permutation test performed using PLABQTL calculated a LOD of >4.5 at the 5% level for a Type I error, but the goal of this study is to compare QTL in the RILs of B73 x De811 with those observed in other studies, and so the lower LOD value (2.5) will be used herein at the risk of increasing the number of Type I errors. Then, the "cov/+ select" option was used to detect closely linked QTL of opposite effects. All QTL were then integrated into a model using the "seq/s" option in PlabQTL. If the Aikake Information Criteria (AIC) values of two models differed by less than 2.0, the model with the fewest parameters was chosen (Jansen, 1993; Cardinal et al., 2001). The QTL x environment option in PlabQTL was used to determine if the QTL x trial interaction was significant and if QTL significant in the mean across trials were also significant in the individual trials.

Digenic epistatic interactions between all pairs of loci were tested by Epistacy (Holland, 1998). Interactions with p < 0.00026 were considered significant. This threshold was based on an estimate of the number of independent linkage groups in maize with each chromosome arm representing one independent linkage group (Holland et al., 1997). Interaction terms were added to a model by PROC REG (SAS Institute, Inc., 1990). Interaction terms that increased the R-square of the model and were significant at p < 0.05 were maintained in the model.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Significant differences between the parents, and genetic variation among the RILs, for stalk tunneling were confirmed by the phenotypic data (Table 1). The stalk tunneling means for B73 and De811 were significantly different in all trials, with the mean of the RILs falling between the two parents. The genetic variation across trials and the heritability [H2 = 0.77; 95% confidence interval (95% CI) = 0.71– 0.80] indicate that significant variability for stalk tunneling is present in this population. The relatively high error and genotype x environment variance ({sigma}2gxe = 9, 95% CI = 7-14) likely reflect the complications of evaluating stalk tunneling, i.e., the screening process and the effect of environment on the expression of resistance.


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  		Table 1. Phenotypic data for stalk tunneling for B73, De811, and the RILs of B73 x De811.

 
The genetic correlation between plant height (H2 = 0.95; 95% CI = 0.94–0.96) and stalk tunneling in the mean across trials (rg = 0.1) was not significant and therefore the plant height data were not analyzed further. However, the high correlation between anthesis and stalk tunneling in the mean across trials (rg = –0.8) necessitated the adjustment of stalk tunneling for anthesis. The correlations between stalk tunneling and anthesis were –0.3 and –0.6 for the 1998 and 1999 trials, respectively. There were significant differences between the parents at all locations for GDD to flowering (Table 2), with De811 consistently flowering later than B73. The heritability for anthesis (H2 = 0.89; 95% CI = 0.86– 0.91) was high, and the genotype x environment variance was not significantly different from zero.


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  		Table 2. Phenotypic data for anthesis for B73, De811, and the RILs of B73 x De811.

 
The phenotypic data for stalk tunneling adjusted for anthesis are similar to the unadjusted data for stalk tunneling, though with an observable reduction in genetic variances for the 1999 trials and the mean across trials (Table 3). As in the unadjusted data, significant differences between the parents, and genetic variation among the RILs, for stalk tunneling were confirmed by the phenotypic data. RILs with tunneling values significantly higher than B73 were observed in all trials, indicative of transgressive segregation and the possibility that B73 carries alleles for resistance to stalk tunneling not present in De811. The genotypic and genotype x environment variances ({sigma}2gxe = 13, 95% CI = 10–18) and the heritability [H2 = 0.68; 95% confidence interval (95% CI) = 0.50–0.76] are lower than but not significantly different from those for the unadjusted data, though for the adjusted data the genotype x environment variance is not significantly different from the genotypic variance for the mean across trials.


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  		Table 3. Phenotypic data for stalk tunneling adjusted for anthesis for B73, De811, and the RILs of B73 x De811.

 
One hundred ninety-one RILs were used for linkage mapping and QTL analysis. Eight RILs were excluded from all analyses because of detection of nonparental alleles at more than 5% of the loci, and one RIL was excluded because of detection of heterozygotes at more than 10% of the loci. Nonparental alleles were observed at 0.5% of the total loci, and the eight lines removed from the study contained 83% of the nonparental alleles observed. These alleles may have resulted from any of a number of possibilities, including contamination of the RILs during inbreeding, mutations, use of parental lines that were still segregating at some loci, or incomplete digestion of the DNA during RFLP analysis. No locus showed greater than 4% nonparental alleles.

Loci associated with resistance to stalk tunneling unadjusted for anthesis were observed on eight chromosomes for the mean across trials. Sixteen QTL associated with 53% of the phenotypic variation were detected on all chromosomes except 4 and 8 (Table 4 and Fig. 1). Most of the QTL had relatively small partial R2 values, with only four at 10% or larger (chromosomes 2, umc8; 6, bnlg1600; 7, umc56; and 9, npi567). The presence of several QTL for which the allele from B73 was associated with a reduction in tunneling may explain the transgressive segregation observed in the phenotypic data. Epistatic effects were not significant in the final QTL model. In concordance with the relatively high genotype x environment variance, different subsets of QTL were observed in the individual trials, with only two QTL (chromosome 2, umc8 and umc4) consistently observed across trials. Seven QTL associated with 30% of the phenotypic variation were observed on chromosomes 2 (2 QTL), 3, 5, 7 (2 QTL), and 8 for the Hinds Farm, while a larger set of 12 QTL associated with 40% of the phenotypic variation were observed for the AAERC 98 on chromosomes 1, 2 (2 QTL), 3 (2 QTL), 4, 5, 6, 7 (2 QTL), 9, and 10. For the AAERC in 1999, 13 QTL on chromosomes 1 (2 QTL), 2 (2 QTL), 5, 6 (2 QTL), 7 (2 QTL), 8, 9 (2 QTL), and 10 were associated with 49% of the phenotypic variation for the AAERC 99A and eight QTL on chromosomes 1, 2 (2 QTL), 4, 7 (2 QTL), 8, and 10 were associated with 36% of the phenotypic variation for the AAERC 99B. The QTL x environment interaction was significant (p < 0.01) with few QTL from the mean across trials significant for the Hinds Farm and AAERC 99B.


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  		Table 4. Location, effect, and partial R2 of QTL for ECB stalk tunneling in RILs of B73 x De811.

 



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  		Fig. 1. Linkage map of B73 x De811 RIL population. Underlined loci exhibited segregation distortion (p < 0.05). Black shapes (e.g., {blacksquare}) denote QTL from the mean across trials for which the allele from De811 is associated with an increase in the trait, while outlined shapes (e.g., {square}) denote QTL from the mean across trials for which the allele from B73 is associated with an increase in the trait.

 
QTL for anthesis detected on nine chromosomes in the mean across trials were associated with 60% of the phenotypic variation, with nine QTL having partial R2 values greater than 10% (Table 5 and Fig. 1). Epistatic effects were not significant in the final QTL model. With the exception of the Hinds Farm, at least 14 QTL accounting for greater than 50% of the phenotypic variation were observed in each of the individual trials. The QTL x environment interaction was significant (p < 0.01); but, for all individual environments except the Hinds Farm a majority of the QTL from the mean across trials were also significant in the individual environments.


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  		Table 5. Location, effect, and partial R2 of QTL for anthesis in RILs of B73 x De811.

 
The adjustment of the stalk tunneling means for anthesis decreased the number of QTL in the mean across trials from 16 to 10 [chromosomes 1, 2 (2QTL), 5, 6 (2 QTL), 7, 9 (2 QTL), and 10] and the percentage of the phenotypic variation explained from 53 to 42% (Table 6 and Fig. 1). The large proportion of QTL (five of 10) for which the allele from B73 was associated with a reduction in tunneling may explain the transgressive segregation observed in the phenotypic data. Epistatic effects were not significant in the final QTL model. In concordance with the relatively high genotype x environment variance, different subsets of QTL were observed in the individual trials, with only one QTL (chromosome 2, umc8) consistently observed across trials. The QTL x environment interaction was significant (p < 0.01) with few QTL from the mean across trials significant for the Hinds Farm or AAERC 99B. Four QTL associated with 24% of the phenotypic variation were observed on chromosomes 2, 5, 7, and 9 for the Hinds Farm, while a larger set of eleven QTL associated with 36% of the phenotypic variation were observed for the AAERC 98 on chromosomes 1, 2 (2 QTL), 3 (2 QTL), 4, 5, 6, 7, 9, and 10. For the AAERC in 1999, nine QTL on chromosomes 1, 2, 5, 6 (2 QTL), 7 (2 QTL), 8, and 10 were associated with 44% of the phenotypic variation for AAERC 99A and five QTL on chromosomes 1, 2, 7, 8, and 10 were associated with 28% of the phenotypic variation for AAERC 99B.


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  		Table 6. Location, effect, and partial R2 of QTL for ECB stalk tunneling adjusted for anthesis in RILs of B73 x De811.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Variability in the phenotypic data was observed across the individual trials, probably due to differences in climate, ecology, and soils. Soil differences between the Hinds Farm and the AAERC have been described (Cardinal et al., 2001) and these may have contributed to the differences observed between the 1998 trials. The majority of the ECB larvae were applied in AAERC 99A during 2 d in which high temperatures reached 34 to 35°C, while in AAERC 99B the larvae were applied 3 d later when the high temperatures had dropped to 24 to 27°C. This may have affected larval survival, with the lower means and variances for the AAERC 99A resulting from increased larval mortality caused by the higher temperatures.

No previous studies have reported adjusting stalk tunneling means for the effects of anthesis, but the genetic correlations reported were all between –0.5 and 0 (Bohn et al., 2000; Cardinal et al., 2001; Krakowsky et al., 2002). The high correlation herein could be the result of the environments used for the evaluation (specifically in 1999), since much lower correlations were reported for the F3 lines of B73 x De811 population (rg = –0.36; Krakowsky et al., 2002) and the RILs of B73 x B52 (rg = –0.42; Cardinal et al., 2001), the latter having been evaluated in the same locations in 1998 with identical infestation and evaluation techniques. Differences in the genotypic variation between unadjusted and adjusted stalk tunneling data for the 1998 trials were much smaller than those of the 1999 environments as expected.

The mean (31 cm) and genetic variance ({sigma}2g = 36) for stalk tunneling in the mean across trials of the F3 lines of B73 x De811 (Krakowsky et al., 2002) were significantly higher than those observed for the mean across trials herein, possibly due to differences in environments and level of infestation (600 larvae per plant in the F3 lines, 300 herein). RILs of B73 x B52 were evaluated at Hinds and the AAERC for two years (1997 and 1998; Cardinal et al., 2001), and the common environments in 1998 enhance comparisons between the two experiments. The genotypic variance and heritability for the mean across trials were not significantly different ({sigma}2g = 21 and H2 = 0.78 for the RILs of B73 x B52).

QTL for anthesis are linked to nine QTL for stalk tunneling (unadjusted for anthesis), in concordance with the high genetic correlation between these traits (Fig. 1). Adjustment for anthesis reduced the overall number of stalk tunneling QTL to ten, with two QTL that do not appear to be linked to anthesis QTL disappearing along with four that are linked to anthesis QTL. Of the remaining stalk tunneling QTL, five are linked to anthesis QTL. The relationship of one parent contributing alleles for delayed anthesis and increased stalk tunneling resistance is present for the five linked QTL. It is possible that these QTL represent different genes related to anthesis and stalk tunneling resistance that happen to be linked in the genome, or that the QTL are regulatory genes that are important for both traits. Infestation with ECB larvae over a longer period of time and grouping materials by flowering date could be useful in future studies to reduce the effects of anthesis on stalk tunneling and to give a clearer indication of the genetic factors involved.

Discrepancies in QTL detection for the same population evaluated in different environments have been reported previously for F3 lines of B73 x De811 (Krakowsky et al., 2002) and B73 x Mo47 (Jampatong et al., 2002), and RILs of B73 x B52 (Cardinal et al., 2001). Some differences in QTL detection across environments are expected due to climatic and soil differences, such as those between the AAERC in 1998 and 1999, but the differences in the 1999 trials indicate that small differences in the environment can affect large differences in the data. The delay in flowering in AAERC 99B as compared with AAERC 99A led to a delay in infestation, which apparently occurred under more favorable conditions and resulted in greater variability for stalk tunneling. These types of problems are difficult to plan for or avoid, and therefore make the use of larger numbers of trials desirable so as to better represent the target environment.

Consistent detection of QTL across generations can provide confirmation for the locations of loci associated with stalk tunneling in a population. The B73 x De811 F3 lines (Krakowsky et al., 2002) furnish the opportunity to compare QTL across studies using the same germplasm and assess the effects of environment and population structure. Three QTL in the F3 lines of B73 x De811 are within 20 cM of QTL for unadjusted stalk tunneling detected in the mean across trials herein on chromosomes 3 (umc10) and 5 (isu92 and npi292; Krakowsky et al., 2002), with only the QTL near isu92 on chromosome 5 also detected in the mean across trials for stalk tunneling adjusted for anthesis. The parental designation of alleles associated with decreased tunneling was consistent across the studies.

Three other studies of stalk tunneling by ECB involving the same susceptible parent (B73) have been reported (Schön et al., 1993; Cardinal et al., 2001; Jampatong et al., 2002). Comparisons of QTL across populations, while complicated by sampling variation and differences in environments and methodology and limited by the number of common genetic loci, can provide an opportunity to assess genetic heterogeneity of a phenotype (van Ooijen, 1992; Jansen and Stam, 1994; Zeng, 1994; Visscher et al., 1996). Herein all comparisons will be made between QTL for stalk tunneling adjusted for anthesis that were detected in the mean across trials and will be based on common marker loci. Nine QTL for stalk tunneling were detected in the B73 x B52 RILs on chromosomes 2, 3, 5, 7, 8, and 9 (Cardinal et al., 2001), three of which are within 25 cM of QTL observed herein on chromosomes 2 (umc8 and umc4) and 9 (npi567). Six and seven QTL were observed in F3 lines of B73 x B52 and B73 x Mo47, respectively (Schön et al., 1993; Jampatong et al., 2002). The QTL herein on chromosome 7 (umc56) and on chromosomes 6 (bnlg1600) and 9 (npi567) are within 25 cM of QTL observed in the B73 x B52 and the B73 x Mo47 F3 lines, respectively. While the comparisons with other studies provide further validation of QTL observed herein, the presence of unique QTL in populations evaluated in common environments (B73 x De811 and B73 x B52 F3 lines in 1989 and RILs in 1998) may indicate that genetic heterogeneity for resistance to stalk tunneling by ECB is present in temperate maize inbreds. Comparisons of pedigrees would provide better information concerning potential genetic heterogeneity for resistance to stalk tunneling between B52 and De811, but the origin of B52 is unclear (Lee et al., 1990).

QTL associated with stalk tunneling by ECB have also been identified in European dent germplasm. There is only one generation of ECB per growing season in Central Europe, but the infestation begins at a similar stage of plant development as the stalk tunneling generation observed in the U.S. Cornbelt (Bohn et al., 2000). Six QTL for tunnel length were observed in F3 lines derived from a cross of D06 (resistant to stalk tunneling by ECB) x D408 (susceptible), three of which are within 25 cM the QTL observed herein in the mean across trials on chromosomes 5 (isu92), 9 (npi567) and 10 (npi105).

Resistance to leaf feeding by ECB in temperate maize has a low genotypic correlation (0.10–0.33; Russell et al., 1974; Sadehdel-Moghaddam et al., 1983; Klenke et al., 1986) with resistance to stalk tunneling. However, six QTL for stalk tunneling in the mean environment herein are in the same chromosomal regions as those detected for resistance to leaf feeding by the southwestern corn borer (SWCB, Diatraea grandisella Dyar) and the sugarcane borer (SCB, Diatraea saccraralis Fabricius) in two tropical maize populations. The QTL in the mean environment herein on chromosomes 2 (umc4), 6 (bnlg1600 and bnl5.47), 7 (umc56), 9 (npi567), and 10 (npi105) are within 25 cM of QTL detected for SWCB and SCB (Groh et al., 1998). Linkage of QTL for stalk tunneling by ECB and leaf feeding by SWCB and SCB has also been reported in other studies (Bohn et al., 2000; Cardinal et al., 2001) and the QTL may represent gene clusters affecting resistance (McMullen and Simcox, 1995) or a common to mechanism of resistance to different insect pests. Resistance to leaf feeding by ECB in temperate maize is usually associated with the chemical 2,4-dihidroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), but exotic germplasm with low levels of DIMBOA and resistance comparable to temperate inbreds with high levels of the chemical has been observed (Sullivan et al., 1974; Abel et al., 1995). Leaf feeding by ECB, SWCB, and SCB were highly correlated in a diallel of nine exotic and one temperate inbreds (Thome et al., 1992), and the mechanisms conferring resistance to leaf feeding these pests may also confer resistance to stalk tunneling by ECB (and possibly by SWCB and SCB).

The linkage of QTL for anthesis and stalk tunneling, and the elimination of several QTL for stalk tunneling when the data were adjusted for anthesis, indicates that anthesis can bias the results of stalk tunneling studies and must be factored into the design of the experiment. Use of the QTL detected herein in a marker-assisted selection program to increase levels of native resistance to stalk tunneling by ECB in temperate maize germplasm may not be efficient due to the small partial R2 associated with most of the QTL and the linkages with QTL for anthesis.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Research conducted in partial fulfillment of the Ph.D. degree by M.D. Krakowsky. This journal paper of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project No. 3134, was supported by Hatch Act and State of Iowa funds and the R.F. Baker Center for Plant Breeding.

Received for publication February 22, 2003.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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M. D. Krakowsky, M. Lee, and J. B. Holland
Genotypic Correlation and Multivariate QTL Analyses for Cell Wall Components and Resistance to Stalk Tunneling by the European Corn Borer in Maize
Crop Sci., March 1, 2007; 47(2): 485 - 488.
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