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Published in Crop Sci. 43:2043-2049 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP BREEDING, GENETICS & CYTOLOGY

Divergent Selection for Resistance to Maize Weevil in Six Maize Populations

Thanda Dhliwayo and Kevin V. Pixley*

International Maize and Wheat Improvement Center (CIMMYT), P.O. Box MP163, Mount Pleasant, Harare, Zimbabwe

* Corresponding author (k.pixley{at}cgiar.org).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Maize weevil (Sitophilus zeamais Motschulsky) is an important pest of maize (Zea mays L.) in the tropics, causing serious losses for many resource-poor farmers who store grain on-farm for use as food and seed. This study evaluated whether weevil resistance of six maize populations could be divergently changed by S1 and S2 selection, and assessed the importance of replicating grain samples when screening for resistance. Weevil resistance was evaluated for unreplicated S1 and for replicated and unreplicated S2 lines by infesting 50-g grain samples with 32 young adult weevils, and then incubating the samples in a controlled temperature and relative humidity (CTH) laboratory. Divergent synthetics were formed by recombining the most resistant 10% and the most susceptible 10% of at least 100 lines screened for weevil resistance for each maize population. Replicated S2 selection was successful for both populations where it was applied, resulting in an average of 16% (P < 0.01), 49% (P < 0.05), and 20% (P < 0.01) difference between divergent synthetics for weevil progeny emerged, grain weight loss, and the Dobie index of susceptibility, respectively. S1 unreplicated selection was successful for two of the six populations, while S2 unreplicated selection was not successful. Reciprocal effects were significant (P < 0.01) for weevil resistance of F1 varietal crosses among the divergently selected synthetics, indicating the influence of maternal effects. Nevertheless, the most resistant crosses were those among the most resistant synthetics, confirming that additive gene action for weevil resistance was important. Our results provide practical insights regarding methodologies and demonstrate that it is possible to improve weevil resistance of maize populations.

Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • CTH, controlled temperature and relative humidity • MDP, median development period (of the weevil)


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
MAIZE WEEVIL (Coleoptera: Curculionidae) is an important pest of stored maize in the tropics, particularly in the lowland and mid-altitude, hot, and humid environments (Longstaff, 1981; DeVries and Toenniessen, 2001; Pingali and Pandey, 2001). Of the 140 million ha of maize grown globally, {approx}96 million are in the developing world, and while 90% of the maize produced in the industrialized countries is grown in temperate regions, only 25% of developing world maize is temperate (Pingali and Pandey, 2001). Also, whereas in the USA and Europe grain is stored in commercial silos, with careful control of moisture content and fumigation to control insects, the vast majority of tropical maize is stored on-farm, usually with no control of moisture content and without chemical protectants. Grain is most susceptible to weevil damage if it is stored at high moisture content (e.g., >15%) (CIMMYT, 2001). A consequence of the above facts is that maize weevil is a greater problem in developing than in developed countries. Studies in Malawi (Golob, 1984) and Zimbabwe (Giga et al., 1991) have reported >20% weight loss caused by weevils for untreated grain of maize hybrids stored in traditional structures, and up to an 80% loss may occur in on-farm stores in tropical countries (Mutiro et al., 1992; Pingali and Pandey, 2001).

Weevil damage results directly in lost food, and may also reduce future maize production for farmers who use saved grain as seed (a practice that accounts for {approx}70% of all maize planted in eastern and southern Africa) (CIMMYT, 1994; Pingali and Pandey, 2001). The use of pesticides for control of weevils is broadly recommended, but resource-poor farmers of the developing world often cannot afford or obtain them. Also, the increasing occurrence of insecticide resistance (Perez-Mendoza, 1999) and environmental concerns about the use of chemical insecticides mean that alternative control methods are required.

Significant genetic variation for resistance to weevil has been found in several studies. Resistance to maize weevil was reported for Mexican landraces, notably Sinaloa 35 and Yucatan-7 (Arnason et al., 1994), for the Tanzanian open-pollinated variety ‘Kilima’ (Derera et al., 1999), for tropical inbred lines Hi41, Hi34, ICA L29, KU1409, Hi39, and ICA L221 (Kim et al., 1988), and for temperate inbred lines B37, B68, R805, and T220 (Tipping et al., 1988). Grain factors reported to contribute to resistance include increased grain hardness and sugar content (Sing and McCain, 1963; Dobie, 1977), and increased phenolic acid content, especially (E)-ferulic acid content (Classen et al., 1990; Arnason et al., 1994, 1997).

Several studies have reported complex inheritance of weevil resistance in maize. Kang et al. (1995) used a diallel mating design among 10 inbreds to investigate combining ability for weevil preference and reported significant additive, nonadditive, and maternal effects, with additive being more important than nonadditive effects. Tipping et al. (1989) investigated inheritance of resistance to oviposition by maize weevil in a 10-parent diallel mating design, and reported that additive gene action and, to a lesser extent, nonadditive gene action were important for this trait. Widstrom et al. (1975) investigated the inheritance of resistance to maize weevil using a North Carolina Design II with 80 inbred lines in a no-choice study and reported that dominance effects of maternal and endosperm genotypes were important and that cytoplasmic effects were unimportant. Derera et al. (2001a)( b), who used a North Carolina Design II involving 18 inbred lines to investigate gene action for weevil resistance in both free-choice and no-choice tests, reported significant additive, nonadditive, and maternal effects.

Despite the progress made in understanding weevil resistance in maize and identifying resistance sources, we are not aware of any commercial maize breeding program that is addressing this objective. The reluctance of maize breeding programs to select for weevil resistance is not without reasons. Conventional methods to evaluate weevil resistance require relatively large quantities of grain, and in pedigree or recurrent selection programs there is usually not enough grain on individual ears (e.g., S1 or S2) to screen for weevil resistance while retaining sufficient remnant seed for continued breeding work. Also, the fact that resistance data are not obtained until approximately three months after harvesting represents a significant time delay for any breeding program. Another disincentive is that little information is available about effective, practical methods to breed maize for improved weevil resistance. We suspect that the main reason maize breeders do not address weevil resistance in their breeding programs, however, is that they do not consider resistance to grain weevil as part of their responsibility; the breeder's job is considered done when a marketable crop is harvested, and it is the farmers' responsibility to either protect the grain or sell it before it deteriorates. These assumptions are not appropriate for resource-poor farmers in developing countries. For these farmers, weevil resistance is an important issue and influences the adoption rate of improved varieties (Kossou et al., 1993). Public and private sector maize breeders should therefore consider breeding for resistance to maize weevil to better meet the needs of, and increase adoption/purchase of improved seed by these farmers.

Our objectives were to determine if maize weevil resistance of six maize populations could be divergently changed by S1 and S2 selection, and assess the importance of replicating grain samples when screening for resistance to weevil in two of these maize populations. Both objectives contribute to elucidating feasible breeding strategies to improve weevil resistance of maize.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Germplasm and Selection Methods
Two synthetic and four biparental populations were chosen for the study (Table 1). The two synthetic populations, SZSYNA99 (S. zeamais synthetic of heterotic group A, made in 1999) and SZSYNB99 (S. zeamais synthetic of heterotic group B, made in 1999), hereafter referred to as SZA and SZB, respectively, were made from lines of opposite heterotic groups from CIMMYT. The lines were selected based on above-average weevil resistance among advanced and elite breeding lines at CIMMYT, Zimbabwe. The four biparental populations were crosses of CML206 (susceptible) with lines of above-average weevil resistance. Two of the biparental populations were reciprocal crosses of the same two lines.


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  		Table 1. Name or pedigree, and type of maize populations used for divergent selection studies for weevil resistance.

 
For the synthetics, grain of 137 S1 ears for SZA and 132 for SZB was evaluated for weevil resistance in the CTH laboratory at CIMMYT's research station at Harare, Zimbabwe. Each S1 ear was shelled individually and a 50-g sample of grain was taken for weevil screening in unreplicated experiments; the remaining seed was kept for future use.

The grain samples were frozen at -20°C for 14 d to control any field infestations (live insects or eggs) by weevils or any other pest. Each sample was then put in a 500-mL glass jar with a brass screen lid that allowed adequate ventilation, and then placed in the CTH laboratory that was maintained at 28 ± 2°C and 70 ± 5% relative humidity. The samples were left in the CTH laboratory for a 3-wk acclimatization period to achieve uniform grain moisture content and grain temperature among all samples. Next, the samples were infested with 32 unsexed weevils aged 7 to 14 d. After an oviposition period of 10 d, the weevils were removed and the number of dead and living weevils was used to calculate parent mortality, or the percentage of weevils that died during the oviposition period. The samples were then left in the CTH laboratory for 45 d (incubation period), after which the number of F1 weevils (progeny) that emerged from each sample was recorded. The number of weevils that emerged from each sample was the criterion for selecting the best and worst lines for recombining (divergent selection). The best 14 lines (10% of 137 lines) and the worst 14 lines were selected for SZA; and the best and worst 13 lines (10% of 132 lines) were selected for population SZB.

While grain of the S1 lines was being assayed for weevil resistance (above), remnant seed of all the 137 lines for SZA and the 132 for SZB was planted and the lines were advanced to S2 by self-pollinating the best plants (based on agronomic aspect) for each line. After eliminating unviable families, for which pollinations failed or ears were severely affected by ear rots, there were 106 remaining families for SZA and 110 for SZB. Three representative S2 ears were kept for each family (originating from the same S1) for weevil screening, such that there were 318 (106 x 3 ears) samples for SZA and 330 (110 x 3 ears) for SZB. The same procedure described above for screening the S1 lines was used for evaluating weevil resistance of the S2's, except that the Dobie index of susceptibility (Dobie, 1977) was also calculated to give an additional parameter of resistance. Weevil resistance evaluation of the 106 and 110 S2 families for SZA and SZB, respectively, was done in a replicated experiment since three S2 sister lines were evaluated per family.

Grain of 100 S1 ears was screened for weevil resistance for each of the four biparental populations, with the procedure described above for the S1 lines from the two synthetic populations. The number of F1 weevils emerged from each S1 grain sample was used to select the best 10 (10% of 100 lines) and the worst 10 lines for each population.

Formation of Synthetics and Agronomic Practices
All selected S1 lines from each population were planted at Harare during the summer of 2000-2001. For each synthetic formation, the lines to be recombined were divided into two arbitrary groups; pollen was collected from representative plants in all rows of each group, bulked, and used to pollinate plants of the other group. The procedure was repeated on several dates to avoid possible bias due to differences in flowering time of the lines. At harvest, all successfully pollinated ears were collected for each family (all pollinations within one line) and an equal amount of seed from each family was used to form a balanced Syn1 bulk of the new synthetic population. This procedure was used to form the 12 synthetics: resistant and susceptible synthetics for each of the original six populations.

In addition to forming the synthetics, at least four plants from each S1 line involved in formation of a synthetic for the four biparental populations were self-pollinated. Harvested S2 seed from the self-pollinated ears within a row was bulked and screened for weevil resistance during winter 2001 in replicated tests (four replicates) with the procedure described for their parental lines. This provided an additional assessment of the success of unreplicated S1 selection for weevil resistance.

A slightly different procedure was used to make synthetics among S2 lines from SZA and SZB. All S2 lines (three ears per S2 family) were planted at Harare concurrent with screening for weevil resistance in the CTH laboratory. Weevil resistance data were available before flowering time and were used to select lines to form eight synthetics, four for each population: (i) SZSYN01BestS2Repl: Made from recombining the best 10% of the families (11 families for each population), based on the mean weevil resistance of the three S2's for each family (replicated); (ii) SZSYN01BestS2Unrepl: Made from recombining the most weevil-resistant 10% of the S2 lines (32 lines for SZA and 33 for SZB), irrespective of source S1 family, based on unreplicated evaluation of resistance for all of the S2 lines; (iii) SZSYN01WorstS2Repl: Made from recombining the worst 10% of the families, based on the mean weevil resistance of the three S2's for each family; and (iv) SZSYN01WorstS2Unrepl: Made from recombining the worst 10% of the lines as in (ii).

All lines selected for recombining into a synthetic were arbitrarily divided into two groups and recombined as described for S1 lines. Again, at harvest, all successfully pollinated ears were collected for each family (all pollinations within one line) and an equal amount of seed from each family was used to form a balanced Syn1 bulk of the new synthetic population.

In addition to forming the synthetics, at least three plants from each row within a selected family were self-pollinated to obtain S3 ears. S3 seed from sister S2 lines (originating from a common S1) was bulked and used for evaluation of weevil resistance in the CTH laboratory with four replications.

All 20 synthetics (12 from SZA and SZB, and 8 from biparental populations) were advanced to Syn2 generation by full-sib, plant-to-plant intermating within each synthetic at Muzarabani during winter 2001. At the same time, all six synthetics from SZA were crossed to their corresponding heterotically complementary synthetics in SZB to form six crosses, plus their reciprocal crosses. Grain of the parent populations (SZA and SZB) was also produced during the same season by full-sib mating within each population. The production in a single environment of all grain for weevil evaluations seemed justified by reports by Tipping et al. (1989) and Kang et al. (1995) of nonsignificant genotype x environment (site or year) interaction for resistance to maize weevil. On the other hand, because environmental effects on weevil resistance of grain can be large, we did not and would not recommend comparing weevil resistance of maize grain produced at different sites, during different seasons, or with different agronomic management.

Agronomic practices were similar at Harare and Muzarabani, except that nurseries at Harare were mainly rain-fed while the winter nurseries at Muzarabani were irrigated. Plot size for nurseries was a 4-m row with 0.75 m between adjacent rows and 0.25 m between plants within a row to give a plant population of {approx}53000 plants ha-1. Plots were fertilized with 32.0, 24.5, 23.2, and 3.2 kg ha-1 N, P, K, and Zn at planting, and supplemented with 69 kg ha-1 N 6 wk after planting at both Harare and Muzarabani. Formation of Syn2 grain for all synthetics and their parent populations was done in 12-row (204 plants) plots, and F1 crosses between complementary synthetics were done in eight-row (136 plants) plots (4 rows of each synthetic to be crossed). Number of ears harvested and bulked for each Syn2 ranged from 130 to 135. For all synthetics, equal amounts of grain were shelled from each ear to form a balanced bulk for each Syn2. An average of 45 ears was harvested for all crosses (including reciprocals) and balanced bulks were formed for weevil evaluation.

Weevil Resistance Evaluation of Synthetics and Crosses
A modification to the Dobie method (Dobie, 1977) was used to evaluate weevil resistance for each synthetic. Five hundred grams of grain was weighed into a 1000-mL jar (with a brass screen lid that allowed adequate ventilation) and placed in the CTH laboratory for a conditioning period of 3 wk. Grain moisture content was then measured before 10 replicates of 50 ± 0.1 g per synthetic were weighed into 500-mL jars. Each replicate was then treated as described for S1 lines above. However, after the 10-d oviposition period, the samples were left undisturbed for 14 d before weevil counts were done every second day until no more weevils emerged. The median development period (MDP) of the weevils was calculated as the number of days by which 50% of the weevils had emerged. An index of susceptibility was calculated for each replicate (Dobie, 1977), where index of susceptibility = 100[loge (total number of adults emerged)/MDP]. Grain weight loss was calculated for each sample as the difference between initial grain weight (50 ± 0.1 g) and final grain weight after vigorous sieving to remove flour produced by weevil feeding and tunneling. A Mexican composite variety (‘Oaxaca 179’) and popcorn (Z. mays subsp. mays) were included as resistant and susceptible checks, respectively. Experimental design in the CTH laboratory was a randomized complete block with 10 replications.

Statistical Analyses
Weevil resistance parameters for synthetics made from biparental populations were analyzed separately from those for SZA and SZB. Progeny emerged (number of F1 weevils hatched), grain weight loss, MDP, and the Dobie index of susceptibility were subjected to ANOVAs. Progeny data were transformed by the logarithm transformation before analyses with the GLM (general linear model) procedure of SAS (SAS Institute, 2001). Population, synthetic, and hybrid effects were considered as fixed, whereas replication effects were considered random in the ANOVAs.

Means were computed for weevil resistance parameters for all synthetics formed by divergent selection for SZA, SZB, and the four biparental populations. The percentage difference for weevil resistance parameters was then calculated for each pair of synthetics formed for each population with each selection method. Percentage differences were calculated as the difference between means for the divergently formed synthetics divided by the mean for the worst synthetic (selected for susceptibility). A percentage difference was considered statistically significant if the means for the two divergently selected synthetics were significantly different from each other according to pair-wise t tests.

Weevil resistance data for S1:2 and S2:3 families from synthetic populations (SZA and SZB), and for S1:2 lines from biparental populations were also subjected to ANOVAs.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
There were highly significant (P < 0.01) differences between entries formed by divergent selection from the two synthetic populations (SZA and SZB) for weevil progeny emerged, grain weight loss, MDP, and the Dobie index of susceptibility to weevil (Table 2). There were no significant differences among entries for parent weevil mortality during the oviposition period (mean mortality was 11%). Variance among synthetics and among crosses (varietal hybrids) was highly significant (P < 0.01) for all parameters except parent mortality. Differences for weevil resistance among synthetics were generally independent of source population, except that synthetics from SZB had somewhat longer MDP (suggesting greater weevil resistance) than those from SZA (Table 3). Synthetics developed by different selection methods (screening of unreplicated S1, unreplicated S2, or replicated S2) differed significantly for all resistance parameters. Selection direction effect (nested within selection method) was highly significant (P < 0.01) for progeny emerged, grain weight loss, and the Dobie index of susceptibility, and significant at P < 0.05 for MDP, indicating that the selection methods differed in their effectiveness for divergently selecting resistant and susceptible materials. Finally, lack of significant variance for population x selection method and population x selection direction within selection method indicated that response to selection method and direction was consistent for synthetics developed from both populations.


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  		Table 2. Analysis of variance for weevil resistance parameters for 12 synthetics formed from two maize populations by divergent selection with three selection methods, and for hybrids (variety crosses) formed among heterotically complementary synthetics from the two populations.

 

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  		Table 3. Means for weevil resistance parameters for maize synthetics formed from two genetically broad-based maize populations by divergent selection for weevil resistance with three selection methods.

 
Variance of weevil resistance parameters was significant among variety (synthetic) crosses and their reciprocals (Table 2). Reciprocal effects (A x B vs. B x A variance) were not significant for weevil progeny emerged or grain weight loss. Reciprocal effects were significant, however, for MDP (P < 0.01) and consequently also for the Dobie index of susceptibility (P < 0.05), indicating influence of maternal effects on resistance to maize weevil. A longer MDP indicates resistance because generation turnover (the time it takes from one weevil generation to another) is longer, which would result in fewer weevils and less kernel damage than for a more susceptible genotype. We were not surprised to find significant reciprocal effects among F1 crosses because the pericarp and endosperm, which are the tissues directly attacked by weevils, are maternal tissue (pericarp) or primarily determined by the maternal genotype (the 3n endosperm has 2n gene complement from the mother plant). The most weevil-resistant F1 hybrids (see Table 4) were crosses among the best parents (see Table 3), which is consistent with reports that gene action determining resistance to maize weevil is predominantly additive (Tipping et al., 1989; Kang et al., 1995).


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  		Table 4. Means for hybrids formed by crossing heterotically complementary synthetics from populations SZA and SZB.{dagger}

 
Replicated S2 selection succeeded to significantly improve weevil resistance and divergently separate the two populations, SZA and SZB (Table 5). Unreplicated S2 selection, however, was not successful for altering weevil resistance for either of these maize populations. Unreplicated S1 selection succeeded in creating divergent (resistant and susceptible) synthetics for one population (SZB) but failed in the other (SZA). We expected replicated selections to be more effective than unreplicated selections because, in our experience, weevil F1 progeny data are inherently variable. Although unreplicated S1 evaluation enabled successful divergent selection for progeny emerged and grain weight loss in one population, it was not consistently effective as replicated S2 evaluation for separating the two populations into susceptible and resistant synthetics (Table 5).


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  		Table 5. Percentage differences{dagger}between best (resistant) and worst (susceptible) synthetics formed by divergent selection for six maize populations by three selection methods.

 
Comparison of synthetic means with their respective parent populations (Table 3) indicates that we failed to make the populations more susceptible to the weevil. This could have been caused by a skewed population distribution such that the mean of the worst 10% was so close to the population mean that selection would yield no response. However, applying the Shapiro-Wilk normality test (Analytical Software, 2000) revealed no evidence of deviation from normal distribution for these data. Another possibility is that, if the maize populations were all effectively susceptible to maize weevil, and alleles for weevil resistance are relatively rare (i.e., most individuals have a preponderance of alleles for susceptibility), our result may simply suggest that it is difficult to make maize more susceptible to weevil than it is already. On the other hand, the parent populations were more resistant than the resistant check, indicating they contain one or more weevil resistance genes, and if the favorable alleles are fixed, this would explain why selection failed to make the populations more susceptible to weevil. Finally, it is possible that significantly increased weevil susceptibility would be achieved after additional cycles of selection for these populations.

Variance among synthetics formed from the four biparental populations was also highly significant (P < 0.01) for all parameters of resistance, except weevil parent mortality (Table 6). Source population did not contribute to variance among the synthetics, but the effect of selection direction within population was significant for progeny emerged, grain weight loss, MDP, and the Dobie index of susceptibility (P < 0.01). These results indicate that although weevil resistance and susceptibility was similar for the four populations, unreplicated S1 selection differed in its effectiveness for divergently selecting for weevil resistance within them. Divergent selection results from the four biparental populations and for SZA and SZB are summarized in Table 5.


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  		Table 6. Analysis of variance for weevil resistance parameters for eight synthetics formed from four biparental populations with S1 unreplicated selection.

 
The general lack of response to selection in three of the four biparental populations again indicated that S1 unreplicated selection was not consistently effective for divergently selecting for weevil resistance (Table 5). There are several possible reasons for differences in selection gain made in the populations; for example, it is possible that some of them had little genetic variation for weevil resistance. If this was the case, then differences for weevil resistance observed among the unreplicated S1 lines were due to experimental error, heritability of weevil resistance was close to zero, and selection could not have been effective. Weevil resistance of the line RA87C3108-B-5-1-1-5-B*3 has been well documented elsewhere (Giga and Mazarura, 1991; Giga et al., 1999; Derera et al., 2001a), whereas the other resistant parents of the biparental populations CML394 and CML442 were classified as moderately resistant only in one report (K. Pixley and N. Kadzere, 1999, unpublished data). Despite the possibility that CML394 and CML442 may not be as resistant to maize weevil as RA87C3108-B-5-1-1-5-B*3, we expected that genetic variation for weevil resistance existed in all of the populations, and that failure to make progress from selection was due to ineffectiveness of unreplicated S1 evaluation of weevil resistance.

The mean weevil resistance (weevil progeny emerged) of S1:2 lines derived from S1's selected to form divergent synthetics differed only for two of the four biparental populations (Table 7). This result was inconsistent with evaluation of the divergent synthetics themselves (Table 5), which revealed significant difference for only one biparental population (RA87C3108-B-5-1-1-5-B*3/CML206), which was not one of the two populations with significant divergence among S1:2 lines. Similarly, we found significant differences for weevil resistance among S1:2 sister lines (families) originating from S1's divergently selected by unreplicated S1 evaluation for only one of the two populations SZA and SZB. By contrast, significant differences were measured for weevil resistance parameters among S2:3 sister lines originating from S2's divergently selected by replicated S2 evaluation for both SZA and SZB (Table 7). Thus, results from evaluating S1:2 and S2:3 sister lines support our earlier finding that replicated S2 selection was more effective than unreplicated S1 selection for weevil resistance. These results provide further evidence that weevil resistance is heritable in these maize populations and that replicated S2 selection was effective in divergently selecting for weevil resistance.


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  		Table 7. Means for weevil resistance parameters for S1:2 sister lines (families) derived from the S1 lines used to constitute S1UNRepl resistant and susceptible synthetics from the six populations (two synthetics and four biparental), and for S2:3 families derived from S2's used to constitute S2Repl resistant and susceptible synthetics from two synthetic populations.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Successful divergent selection by replicated S2 evaluation of weevil resistance indicated that genetic variability for resistance to maize weevil existed in these populations and that the resistance parameters we used are heritable. The inconsistent success of divergent selection with unreplicated assessments of weevil resistance, however, reflects our experience that these assessments are prone to error and indicates that they should be done in replicated experiments. Replicated evaluation of weevil resistance with S2 seed was consistently effective.

This work has established that progress from selection for weevil resistance is possible, and a methodology for population improvement has been found effective. A replicated S2 selection scheme, however, is time and resource intensive and may not be practical for many applied maize breeding programs. Further work is needed and is ongoing, examining recurrent selection schemes with shorter cycle time (e.g., full-sib), and investigating hybrid-oriented breeding methodologies (e.g., line x tester hybrid evaluations). Unless improvement for weevil resistance can be effectively integrated within routine breeding schemes at modest cost, this important objective is unlikely to be adopted by breeders.


   ACKNOWLEDGMENTS
 
Financial support of this work by The Rockefeller Foundation is gratefully acknowledged.

Received for publication January 2, 2003.


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


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