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CROP BREEDING, GENETICS & CYTOLOGY

Identification of Tropical and Temperate Maize Populations Having Favorable Alleles for Yield and Other Phenotypic Traits

Dep. of Crop Sciences, University of Illinois at Urbana-Champaign, 1102 S. Goodwin Ave., Urbana, IL 61801 USA

jdudley{at}uiuc.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
If tropical germplasm is to be useful in increasing maize (Zea mays L.) grain yield, populations containing favorable alleles not present in elite hybrids need to be identified. Thus, a major objective of this study was to identify tropical and temperate populations containing favorable alleles useful for improving the Corn Belt hybrid FR1064 x LH185 (a commercial hybrid representative of the typical Stiff-Stalk x Lancaster heterotic pattern). A group of tropical populations and hybrids from the Germplasm Enhancement of Maize Project (GEM) crossed to either Mo17 or B73 (temperate maize inbreds) was studied. In addition, a group of temperate accessions was evaluated. Grain yield, flowering date, plant height, ear height, and penetrometer reading were evaluated using Dudley's method for identifying populations with favorable alleles. Only three accessions of 41 had positive and significant relative numbers of favorable alleles for yield. For most accessions, unexpectedly, the estimate of l_lµ' (an estimator of relative number of favorable alleles in the accessions not present in the hybrid to be improved) for yield was negative. Negative l_lµ' values for yield resulted from low true values of l_lµ' combined with low frequencies of favorable alleles in the accession for loci having favorable alleles in one inbred and not the other. None of the accessions were identified as having favorable alleles for lowering ear height or increasing stalk penetrometer resistance. These results suggest Dudley's method may not be useful when frequencies of favorable alleles in the populations being evaluated are very low.

Abbreviations: GEM, germplasm enhancement of maize • LAMP, Latin American maize project

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
METHODS FOR SCREENING a large array of materials are necessary in plant breeding programs. For example, the multinational Latin American Maize Project (LAMP) evaluated 12113 accessions. In LAMP, after the first evaluation, adaptation tests were conducted. A total of 813 accessions from 11 countries were sent to 21 different regions and evaluated for agronomic performance. From this evaluation, 270 elite accessions were included in experiments to test combining ability with local testers. In this prebreeding work (Sevilla and Salhuana, 1997) accessions were crossed to an elite single cross, to elite inbreds, or to synthetics. Integrating the most promising materials from this project into breeding programs in several countries is under way. The GEM project is working with a small core set of elite corn materials from LAMP (Pollak, 1997; Salhuana et al., 1998). Because tropical populations can only be evaluated when they have been adapted for flowering date, some of these GEM populations were crossed to temperate maize inbreds B73 and Mo17. Together with the work to collect, acquire, and use exotic materials, developing optimal methods for identifying populations that have favorable alleles not present in elite germplasm and for introgressing these alleles into elite breeding material is important.

Theory for identifying populations containing favorable alleles for improving an elite single cross is available (Dudley, 1987, 1988a). This theory assumes that for any two lines there are four classes of loci. Class i having favorable alleles (++) present in both parents (I1 and I2) of the elite hybrid. Class j being ++ for I₁ and having less favorable alleles (--) for I₂. Class k being -- for I₁ and ++ for I₂, and class l being -- for both I₁ and I₂. The letters i, j, k, and l are also used to represent the number of loci in each class. In the population, the average frequency of favorable alleles in each class is _i, _j, _k, and _l, respectively, and _j and _k are the average frequencies of recessive alleles. In this procedure µ stands for one-half the difference between homozygotes at a locus and is assumed constant across all loci. Complete dominance of favorable alleles and negligible epistasis is assumed. The statistic l_lµ'; the relative frequency of dominant favorable alleles in class l; j_jµ, and k_kµ, the relative frequencies of unfavorable alleles in classes j and k, respectively; and the relationship value - described by Dudley (1987, 1988a) are the primary statistics estimated using this theory.

Applications of this theory and evaluation of its efficacy in detecting populations with favorable alleles have been studied by several authors (Bernardo, 1990; Dudley, 1988b; Dudley et al., 1996; Fabrizius and Openshaw, 1994; Gerloff and Smith, 1988; Hogan and Dudley, 1991; Pfarr and Lamkey, 1992a, 1992b; Stoj0">in and Kannenberg, 1995). Hogan and Dudley (1991), Pfarr and Lamkey (1992b), and Fabrizius and Openshaw (1994) evaluated the model using populations with known relative values of l_lµ'. They identified populations useful for improving yield and other traits of elite hybrids. Dudley et al. (1996) identified improved populations potentially useful in improving elite hybrids. Stojin and Kannenberg (1995) found populations in advanced cycles of selection had favorable alleles for potential improvement for yield of elite hybrids, but unimproved populations did not. Goodman et al. (1990) suggested Dudley's method may be more applicable to U.S. materials than to nonadapted tropical accessions, and Pfarr and Lamkey (1992a) suggested that a low frequency of dominant favorable alleles in exotic populations makes detection of these alleles uncertain.

Dudley (1988a) suggested the appropriate hybrid to be improved was the best hybrid available in an area. To evaluate germplasm with unknown potential, an elite hybrid representative of a high percentage of the types of germplasm being grown in an area would provide the most useful information. The hybrid FR1064 x LH185 was a hybrid representative of the Stiff-Stalk Synthetic x Lancaster heterotic pattern widely used in the U.S. Corn Belt.

As part of an effort to determine the value of GEM germplasm for improving Corn Belt maize, this study had the following objectives: (i) to identify populations that could contribute favorable alleles for yield to the elite hybrid FR1064 x LH185; (ii) to ascertain the possible contribution of populations to the improvement of plant height, ear height, and penetrometer reading; (iii) to determine which parent of the elite hybrid should be improved; (iv) to determine, for populations with favorable alleles, whether to backcross to one of the elite inbreds or to directly self the F₁ cross (elite inbred x exotic) for isolating improved inbreds.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Genetic Material
The elite hybrid to be improved was the single cross FR1064 (I₁) x LH185 (I₂). FR1064 is a B73 type inbred provided by Illinois Foundation Seeds and LH185 is a non-Stiff-Stalk line provided by Holden's Foundation Seeds. Seed of the tropical x B73 and tropical x Mo17 accessions used was provided by Linda Pollak, coordinator of the GEM project. Five accessions were crosses of tropical populations by B73. Ten were tropical population by Mo17 crosses. Seven tropical hybrids were crossed to both B73 and Mo17. Thus, 15 different tropical populations and seven tropical hybrids were evaluated. Twelve temperate accessions were used directly. Thus, 34 different germplasm sources (15 tropical populations, seven tropical hybrids, and 12 temperate accessions) were evaluated. The Ames accession numbers or PI numbers of these germplasm sources are shown in Table 1 .

Evaluation Trials
Each accession crossed to FR1064 or LH185 plus the hybrid FR1064 x LH185 was included in an (0, 1) generalized lattice design with 85 entries, 17 blocks, and three replications. Entries included 41 accessions crossed to FR1064 and to LH185 (a total of 82 entries), the F₁ (FR1064 x LH185), and two other hybrids, for a total of 85 entries. The two inbreds (FR1064 and LH185) were planted in 10 replications of a randomized complete block design in a separate experiment in the same field. Plots were two rows 5.3 m long with 0.76 m between rows. Standard cultural practices were used. Plots were thinned at the four- to five-leaf stage to a stand of 57100 plants ha^-1. Experiments were grown at two locations on the Crop Sciences Research and Education Center of the University of Illinois at Urbana-Champaign in both 1997 and 1998.

Plots were machine harvested with weight and moisture recorded electronically on the combine. Yield of shelled grain was converted to grain yield in tonnes per hectare adjusted to 155 g kg^-1 moisture. Flowering dates were measured as the difference in days between emergence dates (a general date for the whole experiment) and the specific dates when 50% of the plants in each plot were shedding pollen. Flowering dates were recorded for the yield trials and also in a set of disease evaluation trials grown in the same years (See Kraja et al., 2000). Thus, data for flowering date were available from 12 experiments for both inbreds and hybrids. Plant height was measured as the distance from the soil to the flag leaf node. Ear height was measured as the distance from the soil surface to the upper ear-bearing node. Force in kilograms to penetrate the internode below the top ear (penetrometer reading) was measured using a modified Accu Force CADET Force Gage (Ametek, Mansfiled & Green Division, Pittsburgh, PA). Penetrometer reading has been shown to relate to stalk lodging resistance (Dudley, 1994). Ten plants were measured in each plot. Plant height, ear height, and penetrometer data were taken only from the yield trials.

Statistical Analysis
Proc GLM (SAS Institute, 1990) and Proc Mixed (Littell et al., 1996) of SAS were used for analysis of variance for each trait. Years and locations were treated as random effects and hybrids were treated as fixed effects. Replications were treated as random effects and nested in year x location combinations. Blocks were nested in replications. Proc GLM and Proc Mixed procedures of SAS produced the same estimates of fixed effects and almost the same estimates of error variance. Proc Mixed and lsmeans of SAS were used to obtain the adjusted means (Federer and Wolfinger, 1998). A program in C++ was written to calculate l_lµ' and other statistics for evaluating the relative frequency of alleles in populations. Estimates of l_lµ' were considered different from zero if their absolute value was greater than twice the standard error. Differences between l_lµ' values were considered significant if they exceeded twice the standard error of a difference. For yield, favorable alleles were dominant. Thus, positive values of l_lµ' indicated the presence of favorable dominant alleles in the population (P_y) that were not present in the hybrid to be improved. In contrast, dominance is for increased plant height and ear height. Because shorter plants with lower ear height are desirable, interest is in identifying recessive favorable alleles that shorten plant and ear height. When favorable alleles are recessive, the greatest positive values of j_jµ - l_lµ' or k_kµ - l_lµ' (depending on whether P_y is more closely related to I₁ or to I₂), reflect the presence of favorable recessive alleles present in P_y, but not in I₁ or I₂.

The error variances of the hybrids and error variances of the inbreds coming from their respective analyses of variance, were used to calculate standard errors of the linear equations used to estimate l_lµ', j_jµ, k_kµ, and the relationship statistic. The comparison of l_lµ' with j_jµ or l_lµ' with k_kµ was used to determine whether selfing should begin in the F₁ (exotic x elite inbred) or after backcrossing to the specified elite inbred (Dudley, 1987).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Mean and Relationship Values
The elite hybrid FR1064 x LH185 had higher grain yield, was taller, and had greater ear height than its mid-parent value (Table 2) . As expected, none of the accession by FR1064 or LH185 crosses had yields as great as FR1064 x LH185 (Tables 2, 3, 4, and 5) . Some accessions, closely related to FR1064 or LH185, when crossed, respectively, to FR1064 or LH185, had yields close to that of the inbred FR1064. For example, DK888 x B73, closely related to FR1064 (Table 6) , when crossed to FR1064 yielded 4 Mg ha^-1 (Table 3), a value lower than the yield of FR1064, but higher than LH185. B844 x Mo17 (closely related to LH185, Table 6), when crossed to LH185 yielded 5.7 Mg ha^-1 (Table 3). This result was closer to the yield of FR1064 than of FR1064 x LH185 (Table 2). When tropical population x B73 accessions were crossed to FR1064 and to LH185, yields of the crosses to FR1064 (related to B73) were lower than crosses to LH185. Similarly, tropical population x Mo17 crosses had lower yields when crossed to LH185 than when crossed to FR1064. Most of the accessions crossed to one of the elite inbreds flowered later than the elite hybrid and were taller with higher ear heights.

View this table: [in this window] [in a new window]   Table 6 Relative frequency of favorable alleles for yield, flowering date, and penetrometer reading; net values for plant and ear height; and relationship values based on grain yield for those accessions that included the same tropical populations crossed to both B73 and Mo17

View this table: [in this window] [in a new window]   Table 7 Relative frequency of favorable alleles (llµ') for yield, flowering date, and penetrometer reading; net values (jjµ/kkµ - llµ') for plant and ear height; and relationship values based on grain yield for accessions consisting of tropical populations crossed to either B73 or Mo17

View this table: [in this window] [in a new window]   Table 8 Relative frequency of favorable alleles (llµ') for grain yield, flowering date, and penetrometer readings;. net values (jjµ/kkµ - llµ') for plant and ear height; and relationship values based on grain yield for temperate populations

For 22 of the accessions estimates of relative frequency of favorable alleles were negative and significant. In order to understand the reasons for the significant negative estimates of l_lµ', theory behind estimates of l_lµ' was examined.

Dudley (1987) proposed four cases for estimating the relative frequency of favorable alleles. These cases are defined by whether _j and _k needed to be set to 0 or 1 in order to provide realistic estimates of _j or _k.

The expected values of l_lµ' for each case in Dudley's model are as follows (Table 4 in Dudley, 1987):

(1)

(2)

(3)

(4)

where l_lµ* is the true value of l_lµ. Note that, for l_lµ' to be negative, l_lµ* must be small, and depending on the case, _j and _k. must be large relative to _j and _k. To illustrate how negative estimates of relative frequency of favorable alleles can occur and how accession x inbred yields similar to inbred yields can occur, consider the following example.

Assume diploid individuals, with two alleles per locus, complete dominance , and negligible epistasis. For each locus, let the homozygous recessive (bb) have a genetic value and the homozygous dominant (BB) have a genetic value . Let the number of loci in the respective classes i, j, k, and l be . The value of µ was calculated as . The frequencies of favorable alleles in each class for the putative population were assumed to be . Expected means of the accession x inbred crosses were calculated. Using the formula for the mean of specific loci, and keeping z, µ, and constant across all loci, the calculated phenotypic values for I₁, I₂, I₁xI₂, I₁xP_y, and I₂xP_y were 4.94, 4.55, 8.45, 5.14, and 4.96 Mg ha^-1, respectively. Using these values l_lµ' was estimated. Since (I₁xP_y - I₂xP_y) > 0, (I₁xP_y - I₂xP_y + I₁xI₂ - I₁) > 0, and (I₁xP_y - I₂xP_y + I₁xI₂ – I₁) < (I₁xI₂ - I₂), the _k0, _k1 case is appropriate. Using the appropriate estimation equation for this case

(5)

The appropriate formula showing the bias estimation of l_lµ' is

(6)

Equation [6] and the equations for the three other cases (Eq. [1], [2], and [3]) illustrate the point that for estimates of l_lµ' to be negative, _l must be low, _j and/or _k must be large. This means that when the yields of the accessions crossed to elite inbreds are comparable with the yields of the inbreds itself, l_lµ' estimates will probably have negative signs.

The findings of only three accessions with favorable alleles for yield not present in FR1064 or LH185 and of 22 populations with significant negative estimates of l_lµ' support the suggestion of Goodman et al. (1990) that this method may not be useful for identification of exotic germplasm with favorable yield genes.

Pollen Shed Date
Early pollen shed is usually dominant (Dudley et al., 1996). Therefore the largest absolute values of l_lµ' with negative signs identified accessions with the highest frequency of favorable dominant alleles for early pollen shedding. Only Uruguay 377A, ARZM 16035, and CHZM 05027 had negative l_lµ' values that were significantly different from zero (Table 8). Other accessions did not have significant estimates of favorable dominant alleles for early flowering (Tables 6, 7, and 8). The large number of accessions with significant positive estimates of l_lµ' suggests a low frequency of favorable alleles in classes i and j as well as in class l. These results parallel those for grain yield.

Plant Height and Ear Height
Taller plants and increased ear height are conditioned by dominant alleles (Dudley et al., 1996). If breeding efforts are directed to improving yield of the elite hybrid, then choosing the wrong accessions could result in lines with undesirably greater plant and ear height. Favorable alleles for shorter plants and lower ears are usually recessive. Consequently, the selection of accessions with high frequencies of favorable recessive alleles is based on net value (differences j_jµ - l_lµ' or k_kµ - l_lµ' depending on whether FR1064 or LH185 is to be improved) (Dudley et al., 1996). If j_jµ or k_kµ is significantly greater than l_lµ' (which is the same as net value being positive and significantly greater than zero) a high frequency of favorable recessive alleles in class j or k relative to unfavorable dominant alleles in class l of the accession is indicated. Only CHZM05027 had a significant positive net value for plant height, and none of the populations had significant positive values for ear height (Tables 6, 7, and 8). Thus, plant and ear height would need to be monitored if these accessions are to be used for improvement of other traits.

Penetrometer Reading
The relative frequencies of favorable alleles for penetrometer reading were not different from zero for any accession (Tables 6, 7, and 8). Seven accessions did not have solutions for upper and lower limits of _j and _k. Thus, analysis of this trait did not identify any population as having favorable alleles for improving stalk strength of the elite hybrid.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Only accessions XL212 x Mo17, CHZM05027, and B844 x Mo17 had significant positive estimates of favorable alleles for yield. Unexpectedly, 22 accessions had significant negative estimates of l_lµ' for yield. Examination of theory indicated negative estimates of relative frequency of favorable alleles for yield resulted from a high frequency of less favorable alleles in class j and/or k as well as in class l of the donor accession. Because the frequency of favorable alleles for yield in all classes of loci is expected to be low in tropical populations, the theory may have limited application to identifying tropical germplasm containing alleles for yield that can be used to increase yield of temperate hybrids.

Estimates of relationship to FR1064 and LH185 of populations crossed to B73 or Mo17 agreed with pedigree information.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Supported in part by funds from the Illinois Agric. Exp. Stn. and by the Germplasm Enhancement of Maize project.

Received for publication July 19, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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