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PLANT GENETIC RESOURCES

Testcross Performance of Semiexotic Inbred Lines Derived from Latin American Maize Accessions

^a Dep. of Crop Science, Box 7620, North Carolina State Univ., Raleigh, NC 27695-7620
^b USDA-ARS, Plant Science Research Unit, Dep. of Crop Science, Box 7620, North Carolina State Univ., Raleigh, NC 27695-7620

^* Corresponding author (jatarter{at}unity.ncsu.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Tropical maize (Zea mays L.) represents the most diverse readily available source of germplasm to broaden the limited genetic base of temperate maize in the USA. One objective of this study was to determine if exotic-derived alleles contributing to enhanced testcross agronomic performance were maintained in semiexotic lines created by inbreeding and pedigree selection. A second objective was to determine if first-generation semiexotic lines could produce hybrids with agronomic performance comparable to commercial U.S. hybrids. One hundred sixty-four semiexotic inbred lines were developed from crosses between temperate-adapted inbred line Mo44 and 23 Latin American maize accessions. Mo44 and each semiexotic line were testcrossed to temperate hybrid LH132 x LH51 for evaluations. In first-stage replicated yield trials, testcrosses of 18 semiexotic lines, representing six different races, had significantly greater grain yields than the Mo44 testcross. Advanced yield evaluations were performed on check entries and 33 selected semiexotic line testcrosses in three additional environments. Across 10 environments, 12 semiexotic line testcrosses exhibited significantly greater grain yield than the Mo44 testcross, indicating recovery of favorable exotic alleles. Semiexotic testcrosses were not competitive with commercial hybrids for grain yield but were similar to or better than commercial hybrids for grain moisture and lodging resistance. Many superior accessions represent relatively recent introductions into regions from which they were collected. Tropical landraces seem to be a good source of exotic germplasm that can be used to broaden the genetic base of modern U.S. maize production and improve productivity.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
IN THE USA, only limited use is made of the vast supply of available maize germplasm. Only six of the approximately 300 races of maize in the New World are represented in commercial cultivars, and only one of these is used in the USA (Goodman, 1985, 1990). As of 1996, exotic germplasm contributed less than 3% to the pedigrees of U.S. maize cultivars (Goodman, 1999). Interbreeding elite lines derived from a small number of open-pollinated cultivars of the Corn Belt Dent race for 60 yr has further reduced the genetic diversity of commercial U.S. cultivars (Troyer, 1999). For example, a 1984 American Seed Trade Association survey revealed that 16% of new inbred line development came from continually breeding new lines from B73 and 9% from Oh43 (Darrah and Zuber, 1986). Smith (1988) reported that a small number of lines, including B73, A632, Mo17, and Oh43, were the major contributors to commercial U.S. maize hybrids. Subsequently, Smith et al. (1999) found that pedigree diversity of Pioneer brand inbreds and hybrids were lower in the 1990s than any previous decade.

Tropical maize germplasm likely contains unique alleles that would be useful to temperate breeding programs (Crossa and Gardner, 1987; Holland and Goodman, 1995). Successful incorporation of exotic germplasm has resulted in the creation of new breeding stocks exhibiting increased yields in maize (Goodman et al., 2000) as well as in soybean [Glycine max (L.) Merr., Thompson and Nelson, 1998], and wheat (Triticum aestivum L., del-Blanco et al., 2001). However, working with tropical maize in temperate environments is hindered by photoperiod sensitivity (Brown, 1975) and linkage between favorable alleles and alleles contributing to maladaptation (Brown, 1953, 1988). Furthermore, choosing appropriate germplasm sources from among the many thousands of available accessions has been hindered, until recently, by inadequate testing and reporting systems (Stuber, 1978; Goodman, 1983, 1999).

A maize germplasm incorporation program, independent of the Germplasm Enhancement of Maize project (Pollak and Salhuana, 1998) and the Latin American Maize Project (Salhuana et al., 1991), has been conducted at North Carolina State University for about 20 yr. This program began by screening 1300 typical Latin American maize accessions for agronomic utility (ear and plant height, lodging resistance, and pollination success rate) in a nearly daylength-neutral nursery in southern Florida for 2 yr (Goodman, 1983). Castillo-Gonzalez and Goodman (1989) advanced the best accessions to replicated yield trials under short-day and long-day photoperiod conditions and identified a subset of accessions that appeared to have sufficient agronomic value to be useful for breeding. The 40 most promising accessions were selected for conversion to photoperiod insensitivity by crossing them to temperate-adapted line Mo44 and then intermating within each segregating population. Following selection for photoperiod insensitivity, four full-sib families were sampled from each of the 40 populations for agronomic evaluation. Holland and Goodman (1995) reported that 24 of these families ranked higher than Mo44 for grain yield in testcrosses to Corn Belt Dent hybrids and concluded that multiple testers were not needed to efficiently identify superior semiexotic populations. Holland and Goodman (1995) suggested that inbred lines with good combining ability for yield could be extracted from superior semiexotic families. Therefore, they selected 69 semiexotic families representing 29 different Latin American accessions as sources for inbred line development.

We hypothesized that favorable alleles would be maintained in some of these semiexotic lines following inbreeding and visual selection. To test this hypothesis, we developed semiexotic inbred lines of 50% tropical parentage and agronomically evaluated them in crosses to a U.S. Corn Belt hybrid. The semiexotic inbred lines, derived from superior semiexotic families identified by Holland and Goodman (1995) were compared with Mo44 in testcrosses. Our objective was to determine if semiexotic inbred lines inherited exotic alleles superior to Mo44 alleles for productivity in a temperate environment. The semiexotic line testcrosses were also compared with commercial hybrid cultivars to assess their potential as germplasm donors to commercial maize breeding programs.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Semiexotic Inbred Line Development
The source populations for inbred line development were 69 photoperiod-insensitive, semiexotic, full-sib families developed from crosses between Mo44 and tropical accessions, as described by Holland and Goodman (1995). Mo44 was derived from a cross between Mo22 and Pioneer Mexican Synthetic 17 and is largely unrelated to either of the two major heterotic groups of maize, Reid Yellow Dent or Lancaster Sure Crop (Gerdes and Tracy, 1993).

Semiexotic inbred lines were developed from the selected families by two generations of full-sib mating, followed by two generations of self-fertilization. At each generation of inbreeding, visual selection, among and within lines, was conducted for earlier flowering, minimal silk-tassel interval, stay green, lower ear placement, stalk strength, well-filled ears, lower grain moisture at harvest, and resistance to southern corn leaf blight {caused by Cochliobolus heterostrophus (Drechs.) Drechs. [anamorph = Bipolaris maydis (Nisikado) Shoemaker]}, northern corn leaf blight [caused by Exerohilum turcicum (Pass.) Leonard and Suggs], gray leaf spot (caused by Cercospora zeae-maydis Tehon and Daniels), anthracnose stalk rot (caused by Colletotrichum graminicola G. W. Wils.), and Fusarium ear rot {caused by Fusarium verticillioides (Sacc.) Nirenberg [syn. Fusarium moniliforme J. Sheldon]}. Leaf blights were artificially inoculated by applying sorghum seeds infected with pathogenic fungi to the whorl of juvenile maize plants. A toothpick coated with Fusarium spores was inserted into the developing ear approximately 1 wk after pollination to promote Fusarium ear rot. Inoculation for stalk rot was accomplished by penetrating the base of the stalk and injecting a liquid solution of inoculum approximately two weeks after pollination.

The final set of selected semiexotic germplasm consisted of 164 lines derived from 36 source families, with 1 to 36 lines developed from each family (Table 1). The selected lines represent 23 of the original accessions, 14 different races, and nine different countries of origin (Table 1). Semiexotic lines were designated by their race (e.g., Tusón), their five to six character accession code (e.g., BAI III), their family of origin (e.g., a, b, c, or d, corresponding to Holland and Goodman, 1995), and a unique line within family number (1–36). The lines were derived from plants in a generation which had an expected inbreeding coefficient of F = 0.74 and the lines had expected homogeneity approximately equivalent to F_4:5 lines. The plants within the lines had an expected inbreeding coefficient of F = 0.87.

At all locations, plots were two 4.86-m rows sown with 44 seeds, with a 1-m length alley at the end of each plot. At Plymouth and Clayton, rows were spaced 0.97 m, resulting in a population density of approximately 57 000 plants ha^-1 within the planted plot area. In Lewiston and Jackson Springs, interrow spacing was 0.91 m, and population density was 60000 plants ha^-1.

The number of plants, mean plant height (measured from the ground to the tassel tip in 1999 and to the terminal node in later years), and mean ear height (height to node of topmost ear) were recorded for each plot at all seven environments. Days to anthesis and silking were recorded only at Clayton. Anthesis date was the date on which at least 50% of the plants in the plot were shedding at least 50% of the available pollen. Silk emergence date was the date on which 50% of the plants in the plot were displaying visible silks. Counts were taken of root-lodged plants (leaning greater than 30° from vertical with intact stalks) and stalk-lodged plants (broken below the ear or plants with dropped ears) in all environments except Lewiston and Plymouth in 1999. In 1999, hurricanes Dennis and Floyd made landfall on the coast of North Carolina before harvest and resulted in severe lodging at Lewiston and Plymouth only. Therefore, a visual estimate of percent erect plants was made at these coastal plain locations in 1999. At maturity, plots were machine harvested, and grain yield and moisture content were recorded for each plot. Grain yields were adjusted to 155 g kg^-1 moisture content.

Advanced Yield Trials
On the basis of the agronomic evaluations in 1999 and 2000, 33 semiexotic lines with testcross grain yields greater than the Mo44 testcross, acceptable percentage of erect plants, and acceptable grain moisture were selected for additional testing in 2001. The 33 semiexotic entries were tested along with Mo44 x (LH132 x LH51), and eight commercial hybrid checks (Pioneer brand hybrids 3165, 32K61, and 31B13; DeKalb brand hybrids 683, 689, 714, and 743; and LH132 x LH51). The experiment was conducted at Clayton, Plymouth, and Jackson Springs, NC. Experimental design was a 6 x 7 lattice design with two replications per location. Otherwise, the experimental methods were identical to the original experiments conducted in 1999 and 2000.

Statistical Analysis
Data from seven or 10 environments were combined and analyzed by means of Proc MIXED in SAS version 8.0 (SAS Institute, 2000). Entry (experimental or check hybrid) was considered a fixed factor, whereas all other factors (environment, set, set x environment, entry x set, entry x environment) were treated as random. Percent stand was used as a covariate for grain yield only. Tests for significance of genotype by environment interaction were performed with a chi-square test of the difference between –2 times the logs of the restricted maximum likelihoods of the complete model and a reduced model lacking the genotype by environment interaction factor (Littell et al., 1996). The approximate P-value of this test was obtained by dividing the P-value of the one degree of freedom chi-square statistic by two (Self and Liang, 1987; Littell et al., 1996). This analysis permitted estimation of experimental line means adjusted for set effects, facilitating presentation of results from both sets simultaneously.

The effect of missing data of Mo44 x (LH132 x LH51) in three of the seven first-stage yield trial environments was investigated by comparing results based on least square means from all seven environments to those based on least square means from only the four environments in which Mo44 x (LH132 x LH51) was included. The conclusions about which semiexotic line testcrosses were significantly greater yielding than their temperate parent was consistent across the two analyses. We concluded, therefore, that use of the complete data set was appropriate, and all results reported from the first stage yield trials are based on data from all seven environments.

Obvious spatial trends within locations were observed in the data from the advanced yield trials grown in 2001. A variety of spatial analyses was performed on the data from within each environment of the advanced yield trials. Models with up to fourth-order polynomial effects of rows and columns in the field layout were tested. Trend effects were maintained in the model if significant at P < 0.01 (Brownie et al., 1993). Proc MIXED was used to compare the following models within each environment: a model including complete and incomplete block effects, models with row and column trend effects selected as described, a model with correlated errors, and a model with both trend effects and correlated errors (Brownie et al., 1993). Within each environment, the model that minimized Akaike's Information Criterion (Lynch and Walsh, 1997) was chosen. Finally, adjusted entry means for the selected lines were combined from all 10 evaluation environments within each set and analyzed by Proc MIXED.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
First Stage Yield Trials
Significant (P < 0.05) variation among entries was observed for all measured traits except root lodging in the first stage yield trials (1999 and 2000). Overall, 18 semiexotic lines had individual mean testcross grain yields significantly (P < 0.05) greater than the mean grain yield of Mo44 x (LH132 x LH51) (Fig. 1). These superior lines were derived from eight accessions and six of the 14 races. Among these eight accessions, the proportion of derived lines with significantly greater testcross yield than Mo44 ranged from 14% for lines derived from Puya SAN 349 to 45% for lines derived from Tusón CUB 57 (Table 1). The races from which a relatively high proportion of superior semiexotic lines were derived included Tusón, Tuxpeño, and Puya, suggesting that they are excellent sources of germplasm for combining ability with U.S. Corn Belt Dent germplasm. Only one semiexotic line testcross had significantly lower grain yield than Mo44 x (LH132 x LH51).

View larger version (28K): [in this window] [in a new window]   Fig. 1. Histogram of mean grain yields of 164 semiexotic maize lines testcrossed to LH132 x LH51 evaluated in first stage yield trials in seven North Carolina environments in 1999 and 2000. Mean grain yields of Mo44 x (LH132 x LH51) and eight commercial hybrids indicated with arrows. An LSD of 0.7 Mg ha-1 is appropriate to compare means of one semiexotic line testcross to Mo44 x (LH 132 x LH51) at = 0.05. An LSD of 0.6 Mg ha-1 is appropriate to compare means of one semiexotic line testcross to mean of all seven commercial hybrids at = 0.05.

View larger version (31K): [in this window] [in a new window]   Fig. 2. Histogram of mean grain moisture of 164 semiexotic maize lines testcrossed to LH132 x LH51 evaluated in first stage yield trials in seven North Carolina environments in 1999 and 2000. Mean grain moistures of Mo44 x (LH132 x LH51) and eight commercial hybrids indicated with arrows. An LSD 7 g kg-1 is appropriate to compare means of one semiexotic line testcross to Mo44 x (LH 132 x LH51) at = 0.05. An LSD of 5 g kg-1 is appropriate to compare means of one semiexotic line testcross to mean of all seven commercial hybrids at = 0.05.

The mean grain yield of the selected semiexotic line testcrosses was significantly lower than the mean of seven commercial hybrids in both sets (Tables 2 and 3). However, the selected semiexotic lines performed well compared with commercial hybrids for other important agronomic traits. The mean semiexotic line testcross grain moisture was significantly lower than the mean of seven commercial checks in both sets. The semiexotic testcrosses of set one had significantly better lodging resistance than the commercial hybrids and exhibited no other significant differences (Table 2). In addition to a reduction in grain moisture, the selected semiexotic line testcrosses in set two also had fewer days to both anthesis and silk than the commercial hybrids (Table 3).

Relationships among Superior Latin American Accessions
A rational method for choosing better germplasm sources among landrace accessions in the absence of yield trial data would be useful to maize breeders. Identification of common geographic or ecological origins among superior landrace accessions could provide guidance for designing optimal germplasm sampling strategies. The most striking feature in common among many of the superior accessions evaluated in this study was that they represent relatively recent introductions (since the 1920s) into the regions where they were collected. Twelve accessions in the set of semiexotic lines evaluated in this experiment were introduced races (Table 1). All of the accessions from Ecuador that were represented in the final set of semiexotic inbred lines were introduced there, probably from Cuba or Mexico (Timothy et al., 1963). The accessions from Brazil, Bolivia, Peru, and Venezuela represented in our study also were introduced, but their original countries of origins are unknown (Grant et al., 1963; Paterniani and Goodman, 1977; Ramírez et al., 1960; R. Sevilla, personal communication). We suggest that the superior semiexotic lines disproportionately represent introduced races because those races that flourished sufficiently in their introduced ranges have broader adaptability and superior productivity. Introduced accessions with broad adaptability and outstanding productivity may have out-competed local landraces and spread beyond their introduced range. For example, races such as Cubano Amarillo Duro, Cubano Cateto, Cubano Dentado, and Cubano Tusón, which were collected at relatively high frequency outside of Cuba, likely represent highly competitive germplasm. In contrast, introduced races that lacked outstanding adaptability would not have competed adequately with local landraces and would have been extinct or approaching extinction by the time of large-scale maize germplasm collections in the 1940s to 1960s.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The recovery of semiexotic lines with superior combining abilities compared with their temperate-adapted parent, Mo44, supports our hypothesis that superior exotic alleles can be maintained through the inbreeding process even in the absence of selection for testcross performance during line development. The performance of these semiexotic lines indicates that these lines inherited alleles from their tropical accession parents that contribute to enhanced productivity compared with Mo44 alleles. Although none of the semiexotic line testcrosses was competitive with current commercial hybrids for grain yield, neither was their temperate parent testcross, Mo44 x (LH132 x LH51), nor their single-cross tester, LH132 x LH51 (Tables 2 and 3). Furthermore, the commercial cultivars are single-cross hybrids, whereas the semiexotic line testcrosses were three-way crosses, with less potential for heterosis. We hypothesize that semiexotic hybrids competitive with commercial cultivars could be developed by transferring the favorable exotic-derived alleles recovered in these semiexotic lines to superior genetic backgrounds and crossing the resulting lines to elite inbred testers.

The semiexotic lines we evaluated were less homozygous (F = 0.87) than inbreds typically used to produce commercial hybrids. Several more generations of self-fertilization would be required to produce highly inbred lines, and some of the exotic alleles maintained in the semiexotic lines tested in this study could be lost during additional generations of inbreeding. However, it is likely that exotic alleles with strongly deleterious effects were already eliminated in the early generations of inbreeding and selection, so that additional inbreeding and selection would cause only relatively minor changes in allele frequencies. Therefore, it should be possible to develop highly inbred semiexotic lines similar to the lines evaluated in this study in terms of their proportions of exotic germplasm and testcross performances.

Because the semiexotic lines have distinct genetic backgrounds compared with the commercial maize of the USA, it is likely that some of the favorable yield alleles they inherited from their tropical accession parents are not represented in the elite maize breeding germplasm of the USA. Furthermore, Holland and Goodman (1995) reported that the parental semiexotic populations of these inbred lines exhibited similar levels of heterosis when combined with either Reid Yellow Dent or Lancaster Sure Crop testers. This result suggests that the tropical landraces contain alleles for productivity absent from both of the major heterotic groups used in the USA and that lines from most tropical accessions can be considered a third heterotic group with respect to Reid Yellow Dent and Lancaster Sure Crop temperate germplasm. This alternative heterotic group provides maize breeders with a new source of parental breeding lines that can contribute to broadening the genetic base of modern U.S. inbred lines without sacrificing productivity.

Received for publication November 18, 2002.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Tarter, J. A. Articles by Holland, J. B. Search for Related Content PubMed Articles by Tarter, J. A. Articles by Holland, J. B. Agricola Articles by Tarter, J. A. Articles by Holland, J. B. Related Collections Maize Crop Genetics