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Published in Crop Sci. 43:1952-1959 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP BREEDING, GENETICS & CYTOLOGY

Comparing Methods for Integrating Exotic Germplasm into European Forage Maize Breeding Programs1

Domagoj Simicb, Thomas Presterla, Günter Seitzc and Hartwig H. Geiger*,a

a Institute of Plant Breeding, Seed Science, and Population Genetics, D-70593 Stuttgart, Germany
b Agricultural Institute Osijek, HR-31000 Osijek, Croatia
c AgReliant Genetics, Westfield, IN 46074, USA

* Corresponding author (geigerhh{at}uni-hohenheim.de).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
To broaden the genetic base of Central European germplasm of forage maize (Zea mays L.), it is necessary to integrate exotic materials into adapted breeding populations. The aim of the study was to compare several methods of integration by evaluating 18 initial crosses between adapted (recipient) and exotic (donor) elite dent inbred lines with regard to their testcross performance. Specific objectives were (i) to assess the effects of backcrossing the F1 to the recipient and intermating the F2 generation, (ii) to determine the benefit of selection for earliness, and (iii) to examine the influence of the recipient and donor genotype on the integration methods. Three foundation populations were developed from each of the initial crosses: F2, F2–Syn2 (= F2 twice intermated) and BC1. Fifty S1 lines were produced from the 4.4% earliest S0 plants of each foundation population. Bulks of these S1 lines along with the pertinent F1s and recipient parents were testcrossed with an adapted flint line. The testcrosses were evaluated in field trials at three locations in southern Germany in 1994 and 1995. Whole plant quality traits were investigated by near infrared spectroscopy (NIRS). Differences between generation means were significant for all agronomic traits and for starch content and crude protein content. For dry matter yield, the effect of backcrossing varied among the initial crosses, while the influence of intermating was nonsignificant in most instances. Averaged across all generations, initial crosses which had inbred lines B73 or Mo17 as donors were superior to all other crosses for dry matter yield without being later in maturity. For maturity, the effects of backcrossing, intermating, and selection for earliness generally were all germplasm specific. This suggests that the choice of germplasm is more important than the integration method.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
CONTINUED "SECOND CYCLE" BREEDING in maize unavoidably decreases the genetic diversity. Exotic germplasm has been suggested as a source to increase the genetic variability of maize breeding programs (Wellhausen, 1965; Hallauer, 1978; Stuber, 1978; Geadelmann, 1984; Goodman, 1985; Ron Parra and Hallauer, 1997). Hallauer and Miranda (1988) defined exotics as germplasm that does not have immediate use without selection for adaptation. A similar definition was given by Goodman (1985). In temperate areas, potential resources can be indigenous old landraces, temperate late foreign germplasm, or tropical and semi- or subtropical germplasm. Ron Parra and Hallauer (1997) illustrated that in temperate zones the highest short-term gain in selection can be achieved by using unadapted temperate germplasm. Various methods have been proposed to improve the performance of materials containing exotic germplasm, but to date no experimentally verified theory exists on how optimally to integrate unadapted germplasm into adapted breeding populations.

The nonadaptedness of exotic temperate germplasm per se and of its crosses with adapted lines is primarily defined by late flowering and maturity, and by sensitivity to chilling (Frei, 2000). Selection for early flowering has been suggested as a means to reduce grain moisture content in populations derived from exotic x adapted crosses (Troyer and Brown, 1972; Geadelmann, 1984; Hawbaker et al., 1997). The optimal proportion of exotic and adapted germplasm in a foundation population, from which line development is initiated, was theoretically studied by Ho and Comstock (1980), Dudley (1984), Bridges and Gardner (1987), and Crossa (1989), and experimentally investigated by Crossa and Gardner (1987), Albrecht and Dudley (1987), Gouesnard et al. (1996), Holland et al. (1996), and Tallury and Goodman (1999). All these studies showed that the "performance gap" between the adapted and the exotic parent has a crucial influence on the optimal type of foundation population. In experimental studies, backcross foundation populations generally were more suitable than the F2 because of their higher mean performance (Albrecht and Dudley, 1987; Crossa and Gardner, 1987; Gouesnard et al., 1996). The genetic variance, on the other hand, is greater in F2 populations allowing for a faster progress from selection. To reduce unfavorable linkage disequilibria in F2 (would also apply to BC1), advancing the populations by random intermating over several generations has been suggested (Hanson, 1959a, b; Nelson, 1973; and Lonnquist, 1975). All experimental studies published to date used either one particular exotic source or one integration method only. No study compared backcrossing, intermating, and selection across various recipient x donor combinations.

Germplasm broadening of forage maize via integration of exotic germplasm has so far received little attention (Bosch et al. 1994). Thompson (1968) reported that tropical exotics would be useful for achieving maximal forage yield, but the resultant material would display suboptimal feeding quality. Recently, Bosch et al. (1994), Gouesnard et al. (1996), and Mas et al. (1998) evaluated temperate x tropical crosses for forage production in Europe. Information on the suitability of temperate U.S. germplasm for silage production is limited (Coors, 1994; Lundvall et al., 1994), though it could be important for European breeding programs.

The general aim of the study was to compare methods of integrating exotic elite maize lines from U.S. Corn Belt into current central European breeding materials. Specific objectives were (i) to assess the effects of backcrossing to the adapted line (recipient) and of intermating the F2 generation, (ii) to determine the importance of selection for earliness within populations, and (iii) to examine the genotypic influence of recipient and donor (exotic line) on the integration methods.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Materials
The basic parental material was comprised of four adapted (recipients) and nine unadapted elite inbred lines (donors) (Table 1). The recipients represent current Central European dent breeding material of early maturity (about FAO 200 to FAO 250) with commercial relevance. They are designated as D06, D408, D08, and F272. The first three lines were developed at the University of Hohenheim, and inbred F272 originates from the "Institut National de la Recherche Agronomique" (INRA), France. The donors are well known public lines from the USA ranging from FAO 500 to FAO 700. Six of them (A619, H107, Mo17, Va26, H108, and NC258) belong to the ‘Lancaster Sure-Crop’ gene pool and the remaining three (B73, B79, N192) to the ‘Reid Yellow Dent’ pool (MBS Genetics Handbook, 1984, 1990, 1994).


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  		Table 1. Genetic background of the 13 dent parental inbred lines of maize (Zea mays L.) used in the study.

 
All materials were developed in Emmendingen, Germany. This location is located in the South of the upper Rhine valley. The climate there is sufficiently mild to avoid loss of genotypes through insufficient seed maturity. The adapted lines D06 and D408 were each crossed with the first five exotics, and D08 and F272 with the remaining four, resulting in 18 adapted x exotic crosses, respectively. The initial crosses were produced in 1988 and the first backcross (BC1) and F2 generation in 1989. F2 populations were intermated twice by hand pollination in 1990 and 1991 to produce the F2–Syn2. For that purpose, 50 paircrosses were produced from 100 random parent plants. Equal numbers of kernels of all pollinated ears were bulked for the next generation. From each of the 54 segregating populations (generations F2, F2–Syn2, and BC1 from each of the 18 initial crosses), 1150 plants were grown in 1992 in 25 rows with 46 plants per row. The three earliest flowering plants in each row, excluding border plants, were selfed, resulting in a total of 75 selfed plants per population. At maturity, the earliest 50 of the 75 selfed plants were selected for S1 line production.

In 1993, bulks of the selected 50 S1 lines from each foundation population, along with the 18 F1 crosses and three of the recipients (D06, D408, and D08) were topcrossed with the flint inbred line D171 (University of Hohenheim) as a common tester. Line D171 traces back to various European flint lines (D504, D102, DK105 and D107) and is fully adapted to the target area. No seed was available for producing testcrosses of recipient line F272. Figure 1 illustrates the material development for one of the 18 initial crosses. Each of the 18 adapted x exotic cross combinations was represented by four entries: one S1–bulk testcross progeny each of the F2, F2–Syn2, and BC1 foundation populations, and one testcross progeny of the F1. In the following the three different types of foundation populations and the F1s are termed generations.



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  		Fig. 1. Development of testcross progenies for one of the 18 initial crosses.

 
Field Trials
In the field trials, all testcrosses and six checks were grown together in a 9-by-9 lattice design with three replications. Checks were high-yielding commercial hybrids ranging from FAO 250 to FAO 380 maturity units. Experiments were conducted during 1994 and 1995 at three locations in southwest Germany (Eckartsweier near Strasbourg and Emmendingen near Freiburg, both located in the upper Rhine valley, and Stuttgart-Hohenheim). All environments represent cropping areas for FAO 250 to 300 hybrids and are located at 48° N (latitude) and between 7° E and 9° E (longitude). The soils of the experimental fields are sandy loam at Emmendingen, and silty loam at Emmendingen and Hohenheim. From April to October, long-term average daily temperature and annual precipitation were 14.6°C/576 mm at Eckartsweier, 14.3°C/604 mm at Emmendingen, and 13.6°C/478 mm at Hohenheim. The year 1994 was characterized by stress conditions with maximal temperatures of more than 34°C in late July and early August at all three locations, combined with below normal precipitation.

The experimental fields received between 120 and 150 kg fertilizer N ha-1 at the time of planting. Trials were machine-planted in two-row plots, 4 m long with 0.75 m between rows. Plots were overplanted and thinned to a uniform stand of 11 plants m-2 at the five-leaf stage. Weed control, and other cultivation measures were performed as in practical farming. The parasitic wasp Trichogramma evanescens (Westwood) was used as a biological control of the European corn borer Ostrinia nubilalis (Hübner).

Character Assessment
Five agronomic traits were assessed: dry matter content of the whole plant (DMC, %), dry matter yield of the whole plant (DMY, g m-2), plant height (cm), days (after planting) to pollen shed, and days to silking. Flowering dates were measured in four out of the six environments only. Whole plants were harvested with a forage chopper. Chopped plant matter was weighed and a sample of approximately 1 kg was collected from each plot for laboratory analysis.

For the NIRS (near infra red spectroscopy) analyses, the fresh matter samples were dried to a constant weight at 40°C in a forced-air oven. Dried samples were ground with a Brabender hammer mill and reground with a Retsch Ultra-centrifugal ZM 1 mill (Retsch GmbH & Co., Germany) to pass a 1-mm sieve. Two subsamples of each sample were measured. In 1994 a Perstorp NIRSystem 5000, and in 1995 a Perstorp NIRSystem 6500 near infrared analyzer (Perstorp Analytical GmbH, Rodgau, Germany) were used. The same wavelength spectrum was analyzed in both years. Thirty randomly chosen samples from the experiments in 1994 were used as a calibration set for the whole series of experiments. Calibrations, predictions, and laboratory assays of reference analyses were performed as described by Degenhardt (1996). Calibrations and reference analyses were performed at the Institute of Grassland and Forage Research, Federal Agricultural Research Centre, Braunschweig-Völkenrode, Germany. The following four quality traits were determined by NIRS assessment: acid detergent fiber content (ADF), in vitro digestibility, starch content, and crude protein content. In vitro digestibility was expressed as percentage of organic matter, and all other traits as percentage of dry matter. Reference analyses were conducted according to Goering and Van Soest (1970) for ADF, and according to Tilley and Terry (1963) for in vitro digestibility.

Statistical Analysis
In a first step, the individual experiments were analyzed corresponding to the lattice design as described by Cochran and Cox (1957). Outliers were determined according to Anscombe and Tukey (1963) and if significant at P = 0.05, they were declared as missing values and substituted by estimated values using the iterative method of Healy and Westmacott (1956). For each missing value, one degree of freedom was subtracted from the total and error degrees of freedom.

Entry means and error variances from the individual experiments were used for combined analyses of variance of the series of experiments. If the lattice efficiency (Cochran and Cox, 1957) in an individual experiment exceeded 100%, lattice-adjusted entry means and effective error variances were used for the combined analyses of variance. This generally applied to the traits DMC, DMY, and crude protein content. Each of the six location x year combinations was considered as a random (macro-) environment. In all statistical analyses, the entry effects (entries, generations, initial crosses, recipients, donors) and corresponding interaction effects were assumed as fixed, and all other effects (environments, genotype x environment interaction, and error) as random variables.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The two growing seasons varied considerably. The 1994 season was very dry and much hotter than normal, resulting in a particularly low DMY at Emmendingen (Table 2). Growing conditions in 1995 were favorable only for early maturing maize, since the vegetative growth period was delayed due to heavy rains resulting in low DMC in late maturing materials. Averaged across Hohenheim and Eckartsweier, pollen-shedding and silking dates were nine and seven days earlier in 1994 than in 1995. The highest environmental mean for ADF was obtained at Emmendingen 1994, for in vitro digestibility and starch content at Hohenheim 1994, and for crude protein at Eckartsweier 1994.


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  		Table 2. Environmental means including all entries and overall means of testcrosses and checks for five agronomic (A) and four quality (B) traits.

 
On the average, the testcrosses had a significantly higher DMC than the checks (Table 2). The checks outyielded the testcrosses on average by 110 g m-2 and were 9 cm taller (both significant). While pollen shed in checks occurred significantly later (on average two days later), silking was only 0.5 d later. The differences between testcrosses and checks for quality traits were small and nonsignificant.

The combined analyses of variance across environments showed significant differences among the 81 entries for all traits considered (data not shown). Entry means ranged from 25.9 to 34.8% for DMC, from 1303 to 1659 g m-2 for DMY, from 77.2 to 84.9 d to pollen shedding and from 79.0 to 89.2 d for silking date. Partitioning the entry sums of squares revealed significant effects of generations for all agronomic traits, as well as for starch content and crude protein content, and differences among initial crosses were highly significant for all traits (Table 3). There were significant interactions between generations and initial crosses for all traits. The three-fold interactions among generations, initial crosses, and environments were significant for DMC, only.


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  		Table 3. F-statistics and significance levels of the effects of mean squares for generations (G), initial crosses (I), their interaction (G x I), and interactions of G, I, and G x I the environments (G x E, I x E, G x I x E) for five agronomic (A) and four quality (B) traits, combined across six environments.

 
Because of the foregoing complex interactions, the effects of generations were studied separately for each initial cross. For DMC, significant differences between generations were observed in all initial crosses, except D408 x A619 (data not shown). The testcrosses of the F1 (F1·T) generally had the lowest values, while those of BC1s (BC1·T) consistently displayed the highest values (Table 4). Differences between these two generations were significant in every initial cross, except D08 x N192. Averaged across initial crosses, significant differences occurred between the testcrosses of F2 (F2·T) and those of the other three generations.


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  		Table 4. Means and significance levels of differences between the testcrosses of four generations (F1, F2, F2–Syn2, and BC1) derived from 18 adapted x exotic initial crosses for dry matter content (%). Generations F2, F2–Syn2, and BC1 were preselected for early flowering.

 
For DMY, testcross means of the four generations did not differ in most initial crosses (data not shown). Three initial crosses with significant differences among generations for both DMC and DMY are presented in Fig. 2. The F2·T of D06 x B73 was the highest yielding testcross in the whole experiment with 1549 g m-2. However, there were significant differences only between F2·T and BC1·T, and between F2·T and R·T (R = recipient line). The F1·T of D06 x Va26 significantly outyielded the BC1·T only. In D08 x NC258, the BC1·T had a significantly lower DMY than the testcrosses of all other generations. The R·T crosses of D06 x B73 and D08 x NC258 had the highest values for DMC and the lowest for DMY, while R·T of D06 x Va26 had a much higher DMY than BC1·T and F2·T. Testcrosses of generations derived from D08 x NC258 varied greatly for both DMY and DMC. In summary, the three initial crosses showed different groupings of generations: while the series R·T: BC1·T: F2·T, F2–Syn2·T: F1·T of D08 x NC258 showed a linear yield increase, entry F2·T significantly deviated from linearity in D06 x B73, and no linearity at all could be observed in D06 x Va26.



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  		Fig. 2. Relationship between dry matter content and dry matter yield based on testcrosses of four generations (F1, F2, F2-Syn2, BC1) and the respective recipient x tester cross of three recipient x donor initial crosses.

 
Significance levels of the effects of generations and donors, computed for each recipient line separately, greatly varied between recipient lines and also depended on the trait (Table 5). In the testcross groups with recipient lines D06 and D408 in common, donors had a stronger influence than generations for most traits while in the D08 and F272 groups the reverse was true for the agronomic traits whereas no consistent trend was detectable for the quality traits.


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  		Table 5. F-statistics and significance levels of the mean squares for generations (Gen.) and donors in testcrosses of four generations from 18 initial crosses combined over donor lines of a given recipient line for five agronomic (A) and four quality (B) traits, combined across six environments.

 
Averaged across donors with a given recipient line in common, the recipient x donor testcrosses (F1·T) had notably lower DMC than the testcross of the recipient line (R·T) (Tables 6 and 7). Those differences were relatively small for DMY, except for recipient D08, which had a considerably lower testcross yield than its F1s. This was also reflected in a relatively large inferiority of the BC1 testcross means (BC1·T) compared to those of generations F1, F2, and F2–Syn2. For recipient line F272, the F1 testcrosses significantly outyielded the other three generations, which themselves did not differ significantly from each other. For all recipients, the R·T crosses flowered earlier than the respective F1·T crosses. Also, the BC1·T crosses generally flowered earlier than the F2·T and F2–Syn2·T crosses. No significant differences between generations were observed for ADF content and in vitro digestibility. Averaged across all 18 initial crosses, this was also true for starch and crude protein content.


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  		Table 6. Means of testcrosses of recipient lines (R·T) of four generations (F1, F2, F2–Syn2, and BC1) derived from crosses of the recipients D06 and D408 with five exotic donors averaged across the respective donors for five agronomic (A) and four quality (B) traits. Generations F2, F2–Syn2, and BC1 were preselected for early flowering.

 

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  		Table 7. Means of testcrosses of recipient lines (R·T) and of four generations (F1·T, F2·T, F2–Syn2·T, BC1·T) derived from crosses of the recipients D08 and F272 with four exotic donors averaged across donors for five agronomic (A) and four quality (B) traits. Generations F2, F2–Syn2, and BC1 were preselected for early flowering.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Late commercial checks of maturity group FAO 300 through 350 were taken since we expected extreme lateness of entries with a high proportion of exotic germplasm, particularly of the F1·T crosses. However, the mean DMC of the latter was about 1.5% lower than that of the checks. Thus, a proportion of 25% of exotic germplasm in the testcrosses, did not seriously delay maturity in our materials. Holley and Goodman (1988) observed that even a proportion of 50% of tropical germplasm may lead to grain moisture contents within the range of commercial U.S. hybrids.

Contrasting the F1·T with the R·T crosses for yield allows one to select the most promising initial cross(es) before commencing the process of integrating the exotic germplasm. Our results show that the majority of the F1·T crosses outyielded the respective R·T crosses (Fig. 2; Table 5). After backcrossing, intermating and selection for earliness, mean values of the F2·T, F2–Syn2·T, and BC1·T crosses did not change substantially for DMY.

As expected, the BC1·T crosses were earlier than the F2·T crosses in all instances. However, this difference was significant in 10 initial crosses only. The difference did not always reflect the maturity difference between recipient and donor line of a particular initial cross. For example, a much greater deviation in maturity between F2·T and BC1·T was expected for the initial cross D08 x NC258, one of the most extreme parental combinations (very early x very late). The deviation was not greater than that obtained for the combination of the same recipient with the earliest exotic line N192. These results are questioning Dudley's (1984) statement that generation BC1 is preferable to F2 as a foundation population if the parents greatly deviate in adaptation.

The low influence of recombination (F2·T versus F2–Syn2·T) for DMY indicates that either epistatic interaction between linked genes was negligible or that positive and negative epistatic effects largely canceled each other. Results are in accordance with those obtained by Hoffbeck et al. (1995). However, Eagles et al. (1989) found a detectable effect of one generation of random mating (F2·T versus F2–Syn1·T) in two tropical x temperate maize population crosses.

Intense preselection for earliness generally led to a significant increase in DMC. In addition to allele frequency changes, this might have been caused by a shift of the genome proportions in favor of the recipients. Separating the effects of these two confounded phenomena would have been possible with genetic markers, but this was beyond the scope of the present study. On the other hand, no significant yield loss was observed from F1·T to F2·T in most crosses. The phenotypic correlation between DMC and yield of the testcrosses differed between the four recipients. Significant negative correlation coefficients were obtained for D08 and F272 and nonsignificant coefficients for D06 and D408. This indicates that the correlation was partly due to linkage disequilibrium which may vary in size and direction among sets of initial crosses. Similar results were obtained by Lindsay et al. (1962).

The usefulness (Schnell, 1983) of the foundation populations strongly depended on the recipient line. Hence, differences in the suitability of adapted lines as recipients for integrating exotic germplasm should be taken into account in practical breeding. "Matching" of recipient and donor genomes seems to be an important prerequisite for a successful integration.

All exotic donors used in the present study are well-known U.S. public Corn Belt lines. They represent the basic germplasm for many maize hybrids grown in the USA as well as in most other temperate regions in the world. Although the donors belong to only two heterotic groups, their suitability as donors differed considerably. Materials including the donors B73 or Mo17 generally outyielded the materials derived from all other donors (Fig. 2 and data not shown). Obviously, these two lines are outstandingly good as donor sources excelling in high combining ability to the European flint pool.

In conclusion, our results demonstrate that the effects of selection for earliness, and the influence of backcrossing as well as intermating were germplasm specific. Thus, the choice of germplasm seems to be more important than the integration method. Generally, little or no advantage of backcrossing the F1 to the recipient parent or of intermating the F2 population showed up in the testcrosses. Thus, F2 populations appear to be the most promising foundation population material when U.S. elite inbred lines are used as exotic donors for broadening the genetic base of Central European breeding materials.


   ACKNOWLEDGMENTS
 
The authors are indebted to Dr. C. Paul at the Institute of Grassland and Forage Research, Federal Agricultural Research Centre Braunschweig-Völkenrode for NIRS calibrations and reference analyses, and to Prof. Dr. A.E. Melchinger for his advice in designing the experiment and for building up the experimental materials. Many thanks to Dr. D. Klein, T. Schmidt, B. Lieberherr, and the technical staff of the Hohenheim and Eckartsweier experimental stations who carefully managed the field experiments and assisted with the data collection. This research was supported by a grant from Deutsche Forschungsgemeinschaft (DFG), project no. Ge 340/22-2.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
1 Dedicated to Prof. Dr. Dr. h.c. F.W. Schnell on the occasion of his 90th birthday. Back

Received for publication July 22, 2002.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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