Published online 24 January 2006
Published in Crop Sci 46:312-320 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY
Use of Sister-Lines and the Performance of Modified Single-Cross Maize Hybrids
E. A. Lee*,
A. Singh,
M. J. Ash and
B. Good
Dep. of Plant Agriculture, Crop Science Building, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
* Corresponding author (lizlee{at}uoguelph.ca)
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ABSTRACT
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In hybrid maize (Zea mays L.) seed production, yield of the female parent is a major factor affecting production costs. As an alternative to using an inbred line as the female parent, sister-lines (SLs), the F1 between two highly related inbred lines (AxA*), have been used in seed corn production. Hybrids produced using SLs are referred to as modified single-cross (MSC) hybrids. This study examined (i) yield changes associated with MSC hybrids compared with their respective single-cross (SC) hybrid counterparts and (ii) differences in grain yield of inbred lines and SLs. Three families of six inbred lines each were used to produce three diallel groups of 15 SLs. Simple sequence repeat (SSR) primer pairs were used to establish the degree of relatedness between inbred lines. The SLs and the inbred lines were mated to four unrelated inbred lines to form MSC and SC hybrids. The SC and MSC hybrids were evaluated for grain yield, grain moisture, test weight, and broken stalks in six environments. The six inbred lines and 15 SLs from each family were grown in five environments and evaluated for grain yield, grain moisture, test weight, and broken stalks. Most of the MSC hybrids (72–83%, depending on inbred line family) were not significantly different than the "best" SC counterpart. However, a low frequency of the MSC hybrids, 11 out of 180 (6.1%), had grain yields that were significantly lower than both SC hybrid counterparts. And surprisingly, three out of 180 (1.7%) of the MSC hybrids had grain yields that were significantly greater than both SC hybrid counterparts. The SLs used in this study exhibited an average grain yield that was two-fold greater and more stable or predictable than the inbred lines. These results suggest that there are definite advantages in utilizing SLs in hybrid seed production and that, in general, the performance of the resulting MSC hybrids is expected to be similar to the "best" SC hybrid counterpart.
Abbreviations: I, inbred • IBD, identity-by-descent • MSC, modified single-cross • PCR, polymerase chain reaction • RCBD, randomized complete block design • SC, single-cross • SL, sister-line • SSR, simple sequence repeat
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INTRODUCTION
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ESSENTIALLY all of the maize acreage grown in the USA and Canada today is planted to hybrid maize, with an increasing percentage of the acreage worldwide (
65%) moving from open-pollinated populations, improved synthetics, and variety crosses to hybrids (Duvick, 1999). Production of hybrid maize seed is an intricate process as seed quality, seed purity, and cost of production are all critically important factors (for review see Wych, 1988; Beck, 2004). One aspect that influences cost of production is grain yield of the female seed parent. Generally. grain yield of inbred lines is in the range of 3.8 to 5.4 Mg ha–1, which is two- to three-fold lower than hybrid grain yields (Duvick, 1999). Historically, poor inbred seed yield was the impetus behind using double-cross and three-way hybrids (Jones, 1918; 1922; Hallauer and Miranda, 1988; Crow, 1998). With the improvement in performance of inbred lines, SC hybrids began to replace double-cross and three-way hybrids in the marketplace (Wych, 1988; Hallauer and Miranda, 1988; Duvick, 1999). In the late 1980s, it was estimated that SC hybrids represented approximately 90% of the hybrid seed sold in the USA and Canada (Wych, 1988). In the 1960s and early 1970s, MSC hybrids and three-way hybrids were grown extensively in the USA and Canada and in the late 1980s, it was estimated that they still occupied about 10% of the hybrid market (Wych, 1988). Modified single-cross hybrids involve crossing two highly related inbred lines together (A and A*) and using the related-line F1 (AxA*; i.e., SL) as the female parent in the seed corn production fields.
Several factors, whether real or inferred, were behind the shift from double-cross to three-way to SC hybrids. The results of hybrid type comparison studies conclude that SC hybrids are higher yielding and more uniform in appearance (e.g., Eberhart and Russell, 1969; Jugenheimer, 1976; Wych, 1988). When hybrid type comparison studies are summarized, however, the differences in grain yield are not as great as predicted on the basis of quantitative genetic theory [100% for SC, 103.8% for three-way cross, 93.1% for double-cross (weighted averages) (c.f., Hallauer and Miranda, 1988). This expectation of SC performance > MSC > three-way > double-cross may be due more to inferences based on quantitative genetic theory. Single-cross hybrid superiority over MSC, three-way, and double-cross hybrids is expected on the basis of a single-locus model of additive and non-additive genetic action (Hallauer and Miranda, 1988). Double-cross, three-way, and MSC hybrids represent heterogeneous mixtures, with double-cross hybrids potentially being the most heterogeneous (four different parental genotypes) and MSC hybrids being the least heterogeneous (three parental genotypes, two which are >50% identical). Heterogeneous mixtures of genotypes may result in temporal variability in the field. Temporal variability due to emergence has been shown to reduce grain yield by 4 to 6%, depending on the extent of the variability (Liu et al., 2004). While the presence of temporal variability in double-cross, three-way, and MSC hybrids was not discussed in the hybrid comparison studies, it could explain the yield advantage of SC hybrids over these other hybrid types. Regardless of the reason, it is quite common to find in hybrid selection recommendations that type of cross is an important consideration, as "single crosses have the maximum hybrid vigor, and thus the greatest yield potential, followed by modified single crosses, three-way crosses, and double crosses" (Thomison, 1995).
This study re-examines the potential of MSC hybrids, specifically to evaluate their grain yield relative to their corresponding SC hybrids. The objectives of this study were to examine (i) yield changes associated with MSC hybrids compared with their respective SC hybrid counterparts and (ii) differences in grain yield of inbred lines and their corresponding SL.
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MATERIALS AND METHODS
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Genetic Materials
Single-cross and MSC hybrids were generated in controlled pollinations by using four inbred lines as male parents and 63 female parents. The four elite inbred lines used as males represent both Stiff Stalk and non-Stiff Stalk heterotic patterns: CG102 and MBS1236 representing the Stiff Stalk pattern B14 and LH162 and LH176 representing the non-Stiff Stalk patterns C103 and LH82, respectively (Gerdes et al., 1993; Lee et al., 2001b; Anonymous, 2003). The 63 females consist of three families of six inbred lines and 15 SL within each family (Table 1). The female inbred lines from family-3902 and family-3790 belong to the Iodent heterotic pattern, which is distinct from the heterotic patterns represented by the males (Lee et al., 2000a, 2000b, 2001a). Family-3929 represents an unrelated heterotic pattern that is distinct from Iodent and from the heterotic patterns of the males. Sister-lines were generated by mating two inbred lines derived from the same genetic background (i.e., inbred line family). The three families of inbred lines were derived over a 15-yr period through inbreeding three SC hybrids: Pioneer3902, Pioneer3929, and Pioneer3790. During the inbreeding process, initial selections were made in the S2 or S3 generations and again in the S4 or S5 generations by evaluating the performance of the lines in testcross (i.e., hybrid) combinations.
Molecular Marker Genotyping
Initially, DNA was extracted from leaf tissue collected from the three original SC hybrids, Pioneer3902, Pioneer3929, and Pioneer3790 (seed supplied courtesy of Pioneer Hi-Bred International). Leaf disks were freeze-dried and stored at –80°C. Extraction of genomic DNA was conducted by the FastPrep System and protocol (Q-biogene, Carlsbad, CA). From the collection of publicly available simple sequence repeat (SSR) primer pairs, 748 SSRs were screened using the F1s to identify polymorphic (i.e., heterozygous) SSR primer pairs (Taramino and Tingey, 1996; Senior et al., 1998; Sharopova et al., 2002). For family-3902, 269 polymorphic SSRs were identified, 264 were identified for family-3929, and 271 were identified for family-3790. The relative positions of the markers were inferred from previously reported map locations (Lee et al., 2002; MaizeGDB, 2004). The number of markers in the informative SSR primer pair sets for each family was reduced further by eliminating SSR markers that were tightly linked, on the basis of the IBM2 Neighbors maps in MaizeGDB (2004). The final marker sets for each family consisted of 173 SSR primer pairs for family-3902, 185 SSR primer pairs for family-3929, and 175 SSR primer pairs for family-3790. The primer pairs on the basis of previous map positions were distributed throughout the maize genome (Lee et al., 2002; MaizeGDB, 2004). Using the SSR primer pair sets, molecular marker genotypes were established for the six inbred lines from each family. Identity-by-descent (IBD) values for each pair of inbred lines within a family were established by dividing the number of shared alleles by the total number of marker loci. Chi-square goodness-of-fit tests were conducted to test for significant (
= 0.05 and = 0.01) deviations from the expected IBD value of 50% for each pair of inbred lines.
Polymerase chain reaction (PCR) conditions for amplification were 50 ng of genomic DNA, 50 ng of each primer (Invitrogen, Carlsbad, CA), 0.3 units of Platinum Taq Polymerase (Invitrogen), 1x Platinum Taq buffer (Invitrogen), 0.1 mM of each dNTP (Invitrogen), 2.5 mM MgCl2 (Invitrogen), and sterile ddH20 to 15 mL. The PCR cycling conditions were 1 cycle of [1 min at 95°, 1 min at 65°C, 1.5 min at 72°C]; 10 cycles with a 1° decrement in annealing temperature per cycle down to a final annealing temperature of 55°C; and 40 cycles of 1 min at 95°C, 1 min at 55°C, 1.5 min at 72°C. The PCR reactions were performed in thin-walled 96-well microtiter style plates (Diamed Inc., Mississauga, ON) topped with an equal volume of mineral oil (Sigma-Aldrich Canada Ltd., Oakville, ON) and covered with adhesive film (Diamed). Thermal cycling was conducted with a Robocycler 96-well temperature cycler (Stratagene, La Jolla, CA). Electrophoresis of PCR products was performed on 5% Metaphor agarose (w/v) (BioWhitaker Molecular Applications, Rockland, ME) gels in a 1x TBE buffer using Sunrise 96 gel electrophoresis units (Invitrogen) running at 115 V.
Experimental Design
Both the inbred and hybrid trials were machine planted (Wintersteiger precision planter) in May and machine harvested with a New Holland split-plot combine (ALMACO, Allan Machine Company, Nevada, IA) equipped with a HM2200 HarvestMaster high capacity dual GrainGage (Juniper Systems, Inc., Logan, UT) in October or November. Trials were over-planted and thinned at the six-leaf tip stage to uniform stands of 68c000 plants ha–1 (2002) and 64c200 plants ha–1 (2003). The soil type at all Ontario locations is Guelph loam (Typic Hapludalf). Fertilizer was applied on the basis of soil tests at the rate of: 102, 54, and 43 kg ha–1 supplemented with 50c000 L ha–1 liquid swine manure at Alma (2002) (N, P2O5 and K2O, respectively); 100, 39, and 39 kg ha–1 at Alma (2003); 157, 39, and 39 kg ha–1 at Waterloo (2002); 140, 50, and 50 kg ha–1 at Waterloo (2003); and 140, 20, and 50 kg ha–1 at Woodstock (2002). Weeds were controlled by conventional herbicides. Four traits were measured: machine harvested grain yield (Mg ha–1) adjusted to 155 g kg–1 grain moisture, grain moisture at harvest (g kg–1), percentage of broken stalks (plants broken below the ear or inclined more than 45° from the vertical), and test weight (kg hL–1).
Inbred Trial
The inbred yield trial consisted of 63 entries, or six inbred lines and 15 SLs from each of four inbred line families (i.e., family-3902, family-3929, family-3790). Yield trials were grown in a total of five environments: four southwestern Ontario locations, Alma [2550 Ontario Crop Heat Units (OCHUs); Brown and Bootsma, 1993] and Waterloo (2750 OCHUs) in 2002 and 2003, and Woodstock (2850 OCHUs) in 2002, using a randomized complete block design (RCBD) with two replications at each location. The RCBD involved the randomization option for nearest neighbor analysis (Agrobase IV software; Mulitze, 1992) to minimize the number of times that the same treatments are adjacent in each replicate. Experimental units were two-row plots, 11.56 m long, with a spacing of 0.76 m between rows. To meet the minimum plot weights required for the HM2200 dual GrainGage, plot lengths in the inbred trial were twice the length of those used in the hybrid trial. Competition between plots of SLs and inbred lines was viewed to be minimal and therefore two-row, rather than four-row plots were used. This decision was based on 2 yr of observations of the SLs and the inbred lines in the breeding nursery (Cambridge, ON, 2000–2001). The major competition effect between plots in a yield trial is one based on differences in plant height and leaf area. On the basis of visual assessment, the SLs and inbred lines used in this study did not differ substantially in plant height or in leaf area.
Hybrid Trial
The hybrid yield trial consisted of 252 hybrids, which were made by crossing six inbred lines and 15 SLs from each family as females to four inbred testers as males. Yield trials were grown in a total of six environments: four southwestern Ontario locations, Alma and Waterloo (2002 and 2003), and Woodstock (2002), and one Minnesota location (Cannon Falls 2002; courtesy of Holden's Foundation Seeds) using an RCBD (nearest neighbor option) with two replications at each location. Experimental units were two-row plots, 5.78 m long, with a spacing of 0.76 m between rows.
Statistical Analysis
For both the inbred and hybrid trials, individual plot means were adjusted using nearest neighbor analysis (Agrobase IV software; Mulitze, 1992) according to the BEST option, which compares the adjustments made on a longitudinal and latitudinal basis and chooses the option with the greatest precision. The adjusted plot means were then combined across locations and analyzed as an RCBD by PROC GLM (SAS, 1999) according to the linear model:
where Yrge is the measured trait of genotype g in replicate r at environment e; µ is the grand mean; g and ße are the genotype and environment main effects; 
r(ße) is the replicate effect nested within an environment; gße is the interaction between main effects; and 
rge is the random experimental error. Genotypes were considered fixed, while environments and blocks were considered random effects.
Inbred Trial
Genotype variance was partitioned on a plot basis into family and among families, and each family variance was further partitioned into inbred (I), SL and I vs. SL. Genotype partitions were tested using their respective source x environment mean squares, while the pooled error term was used to test sources partitioned from the entry x environment source. Two approaches were used to examine yield stability in the inbred trial, the linear regression (LR) approach of Finlay and Wilkinson (1963) and the mean-coefficient of variation (CV) approach of Francis and Kannenberg (1978). For the plot-based LR, a type 2 stability statistic (Lin et al., 1986), the genotype x environment (GxE) interaction variance, was partitioned into two components, linear trends and deviations from linear, using PROC GLM. The LR on environmental mean index model is
where Ygre is the yield of genotype g in replication r at environment e; µ is the grand mean; g and ße are the genotype and environment deviations from the grand mean; r(ße) is the replicate effect nested within an environment; 
g is the yield sensitivity (regression coefficient) of genotype g to the change in the environmental mean; 
ge is the deviation from linear trends; and rge is the random experimental error. The linear trends and deviations from linear partitioned out of the G x E interaction in the LR model were tested for significance using gre mean squares. For the mean-CV stability analysis, a type 1 stability statistic (Lin et al., 1986), the mean grain yield is plotted against the CV of each individual entry. The mean grain yield and mean CV for the entire trial serve to divide the plot into four quadrants: I—high mean grain yield, low CV; II—high mean grain yield, high CV; III—low mean grain yield, low CV; IV—low mean grain yield, high CV (Francis and Kannenberg, 1978). Entries in quadrant I are considered the most desirable, while entries in quadrant IV are considered the least desirable (Francis and Kannenberg, 1978).
Hybrid Trial
Genotype cross variance was partitioned on a plot basis into male, female, and male x female using the NC Design II partitions (Comstock and Robinson, 1948; 1952). Within the female and male x female partitions, the variance was further partitioned into inbred line family and among inbred line families, and then the variance of each family was partitioned into I, SL, and I vs. SL. Genotype partitions were tested using their respective source x environment mean squares, while the pooled error term was used to test sources partitioned from the entry x environment interaction source.
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RESULTS AND DISCUSSION
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Genetic Relationships within Families
In theory, any two inbred lines derived from the same F1 should be 50% IBD. All of the IBD estimates in the 3902 and 3790 families were significantly greater than 50% (Table 2). The IBD estimates for all possible SL combinations ranged from 52 to 80%. In other words, inbred lines CG42 and CG70, from family-3929, have inherited only 52% of their alleles from the same parent, while inbred lines CG64 and CG85, from family-3902, have inherited 80% of their alleles from the same parent. The lines within family-3902 and family-3790 are more closely related to one another and are considered SLs. Some of the lines within family-3929 show the same levels of IBD observed in the other two families. However, about half of the inbred line combinations within family-3929, particularly those involving CG42, are not considered SLs, on the basis of the lower levels of IBD (52–58%) that were not significantly different from the 50% IBD expectation. This is consistent with molecular fingerprinting data suggesting that CG42 is an Iodent line, while the other five lines belong to a distinct and unrelated heterotic group (Lee and Lukens, unpublished data).
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Table 2. Identity by descent (IBD) estimates for the sister-line (SL) combinations. For family-3902, the estimates are based on 173 simple sequence repeat (SSR) primer pairs; for family-3929, the estimates are based on 185 SSR primer pairs; and for family-3790 the estimates are based on 175 SSR primer pairs. Significant (* = 0.05 and ** = 0.01) deviations from the expected IBD of 50% were determined using Chi-square goodness of fit test.
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Hybrid Performance
The environments chosen for this study range between 2550 OCHUs (Alma) and 2850 OCHUs (Woodstock). The highest average grain yield, 10.3 Mg ha–1, was recorded at the Alma location in 2002 (8.5% broken stalks, 73.3 kg hL–1 test weight, 246 g kg–1 grain moisture) and the lowest average grain yield was recorded at the Woodstock location in 2002, 7.7 Mg ha–1 (15.0% broken stalks, 67.7 kg hL–1 test weight, 208 g kg–1 grain moisture). Hybrids involving the two Stiff Stalk males, CG102 and MBS1236, exhibited average grain yields of 8.9 Mg ha–1 (6.6% broken stalks, 69.1 kg hL–1 test weight, 246 g kg–1 grain moisture), and 9.2 Mg ha–1 (11.8% broken stalks, 69.9 kg hL–1 test weight, 244 g kg–1 grain moisture), respectively. Hybrids involving LH162, the C103-type male, averaged 8.3 Mg ha–1 (9.8% broken stalks, 69.1 kg hL–1 test weight, 221 g kg–1 grain moisture), while hybrids involving LH176 averaged 9.1 Mg ha–1 (6.6% broken stalks, 69.1 kg hL–1 test weight, 236 g kg–1 grain moisture). The female families used in this study represent elite germplasm, with performance levels similar to commercial 2600 to 2700 OCHUs hybrids. Within families, average SC and MSC grain yields ranged from 8.1 to 8.9 Mg ha–1, broken stalks ranged from 7.2 to 11.4%, test weight ranged from 69.2 to 70.3 kg hL–1, and grain moisture ranged from 232 to 244 g kg–1 (Table 3).
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Table 3. Within family means and standard errors for grain yield, test weight, grain moisture, and broken stalks of the inbred (I) and sister-lines (SL) from the hybrid performance trial across six locations.
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Environments, genotypes, and their interaction were significant sources of variation for all four traits in the hybrid performance trial (Table 4). Genotypes were further partitioned, using a NC Design II analysis (Comstock and Robinson, 1948; 1952), into sources of variation due to males, females, and males x females. Males and females were significant sources of variation for all traits. The males x females interaction was significant for grain yield, grain moisture, and test weight. Within females, there were significant differences among the three families of inbred lines for all of the traits. Family-3902 was the only family exhibiting significant variation for grain yield. The within family sums of squares were further partitioned into I, SL, and the contrast I vs. SL. The contrast indicates whether there were differences between the average performance of the two types of females across all males. Family-3902 did not exhibit a significant contrast (i.e., I vs. SL), indicating that when averaged across the four males, there were not significant differences between the two types of hybrids for grain yield. For the other traits examined, the I vs. SL contrast was significant only for grain moisture in family-3902 and family-3790. Within the males x females partition, the sums of squares were further partitioned into among and within families, and the families were further partitioned into I, SL, and the I vs. SL contrast. The contrasts were not significant sources of variation indicating that there were not significant differences between the two types of hybrids for grain yield, grain moisture, and test weight.
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Table 4. Mean squares from the analysis of variance of the hybrid yield trial combined across six environments for grain yield, test weight, grain moisture, and broken stalks.
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Genotype x environment interaction was a significant source of variation for all traits. To examine the interaction further, the GxE interaction variance was further partitioned into males, females, and males x females effects. The underlying cause of the significant GxE interaction is due to males, as only the male partition was significant (Table 4). This is not entirely surprising in that the males range in maturity from 2500 OCHUs for LH162 to 2800 OCHUs for LH176 and MBS1236 (Anonymous, 2003) and the environments used range from 2550 to 2850 OCHUs for Alma and Woodstock, respectively. The females and males x females partitions of GxE interaction were not significant, suggesting that meaningful comparisons between the MSC hybrid and the SC hybrid counterparts can be made across environments.
Inbred Performance
The highest average grain yield, 6.3 Mg ha–1, was recorded at the Alma location in 2002 (7.5% broken stalks, 72.6 kg hL–1, 238 g kg–1 grain moisture), and the lowest average grain yield was recorded at the Alma location in 2003, 5.2 Mg ha–1 (13.3% broken stalks, 66.5 kg hL–1, 314 g kg–1 grain moisture). Genotype and GxE interaction were significant sources of variation for all four traits (Table 5). Environment was a significant source of variation for test weight, grain moisture, and broken stalks; however, it was not a significant source of variation for grain yield. Within genotypes, there were significant differences among and within the three families of inbred lines for all of the traits, except broken stalks within family-3902. The I vs. SL contrast, an indication of heterosis, was significant for grain yield in all three families of lines and for test weight in family-3929 and family-3790. Even though the inbred lines are highly related (i.e., >50% IBD), significant heterosis for grain yield is observed between the inbred lines and SLs (Table 6). Previous work with inbred lines suggested that the major limitation to grain yield is in partitioning dry matter to the ear (i.e., harvest index, HI) (Lee and Kannenberg, 2004; Tollenaar et al., 2004). Perhaps even modest amounts of heterosis improve HI, thereby enhancing grain yield.
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Table 5. Means squares from the analysis of variance of the inbred and sister-line yield trial combined across five locations for grain yield, test weight, grain moisture, and broken stalks.
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Table 6. Within family means and standard errors for grain yield, test weight, grain moisture, and broken stalks of the inbred (I) and sister-lines (SL) from the inbred performance trial across five locations.
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The linear partition of GxE interaction for grain yield was not significant. Hence, the Finlay-Wilkinson (1969) LR-based approach for assessing yield stability was not used. Instead, the mean-CV approach (Francis and Kannenberg, 1978) was used to examine differences in yield stability of the inbred lines and sister-lines (Fig. 1
). The grand mean grain yield was 5.7 Mg ha–1, with a mean CV of 18.9%. Genotypes exhibiting higher than average grain yield and lower than average CVs for grain yield (i.e., quadrant I) are desirable, as they are considered more stable. The least desirable genotypes are those in quadrant IV, exhibiting lower than average grain yield and higher than average CVs. The SLs (squares) generally fell within quadrant I, while the inbred lines (triangles) generally fell with quadrant IV (Fig. 1). Perhaps an additional advantage of using SLs in seed corn production may be more consistent, predictable seed yields of the female parents.
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Fig. 1. Mean-CV plot of the mean grain yield and coefficient of variation (CV) from the inbred line (I, triangles) and sister-line (SL, squares) yield trial. The mean grain yield and CV for the entire trial (5.7 Mg ha–1, 18.9%) serve to divide the plot into four quadrants: I—high mean grain yield, low CV; II—high mean grain yield, high CV; II—low mean grain yield, low CV; IV—low mean grain yield, high CV (Francis and Kannenberg, 1978).
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Epistasis and Its Implications
The average performance (i.e., grain yield) of the MSC hybrids was not significantly different than the SC hybrids (Table 4). However, this was the average of the SC hybrids versus the average of the MSC hybrids. Not all MSC hybrids were statistically equivalent in their performance to their "best" respective SC hybrid counterparts. Of the 180 MSC hybrids evaluated in this study, 14 MSC hybrids (i.e., 7.8%) exhibited grain yields that were significantly (P < 0.05) different than both SC hybrid counterparts (Table 7), with an additional 31 MSC hybrids (17.2%) having grain yields significantly (P < 0.05) less then the "best" SC hybrid counterpart (data not shown). In 11 instances, the grain yield of the MSC hybrid was significantly less than both of the SC hybrid counterparts. Two of these were MSC hybrids from family-3929 involving CG42. CG42, as previously pointed out, should not be considered a SL to the other inbred lines in the family based on the low IBD values and its grouping within the Iodent heterotic pattern. When the MSC hybrids involving CG42 are not considered, only nine MSC hybrids exhibited grain yields that were significantly less than both of the SC hybrid counterparts. Perhaps most surprising, however, were three instances involving family-3790 inbred lines where the grain yield of the MSC hybrid was significantly (P < 0.05) greater than either of the SC hybrid counterparts. These are examples of the quantitative genetic phenomenon called epistasis, seven cases of negative epistasis and three cases of positive epistasis. Epistasis occurs when there are interactions between genes. In the MSC hybrids, allelic combinations exist that did not exist in either SC hybrid, thereby creating an opportunity for new epistatic interactions to occur and affect the phenotype.
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Table 7. Mean grain yield, test weight, grain moisture, and broken stalks across six environments of the modified single-cross (MSC) hybrids that were significantly different than both of their single-cross (SC) counterparts. Identity-by-descent (IBD) estimates indicate the degree of relatedness between the two inbred lines.
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Epistasis is potentially the source of the greatest difficulties when attempting to develop MSC hybrids. CG44 and CG85, which are 69% IBD, illustrate the difficulties in generalizing about MSC hybrids (Table 8). CG44 and CG85 have a per se grain yield of 3.7 and 3.1 Mg ha–1, respectively. The SL's grain yield is nearly double that of either inbred, 7.1 Mg ha–1, making the MSC option very attractive from the standpoint of hybrid seed yield. The SL version, when crossed to LH176, exhibited positive epistasis for grain yield, 9.5 versus 8.4 Mg ha–1 (CG44) and 8.1 Mg ha–1 (CG85). Yet, when crossed to MBS1236, a Stiff Stalk line, the MSC exhibited negative epistasis for grain yield, 8.4 versus 9.5 Mg ha–1 (CG44) and 9.2 Mg ha–1 (CG85). When crossed to the other Stiff Stalk line, CG102, or the Lancaster line, LH162, the MSC hybrid was not significantly different from the "best" SC hybrid counterpart (Table 8). This illustrates the need to examine all potential MSC hybrids even when the change in hybrid formula is within the heterotic pattern.
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Table 8. Mean grain yield, test weight, grain moisture, and broken stalks of the hybrids involving inbred lines CG44 and CG85, and the sister-line CG44xCG85 across six environments. CG44 and CG85 are 69% identical by descent (IBD).
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CONCLUSIONS
The use of MSC hybrids is an attractive option, in terms of a strategy, for reducing hybrid seed production costs. The most immediate impact on seed production is that less acreage would be required for the production of the same amount of hybrid seed. In terms of seed production, the use of SLs rather than inbred lines as females resulted in a significant, two-fold increase in grain yield (Table 6). Furthermore, the performance of the SLs was in general more stable or predictable than the inbred lines, as indicated by the mean-CV stability plot (Fig. 1). In most cases (75.6%), there was not a "yield penalty" associated with the MSC hybrids, particularly when attention was paid to degree of relatedness of the SLs. However, a low frequency of the MSC hybrids, 10 out of 180 (5.6%), had grain yields that were significantly lower than both of the SC hybrid counterparts. Significant epistatic interactions, while rare in this data set, can potentially present problems and, in extremely rare cases, opportunities. When considering the SL option in seed corn production, other "opportunity costs" should be factored in, such as the additional expense, land area, and time required for maintaining the two inbred lines and in producing the SL, and the longevity of a commercial hybrid or inbred line family.
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ACKNOWLEDGMENTS
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Technical support by C. Grainger; original single-cross seed of Pioneer3902, Pioneer3929, and Pioneer3790 supplied by Pioneer Hi-Bred International; seed of LH162 and LH176, and the Minnesota hybrid yield trial location supplied by Holden's Foundation Seeds; and seed of MBS1236 supplied by MBS, Inc. is gratefully acknowledged.
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NOTES
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Financial support, in part, from the Ontario Ministry of Agriculture and Food, Natural Science and Engineering Research Council, Canadian Foundation for Innovation, Ontario Innovative Trust, and Ontario Corn Producers' Association. A. Singh is supported by a graduate scholarship from Pioneer Hi-Bred International.
Received for publication February 1, 2005.
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REFERENCES
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E. A. Lee, M. J. Ash, and B. Good
Re-examining the Relationship between Degree of Relatedness, Genetic Effects, and Heterosis in Maize
Crop Sci.,
March 1, 2007;
47(2):
629 - 635.
[Abstract]
[Full Text]
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ABSTRACT
INTRODUCTION
