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J. Agric. Sci. Technol. (2006) Vol. 8: 153-169
153
Generation Mean Analysis to Estimate Genetic Parameters for Different Traits in Two Crosses of Corn Inbred
Lines at Three Planting Densities
F. Azizi1, A. M. Rezai1*, and G. Saeidi1
ABSTRACT
The choice of an efficient breeding procedure depends to a large extent on knowledge of the genetic system controlling the character to be selected. The objective of this study was to determine genetic parameters for yield and other traits including some of the yield components under three planting densities, using analysis of generation means (P1, P2, F1, F2, BC1 and BC2) derived from crosses of B73 with Mo17 and K74/1 inbred lines of corn. Analysis of variance reinforced the hypothesis that interaction of plant density on genera-tion means depends on evaluating genotypes and the kind of trait. Generation mean analysis suggested that both additive and dominance effects were important for most of the traits evaluated in this study, but dominance had a more pronounced effect. Epistasis affected the expression of nine traits in both crosses at three planting densities. Expres-sion of epistasis and genetic parameters differed in the two crosses and were influenced by plant density. Plant densities interacted more strongly with epistasis gene action than with additive or dominance gene action in both crosses.
Keywords: Additive, Dominance, Epistasis, Gene action, Heritability, Maize, Variance components.
_____________________________________________________________________________ 1. Department of Agronomy and Plant Breeding, College of Agriculture, Isfahan University of Technology, Isfahan, Islamic Republic of Iran. * Corresponding author.
INTRODUCTION
The choice of an efficient breeding pro-gram depends to a large extent on knowl-edge of the type of gene action involved in the expression of the character. Whereas dominance gene action would favor the pro-duction of hybrids, additive gene action in-dicates that standard selection procedures would be effective in bringing about advan-tageous changes in character (Edwards et al., 1975). Information on genetic variances, levels of dominance, and the importance of genetic effects have contributed to a better understanding of the gene action involved in the expression of heterosis (Wolf and Hal-lauer, 1997). Maize breeders have success-fully exploited heterosis for grain yield by crossing inbred lines to develop desirable
hybrids. However, the nature of gene action involved in the expression of heterosis for the grain yield of elite maize hybrids re-mains unresolved.
The frequent occurrence of a nonallelic in-teraction in quantitative traits reveals their existence in the inheritance of quantitative characters. Much of the information on epis-tasis stems from studies in cross-pollinated crops probably because of the major role of heterosis in these crops and the possible re-lationship between hybrid vigor and epista-sis (Ketata et al., 1976). The importance of epistasis for gene controlling grain yield in the breeding population of maize is not well understood. Most statistical models for esti-mating gene effects assume epistasis to be of limited importance. This assumption has
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_________________________________________________________________________ Azizi et al.
154
been used in the estimation of heritability and the number of genes affecting quantita-tive traits. Theoretical comparisons have shown that estimates of genetic parameters may be biased greatly if epistasis is present, and expectations based on such parameters may lead to erroneous expectations of re-sponse to selection (Eta-Ndu and Openshaw, 1999; Templeton, 2000).
The importance of nonallelic interaction on the expression of several agronomic traits has been reported in a number of instances. Wolf and Hallauer (1977) reported that an epistatic effect could contribute to the ex-pression of heterosis for specific hybrids. They showed that additive by additive ef-fects were not significant for grain yield whereas additive by dominance and domi-nance by dominance effects were signifi-cant. In the study of Darrah and Hallauer (1972), the additive by additive and domi-nance by dominance effects for yield com-ponents (ear length, ear diameter and num-ber of kernel per row) were greater than plant height and ear height. Hallauer (1990) reported that since inbreeding is conducted simultaneously with hybrid evaluations, fa-vorable epistatic gene combinations can ul-timately be fixed in the inbred lines. Also, since maize breeders use related inbreds or at least inbreds from the same heterotic pat-tern as the parents of source populations, they would tend to maintain favorable epistatic gene combinations, especially linked epistatic combinations. Epistasis could also explain why it has been difficult to develop improved recoveries from some maize inbreds (Melchinger et al., 1988; Lamkey et al., 1995).
A few studies have indicated that epistasis was not a significant component of genetic variability in the maize population (Silva and Hallauer, 1975; Ketata et al., 1976; Hinze and Lamkey, 2003). Other studies, however, have shown that epistatic effects are important for the specific combination of inbred lines (Darrah and Hallauer, 1972;
Wolf and Hallauer, 1977; Moreno-Gonzalez and Dudley, 1981; Lamkey et al., 1995; Chen et al., 1996; Hinze and Lamkey, 2003). Hallauer and Miranda (1988) con-cluded that epistasis variance is not an im-portant contributor to the genetic variance for yield in maize. It seems that epistasis for complex traits, such as yield, must exist, but realistic estimates of additive by additive epistasis have not been obtainable. Hence, either the genetic models used are inade-quate or epistasis variance is small relative to the total genetic variance of the maize population (Hallauer and Miranda, 1988). Biometric methods that use mean compari-son rather than variance component estima-tion (for example, generation mean analysis and triple test cross) have regularly indicated that epistatic effects are important for grain yield in maize (Eta-Ndu and Openshow, 1999; Lamkey et al., 1995; Moll and Stuber, 1971; Wolf and Hallauer, 1977).
Genotype × environment interaction is a major factor in the genetic study of quantita-tive traits because it complicates the inter-pretation of genetic experiments and makes predictions difficult. The bias caused by these interactions in the estimates of the various genetic parameters is of unknown magnitude and direction and may not be the same for each parameter (Gamble, 1962a, c). Planting densities, which can be considered as different environments, could affect inter-relationships among agronomic traits meas-ured on generations of the same or different crosses, and bias the estimates of genetic parameters (Adetimirin et al., 2001; Hal-lauer and Miranda, 1988)
The objectives of this study were: (1) To estimate and compare genetic parameters for different traits using P1, P2, F1, F2, BC1 and BC2 generations of crosses between B73 and Mo17, and between B73 and K74/1 inbred lines, evaluated over three planting densities (environments), and (2) to study the effect of different environments (planting densities) on the estimates of genetic parameters.
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Generation Mean Analysis in Two Crosses of Corn Inbred Lines_____________________
155
MATERIALS AND METHODS
Genetic Materials and Experimental
Procedure
Generation means of two crosses, i.e. B73×Mo17 and B73×K74/1 were analyzed to estimate the genetic parameters for differ-ent traits in three plant densities. Inbred B73 was a selection from Iowa Stiff Stalk Syn-thetic (BSSS) after five cycles of half-sib recurrent selection for grain yield (Russell, 1972). Inbred Mo17 was derived by selec-tion from the single cross of inbred lines CI187-2 and C103 (Zuber, 1973). Inbred K74/1 is an Iranian Inbred line derived from introduced germplasms from Yugoslavia and is widely used as parent of single crosses grown in Iran. The six generations (P1, P2, F1, F2, BC1 and BC2) of each cross were evaluated in a separate randomized complete block design with 3 replications at 3 plant densities (70000, 105000, 140000 plant ha-1) at the Experimental Field of Seed and Plant Improvement Institute at Karaj, Iran, in 2001. The site is at 35°, 50′ N lati-tude; 50°, 58′ E longitude; and 1300 m ele-vation, with maximum and minimum tem-peratures of 38°C and 25°C during the growing season. The experimental units con-tained four rows for non-segregating genera-tions (P1, P2 and F1), and six rows for F2, BC1 and BC2 generations. Each row was 4 m long and 0.75 m wide. The planting date was May, 11, 2001. Fertilizer treatments were 150 kg ha-1 of N applied prior to planting, plus and additional of 100 kg ha-1 of N top-dressing after ear emergence. In each repli-cation, observations were recorded on 10 random plants of P1, P2 and F1 and on 50 plants of F2, BC1, and BC2. Nine traits in-cluding anthesis (days from planting to an-thesis), plant and ear heights (from soil sur-face to the collar of the flag leaf and primary ear node in cm, respectively), kernel rows, kernel per row, kernel depth (difference be-tween ear and cob diameters in mm), grain yield per plant (14% moisture in g), 100-seed weight(g), and cob dry weight(g), were
measured.
Data Analysis
Data were subjected to combined analysis of variance over planting densities for each cross using a general linear model and gen-eration and generation × plant density sum of squares were partitioned to different or-thogonal contrasts (Tables 1 and 2). A quan-titative generation mean analysis was per-formed separately for each plant density. The trait means for each generation, across replication within each density, were ob-tained and different 2, 3, 4, and 5 parameter models were fitted by weighted least square analysis or joint scaling test (Mather and Jinks, 1982). Mather and Jinks (1982) model describes the phenotype in terms of the mid-parental values [m], additive effects [d], dominance effects [h], and additive by addi-tive [i], additive by dominance [j], and dominance by dominance [l] epistatic inter-action effects. (Mather and Jinks, 1982; Shonnard and Gepts, 1994). The generation means and their expectations were weighted by using the reciprocal of the variance of generation means (1/ VX) (Warnock et al., 1998; Mansure et al., 1993; Mather and Jinks, 1982; Shonnard and Gepts, 1994). The goodness of fit was tested by a chi-square with 4, 3, 2 and 1 degrees of free-dom; i.e. the number of available genera-tions minus the number of estimated pa-rameters (Cukadar-Olmedo and Miller, 1997). The significance of parameters was tested with related standard errors at 1% and 5% probability levels.
Broad-sense (hb2) and narrow-sense (hn
2) heritabilities were estimated using the vari-ance component method (Wright, 1968) and variances of F2 and back cross generations (Warner, 1952), respectively, as:
h b2 = {VF2 – [(VP1 + VP2 + 2VF1) /4]}/ VF2 h n2 = [VF2 – (VBC1 + VBC2) /2] / VF2 Response to selection was estimated with
5% selection intensity (i) (Selection differ-ential, k=2.06) as:
R = i × h2n
× 2FV
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_________________________________________________________________________ Azizi et al.
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Heterosis was calculated as F1 mean devia-tion from mid-parental performances. Vari-ance components (additive, dominance and environment) were estimated as described by Mather and Jinks (1982) using the fol-lowing equations; D = 4 VF2 – 2 (VBC1 + VBC2) H = 4 (VB1 + VB2 - VF2 - VE) EW = 0.25 (VP1 + VP2 + 2VF1)
In these formulae, V stands for variance and the subscripts refer to generations. EW, D, and H are variances of environment, ad-ditive and dominance effects, respectively.
RESULTS
The combined analysis of variance over plant densities indicated highly significant differences (P < 0.01) among plant densities for anthesis, kernel per row, seed yield per plant and cob weight in both crosses (Tables 1 and 2 ). Also differences among plant den-sities for plant height, kernel rows and 100-seed weight were significant (P< 0.05) in cross B73×Mo17 but not significant in cross B73×K74/1. Plant density differences for ear height and kernel depth were non-significant in both crosses. There were significant dif-ferences among generations (P < 0.01) for all traits in both crosses.
Significant generation × plant density in-teraction effects were found for anthesis, plant height, kernel per row, seed yield per plant, cob weight and ear height in cross B73×Mo17 (Table 1). None of the charac-ters showed significant generation × plant density interaction in cross B73×K74/1, ex-cept anthesis (Table 2). Therefore, differ-ences between parents (P1 vs P2) that reveal additive effects were significant for all traits except ear height and kernel depth in cross B73×Mo17 and anthesis and 100-seed weight in cross B73×K74/1. The interaction effects (P1 vs P2) × plant density were non-significant for all traits in both crosses, ex-cept for anthesis in cross B73×K74/1 which resulted in a non-significant P1 vs P2 mean square for this trait. Heterosis was signifi-cant for all traits in both crosses except 100-
seed weight in cross B73×Mo17 and kernel row number in cross B73×K74/1. The inter-action effects of heterosis × plant density were non-significant for all traits in both crosses, except kernel per row in cross B73×Mo17. In other words, ranking of the estimates of heterosis was the same at dif-ferent planting densities. Generally, most of the interaction mean squares involving planting density were significant. Therefore, generation mean analysis was performed separately for each plant density.
The generations' performances for cross B73 × Mo17 and cross B73 × K74/1 at dif-ferent planting densities are presented in Tables 3 and 4. Traits responded differen-tially to planting densities. Also some of the characteristics studied showed more varia-tion among generations. For example, grain yield per plant and the number of kernels per row had relatively more variations. With a few exceptions, the trend of decreased per-formance with increased planting density was consistent for all characteristics in all generations of the two crosses. Mo17 had more grain yield per plant, kernel per rows and 100-seed weight, but B73 was superior with respect to the other traits. K74/1 out-performed B73 with respect to grain yield per plant, kernel per row, kernel rows and cob weight, but performed almost similar to B73 for the other traits at all planting densi-ties, except for plant height.
For cross B73 × Mo17 at all planting densi-ties, F1 and F2 mean performances were greater than the top parents for all traits ex-cept kernel rows, 100-grain weight and an-thesis. Both F1 and F2 means were close to superior parents for kernel rows and to infe-rior parents for 100-seed weight. For anthe-sis, F1 mean was lower than the earlier ma-turing parent but F2 mean was greater than F1. All the generation means for 100-seed weight were close to the inferior parent. Both BC generation means were greater than the superior parent for all the traits, except the BC1 grain yield per plant and kernel per row means which were close to superior parent.
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Generation Mean Analysis in Two Crosses of Corn Inbred Lines
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Generation Mean Analysis in Two Crosses of Corn Inbred Lines
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Azizi et aL
160
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Generation Mean Analysis in Two Crosses of Corn Inbred Lines_____________________
161
For cross B73×K74/1, the F1 means for all the traits at all planting densities were greater than the superior parent with the ex-ception of kernel rows, cob weight and an-thesis. For anthesis all the generation means were lower than or close to the earlier ma-turing parent. For kernel rows, cob weight, grain yield per plant, and plant height all the generation means (except F1 for two last traits) were between parental means. For kernel depth, all the generation means ex-ceeded the superior parent means. For ear height and kernel per row F2, BC1 and BC2 means were close to the superior parent.
Different 3 to 6 parameter models showed the best fits to generation means of different traits, planting density, and cross combina-tions (Tables 5 and 6). In cross B73×Mo17, additive effects were significant for all traits in all plant densities, except ear height in all plant densities, and kernel depth in low plant density. Non-significancy in those cases may be ascribed to large error variance (Ed-wards et al., 1975). In cross B73×K74/1, except for ear height in high plant density, kernel depth in low and intermediate plant densities, and 100-seed weight in low plant density, the other additive effects were significant. As is shown in Tables 3 and 4, some of the additive effects were negative. The negative or positive signs for additive effects depend on which parent is chosen as P1 (Cukadar-Olmedo and Miller, 1997; Ed-wards et al., 1975). The additive effects for grain yield per plant were much greater in cross B73×K74/1 than cross B73×Mo17, but for the other traits were almost of the same magnitude.
Dominance effects were positive and sig-nificant in cross B73×Mo17 for all traits at all planting densities, except for anthesis in high planting density, plant height in low and high planting densities, ear height and 100-seed weight at low planting density. Also, in this cross, negative and significant dominance effects were estimated for plant height and kernel per row at an intermediate planting density, grain yield per plant, 100-seed weight in intermediate and high plant-ing densities. In cross B73×K74/1, domi-
nance effects were significant for all traits at all planting densities, except ear height at low and intermediate planting densities and kernel depth at high planting density. Also in this cross, dominance effects were signifi-cant and negative for anthesis, plant height and 100-seed weight at all planting densities, grain yield per plant and cob weight at low and high planting densities, and ear height at high planting density.
As it is shown in Tables 5 and 6, different types of epistasis interaction effects were found for different trait, cross and planting density combinations. With the exception of anthesis for B73×Mo17 at high planting density, all the other signs of [h] and [l] type of epistasis were opposite, indicating dupli-cate non-allelic gene interactions. For plant height at all planting densities of cross B73×Mo17 and anthesis at high planting densities of both crosses, a six parameter model had the best fit to the data. This find-ing suggested that more generations are needed for a more exact estimate of genetic parameters for plant height. The estimates of additive, dominance, and environment components of variance, broad- sense and narrow-sense heritabilities, ge-netic gain from selection and heterosis for different traits in different planting densities are presented in Table 7. By increase plant-ing density, in cross B73×Mo17, for grain yield per plant, 100-seed weight and ear height, and in cross B73×K74/1 for 100-seed weight, the additive variance was decreased but the dominance variance was increased for grain yield per plant, 100-seed weight, and ear height, causing lower broad and nar-row sense heritability estimates and also re-sponse to selection. Likewise the average degree of dominance was decreased by the increase of plant density. In cross B73× Mo17 the additive variances for anthesis and kernel rows were increased in higher plant-ing densities, but the dominance variances were decreased. The same results were ob-served for anthesis, ear height and kernel depth in cross B73×K74/1. In cross B73× K74/1 both additive and dominance
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variances for kernel per row and cob weight were decreased with increase of planting densities. For the other traits very small changes were detected in genetic variances with an increase in planting densities.
Broad sense heritability estimates ranged from 0.24 (100-seed weight at high planting density) to 0.97 (anthesis at low planting density) in cross B73×Mo17, and from 0.20 (cob weight at a high planting density) to 0.97 (anthesis at all plant densities) in cross B73×K74/1.
Narrow-sense heritabilities ranged from 0.02 (kernel per row at low planting density) to 0.72 (ear height at low planting density) in cross B73×Mo17, and from 0.04 (100-seed weight at high planting densities) to 0.68 (kernel depth at high planting density) in cross B73×K74/1. For grain yield per plant, kernel rows, and ear height, greater estimates of narrow-sense heritability and consequently greater gain from selection were found in cross B73×Mo17. In contrast, these estimates were greater in cross B73×K74/1 for anthesis and kernel per row. Genetic advance ranged from 0.27 (anthesis at low planting density) to 36.70 (grain yield per plant at low planting density) in cross B73×Mo17, and from 1.41 (plant height at high planting densities) to 36.30 (kernel depth at high planting density) in cross B73×K74/1.
Based on variations in additive and domi-nance variance, broad sense heritability de-creased at higher planting densities for both crosses, except for kernel depth and cob weight in cross B73×Mo17 and anthesis, kernel depth and kernel row number in cross B73×K74/1.
Narrow sense heritability and also genetic advance decreased at higher planting densi-ties in cross B73×Mo17 except for anthesis and kernel per row but it increased in cross B73×K74/1 except for plant height, 100-seed weight and cob weight.
Absolute estimates of heterosis ranged from 0.02 (anthesis at high planting density) to 0.69 (grain yield per plant at low planting density) in cross B73×Mo17, and from 0.00 (kernel rows at low planting density) to 0.78
(grain yield per plant at low planting den-sity) in cross B73 × K74/1.
DISCUSSION
In both crosses, the dominance effects were greater than the additive effects for all char-acters at all planting densities, except kernel rows at high planting density of cross B73×Mo17 and low planting density of cross B73×K74/1. Some studies have indi-cated the importance of the dominance ef-fect for yield in corn (Guei and Wassom, 1992; Malvar et al., 1996). Gamble (1962a and b) reached the same conclusion in evaluating some ear characteristics in differ-ent crosses. The contribution of the parents to dominance effects varied according to trait and planting density. The sign for domi-nance effect is a function of the F1 mean value in relation to the mid-parental value and indicates which parent is contributing to the dominance effect (Cukadar-Olmedo and Miller, 1997).
The possibility that epitasis accounts for a significant proportion of the genetic variance of quantitative traits has been investigated extensively. Our results showed that, besides the additive and dominance genetic effects, epistatic components have also contributed to genetic variations for most of the charac-ters studied. However, their relative magni-tudes vary for different traits and under dif-ferent plant densities. In such a situation, the appropriate breeding method is the one that can effectively exploit the three types of gene effects simultaneously. Lamkey et al. (1995) found that unlinked additive by addi-tive epistasis accounted for at least 21% of the variation among test cross generation means derived from elite maize inbred lines. Under highly productive environmental conditions, dominance effects have ac-counted for most of the variability in yield, with epistasis having a small and significant influence on the final performance of differ-ent generations.
Specific combining ability is more impor-tant for selected lines than for unselected
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lines, indicating the importance of domi-nance and epistatic effects in elite germ-plasm (Hallauer and Miranda, 1988). Spe-cific crosses with epistatic effects probably have unique combinations of genes contrib-uting to heterosis. These unique combina-tions are restricted to the specific cross and may be of small importance in any maize population (Hallauer and Miranda, 1988). The hybrid B73×Mo17 was a widely grown hybrid in the late 1970s and early 1980s and it is possible that favorable epistasis effects contributed to the exceptional performance of this hybrid (Lamkey et al., 1995). The evidence indicates that there are net positive epistasis effects fixed in B73. This may ex-plain why B73 has been such a widely used and successful inbred in maize breeding programs (Lamkey et al., 1995; Ceballos et al., 1998). Kearsey and Jinks (1968) sug-gested that the two parental inbreds (B73 and Mo17) have equal opportunity to con-tribute to the expression of additive by addi-tive effects when averaged across all possi-ble F2 genotypes.
Confounding epistatic effects in the models suggested that inheritance of these traits is complex and polygenic (Warnock et al., 1998; Upadhyaya and Nigam, 1998). Be-cause one or more kinds of epistatic effects were detected for all the traits, estimates of the additive and dominance components for these traits would have been biased because of nonorthogonality if they had been esti-mated using procedures that assume no epis-tasis (Upadhyaya and Nigam, 1998). For this reason estimates of epistasis obtained are likely to be minimum value. The assumption of no epistasis is one of the most common made in quantitative genetic models (Weir and Cockerham, 1977). The amount and type of epistasis present in crop species can have major consequences on both the reli-ability of prediction and the design of breed-ing programs.
The presence of epistasis has important implications for any plant breeding program. The [i] type interaction can be fixed in in-bred lines. A recurrent selection scheme, in which large populations are carried forward
to later generations to allow favorable gene combinations to be in a homozygous state before practising final selection, would be the most appropriate. The other digenic in-teractions can be effectively exploited through the selection of lines that exhibit high levels of the trait in crosses with other inbred lines.
The signs associated with estimates of [i], [j] and [l] types of epistasis indicate the di-rection in which the gene effect influence the mean of the population. For [i] and [j], the sign also provides information on the association or dispersion of genes in the par-ents (Mather and Jinks, 1982). With two ex-ceptions, all the other signs of [i] and [j] type of detected epistasis were negative. Also, a negative sign for any of the two pa-rameters suggests an interaction between increasing and decreasing alleles, thus pro-viding evidence for some level of dispersion in the inbred parents. A negative sign for each of these two parameters suggests that it should be possible further to improve the level of the corresponding traits. With one exception (anthesis at low planting density in cross B73×Mo17) all the other signs of the estimates of [l] were opposite to that of [h] in both crosses, indicating duplicate epis-tasis. This kind of epistasis generally hinders the improvement through selection and, hence, a higher magnitude of dominance and [l] type of interaction effects would not be expected. It also indicated that selection should be delayed after several generations of selection (single seed descent) until a high level of gene fixation is attained. Subsequent intermatings between promising lines may be important in accumulating favorable genes. Since none of the signs of the [h] were similar to the [l] type of epistasis, it was concluded that no complementary type of interaction was present in the genetic con-trol of the studied traits.
Similarity in estimates for three planting densities was observed for additive effects in B73×K74/1 cross for most of the traits. This was true to some extent for the B73×Mo17 cross. On the other hand, the estimates of dominance effects showed considerable
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variations in magnitude and sign depending on cross, trait and planting density. Non-consistency in estimates was more pro-nounced for epistasis effects for most of the traits at different cross-planting densities combinations. Martine and Hallauer (1976) reported that interaction between epistatic effects and environment in maize is very important. Interactions between environment and genetic parameters depends on the num-ber of genes involved in the inheritance of the trait and, as the number increases the influence, of the environment becomes greater (Gamble, 1969c ; Upadhyaya and Nigam, 1998). This could be one of the pos-sible reasons for the epistasis × planting density interaction in this study. Therefore, widespread and unpredictable epistasis caused by environmental interaction rein-forces the need for wide and repeated testing of maize hybrids.
By considering the three digenic epistatic effects, it was evident that epistasis was a major factor in generation × planting density interactions especially for cross B73×Mo17. This interaction was more evident for [i] type epistasis since, when generation mean analysis was conducted across planting den-sities, the averaging of gene effect estimates resulted in adequacy of the models without incorporating [i] type epistasis (data is not shown). Similar results that epistatic effects interact more strongly with the environment than additive and dominance gene effects have been reported for maize (Adetimirin et al., 2001; Eta-Ndu and Openshaw, 1999; Gonzalez-Moreno and Dudley, 1981).
Narrow-sense heritability estimates were generally lower than broad-sense heritabili-ties indicating the presence of non-additive gene action. The departure from the addi-tive-dominance model indicates that multi-ple genes interact to affect most of the stud-ied traits. The low hn
2 estimates for most traits suggested that the inheritance is com-plex. Although the results of this experiment may be applicable only to the germplasm used herein, the identification of dominance and epistatic effects suggest that additional research is necessary.
One advantage of generation mean analy-sis, compared with other mating designs such as diallel, is an increased level of sensi-tivity through a decreased error rate (Hal-lauer and Miranda, 1988). However, envi-ronmental differences may cause averages to cancel out effects for opposing directions. This may explain why the results of our ex-periment support the importance of nonaddi-tive effects such as dominance.
The results of this study demonstrated that gene effects obtained by generation mean analysis differed with the different genetic backgrounds of the inbred crosses, and were also influenced by environmental conditions (planting densities). Also our results re-vealed the involvement of epistasis in ge-netic control of some of the studied planting density-traits combinations. The involve-ment of gene interactions for quantitative characteristics in maize has been reported previously (Darrah and Hallauer, 1972; Eta-ndu and Openshaw, 1999, Lamkey et. al, 1995; Morenzo-Gonzalez and Dudley, 1981; Stuber and Moll, 1971; Wolf and Hallauer, 1997).
REFERENCES
1. Adetimirin, V. O., Aken’Ova and Kim, S. K. 2001. Detection of Epistasis for Horizontal Resistance to striga bermonthica in Maize. Maydica, 46: 27-34.
2. Ceballos, H., Pandey, S., Narro, L. and Perez-Velazquez, J. C. 1998. Additive, Dominant, and Epistatic Effects for Maize Grain Yield in Acid and Non-acid Soils. Theor. Appl. Genet., 96: 662-668.
3. Chen. L., Cui, S., Su. Y. and Chen, L. 1996. Analysis of the Gene Effect on Ear Charac-ters in Maize. Acta Agr. Boreali Sinica, 11(2): 28-32.
4. Cukadar-Olmedo, B. and Miller, J. F. 1997. Inheritance of the Stay Green Trait in Sun-flower. Crop. Sci., 37: 150-153.
5. Darrah, L. L. and Hallauer, A. R. 1972. Ge-netic Effects Estimated from Generation Means in Four Diallel Sets of Maize Inbreds. Crop Sci., 12: 615-621.
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در دو ها براي تخمين پارامترهاي ژنتيكي صفات مختلف تجزيه و تحليل ميانگين نسل در سه تراكم كاشتهاي ذرت تلاقي اينبرد لاين
سعيدي. ق و رضائي. عزيزي، ع.ف
دهيچك
هدف . نژادي به دانش كافي از نظام ژنتيكي كنترل كننده صفت بستگي دارد انتخاب موثرترين روش بهاين بررسي تعيين پارامترهاي ژنتيكي براي عملكرد دانه و صفات وابسته از جمله برخي از اجزاء عملكرد در
درتلاقيهاي BC2و P1 ، P2 ، F1 ، F2 ، BC1بدين منظور از تجزيه ميانگين نسلهاي . سه تراكم كاشت بودتجزيه واريانس نشان داد كه اثر متقابل تراكم كاشت و . استفاده شد K74/1و Mo17با B73اينبرد لاين
و غالبيت ژنها يتجزيه ميانگين نسلها نشان داد كه آثار افزايش. نسل به ژنوتيپ و نوع صفت وابسته استاثر متقابل غير آللي بر نمود تمام . براي همه صفات مورد بررسي اهميت دارند، ولي اثر غالبيت بارزتر بود
اثر متقابل . صفات در هر دو تلاقي و سه تراكم كاشت اثر داشت و اين اثر در تلاقيها و تراكمها متفاوت بود . پذيري و غالبيت بود آن با آثار جمعتر از اثر متقابل تراكم كاشت با اپيستازي شديد
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