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CELL BIOLOGY & MOLECULAR GENETICS

Quantitative Trait Loci for Tassel Traits in Maize

^a Department of Crop Sciences, Univ. of Illinois, Urbana, IL 61801 USA

berke{at}ms21.hinet.net

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
The inheritance of tassel traits in maize (Zea mays L.) is not well understood. This study was conducted to estimate the chromosomal location and magnitude of effect of major quantitative trait loci (QTL) affecting tassel branch angle (ANGLE), branches per tassel (BRANCH#), and tassel weight (TASSWT) in 200 S₁ lines derived from a single F₁ plant from a cross of Illinois High Oil (IHO) by Illinois Low Oil (Early Maturity) [ILO(EM)]. The restriction fragment length polymorphism (RFLP) genotypes of the 200 S₁ lines were determined at 80 polymorphic loci. These lines were measured for ANGLE, BRANCH#, and TASSWT in replicated field trials during 1992 and 1993 at Urbana, IL. Composite interval mapping using selected markers as cofactors detected several QTL for each tassel trait. Six QTL located on chromosomes 2, 3, 5, 6, and 10 were significantly associated with ANGLE. They showed primarily dominant gene action and explained 43.1 and 50.7% of the phenotypic and genetic variance, respectively. Three QTL located on chromosomes 2, 4, and 7 were significantly associated with BRANCH#. They showed primarily additive gene action and explained 44.3 and 49.3% of the phenotypic and genetic variance, respectively. The QTL for BRANCH# on chromosome 7 explained 35.3% (R²) of the phenotypic variance. Seven QTL located on chromosomes 1, 2, 3, 4, and 7 were significantly associated with TASSWT. They showed primarily negative dominant gene action and explained 35.1 and 43.4% of the phenotypic and genetic variance, respectively. Most of the major QTL detected for these tassel traits were not associated with anthesis date (ANTH) in this population.

Abbreviations: ANGLE, tassel branch angle • ANTH, anthesis date • BRANCH#, number of branches per tassel • cM, centimorgan • h², narrow-sense heritability • IHO, Illinois High Oil • ILO(EM), Illinois Low Oil (Early Maturity) • LOD, log₁₀ odds ratio • MS, mean square • QTL, quantitative trait locus or loci, depending on context • RFLP, restriction fragment length polymorphism • S, short arm of chromosome • TASSWT, tassel weight

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
THE TASSEL is the male inflorescence in maize. Four different reproductive meristem types have been identified in maize: the inflorescence meristem, the spikelet pair meristem, the spikelet meristem, and the floret meristem (Irish, 1997). Several loci with qualitative effects, such as ramosa1 (ra1), ramosa2 (ra2), ramosa3 (ra3), barren stalk2 (ba2), Tassel seed6 (Ts6), and branched silkless1 (bd1), are known to affect tassel characteristics such as BRANCH# and TASSWT (Coe et al., 1988) by causing changes in the growth of one or more meristem types. For example, Ts6 causes extra branches to form in the tassel. Variation in tassel morphology may be an important indicator of genetic diversity in both wild and cultivated maize (Orr and Sundberg, 1994).

Geraldi et al. (1985) studied the inheritance of tassel characters in three broad-based maize populations. They found heritabilities (h², single plant basis) of 36.1% for TASSWT and 45.8% for BRANCH# averaged over the three populations. They also found a high negative correlation (r = -0.65) between branch number and grain yield. Mock and Schuetz (1974) studied BRANCH# inheritance using a generation means analysis with two inbred lines differing in BRANCH# as parents. Heritability (single plant basis) was 0.50, with predominantly additive gene effects and some dominance for high BRANCH#. Fischer et al. (1987) conducted six cycles of selection for reduced BRANCH# in three tropical maize populations. They reduced BRANCH# by 7.7% cycle^-1 averaged over the three populations. Bolanos et al. (1993) studied the effect of eight cycles of selection for drought tolerance on BRANCH#. Selection for drought tolerance decreased BRANCH# by 2.6% cycle^-1, from 19.1 branches in Cycle 0 to 14.8 branches in Cycle 8.

The Illinois Long-Term Selection experiment in `Burr's White' maize variety was begun in 1896. Ninety generations of mass selection for high percentage kernel oil had been completed in the Illinois Long-Term Selection strains by 1989, and the experiment is still in progress (Dudley and Lambert, 1992). Mass selection for kernel oil has been accompanied by divergent changes in ANGLE, BRANCH#, and TASSWT in Illinois High Oil (IHO) and in Illinois Low Oil (Early Maturity) [ILO(EM)] (J. Dudley, 1990, personal communication). These tassel traits are commonly thought to be quantitatively inherited.

The detection of a QTL for a quantitative trait depends on the population sample size (N) and the heritability of the trait (Beavis et al., 1994). According to Lande and Thompson (1990), as cited in Melchinger et al. (1998), the proportion of the additive genetic variance explained by detected QTL is inversely related to the product h²N. Consequently, for traits with moderate or low h², the chances of detecting a QTL in a population with sample size in the range N = 100 - 200 is fairly low unless it explains a substantial portion of the genetic variance (a major QTL).

The objective of this research was to estimate the chromosomal location and types of genetic effects of major QTL affecting tassel traits in a segregating population of 200 S₁ lines derived from a cross of IHO by ILO(EM).

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
A population of 200 S₁ lines was derived from a single F₁ plant from a cross of one IHO plant by one ILO(EM) plant, as described by Berke and Rocheford (1995). The 200 S₁ lines were grown in a randomized complete block design in 1992 and 1993 at Urbana, IL, with two replications each year. The one-row plots were 4.6 m long and 76 cm apart, with approximately 15 plants plot^-1. When the latest S₁ line had finished shedding pollen, ANGLE was scored visually on each plot. A scale of 1 to 5 was used, with 1 representing approximately vertical tassel branches ( 15 degrees from the main tassel spike), 3 representing tassel branches at a 45 degree angle from vertical, and 5 representing approximately horizontal tassel branches (90 degrees from vertical). Five tassels per plot were scored at random. The average of all branches on a tassel was visually estimated to determine the angle score for a given tassel. Five tassels were harvested at random from each plot three or four days after the latest S₁ line finished shedding pollen by cutting the tassel one cm below the first tassel branch. The tassels were dried in a forced-air dryer at 25°C for 7 d and then weighed to determine the average TASSWT for each plot. Tassels from 5 to 6 plots were then dried in an oven at 90°C for 3 to 4 d and weighed again; there was no decrease in weight, indicating all moisture had been removed from the tassels. The number of primary tassel branches (branches attached to the main spike) on each tassel was counted to determine the average BRANCH# for each plot. To obtain an estimate of parental means, the parental populations [IHO and ILO(EM)] were grown in separate-but-adjacent experiments in 1992 and 1993 in a randomized complete block design with four replications. Tassel traits were measured on the parents as described previously.

DNA isolation, gel electrophoresis, and Southern blotting procedures were outlined by Berke and Rocheford (1995) and Goldman et al (1993). Genomic and cDNA clones were selected from collections of mapped maize clones provided by the University of Missouri-Columbia (umc), Brookhaven National Laboratory, Long Island, NY (bnl), and Pioneer Hi-Bred International, Johnston, IA (php). A set of 74 clones identified 80 polymorphic loci spaced approximately 24 cM apart on 19 of the 20 chromosome arms, with the exception of 7S. The map covered 1896 cM. Map construction details were outlined in Berke and Rocheford (1995).

An analysis of variance (ANOVA) was calculated for the 200 S₁ lines for each trait for each year, and then a combined analysis over environments was computed. Lines, environments, and replications were considered random effects. Heritability of each trait was estimated on a family mean basis (Hallauer and Miranda, 1981). Exact 90% confidence intervals (CI) of h²_f were calculated according to Knapp et al. (1985). Standard errors of means were derived from the formula (MS_g / n)^1/2, where MS_g is the genotype mean square and n = entries x replications x environments. Pearson correlation coefficients (r_p) among tassel traits and ANTH were calculated by the PROC CORR procedure of SAS (SAS, 1988). Genotypic (r_g) correlation coefficients were calculated for a given pair of traits x and y by the formula where cov_xy is the pooled covariance of observed family means for two traits (Mode and Robinson, 1959). An estimated genetic correlation was considered to differ significantly from zero if its absolute value exceeded twice its standard error. A separate analysis of variance was calculated for the separate-but-adjacent experiment involving the parents.

Composite interval mapping (Jansen and Stam, 1994; Zeng, 1994) was performed by PLABQTL (Utz and Melchinger, 1995), which employs interval mapping via regression (Haley and Knott, 1992) in combination with the use of selected markers as cofactors. The statistical model employed follows that of Bohn et al. (1996), where

Here, y_j denotes the phenotypic trait value of the jth S₁ line; m is the mean phenotypic value of S₁ lines with genotype QQ at the putative QTL; b^*₁ and b^*₂ are the additive (a) and dominance (d) effects, respectively, of the putative QTL in the marker interval under consideration; x^*_ajl and x^*_bjl are conditional expectations of the dummy variables A and D given the observed genotype at the flanking marker loci, where A assumes values 0, 1, and 2, and D assumes values 0, 0.5, and 0, when the genotype at the putative QTL is QQ, Qq, or qq, respectively (D = 0.5 rather than 1.0 for heterozygotes because phenotypic traits were evaluated on S₁ lines and not S₀ plants); b_k is the partial regression coefficient of phenotype y_j on the kth marker; x_jk is a dummy variable (cofactor) taking values 1, 0, or -1 depending on whether the marker genotype of individual j at marker locus k is M_kM_k, M_km_k, or m_km_k, respectively; _j is a residual variable for the jth S₁ line. Cofactors were selected by stepwise regression. The final model was the one that minimized Akaike's information criterion with penalty = 3.0 (Jansen, 1993). The threshold of the LOD score for declaring a putative QTL significant was 2.5, which has an approximate experiment-wise Type I error of P < 0.01 and reduces the chances of a Type II error (Jansen, 1994). Following common practice, estimates of QTL positions were obtained at the point where the LOD score assumes its maximum in the region under consideration.

The phenotypic variance explained by a single QTL was estimated by the square of the partial correlation coefficient (R²). Estimates of the additive and dominance effects of each QTL and the total ²_p explained by all QTL, as well as the total LOD score, were obtained by fitting the final model including all putative QTL for a given trait. Following Stuber et al. (1987), the ratio DR = (|d|/|a|) was used to determine the type of gene action at each QTL. If DR < 0.2, gene action is additive; if 0.2 < DR < 0.8, gene action is partially dominant; if 0.8 < DR < 1.2, gene action is dominant; and if DR > 1.2, gene action is overdominant. Since only two environments were used to evaluate the 200 S₁ lines, no QTL x environment interactions were estimated. An estimate of the proportion of the genotypic variance of each trait explained by the QTL in the model was obtained from the ratio (Schon et al., 1994).

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
The ILO(EM) population had fewer branches, approximately vertical branch angle, and lighter tassels compared with the IHO population (Table 1) . The 200 S₁ lines showed a wide range for each trait, and transgressive segregation occurred for BRANCH# and TASSWT (Table 1). The analysis of variance combined over environments revealed highly significant differences (P 0.01) among S₁ lines for each trait, and the genotype x environment interaction was highly significant for BRANCH# and TASSWT but not ANGLE (data not shown).

View this table: [in this window] [in a new window]   Table 3 Chromosomal locations, maximum LOD values, R2, and estimated gene effects of QTL significantly associated with three tassel traits in two environments

Seven QTL located on chromosomes 1, 2, 3, 4, and 7 were significantly associated with TASSWT. One showed only additive gene effects, three showed partially dominant gene effects, two showed dominant gene effects, and one showed overdominant gene effects (Table 3). The locus with additive effects did not follow the parents (i.e., ILO(EM) contributed the allele for heavy TASSWT). The two loci with dominant effects both showed dominance for low TASSWT. Negative dominance effects for traits related to weight or size (e.g. yield, plant height) are unusual in maize, but in this population all dominant QTL affecting tassel weight had negative effects. For example, for bnl5.62A the IHO, ILO(EM), and heterozygous classes had mean TASSWT of 7.3, 7.1, and 7.0 g, respectively.

One QTL on chromosome 3 was associated with both ANGLE and TASSWT. The allele associated with more vertical ANGLE was also associated with heavier TASSWT. This contrasts with the parents, since ILO(EM) had a more vertical ANGLE (1.0) but lighter TASSWT (4.4 g) compared with IHO, with a 5.0 ANGLE and 6.8 g TASSWT. The QTL map locations are slightly different (222 and 214 cM from the terminal marker, respectively) suggesting they may be linked loci rather than one locus with pleiotropic effects. Further studies are needed to resolve this question.

One QTL on chromosome 2 was associated with both ANGLE and BRANCH#. The ILO(EM) allele associated with more vertical ANGLE was also associated with fewer BRANCH#. The QTL map locations are virtually identical (142 and 140 for ANGLE and BRANCH#, respectively), and further studies are needed to determine if it is one QTL with pleiotropic effects or two tightly linked QTL.

Two QTL on chromosomes 4 and 7 were associated with both BRANCH# and TASSWT. In both cases, the allele for higher BRANCH# was also associated with higher TASSWT, as would be expected if the same QTL had pleiotropic effects on both traits. The allele for higher BRANCH# and TASSWT on chromosome 4 came from ILO(EM), contrary to expectations based on parental means. The allele for higher BRANCH# and TASSWT on chromosome 7 came from IHO, as expected.

A reanalysis of the ANTH data from this population with PLABQTL revealed that two QTL on chromosomes 3 and 7 were associated with both ANTH and TASSWT, and the QTL on chromosome 3 was also associated with ANGLE. In both cases, the allele for earlier ANTH was associated with lower TASSWT. On chromosome 3, the QTL map locations for TASSWT, ANGLE, and ANTH were 214, 222, and 232, respectively. Further tests are needed to determine if they are the same QTL with pleiotropic effects, or two or more closely linked loci. On chromosome 7, the QTL map location for TASSWT and ANTH was 108, and likely represents one QTL with pleiotropic effects.

Putative QTL were used to develop a predictive model for genetic control of each trait via multiple regression. Six RFLP loci from chromosomes 2, 3, 5, 6, and 10 accounted for 43.1 (R²) and 50.7% of the phenotypic and genotypic variation for ANGLE (Table 3). Three RFLP loci from chromosomes 2, 4, and 7 accounted for 44.3 (R²) and 49.3% of the phenotypic and genotypic variation for BRANCH#, while seven RFLP loci from chromosomes 1, 2, 3, 4, and 7 accounted for 35.1% (R²) and 43.4% of the phenotypic and genotypic variation for TASSWT. No significant epistatic interactions were detected for these three traits via multiple regression analysis. Previous QTL mapping studies in maize have observed relatively few epistatic interactions (Stuber et al., 1992; Schon et al., 1994; Berke and Rocheford, 1995).

Both additive and dominant gene effects of QTL were detected. The direction of dominance and type of gene effects of QTL in different genomic regions varied for each trait. Similar results were observed by Beavis et al. (1991), Edwards et al. (1992), and Berke and Rocheford (1995), who found that the type of gene effects and direction of dominance varied for a single trait in different regions of the genome. In this study, one parent could contribute both high and low alleles for a tassel trait, in contrast to the results reported by Berke and Rocheford (1995), where IHO only contributed alleles for high oil, and ILO(EM) only contributed alleles for low oil. This is not surprising since these populations were selected for their oil content, not their tassel traits. Evaluation of the regions significantly associated with tassel traits in this population should be done in other germplasm to determine their effects in different backgrounds. Crosses between IHO, ILO(EM), and tassel mutant stocks such as Ts6 should be made to test for allelism between QTL for tassel traits in this population and known mutant loci.SAS Institute 1988

	ACKNOWLEDGMENTS

 
The authors thank J.W. Dudley for pointing out the tassel morphology variation in the IHO x ILO(EM) mapping population and helpful assistance in several aspects of the research, and R.J. Lambert for assistance in the fieldwork. This work was performed in conjunction with research supported by a grant from the Consortium for Plant Biotechnology, Inc., with matching support from the University of Illinois Agricultural Experiment Station, Cargill Seeds, Inc., and Pioneer Hi-bred International, Inc.

Received for publication June 22, 1998.

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