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CROP BREEDING, GENETICS & CYTOLOGY

Genetic Mapping of Factors Affecting Quantitative Variation for Flowering in Sunflower

^a Zeneca Semillas, Balcarce Research Station, CC: 30, (7620) Balcarce, Pcia. de Bs. As., Argentina
^b Mar del Plata University, (7620) Balcarce, Pcia. de Bs. As., Argentina
^c Dep. of Agronomy, Iowa State University, Ames, IA 50011-1010 USA

mlee{at}iastate.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
The number of days from seedling emergence to flowering (DTF) is a major consideration in sunflower (Helianthus annuus L.) breeding because the maximum yield of the crop can only be achieved if the cultivars are phenologically adapted to the production environment. Identification of genetic factors that affect flowering could create opportunities for improved breeding methods and for more fundamental investigations of this important trait and its interactions with the environment. The objectives of this study were to locate quantitative trait loci (QTL) for DTF in an elite sunflower population evaluated in four environments. Two hundred thirty-five F₂-generation plants and their F₃ progeny of a single-cross population of two divergent inbred lines were evaluated in four environments (Fargo, ND and Venado Tuerto, Daireaux, and Balcarce in Argentina). Detection of QTL was facilitated with a genetic linkage map of 205 loci defined by restriction fragment length polymorphism (RFLP) and composite interval mapping. Five QTL of five linkage groups accounted for 89% of the genetic variation for DTF. Gene action was additive at four QTL and dominant at the other locus. Three QTL were detected in all environments and generations. The parental effects and the relative magnitudes of the genetic effects of those QTL were consistent across generations and environments.

Abbreviations: cM, centimorgans • DTF, days to flowering • LOD, log₁₀ of the odds • QTL, quantitative trait loci • RFLP, restriction fragment length polymorphism

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
THE GENETIC AND ENVIRONMENTAL CONTROLS of flowering in sunflower are certainly complex and mostly undefined. Our abilities to investigate and manipulate the phenotype in selection programs could be enhanced with improved resolution of genetic factors that influence flowering and the rate at which genotypes proceed from seedlings to anthesis. Most genetic studies of flowering in sunflower have assessed the phenotype as the number of days from seedling emergence to anthesis (DTF). Polygenic inheritance patterns have been reported in most studies (Stoenescu, 1974; Machacek, 1979), although there is some evidence of genetic factors with major, qualitative effects (Jan, 1986). Additive gene action has the greatest influence on flowering (Miller et al., 1980; Roath et al., 1982; Reid, 1992; Alvarez et al., 1992; El-Hity, 1992), but dominant effects have been noted (Jan, 1986). Estimates of broad-sense heritability have ranged from 0.62 to 0.95 (Shabana, 1974; Alvarez et al., 1992; Berretta de Berger and Miller, 1984; Miller and Fick, 1997).

The genetic components of flowering in sunflower have not been described within the context of contemporary genetic analysis and molecular linkage maps. Thus, there is very limited information on genetic factors affecting flowering or DTF, their locations in the genome, and their linkage and interaction with other genes, traits, and environmental cues. To our knowledge, genetic analysis of DTF in sunflower with a comprehensive genetic map based on DNA marker loci has not been done. Therefore, our understanding of this complex trait would be advanced through genetic mapping of QTL with DNA markers. Ultimately, such information could facilitate marker-assisted selection in breeding programs and other more fundamental inquiry. The objectives of this study were (i) to locate QTL for DTF using replicated progeny evaluated in four environments and (ii) to compare detection of QTL for DTF using individual plants in the F₂ generation and their F₃ families.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Germplasm and Field Design
A cross was made between nonrestorer inbred lines (B lines) ZENB8 (female) and HA89 (male). A single plant of the F₁ generation was self-pollinated to create seed of the F₂ generation. ZENB8, a proprietary inbred line, flowers 75 d after planting at photoperiods (15–16 h) and temperatures typical of growing seasons of the locations used in this study (Fargo, ND and Venado Tuerto, Daireaux, and Balcarce in Argentina). HA89, released by the USDA, flowers 65 d after planting under the same conditions at these locations.

The F₂ generation was planted at Fargo on 14 May 1992. Two seeds per hill were sown with a hand planter and thinned to one plant per hill. The space between rows was 75 cm and the distance between hills within a row was 30 cm. Five rows of each parent and the F₁ were planted at different periods (-10, -5, 0, +5, +12 d relative to the F₂ planting date) to estimate the within-row error variance (Leon et al., 1995). Before anthesis, individual heads of the F₂-generation plants were covered with pollination bags to ensure self-pollination and production of F₃ generation seed. Two hundred thirty-five F_2:3 families were planted with a hand planter at Daireaux, Venado Tuerto, and Balcarce on 17, 18, and 20 Nov. 1992. One row per family was planted at each location. Fifteen replicates of each parent and the F₁ hybrid were included to provide an estimate of the error variance within and across locations. Rows were 3 m long and contained ten hills. The space between rows was 70 cm. Three seeds per hill were planted and seedlings were thinned to one plant per hill. The families, parents, and F₁ were randomly assigned to plots at each location.

The number of days from emergence (VE) to 50% flowering (R5.5) was recorded for individual F₂ plants and their corresponding progenies (growth stages as defined by Schneiter and Miller, 1981). The day of flowering of an F₃ progeny row was the day when 50% of the plants reached the R5.5 stage.

The RFLP map and segregation data have been described previously (Berry et al., 1995; Leon et al., 1995, 1996). The 205 RFLP loci covered 1380 centimorgans (cM) and were arranged in 17 linkage groups, the haploid number of chromosomes in this species. The average interval size was 5.9 cM. The genetic map was constructed using MAPMAKER version 3.0 (Lander et al., 1987). Genotypic classes at 23 loci deviated significantly from the expected ratios. Those loci exhibited a deficiency in the ZENB8 homozygous class. The majority of the loci with deviant ratios (18 of 23 loci) mapped to four regions, representing Linkage Groups G, L, and P (Berry et al., 1995).

Statistical Analysis
To estimate the total phenotypic variability due to genetic effects, the broad-sense heritability was estimated according to Allard (1966)(p. 96) for individual plants in the F₂ generation (Leon et al., 1995). The within-row variance in the F₂ generation was estimated by pooling within-row variances of the parent and F₁ rows. The error variance among rows was estimated in the F₂ generation. Genetic variation was then estimated by subtracting the within- and among-row variances from the phenotypic variance (Leon et al., 1995). For the F₃ families, broad-sense heritabilities were estimated using variance components according to Fehr (1987)(p. 96). The heritability on a plot basis is given by (for each location) and heritability on an entry-mean basis (across locations) by are genotypic variance, experimental error variance, and genotype x environment variance, respectively, with t and r the number of environments and replications within environments, respectively. Estimates of ²_e within and across locations were obtained from the parents and F₁, according to Hallauer and Miranda (1988)(p. 113). The significance of the genotype x environment interaction was tested according to Hallauer and Miranda (1988)(p. 113), using the ²_e estimated from the parents and F₁ across locations.

Composite interval mapping (Zeng, 1994) was used for mapping QTL. Phenotypic data consisted of trait values for each F₂ plant or F₃ family evaluated at each location and the average value of the F₃ families across locations (the mean environment). The use of single replicates of each family in each environment has been described previously for QTL mapping in maize for grain yield (Stuber et al., 1992; Beavis et al., 1994) and plant height (Beavis et al., 1991). Computations were performed with PLABQTL Version 1.1 (Utz and Melchinger, 1996) as described in detail by Bohn et al. (1996) and Austin and Lee (1998). The initial analysis was made with the "first" statement to check the database for errors and outliers. A second analysis was conducted to select cofactors using the "model D" and "scan" statement with a log₁₀ of the odds (LOD) threshold value of 2.5. The third analysis was done adding the preselected cofactors in the "cov" statement and the "smodel" statement for detection of digenic epistatic interactions between QTL that had significant main effects. The coefficient of determination (R²) of the model for the mean environment (the average of the other environments) was compared with the broad-sense heritability to calculate the amount of genetic variation associated with the RFLP loci.

The QTL and their positions were used in simultaneous multiple regression to estimate the additive (a) and dominance (d) effects for the F₂ and F₃ generations. The DTF data identified the median of a row (when 50% of the plants in a row have flowered) in the F₃ generation and not the mean value of the trait because the DTF of the later plants of the row were not recorded (per all other studies of QTL for flowering in crop species). Therefore, dominance effects for DTF were not multiplied by a factor of two. Under the assumption of |d| |a|, when we measure the median DTF of an F₃ family derived from an F₂ plant heterozygous at a specific locus (QTL), we are directly assessing the DTF of heterozygous genotypes in the row. For F₃ progeny from a heterozygous F₂ plant for a specific locus, the expected mean is (Falconer, 1981) and , where are the expected average values of an F₃ progeny from self-pollinating an F₂ plant heterozygous at a given locus, homozygous for the allele from parent A at a given locus, and homozygous for the allele from parent B at a given locus, respectively; µ is the population average; and d is the dominance deviation. Thus the difference between the heterozygous and the midparent mean values needs to be multiplied by a factor of two. When a median value is used instead of a mean value, the previous approach yields upwardly biased estimates of dominance because E(_F3,H) = µ + d(1 - 2p), where _F3,H is the median value, p is the probability of having at least 50% of the n F₃ plants in a row (from a heterozygous F₂ plant at a given locus) homozygous (AA) at that locus. For an n value of 10 plants in a row, p is equal to 0.02; then E(_F3,H) = µ + 0.96d (µ + d), and is a direct estimate of dominance effects in the F₂ generation. The above expression can be extended to other generations and different numbers of plants in a progeny by adjusting the p value.

The d/a (dominant/additive) ratio scale described by Edwards et al. (1987) was used to classify gene action [A = additive or partial dominance (0 < |d/a| < 0.55); D = partial dominance or dominance (0.55 < |d/a| < 1.20); OD = overdominance (|d/a| > 1.20)].

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
ZENB8 flowered later than HA89 at each location by 5 to 12 d. The average difference was 8 d. Directional dominance for earliness was indicated, as the F₁ had similar values to HA89 and the means of the F₂ and F₃ generations were between the midparent value and HA89 (Table 1) . Coefficients of skewness were positive: 0.61, 0.85, 0.73, 2.14, and 1.17 for Venado, Daireaux, Balcarce, Fargo, and the mean environment, respectively. Broad-sense heritabilities ranged from 0.60 in the F₂ generation at Fargo to 0.92 for the F₃ families at Venado. The heritability estimated on an entry basis in the mean environment was 0.82 (Table 1). These values are similar to those obtained with other populations in other environments (Shabana, 1974; Alvarez et al., 1992; Miller and Fick, 1997). The genotype x environment interaction was not significant (Table 2) .

In sunflower, DTF is controlled primarily by the genotype, photoperiod, and temperature (Goyne et al., 1977; Marc and Palmer, 1981; Goyne and Hammer, 1982). The lack of a genotype x environment interaction could be explained by the similarity of photoperiods among environments. With the exception of the cool temperatures toward the end of the growing season at Fargo, temperatures among the locations were also very similar throughout the growing season.

The QTL identified in this study could be used for marker-assisted selection for these and related environments. Since it is known that the inbred lines ZENB8 and HA89 are photoperiod sensitive (A. Leon, 1991, unpublished data), further research is being conducted to genetically resolve that component of flowering. Further understanding of the components of DTF and the interaction with the environment will refine the use of marker-assisted selection for modifying DTF for a wider range of environmental conditions and for understanding the influence of DTF on the expression and perception of other traits such as grain quality.

	ACKNOWLEDGMENTS

 
The authors thank Dr. Florin Stoenescu, Nora Costa, and Abelardo de la Vega for supervising the experiments at their research stations; Dr. Simon Berry for reviewing the manuscript; Dr. Martin Grondona for statistical support; and Dr. Ian Bridges and Zeneca Semillas (ADVANTA group) for making this research possible.

Received for publication December 4, 1998.

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