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Published online 23 September 2005
Published in Crop Sci 45:2346-2354 (2005)
© 2005 Crop Science Society of America
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GENOMICS, MOLECULAR GENETICS & BIOTECHNOLOGY

RAPD and SCAR Markers Linked to an Introgressed Gene Conditioning Resistance to Peronospora tabacina D.B. Adam. in Tobacco

S. R. Millaa, J. S. Levina, R. S. Lewisa,* and R. C. Ruftyb

a Dep. of Crop Science, North Carolina State Univ., Campus Box 7620, Raleigh, NC 27695-7620
b Office of the Graduate School, North Carolina State Univ., Campus Box 71-2, Raleigh, NC 27695

* Corresponding author (ramsey_lewis{at}ncsu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Blue mold, caused by the fungal pathogen Peronospora tabacina D.B. Adam, is one of the most important foliar diseases of tobacco (Nicotiana tabacum L.). Identification of molecular markers linked to genetic factors controlling resistance would facilitate development of resistant cultivars. Bulked segregant analysis was used to screen 1216 random amplified polymorphic DNA (RAPD) primers for their ability to reveal polymorphism between DNA bulks from susceptible doubled haploid (DH) lines and resistant DH lines possessing resistance derived from cultivar Ovens 62. Fifteen RAPD markers were tentatively identified as being linked to a major gene conditioning resistance to blue mold. These 15 markers (12 in coupling phase linkage with resistance and three in repulsion phase) were found to lie within a single linkage group of 36.6 cM and were subsequently tested on 122 DH lines derived from crosses between resistant and susceptible parents. F tests revealed statistically significant associations between resistance and each of the 15 RAPD markers. Interval mapping was used to more accurately place the quantitative trait locus (QTL) controlling resistance on the linkage map. The RAPD markers were screened on a set of 45 resistant and susceptible cultivars or breeding lines and four Nicotiana species. At variance with previous reports, marker genotypes indicated that resistance in Ovens 62 and most other blue mold resistant lines likely originated from N. debneyi Domin. Two RAPD markers flanking the most likely QTL position were converted to sequence characterized amplified region (SCAR) markers. These markers should aid in development of blue mold-resistant tobacco cultivars worldwide.

Abbreviations: BSA, bulked segregant analysis • DH, doubled haploid • QTL, quantitative trait locus • RAPD, random amplified polymorphic DNA • SCAR, sequence characterized amplified region


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
BLUE MOLD is one of the most important foliar diseases of tobacco (Lucas, 1980). The disease is capable of infecting tobacco of all types in many tobacco growing regions of the world. The pathogen can attack plants throughout the growing season (including transplant production) and can spread rapidly under favorable weather conditions (Main, 1991). Depending on environmental conditions, losses from the disease can range from only slight to complete destruction of the crop. Use of fungicides is recommended to reduce potential for economic loss from blue mold (Shoemaker, 2004). The fungicide Ridomil (Syngenta Crop Protection, Greensboro, NC; methyl benzimidazol-2-ylcarbamate) has been used effectively in the past, but resistant isolates have developed in several countries in recent years (Main, 1991; Shoemaker, 2004). Deployment of host-plant resistance is the most economic and environmentally sustainable method for controlling blue mold. Naturally occurring resistance within N. tabacum is generally very low, however (Rufty, 1989). Resistance can be found within several Nicotiana species of Australian origin where blue mold is endemic and has reportedly been introgressed into cultivated tobacco from N. debneyi (Clayton, 1967; Clayton et al., 1967; Lea, 1963), N. goodspeedii Wheeler (Wark, 1963, 1970), and N. velutina Wheeler (Wark, 1963, 1970). Resistance induced by treatment of flue-cured cultivar Virginia Gold with the mutagen triethylene iminotrazine has also been reported (Marani et al., 1972). Few resistant cultivars are available to growers worldwide, however (Shoemaker, 2004).

Several studies have been conducted on the control of genetic resistance derived from different sources, but many discrepancies exist (Rufty, 1989). Although some reports have indicated that introgressed resistance may be controlled by several genetic factors (Clayton, 1968), it seems likely that single genes would be of primary importance in controlling resistance in species-derived lines. This presumption is based on the low probability of transferring several unlinked genes to N. tabacum from wild Nicotiana species not closely related to probable progenitor species N. sylvestris Speg. or N. tomentosiformis Goodsp. Incorporation of blue mold resistance into new cultivars has been complicated by the complex interaction between P. tabacina and the tobacco host plant. Peronospora tabacina is an obligate parasite, and selection for resistance is best performed under natural or induced field epidemics. Disease reactions are highly dependent, however, on factors such as plant age, physiological status of the plant, and environmental conditions. These factors can cause field experiments to be highly variable and unpredictable (Rufty, 1989). Identification of molecular markers linked to genes contributing to blue mold resistance would be valuable for increasing the capacity for eliminating susceptible individuals and/or lines during early stages of a breeding program.

Bulked segregant analysis (Michelmore et al., 1991), simple F tests, and interval mapping (Lander and Botstein, 1989) are useful techniques for identifying molecular markers linked to a specific gene or region of the genome. Although DNA polymorphism as revealed by molecular markers is generally low for N. tabacum, identification of markers linked to resistance genes transferred to tobacco from wild relatives has been successful (Bai et al., 1995; Yi and Rufty, 1998; Yi et al., 1998; Johnson et al., 2002). The first objective of this work was to use bulked segregant analysis, F tests, and interval mapping to identify a set of RAPD markers linked to a gene contributing to blue mold resistance found in flue cured tobacco cultivar Ovens 62. Resistance in this cultivar has been reported to be controlled by a major single gene introgressed from either N. velutina or N. goodspeedii in addition to N. tabacum genes with smaller modifying effects (Powell, 1979; Rufty, 1989; Rufty et al., 1990a).

Transformation of RAPD markers into SCAR markers is usually considered desirable before application in marker assisted breeding due to their relative increased specificity and reproducibility (Paran and Michelmore, 1993). We were able to convert two RAPD markers flanking an introgressed QTL influencing blue mold resistance to SCAR markers on the basis of specific forward and reverse primers of at least 21 base pairs in length. We also determined the presence/absence of identified RAPD markers in a set of 45 cultivars or breeding lines and four Nicotiana species of Australian origin to determine the origin and distribution of the Ovens 62 type of blue mold resistance.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Population Development
Genetic materials used in this research were generated from an applied breeding program with the objective of developing blue-mold resistant burley tobacco cultivars using flue-cured cultivar Ovens 62 as a donor of genetic resistance (Rufty et al., 1990a). This cultivar was believed to contain an introgressed gene from either N. velutina or N. goodspeedii that has a major effect on blue mold resistance (Powell, 1979; Rufty, 1989). The materials were developed over three cycles of doubled haploid (DH) breeding (Fig. 1) . In the first cycle, Ovens 62 was hybridized with blue mold susceptible cultivars Ky 15, Ky 17, and McNair 944 (Rufty, 1985). A series of F1–derived DH lines for each cross were evaluated for blue mold resistance, and DH lines NC-BMR 42 and NC-BMR 90 were selected for release as germplasm lines (Rufty et al., 1990b). In the second cycle, NC-BMR 42 and NC-BMR 90 were crossed with Ky 17 and cultivar Ky 14. Derived DH lines were evaluated for blue mold resistance, and lines NC-BMR 113 and NC-BMR 114 were selected for use in crosses with cultivars Ky 14 and TN 90 to develop a set of third-cycle DH breeding lines.



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  		Fig. 1. Development of lines or populations utilized for identification of markers linked to blue mold resistance. Three cycles of doubled haploid breeding were used to develop burley tobacco germplasm possessing blue mold resistance derived from Ovens 62. Selected Cycle 1 and Cycle 2 DH lines were used to generate the second and third cycles, respectively. NC-BMR 42 and NC-BMR 90 were Cycle 1 selections released as germplasm lines by the North Carolina Agricultural Research Service. NC-BMR 113 and NC-BMR 114 were Cycle 2 selections that were also released as germplasm lines.

 
Bulked Segregant Analysis
Two different DNA bulks were prepared from equal volumes of standardized DNA from seven blue mold susceptible tobacco cultivars and seven lines that exhibited excellent blue mold resistance in field experiments. The susceptible bulk was comprised of DNA from blue mold susceptible cultivars Ky 14, Ky 17, TN 86, TN 90, McNair 944, Speight G-28, and Speight G-70. The resistant bulk was comprised of DNA from Ovens 62 and six DH lines derived from the breeding program outlined above: NC-BMR 42, NC-BMR 90, NC-BMR 113, NC-BMR 114, DH 17, and DH 62. DNA was extracted according to Johnson et al. (1995) and was quantified with a Hoefer TKO 100 fluorometer (Hoefer Scientific, San Francisco, CA).

A total of 1216 random 10-mer primers obtained from Operon Technologies Inc. (Alameda, CA) and the Oligonucleotide Synthesis Laboratory of the University of British Columbia, Canada, were screened for their ability to reveal polymorphism between the two bulks following procedures outlined by Williams et al. (1990). PCR reaction components have been described previously by Johnson et al. (2002). Amplifications were performed in a 96-well PTC 100 thermal cycler (MJ Research, Watertown, MA) using the following reaction conditions: 1 cycle 94°C/1 min; 16 cycles 92°C/30 s, 50°C/30 s, 0.5°C/cycle, 72°C/2 min; 21 cycles 92°C/1 min, 40°C/1 min, 72°C/2 min; 1 cycle 72°C/5 min. Reaction products were subjected to electrophoresis in 1.5% (w/v) agarose gels containing 0.15 µg ethidium bromide per liter. Primers generating marker polymorphisms between the DNA bulks were subsequently tested on individual lines comprising the bulks to tentatively identify those that were linked to blue mold resistance.

Verification of Associations between Markers and Blue Mold Resistance
RAPD markers tentatively identified as being linked to blue mold resistance were subsequently tested to verify their association with resistance. Disease resistance data collected for a total of 122 DH lines derived from three crosses and evaluated in four independent field experiments conducted at Papantla, Mexico, was used. Blue mold is endemic in Mexico, and weather conditions were favorable for disease development in these experiments. Experiments were conducted as randomized complete block designs with the number of replications ranging from to two to four. Experimental units consisted of one-row plots containing at least 5 plants per row. Plant spacing was 46 cm within rows and 122 cm between rows. Seed was sown in September, and plants were transplanted in November of each year. The number of DH lines used for marker-trait associations in each experiment ranged from 17 to 50. The pedigrees, population sizes, and parameters for each of the four field experiments are provided in Table 1. Ovens 62 and Ky 14 were included in each experiment as resistant and susceptible checks, respectively. Disease evaluations based on the quantitative Horsfall-Barrat rating scale (Horsfall and Barratt, 1945) were made approximately three months after transplanting.


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  		Table 1. Experiments conducted at Papantla, Mexico, to evaluate experimental doubled haploid (DH) lines for resistance to blue mold.

 
Data for each experiment were analyzed by the GLM procedure of SAS (SAS Institute, Inc., Cary, NC), and mean disease ratings were calculated for each experimental entry. DH lines from each of the four experiments were genotyped with the RAPD markers tentatively identified as being linked to blue mold resistance using BSA. Chi-square tests were performed to determine if marker segregation differed significantly from an expected 1:1 ratio. For each of the four field experiments, the GLM procedure of SAS was used to conduct single degree of freedom F-tests for each marker (Liu, 1998) to determine if mean blue mold resistance ratings differed significantly for the two RAPD genotype classes. Significance at the P = 0.05 level was considered evidence of linkage between the marker and a gene involved in blue mold resistance. R2 values were also calculated for each marker locus to determine that marker's contribution to the total variation for blue mold resistance ratings.

Linkage and Interval Mapping
Data from experiment BM1 contained the largest number of lines (50) derived from a single cross and were utilized to generate a linkage map by Mapmaker/EXP 3.0 (Lander et al., 1987) and to localize resistance QTL by interval mapping. Linkage was determined on the basis of a minimum logarithm of odds (LOD) score of 3.0 and a linkage threshold of 0.4 by the Group command. Linkage order was established by first selecting a subset of eight markers on the basis of LOD scores and pairwise linkages and then identifying the best order using the Compare command. Remaining markers were added by the Try command. The final marker order was checked by the Ripple command with the default log-likelihood threshold value of 2.0. Map distances (centimorgan, cM) were estimated from recombination fractions and the Kosambi mapping function (Kosambi, 1944).

Interval mapping was performed by Windows QTL Cartographer (Wang et al., 2001–2004). Log-likelihood (LOD) plots for statistically significant associations between genotype and resistance were generated by calculating LOD scores at 2-cM intervals along the linkage group. The LOD threshold significance level was determined using 1000 permutations of the procedure of Churchill and Doerge (1994). A 1 – LOD empiric QTL confidence interval for the likely position of a QTL was also determined by Windows QTL Cartographer.

Origin and Distribution of Resistance
The RAPD markers were also tested to determine their presence/absence in a set of 45 tobacco cultivars/breeding lines and 4 Nicotiana species of Australian origin (N. velutina, N. goodspeedii, N. debneyi, N. excelsior J.M.Black). These species had all previously been reported as donors of blue mold resistance. Eight of the cultivars/breeding lines reportedly carried blue mold resistance transferred from either N. goodspeedii, N. velutina, or N. debneyi (Rufty, 1989) (Table 2). Two were reported to possess resistance induced by chemical mutation (Marani et al., 1972; Rufty, 1989).


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  		Table 2. RAPD genotypes for tobacco breeding lines and cultivars classified as being resistant or susceptible.

 
Conversion of RAPD Markers to SCAR Markers
Coupling phase RAPD markers UBC180.328 and OPR06.268 were selected for conversion to SCAR markers on the basis of their significant associations with blue mold resistance and for the ease at which the amplified bands could be purified from agarose gels. RAPD bands were excised from an agarose gel and purified with the Promega Wizard Gel and PCR CleanUp Kit (Promega, Madison, WI). RAPD products were reamplified from the 10-mer primers and cloned with a TOPO TA cloning kit (Invitrogen, Carlsbad, CA). RAPD fragments were sequenced and specific forward and reverse primers of at least 21 bp in length were designed on the basis of forward and reverse sequences of the RAPD fragment by the program Primer 3 (Rozen and Skaletsky, 2000).

SCAR markers were then tested on 73 DH lines evaluated in experiments BM1 and BM2 described previously. SCAR amplifications were performed in 20-µL volumes with 150 ng genomic DNA, 200 nM each primer, 200 nM each dNTP, 1x PCR buffer, 1.5 mM MgCl2, and 0.5 Units Taq polymerase. Reaction conditions for the UBC180.328 SCAR conversion were 1 cycle 94°C/2 min; 29 cycles 94°C/30 s, 60°C/30 s, 72°C/40 s; 1 cycle 72°C/7 min. Reaction conditions for the OPR06.268 SCAR conversion were the same except that the annealing temperature was 61°C.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Bulked Segregant Analysis
Out of 1216 RAPD primers that were screened, 40 primers (3.29%) detected 40 polymorphisms between the resistant and susceptible bulks. When these primers were tested on individuals comprising the two bulks, four primers produced putative repulsion phase markers that were present for all members of the susceptible bulk and absent for all members of the resistant bulk. Seventeen markers were found to be present for all members of the resistant bulk except DH 62 and absent for all members of the susceptible bulk. It was speculated that DH 62 was incorrectly classified as resistant before its inclusion in the resistant bulk, and these 17 markers were tentatively assigned as markers linked in coupling with blue mold resistance. Amplifications of tentative markers were performed twice to determine reproducibility and 15 markers (12 coupling and three repulsion) were chosen for further study (Table 3). Primers with the prefix OP were obtained from Operon Technologies Inc. (Alameda, CA), and primers with the prefix UBC were obtained from the Oligonucleotide Synthesis Laboratory of the University of British Columbia, Canada.


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  		Table 3. Chi-Square tests for segregation distortion and F tests for associations between marker genotype and blue mold resistance for four experiments conducted at Papantla, Mexico.

 
Verification of Associations between Markers and Blue Mold Resistance
Good disease pressure was present for each of the field experiments conducted during 1989, 1992, and 1996, and significant differences were present among entries in each experiment. Variability for disease resistance ratings in the four experiments is indicated by the histograms presented in Fig. 2 . All 15 markers were found to segregate in a 1:1 fashion in two of the four sets of DH lines (Table 3). Segregation of one marker (UBC610.528) in experiment BM3 and two markers (UBC610.528 and UBC528.528) in experiment BM4 deviated significantly from a 1:1 ratio. F tests indicated significant marker–resistance associations for all 15 markers in experiments BM1, BM2, and BM4 (Table 3). Significant associations were observed for 14 of 15 markers in experiment BM3. R2 values for markers associated with resistance ranged from 0.118 to 0.412. R2 values were highest for experiment BM2, where the range was from 0.213 to 0.412.



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  		Fig. 2. Histograms showing frequency distributions for blue mold resistance of doubled haploid lines evaluated in experiments BM1, BM2, BM3, and BM4. Ratings for resistant check cultivar Ovens 62 and susceptible check cultivar Ky 14 are indicated by arrows.

 
Linkage and Interval Mapping
A population of 50 DH lines derived from the cross NC-BMR 113/Ky 14 was used to establish a single linkage group covering 36.6 cM that included all 15 RAPD markers (Fig. 3) . The 12 RAPD markers linked in coupling phase with blue mold resistance were all positioned within a contiguous group of 14.2 cM. The three repulsion phase markers were placed at the end of the linkage group.



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  		Fig. 3. QTL map for chromosome segment associated with blue mold resistance produced using interval mapping. Markers are indicated beneath the X axis with intervals between them indicated in centimorgans immediately above the X axis. The 1 – LOD confidence interval for the position of the QTL is indicated by the bold horizontal line above the X axis. The experiment-wise LOD threshold significance level determined using 1000 permutations of the procedure of Churchill and Doerge (1994) as implemented by WinQTLCart is indicated by the horizontal line crossing the Y axis at LOD = 1.23.

 
The interval mapping approach indicated that all 15 RAPD markers within the linkage group had LOD scores that exceeded the LOD threshold significance level of 1.23 determined by the permutation procedure of Churchill and Doerge (1994) (Fig. 3). All but three markers had LOD scores that exceeded 3.0. A 1 – LOD confidence interval of 8.7 cM was determined that included a LOD peak of 4.12 very near the position of RAPD marker UBC544.499. Nine RAPD markers were included within this interval, and the QTL locus at the given LOD maximum explained 31.7% of the variance in disease ratings.

Conversion of RAPD Markers to SCAR Markers
RAPD amplification products for markers UBC180.328 and OPR06.268 were selected for conversion to SCAR markers. These markers flanked a 6.1-cM region that included the LOD peak generated by interval mapping. The intensity of the amplification products and their good separation from neighboring RAPD bands facilitated their isolation and cloning. The bands were sequenced, and the size of the UBC180.328 RAPD fragment was found to be 279 bp on the basis of sequence data, while the size of the OPR06.268 RAPD fragment was found to be 268 bp. BLAST searchers were performed on these sequences, but no significant similarities were found in the Genbank nucleotide sequence database. Forward primer 5'-CTGAGTTTGGCCGAATAGCAT-3'and reverse primer 5'-CAAACGTCCTAAATGGGGTATAA-3' were designed to amplify a 251-bp SCAR marker corresponding to RAPD marker UBC180.328. Forward primer 5'-GTCTACGGCAAGGGGAGATATTA-3' and reverse primer 5'-GTCTACGGCAGCAATCAACATG-3' were designed to amplify a 268-bp SCAR marker corresponding to RAPD marker OPRO6.268. These SCAR markers are referred to as SUBC180.251 and SOPR06.268, respectively, and are shown in Fig. 4 . The SCAR markers were present in the blue mold resistant bulk and absent for the susceptible bulk. The SCAR markers were used to Genotype 73 DH lines evaluated in experiments BM1 and BM2 and perfect agreement was observed between the RAPD and SCAR genotypes, thus confirming that the SCAR products were, in fact, derived from the RAPD markers, and verifying their value for marker assisted breeding.



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  		Fig. 4. (a) SCAR marker SOPR06.268, (b) SCAR marker SUBC180.251. Lane 1 = molecular size marker, Lane 2 = water, Lanes 3–9 are blue mold susceptible varieties Ky 14, Ky 17, TN 86, TN 90, McNair 944, Speight G-70, and Speight G-28. Lanes 10–14 are blue mold resistant lines Ovens 62, BMR 113, BMR 114, NC BMR 42, and NC BMR 90.

 
Origin and Distribution of Resistance
We investigated the presence or absence of the 15 RAPD markers in 45 tobacco cultivars or breeding lines. Eight of the cultivars or breeding lines reportedly possessed resistance from either N. goodspeedii, N. velutina, or N. debneyi (Delon, 1992; Rufty, 1989). Each of the 12 coupling phase RAPD markers was found to be present for eight cultivars or breeding lines with resistance reportedly derived from a wild Nicotiana species (Table 2). Each of the repulsion phase markers was absent in these lines. The origin of resistance in lines Sota 6505, ZA 684, and ZA 692 was previously unknown to the authors. These lines possessed 11, 12, and 9 of the coupling phase RAPD markers, respectively. Interestingly, Chemical Mutant and NC11-51 (reportedly possessing mutation induced resistance mechanisms) had RAPD genotypes that were identical to those for lines possessing resistance derived from Ovens 62.

We also screened the species N. goodspeedii, N. velutina, N. debneyi, and N. excelsior for the presence of eight coupling phase RAPD markers and the SCAR markers SUBC180.251 and SOPR06.268. A high amount of polymorphism was observed between the four wild Nicotiana species accessions that represented reported possible donors of resistance. The data indicated that N. debneyi is the most likely donor of resistance in all of the cultivars or breeding lines that were genotyped. Nicotiana debneyi (PI 503320) shared in common with the resistant N. tabacum material almost all of the coupling phase RAPD markers and was the only species that possessed the two SCAR markers (Table 4).


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  		Table 4. RAPD and SCAR genotypes for Nicotiana species accessions respresenting possible donors of resistance.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This report describes identification of RAPD and SCAR markers linked to blue mold resistance derived from flue cured tobacco cultivar Ovens 62. The identification of 15 markers within a single linkage group significantly associated with resistance supported the hypothesis that a single genetic locus with a large effect was of primary importance in controlling resistance in Ovens 62. The apparent close linkage of several molecular markers with this locus suggests their value for marker assisted selection. The precise origin of blue mold resistance in this cultivar was not entirely clear on the basis of available pedigree information. The cultivar was described as being derived from a cross of N. velutina with N. tabacum var. Praecox with subsequent backcrosses to tobacco cultivars Hicks, Sirogo, and Sirone (Powell, 1979). Both Sirogo and Sirone reportedly possess resistance derived from N. goodspeedii (Rufty, 1989). Our marker data suggest that N. debneyi may be the true donor of resistance in Ovens 62 as well as in Sirone, Sirogo, ZA 684, ZA 692, Sota 6505, and Bel 6-10. It was interesting that all 12 coupling phase RAPD markers linked to a factor controlling blue mold resistance in Ovens 62 were also present in lines with resistance supposedly achieved via induced mutation (Marani et al., 1972). These data suggest that resistance in Chemical Mutant may be due to alien gene introgression rather than induced mutation. Indeed, Marani et al. (1972) found no segregation for blue mold resistance in progeny derived from the cross Bel 61-10/Chemical Mutant, a result suggesting that genetic factors conditioning resistance in these two lines were identical. As noted by Rufty (1989), it is unlikely that a resistance allele derived via mutation breeding would occur at the same genome position as that which possesses a gene introgressed from a wild Nicotiana relative. Marani et al. (1972) noted the possibility that accidental mixing of seed may have occurred during the development of Chemical Mutant.

Because of low levels of marker polymorphism, it has been difficult to find markers linked to disease resistance genes originating from within N. tabacum (Nishi et al., 2003; unpublished observations). The work reported here, however, adds to the number of cases in which molecular markers have been identified for resistance genes introgressed from wild Nicotiana relatives (Bai et al., 1995; Yi and Rufty, 1998; Yi et al., 1998; Johnson et al., 2002; Lewis, 2005). The close linkage of a set of nine RAPD markers and two derived SCAR markers to a major gene influencing blue mold resistance indicates their potential value for marker-based selection. Field evaluation of experimental breeding materials for blue mold resistance can be difficult in many countries because of the stringent environmental requirements required for disease development. The set of RAPD and SCAR markers should be a valuable asset to breeding programs worldwide wishing to incorporate resistance into new cultivars.

Markers have also been identified that are linked to genetic resistance to Tobacco mosaic virus (Whitham et al., 1994), wildfire [caused by Pseudomonas syringae pv. tabaci (Wolf & Foster) Young et al.; Yi et al., 1998], root-knot nematodes (Meloidogyne spp.; Yi and Rufty, 1998), black shank [caused by Phytophthora parasitica Dastur var. nicotianae (Breda de Haan) Tucker; Johnson et al., 2002, and Potato virus Y (Lewis, 2005). Molecular marker technology can be effectively integrated into a tobacco breeding program to allow selection of multiple disease resistance genes with a single DNA extraction. Coupling-phase, dominant markers linked to resistance genes are generally more useful than repulsion phase markers in a breeding program. Doubled haploid breeding methodologies are frequently used in tobacco breeding (Legg and Smeeton, 1999). DNA markers for disease resistance could be used very effectively in a doubled haploid breeding program by allowing identification of haploid individuals possessing desired resistance genes before the chromosome doubling step.


   ACKNOWLEDGMENTS
 
This research was supported in part by the North Carolina Tobacco Research Commission.

Received for publication December 22, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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