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a Dep. of Agronomy and Plant Genetics, 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108
b USDA-ARS, Cereal Disease Lab., 1551 Lindig Ave., Univ. of Minnesota, St. Paul, MN 55108
* Corresponding author (tsilo001{at}umn.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: CS, Chinese Spring • DH, double haploid • HR, homozygous resistant • HS, homozygous susceptible • IT, infection type • PCR, polymerase chain reaction • RFLP, restriction fragment length polymorphism • seg, segregating • SSR, simple sequence repeat
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication April 12, 2007.
a Dep. of Agronomy and Plant Genetics, 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108
b USDA-ARS, Cereal Disease Lab., 1551 Lindig Ave., Univ. of Minnesota, St. Paul, MN 55108
* Corresponding author (tsilo001{at}umn.edu).
The wheat stem rust resistance gene Sr36, derived from Triticum timopheevi, confers a high level of resistance against a new race (TTKS, or commonly known as Ug99) and many other races of Puccinia graminis f. sp. tritici. Because Sr36-virulent races exist, breeding for durable resistance would require pyramiding Sr36 with other genes, a process that can be facilitated by DNA markers. The aim of this study was to identify and validate microsatellite markers for the detection of Sr36 in wheat breeding programs. Two populations of 122 F2 (LMPG x Sr36/9*LMPG) and 112 F2 (‘Chinese Spring’ x W2691Sr36-1) were evaluated for stem rust reaction. Both populations exhibited distorted segregation with a preferential transmission of the Sr36-carrying segment. Three markers, Xstm773-2, Xgwm319, and Xwmc477, were in complete linkage with Sr36 in the LMPG x Sr36/9*LMPG population. In the Chinese Spring x W2691Sr36-1 population, Xgwm319 was 0.9 cM away from Xstm773-2, Xwmc477, and Sr36. These codominant markers were easy to score and diagnostic for Sr36 in a set of 76 wheat cultivars and breeding lines developed in 12 countries. Together, these markers can be used in marker-assisted selection of Sr36.
Abbreviations: CS, Chinese Spring • DH, double haploid • HR, homozygous resistant • HS, homozygous susceptible • IT, infection type • PCR, polymerase chain reaction • RFLP, restriction fragment length polymorphism • seg, segregating • SSR, simple sequence repeat
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The two hard red spring cultivars, CItr 12632 (= W1656) and CItr 12633 (= W1657), carry the stem rust resistance gene Sr36, derived from T. timopheevi (Allard and Shands, 1954). These cultivars served as the original sources of Sr36 in wheat breeding programs worldwide (Roelfs, 1988a; Knott, 1989; McIntosh et al., 1995). The Sr36 gene is one of the 18 stem rust resistance genes that provide a major source of resistance to TTKS (Singh et al., 2005; Wanyera et al., 2006). However, none of the 18 genes occurs at a high frequency in breeding materials, except Sr36. In the United States, Sr36 provides resistance against QFCS, which was the most predominant race in recent surveys (Jin, 2005) and was also found in previous surveys (McVey et al., 1996, 2002). To some races of stem rust, Sr36 conditions unusual (mixed) infection types (Ashagari and Rowell, 1980), which can make it difficult to distinguish cultivars carrying this gene.
Because Sr36-virulent races exist (Knott, 1989), this gene is best deployed when pyramided with other Sr genes (Knott, 1988), a process that cannot easily be achieved through the conventional phenotypic screening methods. The drawback of classical breeding methods is that the process of pyramiding genes in a single line can be time consuming or impossible, especially when more than one gene confers resistance against known races of P. graminis f. sp. tritici; hence, it becomes difficult to identify genotypes carrying combinations of more than one gene. Pyramiding of resistance genes could be facilitated by marker-assisted selection.
Nyquist (1957) used monosomic analysis to locate Sr36 on chromosome 2B; it was later mapped on the short arm of chromosome 2B (Gyarfas, 1978; McIntosh and Luig, 1973). Bariana et al. (2001) identified 10 molecular markers linked to the Sr36 locus. Eight were restriction fragment length polymorphism (RFLP) or amplified fragment length polymorphism (AFLP) markers, and two were microsatellites (STM773 and GWM271). These authors reported that STM773 gave a better amplification than GWM271. However, even though the STM773 marker could be directly used to identify homozygous genotypes for Sr36, this marker requires careful scoring because the primers also amplify other fragments, making it difficult to distinguish heterozygous from homozygous genotypes. Therefore, more robust, codominant, and easy-to-detect microsatellite markers are needed for Sr36.
The objectives of this study were (i) to identify codominant microsatellite markers closely linked to Sr36; and (ii) to validate their potential use in marker-assisted selection of Sr36 using a set of diverse wheat germplasm.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In addition to the four wheat lines used for genetic analysis of Sr36, a diverse set of 76 wheat cultivars and breeding lines were obtained from the USDA-ARS National Small Grains Collection, Aberdeen, ID. These accessions and breeding lines were selected on the basis of previously published reports that indicated whether they possess Sr36 (accessions with and without Sr36) (Table 1 ). The information on the pedigree and the presence of Sr36 was obtained from two USDA Websites (http://www.ars-grin.gov/npgs/acc/acc_queries.html, http://wheat.pw.usda.gov) and McIntosh et al. (1995). It was supplemented with information from previous surveys of seedling resistance conducted at the USDA-ARS Cereal Disease Laboratory. The Chinese Spring nullisomic-tetrasomic (N2B-T2D) line (Sears, 1966) was used to verify the location of the amplified bands of microsatellite markers.
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7 d after planting, when the primary leaves were fully expanded) following protocols described by Jin (2005). Inoculated seedlings were kept overnight in a dew chamber for 16 h with no light and then exposed to 2 to 4 h of light to complete infection. After infection, plants were placed either in a growth chamber with 16 h of light at 20 to 22°C and 8 h of dark at 18 to 20°C or in a greenhouse set at 18 to 21°C under 160-W very high output (VHO) fluorescent tubes with a 16-h photoperiod. Infection types (ITs) were scored from primary leaves approximately 14 d after inoculation based on the scale of 0 to 4 as stipulated by Stakman et al. (1962) and modified by Roelfs (1988b). The presence and absence of Sr36 in 76 wheat cultivars and breeding lines was verified on the basis of low infection response (0 = immunity) to QFCS and low infection to MCCF (Table 1). All races used for inoculation were verified on the basis of their avirulence/virulence formula using 16 Sr differential lines (Roelfs and Martens, 1988; Roelfs et al., 1993) as checks to verify the standard race designations of all the races.
Molecular Analysis
For molecular mapping of Sr36, 2 to 3 cm of leaf tissues were collected from seedlings of parental lines, 122 F2 (LMPG x Sr36/9*LMPG) lines, 112 F2 (CS x W2691Sr36-1) lines, and 76 wheat cultivars and breeding lines. Total genomic DNA was extracted from the ground tissues following protocols described by Riede and Anderson (1996) and modified by Liu et al. (2006). Since Sr36 was mapped on the short arm of chromosome 2BS (Gyarfas, 1978; Bariana et al., 2001), microsatellite markers used in this study were based on previously published wheat genetic maps of chromosome 2BS (Röder et al., 1998; Somers et al., 2004; Song et al., 2005). A total of 51 microsatellite primer pairs were screened for polymorphisms between near-isogenic lines LMPG and Sr36/9*LMPG, and between two diverse lines, Chinese Spring and W2691Sr36-1. Two microsatellite markers (STM773 and GWM271) were previously reported to be linked to Sr36 in a double haploid population of ‘Sunco’ x ‘Tasman’ (Bariana et al., 2001). The marker STM773 has since been converted into two SSR markers, STM773-1 and STM773-2, and were included in the analysis. The sequences of STM773-1 and STM773-2 were kindly provided by Dr. Matthew Hayden, University of Adelaide, South Australia.
Polymerase Chain Reaction and Electrophoresis
Polymerase chain reaction (PCR) was performed in a 96-well plate with 10 µL of final reaction mixture containing 2.75 µL ddH2O, 1 µL 10X PCR buffer, 0.6 µL of 25 mM MgCl2, 1.6 µL of 1.25 mM dNTPs, 1 µL of each 1 µM primer, 0.05 µL of 5U µL–1 Taq DNA polymerase (Applied Biosystems, Branchburg, NJ), and 3 µL of 15 ng µL–1 genomic DNA. For all the SSR markers, except STM773-1 and STM773-2, the PCR reaction mixture was initially denatured at 94°C for 10 min, followed by 35 cycles of 94°C for 1 min, 48 to 61°C (depending on annealing temperature specific to individual primer pairs) for 1 min, and 72°C for 2 min, with a final extension step of 72°C for 10 min and 4°C indefinitely. The PCR reaction protocol for STM773-1 and STM773-2 primers was provided by Dr. Matthew Hayden as a touchdown PCR program with 94°C for 10 min, followed by touchdown PCR program with 7 cycles of 60 s at 92°C, 60 s at 64°C, 60 s at 72°C, and then five cycles with 60 s at 92°C, 60 s at 57°C, and 60 s at 72°C (conditions identical to the previous cycles, but with an annealing temperature of 57°C). The program also included an additional 10 to 25 cycles each of 30 s at 92°C, 60 s at 55°C, and 60 s at 72°C. The PCR was ended by an extra incubation for 10 min at 72°C and 4°C indefinitely. Polymerase chain reaction thermal cycling was performed in PerkinElmer/Applied Biosystems (Foster City, CA) thermo cyclers. About 5µL of 3X loading dye (0.02 g bromophenol blue, 0.02 g xylene cyanol, 1.6 mL 0.5 M EDTA, 38.4 mL formamide) was added to the PCR products to make a final volume of 15 µL. All samples were denatured for 5 min at 95°C. The PCR products were subjected to electrophoresis in a polyacrylamide gel (6% [w/v] acrylamide/bisacrylamide, 20:1, 8 M urea in TBE, pH 8.3) in 1X TBE buffer (90 mM Tris-borate [pH 8.3], 2 mM EDTA) at a constant power of 110 W for 90 min. Gels were silver stained (Bassam et al., 1991) and photographed.
Genetic Linkage Analysis
For the genetics analysis of Sr36, F2 genotypes inferred from seedling reactions of F2:3 and F3:4 families were classified as homozygous resistant (HR), segregating (Seg) and homozygous susceptible (HS). Chi-squared (
2) distribution analyses were used to test if the observed segregation ratios for Sr36 and marker loci fit the Mendelian ratio of 1:2:1. Genetic linkage analysis was performed between polymorphic microsatellite markers and the Sr36 segregation data using Mapmaker computer program version 3.0b (Lander et al., 1987).
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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2 = 7.36, P = 0.025; and 2 = 47.6, P < 0.001).
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Many cultivars and breeding lines that did not carry Sr36 displayed ITs other than immunity against race QFCS (Table 1). In ‘Excel’, the marker data reveals Sr36-associated alleles in a heterozygous state. In 2 out of 80 wheat cultivars and breeding lines, ‘W 3496’ and ‘CK 9877’, we found that the three markers, Xgwm319, Xmwc477, and Xstm773-2, were not in agreement with the stem rust screening results. The W3496 line was developed in Australia and traces back to Verntein/CItr 12632 as ultimate parents in its pedigree. This line showed zero IT to QFCS, meaning that it could be carrying Sr36 from CItr 12632, a known carrier of this gene. CK 9877 showed 2+/3+ against QFCS, indicating that this line did not have Sr36, and only marker Xwmc477 showed the absence of the Sr36-specific marker allele, whereas Xgwm319 and Xstm773-2 showed the presence of Sr36-specific marker alleles, meaning that there could have been recombination between Sr36 and the two markers Xgwm319 and Xstm773-2.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In this study, we found that GWM319, STM773-2, and WMC477 were diagnostic for Sr36. Two marker loci, Xstm773-2 and Xwmc477, were in complete linkage with Sr36 in both populations. Alleles at the Xgwm319 locus cosegregated with Sr36 in one population and were tightly linked (0.9 cM) with Sr36 in another population. According to the genetic map of Somers et al. (2004), Xgwm319 and Xwmc477 were mapped near the centromere and showed no recombination, confirming that these markers are closely linked. Therefore, these three markers would serve as a first step toward the detection of Sr36 in breeding populations. Preferential transmission of Sr36-carrying T. timopheevi segment was observed (Table 2). The exact mechanism causing preferential transmission of T. timopheevi chromosome segment is unknown. However, Nyquist (1962) hypothesized several possible causes. In our laboratory, studies are in progress to determine the exact cause.
Validation of Sr36-Linked Microsatellite Markers
Based on the previous studies, all seven reference stocks that were widely used as sources of Sr36 in wheat breeding programs (McIntosh et al., 1995) were used in this study: two breeding lines, Sr36/9*LMPG (Knott, 1990) and W2691Sr36-1, and six cultivars, CItr 12632 (= W1656) and CItr 12633 (= W1657) (Allard and Shands, 1954), Idaed 59, Mengavi, Timvera (PI 351987 and PI 237648) (Pridham, 1939), and CItr 14050. In addition to these reference stocks, we examined a range of international germplasm carrying Sr36 as listed by Roelfs (1988a) and McIntosh et al. (1995), including cultivars developed in Australia, Canada, Mexico, South Africa, and the United States (Table 1). The list included cultivars Songlen, Timgalen, Zaragosa 75 (PI 433770), Gouritz, Hand, Kenosha, Roughrider, Shortim, Timson, Arthur, and Arthur 71. Some of the newly developed Sr36-carrying cultivars from the U.S. germplasm were also included—GA-Stuckey, Jaypee, Sisson, NC-Neuse, NE 73843, Vista, Ernie, CK 9803, and Rosen (Table 1). All these cultivars and reference stocks were immune to QFCS and were characterized using Sr36-linked marker alleles of Xgwm319, Xwmc477, and Xstm773-2 (Table 1; Jin, unpublished data). Other cultivars and breeding lines that were known to carry Sr36 were developed in Zambia and the United Kingdom, including Idaho 1877 NR AE and Maris Fundin. Both the marker analysis and stem rust screening indicate that the accession of Maris Fundin (PI 410869) obtained from the National Small Grains Collection was incorrect (Table 1). However, another possibility is that Maris Fundin does not carry Sr36 and that incorrect information about this germplasm exists at the GrainGenes database (http://wheat.pw.usda.gov). Idaed 59 was heterogeneous for both the Sr36 resistance and Sr36-linked marker alleles. The heterogeneity could be the result of seed contamination. The W3496 line, commonly known as Combination III, did not carry any of the Sr36-associated marker alleles. This is in agreement with an Australian study based on a recombinant inbred line population derived from Yarralinka/Schomburgk (H.S. Bariana and coworkers, personal communication, 2007). A rare recombinant combining T. timopheevi segment with stem rust resistance gene Sr9e was present in W3496 and Yarralinka (H.S. Bariana, personal communication, 2007). The CK9877 line does not have Sr36, and only marker Xwmc477 was in agreement; however, loci Xgwm319 and Xstm773-2 showed the presence of Sr36-associated marker alleles. Therefore, based on these data, Xwmc477 appears to be the most diagnostic compared with Xgwm319 and Xstm773-2. However, it is important to mention that although WMC477 amplifies 158- and 190-bp fragments in materials containing Sr36 (Fig. 1C), we consistently observed the 190-bp fragment. We suspect that the quantity of individual PCR products will be lower for one of the fragments; hence, the 158-bp fragment was faint, and only the 190-bp fragment was consistently visible in all accessions that carried Sr36 (Supplementary Fig. 1). Different PCR-reaction protocols might lead to preferential amplification of one of the two bands (158 and/or 190 bp). A similar PCR discrepancy involving amplifications of multiple fragments was described by Bercovich et al. (1999).
Many accessions with similar names may be confusing, especially when the name is widely used instead of the accession number. In this study, we analyzed four accessions of Zaragoza 75, and only PI43370 carried Sr36. The other three accessions could be carrying different stem rust resistance genes. According to Roelfs (1988a) and McIntosh et al. (1995), Purdue carries Sr36; however, there were many accessions of Purdue at the National Small Grains Collection, and the one used in this study did not carry Sr36 according to the results of reaction to QFCS and Sr36-linked markers.
In this study, the information obtained from using races of P. graminis f. sp. tritici alone was not diagnostic of Sr36 in the presence of other Sr genes. This situation was observed in two cultivars, RL 6044 and Kenya 58, which showed immunity to QFCS, and low IT to TPMK and MCCF (Table 1). However, immunity in these cultivars was due to genes or combination of genes other than Sr36. RL 6044 is known to carry Sr33 from Tetra Canthatch//Aegilops squarrosa, whereas Kenya 58 carries Sr6 and other genes from Red Egyptian and Kenyan cultivars (McIntosh et al., 1995). With conventional screening tests, it would require extensive seedling tests and testcrosses to perform gene postulation in these cultivars by using the appropriate stem rust races—a potentially time-consuming process if two or more genes confer resistance to a particular race. However, both the rust screening results and previously published information were successful in validating the cultivars that carried Sr36, and the results were in agreement with the Sr36-specific marker alleles. From these results, it is clear that the Sr36-linked markers are diagnostic for this gene and can be used to detect its presence during cultivar development.
Even though Sr36 does not provide a high level of resistance against a wide range of stem rust races, it is still a valuable gene because it is the best available source of resistance to the new race of stem rust, Ug99. Therefore, tightly linked markers identified in this study should be useful in marker-assisted selection of Sr36 and can be used in selecting for genotypes possessing Sr36 during cultivar development. These markers will accelerate the use of Sr36 in commercial cultivars by allowing pyramiding of Sr36 with other effective genes to confer a more durable resistance. Our results show that these markers are applicable across different genetic backgrounds.
We are thankful to Dr. Matthew Hayden for providing the information on the STM773-1 and STM773-2 markers, and Dr. Harbans Bariana for his useful comments on the manuscript. This research was supported in part by the Minnesota Annual Conference of the United Methodist Church through the Project AgGrad fellowship awarded to T.J. Tsilo, the Agricultural Research Council of South Africa, and the USDA Cooperative Research, Education and Extension Service, Coordinated Agricultural Project grant number 2006-55606-16629.
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Received for publication April 12, 2007.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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