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Published online 23 September 2005
Published in Crop Sci 45:2273-2280 (2005)
© 2005 Crop Science Society of America
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CROP BREEDING, GENETICS & CYTOLOGY

Molecular Mapping and Allelic Relationships of Russian Wheat Aphid–Resistance Genes

X. M. Liua, C. M. Smitha,*, B. R. Friebeb and B. S. Gillb

a Dep. of Entomology
b Wheat Genetics Resource Center and Dept of Plant Pathology, Kansas State Univ., Manhattan, KS 66506

* Corresponding author (cmsmith{at}ksu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious economic pest of wheat (Triticum aestivum L.) worldwide. Although resistant wheat cultivars provide good control of RWA, genetic characterization is critical to the continual use of aphid-resistance genes. The objective of this study was to determine the chromosome location and the genetic relationships of the RWA-resistance genes Dn1, Dn2, Dn4, Dn5, Dn6, Dnx, and several uncharacterized Dn genes. Molecular mapping demonstrated that Dn genes in four uncharacterized wheat lines (PI 47545, PI 222666, PI 222668, and PI 225245) as well as Dn1, Dn2, Dn5, Dn6, and Dnx are tightly linked to microsatellite markers Xgwm44 and Xgwm111. Deletion bin mapping assigned Xgwm44 physically in the region distal to the breakpoints 7DS-4 (FL 0.61) and Xgwm111 distal to 7DS-5 (FL 0.36) of wheat chromosome arm 7DS. Allelism tests detected no segregation for susceptibility among TC1F1 plants from the test crosses or among F2 plants derived from intercrosses between wheat lines containing each of these known Dn genes. Both marker-linkage analyses and allelism tests revealed that all these Dn genes are clustered in a region linked to Xgwm111 and are either alleles of a single locus or are closely related members of a Dn gene family. Dn4 and the uncharacterized Dn gene in PI 151918 are either allelic or linked on wheat chromosome 1DS. The linked markers and genetic relationships of these Dn genes will greatly facilitate their use in wheat breeding and the deployment of aphid-resistant cultivars.

Abbreviations: bp, base pair • cM, centimorgans • Dn, wheat genes expressing resistance to RWA (Diuraphis noxia) • RWA, Russian wheat aphid


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE RUSSIAN WHEAT APHID is a destructive pest of small grain crops worldwide and causes significant cereal crop losses (Walters et al., 1980; Kovalev et al., 1991; Massey and Amosson, 1991; Webster et al., 2000). The use of cultivars expressing RWA resistance is an efficient, economical, and environmentally safe approach to protect wheat from RWA while minimizing the use of insecticides. It has proven necessary to use diverse resistance genes to develop durable insect resistant wheat cultivars (Smith, 1999). During the past 20 yr, much research has focused on identifying sources of RWA resistance from wheat and closely related cereals. More than 30000 accessions of wheat and related Triticeae have been evaluated for RWA resistance worldwide since 1987. Many sources of resistant wheat and related germplasm from Central Asia and the former Soviet Union have been identified (Smith et al., 1991; Souza et al., 1991). Souza (1998) summarized RWA resistance in 74 accessions of wheat and related cereal species. But the gene(s) and inheritance of resistance in most of these sources are still unknown or uncharacterized. Many of these resistant phenotypes are similar and may be controlled by identical genetic factors (Souza et al., 1991).

Ten Diuraphis noxia (Dn) resistance genes from wheat and closely related cereals have been identified and described. These include Dn1 in common wheat accession PI 137739; Dn2 in PI 262660 (Du Toit, 1987, 1988, 1989a, 1989b); Dn3 in the goatgrass [Aegilops tauschii (Coss.) Schmal] line SQ24 (Nkongolo et al., 1991a); Dn4 in PI 372129 (Nkongolo et al., 1991b; Ma et al., 1998); Dn5 in PI 294994 (Du Toit, 1987; Du Toit et al., 1995; Marais and Du Toit, 1993; Elsidaig and Zwer, 1993); Dn6 in PI 243781(Saidi and Quick, 1996); Dn7 derived from rye and transferred to the 1RS·1BL translocation in ‘Gamtoos’ wheat (Marais et al., 1994, 1998); Dn8 and Dn9 in PI 294994; and Dnx in PI 220127 (Liu et al., 2001).

RWA-resistant genes were localized on individual chromosomes and arms by using monosomic and ditelosomic analyses and, more recently, through the use of molecular markers. Dn1 was previously located on chromosome 7D by monosomic analysis (Schroeder-Teeter et al., 1994). Dn4 was located on chromosome 1DS (short arm) by using the linked RFLP locus Xabc156 at 11.6 cM (Ma et al., 1998) and the linked microsatellite markers Xgwm106 at 7.4 cM and Xgwm337 at 12.9 cM (Liu et al., 2002). Dn5 was located on chromosome 7DL (long arm) by monosomic and ditelosomic analyses (Marais and Du Toit, 1993; Du Toit et al., 1995). Dn2 was mapped to chromosome 7DL, because of linkage to RFLP locus XksuA1 at 9.8 cM (Ma et al., 1998), and to microsatellite markers Xgwm44, Xgwm111, and Xgwm437 (Miller et al., 2001) by using the same mapping population. But there is confusion concerning the locations of XksuA1 and Xgwm437 in relation to Dn2. RFLP analyses of wheat deletion lines by Gill et al. (1991) and Hohmann et al. (1995) indicated that XksuA1 maps to chromosome 7DS. Xgwm437 is located on chromosome 7DL (Röder et al., 1998; M.S. Röder, personal communication, 2000; Liu, unpublished data, 2000).

Previous studies on the allelic relationships among Dn genes have provided inconsistent results. Dn1 and Dn2 were regarded as unlinked by Du Toit (1989b), but were reported to be allelic by Saidi and Quick (1996). Marais and Du Toit (1993) reported that Dn1 and Dn5 were linked loci, whereas Saidi and Quick (1996) proposed that Dn5 was allelic to Dn1 and Dn2. In addition, Saidi and Quick (1996) reported that Dn6 was nonallelic and unlinked to Dn1, Dn2, or Dn4.

Our previous mapping results demonstrated that Dn1, Dn2, Dn5, Dn6, and Dnx are tightly linked to microsatellite markers Xgwm44 and Xgwm111 on wheat chromosome 7D (Liu et al., 2001, 2002). Several other pest-resistance genes and drought-tolerance genes in wheat have also been linked to microsatellite markers Xgwm44 and Xgwm111 on wheat chromosome 7D (Jing et al., 2000; Arraiano et al., 2001; Miller et al., 2001; Weng and Lazar, 2002). However, discrepancies about the locations of Xgwm44 and Xgwm111 exist in several publications, complicating the understanding of the chromosome arm locations of genes linked to these markers. Xgwm44 was first located on wheat chromosome 7DS (Röder et al., 1995; Korzun et al., 1997) but later was placed on the centromere of chromosome 7D (Röder et al., 1998). In similar fashion, Xgwm111 was originally placed on wheat chromosome 7DS (Plaschke et al., 1995, 1996) but was later moved to chromosome 7DL (Röder et al., 1998). Although our previous results indicated that Xgwm111 is located on 7DS (Liu et al., 2001), several studies have incorrectly cited the chromosome arm locations of Xgwm111 and Xgwm44, resulting in misinterpretation of experimental results. Arraiano et al. (2001) mapped the Septoria tritici blotch [caused by the perfect state Mycosphaerella graminicola (Fuckel) J. Schröt.] resistance gene Stb5 to 7DS, yet placed the tightly linked Xgwm44 and Xgwm111 on 7D centromere and 7DL, respectively. Miller et al. (2001) mapped Dn2 on 7DL with linkage to Xgwm44, Xgwm111, and other 7D markers. Weng and Lazar (2002) reported linkage between Xgwm111 and the greenbug (Schizaphis graminum Rondani) resistance gene Gb3 on 7DL. Because of these conflicting reports, the objective of the present study was to re-examine and clarify the chromosome locations and the genetic relationships of the RWA-resistant genes Dn1, Dn2, Dn5, Dn6, Dnx, and several uncharacterized Dn genes.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Wheat plant introductions (PI) containing Dn genes (Table 1) were provided by the USDA-ARS National Small Grains Collection, Aberdeen, ID. The near-isogenic wheat lines (Table 2) carrying Dn genes were provided by the Small Grain Institute, Bethlehem, South Africa. All other wheat genetic materials were provided by the Wheat Genetics Resource Center at Kansas State University. Wheat genomic DNA was extracted from leaf tissue of each line by the modified CTAB procedure as described by Gill et al. (1991).


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  		Table 1. Dn genes in different wheat plant introductions, linked markers, sizes of PCR amplified fragments, chromosome location, linkage distance between markers and Dn genes.

 

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  		Table 2. Segregation of F2 or TC1F1 plants as resistant or susceptible, from crosses involving various Dn genes of wheat conferring resistance to Russian wheat aphid.

 
Eleven mapping populations, each consisting of about 100 F2 plants, were derived from crosses between RWA-resistant wheat accessions (male parent) and the susceptible cultivars Wichita or Thunderbird (female parent). All parents were evaluated for homogeneity of phenotypic reaction to RWA. F1 plants were evaluated for phenotypic reaction to RWA infestation and it was confirmed that they expressed heterozygous resistance.

The resistant wheat lines, each carrying one of the known Dn genes, were intercrossed in pairs. F1 plants were selfed to obtain F2 populations or crossed to the susceptible wheat cultivar Wichita to produce TC1F1 testcross populations (Table 2).

The RWA biotype-1 population used in this study was described by Liu et al. (2002). Phenotypic plant reaction to RWA was rated according to the method of Smith et al. (1991) with modifications of Liu et al. (2002). The chi-square test was used to test the goodness of fit of the data to the theoretically expected Mendelian segregation ratios.

Microsatellite markers from wheat chromosomes 1D and 7D (Röder et al., 1998; Pestsova et al., 2000) were screened for linkage to Dn genes. Microsatellite primer sequences and PCR protocols used were as described by Röder et al. (1995)(1998), and Pestsova et al. (2000). The microsatellite PCR amplifications were performed as described by Röder et al. (1998).

To determine the physical location of the microsatellite markers and the linked Dn genes, primers GWM44 and GWM111 were used to amplify genomic DNA from ‘Chinese Spring’ (CS); ‘Thatcher’ (TH); CS nullisomic–tetrasomic (NT) stock N7DT7A; ditelosomic (Dt) stock Dt7DS of CS (no true CSDt7DL line was available); Dt7DS and Dt7DL stocks of TH; and deletion stocks del7DS-4 (FL0.61), -5 (FL0.36), and -6 (FL0.73) of CS (Sears, 1954, 1966; Endo and Gill, 1996). The presence or absence of a specific fragment amplified from a deletion stock indicates that the corresponding marker is located proximal or distal to the breakpoint of the tested deletion stock. In this manner, the markers and linked genes were physically localized into chromosome interval regions (bins) of 7DS defined by deletion breakpoints.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Phenotype and Biotype Reactions of Wheat to Russian Wheat Aphid
Between 14 to 21 d after infestation with RWA, susceptible wheat plants began dying (from severe leaf streaking or bleaching and/or tight leaf rolling or folding) or were dead, but resistant plants were relatively healthy or showed little damage. Two different phenotypic symptoms were observed in resistant plant materials evaluated for reaction to RWA infestation. Wheat plants or their progenies containing Dn1, Dn2, Dn5, Dn6, Dnx, and in germplasm PI47545, PI 222666, PI 222668, and PI 225245 containing unnamed Dn genes, frequently exhibited chlorotic spots or slight chlorotic streaking but had no leaf rolling. Plants of PI 372129 (Dn4), PI 151918, and their derived progeny also exhibited sporadic chlorotic spots but sustained slight to moderate leaf rolling as well. In addition to displaying a unique phenotypic reaction to RWA Biotype-1, Dn4 plants react differently to RWA populations from different geographic areas. RWA populations from Syria, Russia (Puterka et al., 1992), Chile, the Czech Republic, and Ethiopia (Smith et al., 2004) are virulent to Dn4 but avirulent to Dn1, Dn2, Dn5, and Dn6. The two categories of phenotypic reactions of wheat correspond to two groups of Dn genes clustered on wheat chromosome arms 7DS and 1DS, which were revealed in marker linkage and allelic tests described below.

Microsatellite Markers Linked to Dn Genes
Preliminary marker evaluations with 20 randomly selected resistant and susceptible families from each F2:3 population containing the uncharacterized Dn genes (Table 1) indicated that Xgwm44 and Xgwm111 are linked to dominant Dn genes in PI 47545, PI 222666, PI 222668, and linked to one of two dominant Dn genes in PI 225245. Because previous results have demonstrated that markers Xgwm44 and Xgwm111 are tightly linked to Dn1, Dn2, Dn5, Dn6, and Dnx on wheat chromosome arm 7DS (Table 2) (Liu et al., 2001, 2002), the Dn genes in PI 47545, PI 222666, PI 222668, and PI 225245 are either allelic or tightly linked to them, even though the map distances and standard errors may differ in different F2 populations.

The uncharacterized Dn gene in PI 151918 was shown to be linked to the Dn4-linked markers Xgwm106 and Xgwm337 on wheat chromosome arm 1DS, indicating that these genes are either tightly linked or allelic and are independent of the Dn genes on chromosome arm 7DS. The two groups of Dn genes on wheat chromosome arms 1DS and 7DS coincide with the two categories of phenotypic reactions to RWA described above.

Microsatellite Characterization: Functional and Additional Amplified Fragments
It is critically important to characterize the specific DNA fragments amplified with microsatellite primers, especially when the primer pair amplifies multiple fragments. Primer pair GWM44 amplified four fragments (80–182 bp) from DNA of CS (Fig. 1A) and TH, as well as RWA-resistant plants (Liu et al., 2002). GWM111 amplified three fragments (130–205 bp) from DNA of CS (Fig. 1B) and TH, as well as three fragments (130–200 bp) from wheat expressing Dn2, but amplified four fragments (130–225 bp) from wheat expressing Dn1, Dn5, Dn6, or Dnx (Fig. 2) .



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  		Fig. 1. Comparison of DNA fragments amplified from DNA of euploid, aneuploid, and deletion lines of wheat by using primer pairs (A) GWM44 and (B) GWM111, electrophoresed in a 2% agarose gel. Arrow indicates the characteristic fragments present or absent in the corresponding wheat lines. CS = ‘Chinese Spring’, NT = CS N7DT7A, Dt7DS = CS Dt7DS, 7DS-4,-5,-6 = deletion CS lines del7DS-4(FL0.61), del7DS-5(FL0.36), and del7DS-6(FL0.73), respectively. L100 = 100-bp DNA ladder.

 


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  		Fig. 2. Polymorphic bands (within rectangle) amplified by primer pair GWM 111 from DNA of RWA-resistant sources PI 137739 (Dn1), PI 262660 (Dn2), PI 294994 (Dn5), PI 220127 (Dnx), and PI 243781 (Dn6), electrophoresed in a 2% agarose gel. The top band in each lane, corresponding to the characteristic fragment of CS, is associated with Dn1, Dn2, Dn5, Dnx, and Dn6 (as described in text); L = 100-bp DNA ladder.

 
GWM44 and GWM111 amplified fragments from CS DNA with the expected sizes of 182 and 205 bp, respectively, shown as the target CS bands in Fig. 1A and 1B. These specific fragments correspond to the sequences present in the original cloned CS DNA used to screen microsatellite primers and referred to as characteristic or functional fragments (Plaschke et al., 1995, 1996). GWM111 amplified functional fragments from DNA of RWA-resistant wheat sources with expected sizes of 200 to 225 bp (Table 1 and top bands in Fig. 2) that are associated with RWA resistance (Liu et al., 2001, 2002).

Microsatellite primers usually amplify multiple nonspecific fragments from additional loci on the same chromosome, homoeologous chromosomes, or nonhomoeologous chromosomes. These additional fragments are generally smaller than the characteristic fragments from the cloned microsatellite loci (Plaschke et al., 1995, 1996), such as the lower bands as shown in Fig. 1 and 2 amplified with GWM44 and GWM111. Plaschke et al. (1996) amplified CS DNA with primer pair GWM111 and placed the loci of the resulting 205-, 133-, and 129-bp fragments on wheat chromosomes 7DS, 7DS, and 2B, respectively. However, Messmer et al. (1999) placed the aforementioned fragments on chromosomes 7DL, 7BL, and 4AL. Nevertheless, only the 205-bp fragment corresponds to the original sequenced microsatellite clone of CS (Plaschke et al., 1996). In most instances, only the expected functional fragment amplified from the expected locus shows a high level of polymorphism among different wheat varieties (Plaschke et al., 1995, 1996), such as the target DNA bands shown in Fig. 2. The additional fragments usually show low levels of polymorphism, because they have shorter lengths of microsatellite repeats, or have no repetitive sequences, or have less variation in insertions or deletion events in the additional loci than those present in the original loci (Röder et al., 1995; Plaschke et al., 1996; Bryan et al., 1997).

Microsatellite fragment length polymorphisms usually result from deletions or insertions of repetitive microsatellite sequences, which alter the numbers of repeats at given loci. Microsatellite polymorphism and allelic diversity are generated by replication-slippage rather than by single nucleotide mutations (Tautz, 1989; Bowcock et al., 1994). Microsatellite lengths may be altered very rapidly, and it is 10000 times more likely for a microsatellite locus to gain or lose a repeat in one generation than for a gene to undergo a single-base mutation (Moxon and Wills, 1999). As a result, a high level of microsatellite polymorphism may occur between allelic loci from different sources within the same taxon. Because of their rapid evolution, it has been suggested that microsatellite markers may not be suitable for estimating genetic similarities or diversities except between very closely related taxa (Bowcock et al., 1994). Microsatellite polymorphisms detected by primer pair GWM111 from DNA of different sources of allelic series of Dn genes demonstrate such changes in microsatellite length.

Deletion Mapping and Physical Localization of Xgwm44 and Xgwm111
The characteristic fragments (Fig. 1A) amplified from CS (182 bp) and TH (175 bp, not shown) with primer pair GWM44 were also present in the CS Dt7DS and TH Dt7DS lines (lacking the long arm of chromosome 7D) but were absent in the TH Dt7DL line (lacking the short arm of 7D) and in the CS N7DT7A line (lacking 7D, but possessing an additional pair of 7A chromosomes). These results clearly indicate that Xgwm44 is located on the short arm of wheat chromosome 7D. Furthermore, the 182-bp-specific fragment was amplified from DNA of CS del7DS-6, but not from DNA of CS del7DS-4 or del7DS-5 (Fig. 1A), which demonstrated that Xgwm44 is physically located within the bin del7DS4-0.61-0.73 (Fig. 3A) .



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  		Fig. 3. (A) Physical and (B) adjusted genetic maps of microsatellite markers Xgwm44, Xgwm111, and the linked RWA-resistance genes (Dn) on wheat chromosome arm 7DS. FL = Relative fraction-length value of the remaining chromosomal arm from the physical breakpoint (indicated as arrow) to the centromere. S, L = short or long chromosome arm.

 
The characteristic 205-bp fragment (Fig. 1B) amplified from CS and TH using primer pair GWM111 was also present in the CS Dt7DS and TH Dt7DS lines but was absent in the TH Dt7DL line and the CS N7DT7A line. These results clearly indicate that Xgwm111 is also located on the short arm of wheat chromosome 7D, instead of 7DL. The 205-bp-specific fragment was also amplified from DNA of CS del7DS-4 and-6 but was absent from CS del7DS-5 (Fig. 1B), further demonstrating that Xgwm111 is physically located within the bin del7DS5-0.36-0.61 (Fig. 3A).

A physical map of Xgwm44 and Xgwm111 (Fig. 3A) indicates that the previous genetic order of Dn6, Xgwm111, and Xgwm44 described by Liu et al. (2002) should be reoriented as Xgwm44 (distal), Xgwm111 (middle), and Dn6 (proximal) on wheat chromosome arm 7DS. A corrected genetic map (Fig. 3B) of the Dn genes on 7DS was constructed according to the physical map and the linkage relationships among Dn genes, and the markers Xgwm44 and Xgwm111. The location of XksuA1, linked to Dn2 by Ma et al. (1998), was assigned to 7DS and placed in bin 7DS4-0.61-0.73 according to Hohmann et al. (1995).

Genetic Relationships among Dn Genes
Segregations for RWA resistance in wheat seedlings in F2 and TC1F1 populations are listed in Table 2. No susceptible progeny were observed in the F2 populations, ranging in size from 814 to 1129 individual plants, resulting from the cross combinations of Dn1/Dn2, Dn5/Dn1, Dn5/Dn2, Dnx/Dn1, Dnx/Dn2, and Dn6/Dn1. In addition, TC1F1 plants derived from the testcrosses Wichita//(Dn1/Dn2) (105 plants), Wichita//(Dn5/Dn2) (57 plants), and Wichita//(Dn6/Dn1) (67 plants) showed no segregation for RWA susceptibility.

The F2 population resulting from the cross [PI 243781(Dn6)/PI 372129(Dn4)] segregated at 319:19 resistant/susceptible plants (16.8:1 R/S), a good fit to the mode of inheritance for two independent dominant genes ({chi}2 = 0.228, P = 0.659 > 0.05). This allelism test result is consistent with the map locations of Dn4 on 1DS being independent from Dn6 on 7DS.

Both the marker-mapping and allelism test results also demonstrated that Dn1, Dn2, Dn5, Dn6, and Dnx, and the uncharacterized Dn genes in PI 47545, PI 222666, PI 222668, and PI 225245, are either allelic at the same locus on wheat chromosome arm 7DS, or are tightly linked to one another. These results further confirm those of Marais and Du Toit (1993), who reported that Dn1 and Dn5 were linked loci, as well as confirm results of Saidi and Quick (1996), who observed no segregation in three F2 populations (approximately 200 progeny each) from crosses between sources of Dn1, Dn2, and Dn5. In contrast, our results contradict those of Du Toit (1989b), who concluded that Dn1 and Dn2 were independent (unlinked) and contradict the result of Saidi and Quick (1996), who reported that the resistance gene in PI 243781 was independent of Dn1 and Dn2 and designated it as Dn6. Our allelism results also differ from those of Dong and Quick (1995) and Dong et al. (1997), who determined that the RWA resistance genes in PI 225245, PI 222666, and PI 222668 were independent from Dn5 or Dn6.

Critical Factors Affecting the Precision of Allelism Tests
Our experience suggests that three factors are critical to the success of allelism tests between two resistant sources.

  1. Homogenous and homozygous seed stocks: The parents used to test for allelic relationships must be from pure, homozygous seed stocks. Otherwise, conclusions about allelism may be incorrect. Dn1 and Dn2 were first proposed as separate genes present in PI 137739 and PI 262660, respectively, based on a F2 segregation ratio of 266:20 R/S (15:1) from the cross PI 137739/PI 262660 (Du Toit 1989b). But Du Toit (1989b) found that some PI 262660 parent plants, as well as some F1 plants from the cross (PI 262660/Tugela), were susceptible, indicating that the PI 262660 source was either heterozygous or heterogeneous (mixed). The homogeneity of wheat stocks used in the present study was confirmed by progeny testing and infesting the resulting plants with Russian wheat aphids.
  2. Pollination control: Strict measures (such as the separation of extraneous pollen sources) must be taken to prevent contamination of pollen from RWA-susceptible wheat plants during the crossing of two resistant parents. To avoid contamination, we covered wheat spikes in a timely manner with head bags after emasculation and pollination.
  3. Correct scoring of the plant phenotypic reactions: Resistant and susceptible check plants should always be included in the resistance evaluations. When scoring the phenotypic reactions of wheat seedlings to RWA infestation, it is often difficult to distinguish seedling rot or disease symptoms from RWA damage (especially when a plant is dying or dead). Weak, stunted, or rotten seedlings should be removed from the test populations before artificially infesting plants with aphids. After infestation, it is necessary to record the plant damage symptoms every 4 to 6 d before the death of susceptible control plants.

Discrepancies in Previous Chromosome-Arm Determinations
Discrepancies in the chromosome-arm locations of Xgwm44 and Xgwm111 (Röder et al., 1995, 1998; Plaschke et al., 1995, 1996; Korzun et al., 1997), and contradictions about the chromosome-arm locations of Dn2 and Dn5 between previous reports (Du Toit et al., 1995; Ma et al., 1998) and our present data, are most likely due to the previous use of a misidentified CS Dt7DL stock. Werner et al. (1992) demonstrated that the physically or cytologically longer arm of wheat chromosome 7D is actually the genetically shorter arm (7DS) that is homoeologous to chromosome arms 7AS and 7BS. Because of this, the arms of chromosome 7D were incorrectly inverted, and 7DS was placed in the long arm position in the figures of many previous publications. Friebe et al. (1996), using chromosome banding, and Devos et al. (1999), using RFLP hybridization, have verified that all previously available CS ditelosomic 7D stocks (designated as either CS Dt7DS or CS Dt7DL) are actually CS Dt7DS. Thus, there had been no true, previously existing CS Dt7DL stock.

Röder et al. (1998) constructed a wheat microsatellite marker map through the use of an ITMI reference population from ‘Opata 85’ x ‘W7984’. In instances in which microsatellites such as GWM44 and GWM111 mapped near the centromeric region, chromosome-arm locations were determined by CS ditelosomic line analysis (Röder et al., 1995, 1998). Xgwm44 was assigned to the centromere because it amplified with both ditelosomic CS lines Dt7DS and "Dt7DL," which actually was also Dt7DS (M.S. Röder, personal communication, 2000). As a result, the arm locations of Xgwm437 (7DL) and other 7D Xgwm markers (M.S. Röder, personal communication, 2000; Liu, unpublished data, 2000) were confirmed as being consistent with the microsatellite map of Röder et al. (1998).

Dn2 was previously located on the long arm of chromosome 7D, because it was linked to the RFLP locus XksuA1 at 9.8 cM and because the resistant parent expressing Dn2 and the so-called CS "Dt7DL" stock displayed the same specific hybridization banding patterns with marker XksuA1 (Ma et al., 1998). Miller et al. (2001) also mapped Dn2 to 7DL because of its linkage with Xgwm111, referring to the map of Röder et al. (1998). Dn5 was previously located on chromosome 7DL on the basis of a test cross using "CS Dt7DL" (Du Toit et al., 1995). The "CS Dt7DL" stocks used by both Ma et al. (1998) and Du Toit et al. (1995) were most likely CS Dt7DS stocks, according to Werner et al. (1992), Friebe et al. (1996), and Devos et al. (1999).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The present study clarifies previous confusion concerning the chromosome arm location of Xgwm44 and Xgwm111, as well as the location of the linked genes Dn1, Dn2, Dn5, Dn6, and Dnx. These corrections will improve the wheat microsatellite map of Röder et al. (1998), benefit studies of wheat genes linked to these markers, and help avoid misinterpretation of future experimental results. Markers Xgwm44 and Xgwm111 should greatly facilitate the use of Dn genes on wheat chromosome arm 7DS in marker-assisted breeding for RWA resistance in wheat. The results also correct previous discrepancies about the genetic relationships among several Dn genes. On the basis of mapping and allelic results as well as phenotypic reactions, we conclude that Dn1, Dn2, Dn5, Dn6, and Dnx, as well as the uncharacterized Dn genes in PI 47545, PI 222666, PI 222668, and one of two Dn genes in PI 225245, are either allelic at the same locus on wheat chromosome 7DS, or are tightly linked to one another and located in a Dn gene cluster tightly linked to Xgwm111. Dn4 and the uncharacterized Dn gene in PI 151918 are either allelic or linked on wheat chromosome 1DS.


   ACKNOWLEDGMENTS
 
The authors thank Drs. Scot Hulbert, John C. Reese, Ming-Shun Chen, Guihua Bai, W. Jon Raupp, Duane L. Wilson, Renu Malik, Sharon R. Starkey, and Lieceng Zhu for their kind assistance in the research and the editing of the manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution No. 05-82-J, Kansas Agric. Exp. Stn., Kansas State Univ. This research was jointly funded by USDA RSED Grant 58-3148-8044, Kansas State Univ. Wheat Research Center, and CSREES Grant KAN 493. Mention of a proprietary name in this article does not imply approval to the exclusion of other suitable products.

Received for publication December 5, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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Crop Science 2005 45: vii. [Full Text]  



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