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Published online 1 February 2006
Published in Crop Sci 46:630-636 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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CROP BREEDING, GENETICS & CYTOLOGY

Chromosomal Location of the Russian Wheat Aphid Resistance Gene, Dn5

Ian Heynsa, Ewald Groenewalda,b, Francois Marais*,a, Francois du Toitc and Vicky Tolmayd

a Dep. of Genetics, Univ. of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
b Centraalbureau voor Schimmelcultures, P.O. Box 85167, 3508 AD Utrecht, the Netherlands
c (retired), PANNAR, P.O. Box 17164, Bainsvlei 9338, South Africa
d Small Grain Institute, Private Bag X29, Bethlehem 9700, South Africa

* Corresponding author (gfm{at}sun.ac.za)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Recent reports suggested that the Russian wheat aphid (Diuraphis noxia Mordvilko, RWA) resistance gene Dn5 is located on wheat (Triticum aestivum L.) chromosome arm 7DS rather than on 7DL. A further attempt was therefore made to physically map the gene employing the same source material previously used to describe, name, and map it. It was possible to derive monotelosomic 7DL plants carrying Dn5 on the telosome. Such plants could not be developed for the short arm. The identities of the 7DS and 7DL telosomes were confirmed using mapped microsatellite a-nd endopeptidase markers and showed unequivocally that Dn5 occurs on 7DL. It appears that use of the wrong Dn5 source material could explain some of the inconsistencies in earlier genetic studies involving this gene. In an attempt to also genetically map Dn5, a doubled haploid (DH) mapping population was derived from the cross PI 294994 x ‘Chinese Spring’ (CS). A single RWA resistance gene, believed to be Dn5, segregated in the population and was mapped together with 12 microsatellite loci and the Ep-D1 locus. The Dn gene mapped near the centromere on 7DS. However, closer inspection of the data suggested that segregation distortion with opposite effect occurred within two areas of the region that was being mapped. The combined distortion effect rendered the data unreliable and it was not possible to unambiguously determine the chromosome arm on which the Dn gene resides. It appeared that the chromosome 7D proximal regions may be problematic in terms of genetic analysis based on segregation data.

Abbreviations: cM, centimorgan • DH, doubled haploid • RWA, Russian wheat aphid • CS, ‘Chinese Spring’


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
THE CATALOGUE OF GENE SYMBOLS for Wheat (McIntosh et al., 2003) lists nine designated RWA resistance genes (Dn1–Dn9). An undesignated gene, Dnx, was reported by Harvey and Martin (1990). Two of the genes (dn3 from Triticum tauschii (Cosson) Schmalh, and Dn7 from Secale cereale L.) were introduced from related species while the remainder were identified in T. aestivum accessions from the USDA/ARS national small grains collection. Dn4 (Saidi and Quick, 1996), maps to 1DL (Ma et al., 1998) as does Dn9 (Liu et al., 2001). The six remaining genes map to chromosome 7D.

Accession PI 137739 is the source of Dn1 (Du Toit, 1987, 1989) which occurs on 7D (Schroeder-Teeter et al., 1994) whereas Dn2 derives from PI 262660 (Du Toit, 1987, 1989). Dn1 and Dn2 segregated independently in two sets of crosses between PI 137739 and PI 1262660 made by Du Toit (1989) and Marais and Du Toit (1993); however, when Saidi and Quick (1996) made the same cross, their results suggested that Dn1 and Dn2 are allelic. Such discrepancy could have been due to seed mixture or outcrossing in the Dn2 parent (Du Toit, 1989), epistasis, environmental effects or biotype variation (Saidi and Quick, 1996), or fallibility of the bioassay (Miller et al., 2001). Ma et al. (1998) found that the restriction fragment length polymorphism locus XksuA1, mapped to 7DL through aneuploid analysis, is linked to Dn2 at a distance of 9.8 centimorgans (cM). However, Boyko et al. (1999) mapped XksuA1 to 7DS. In a study conducted by Miller et al. (2001), five microsatellite markers, Xgwm44, Xgwm111, Xgwm437, Xpsp3113, and Xpsp3123 mapped close to Dn2 on chromosome 7D. The closest marker, Xgwm437, was linked at 2.8 cM. Röder et al. (1998) mapped Xgwm111 and Xgwm437 on 7DL, whereas Liu et al. (2001) placed Dn2 and Xgwm111 on 7DS.

PI 294994 is the source of Dn5 (Marais and Du Toit, 1993). When crossed with CS, F2 and backcross data suggested segregation of a single dominant resistance gene, which could be associated with chromosome 7D by monosomic analysis. Segregation data from crosses of PI 294994 (Dn5) with PI 137739 (Dn1) and PI 262660 (Dn2), suggested that Dn5 is not allelic to Dn1 and Dn2, yet may be linked to Dn1. Dn5 was located on chromosome arm 7DL by telosomic analysis (Du Toit et al., 1995) using the backcross derivative, 92RL28, as Dn5 single gene source material. Dn5 appeared to segregate independently from the centromere, yet the distance estimate was probably affected by segregation distortion, as the transmission of resistance through gametes was 27% (113 progeny) rather than 50%. Marais et al. (1998) found Dn5 to be loosely linked to Ep-D1b (32 ± 5 cM) and cn-D1 (37 ± 6.3 cM) and the testcross analysis to determine linkage with cn-D1, reflected strong preferential transmission (73% of gametes) of Dn5. Thus, non-Mendelian segregation of the particular chromosome region may have confounded the linkage estimates. Elsidaig and Zwer (1993) crossed PI 294994 with two susceptible club wheat cultivars, Moro and Hyak, and concluded from the F2 and F3 data that resistance in PI 294994 is conferred by a homozygous recessive allele at one locus and a dominant allele at a second locus. This result was confirmed by Dong and Quick (1995). Saidi and Quick (1996) crossed PI 294994 with the susceptible cultivar Carson and concluded that two dominant resistance genes segregated in the progeny. When PI 294994 was crossed to PI 137739 (Dn1), PI 262660 (Dn2), PI 372129 (Dn4), and PI 243781 (Dn6), the F2 progenies of these crosses were all resistant, indicating that PI 294994 has at least one RWA resistance gene in common with each line. Thus, PI 294994 had to contain more resistance genes than the two suggested by the results from the cross with Carson. Segregation ratio data obtained by Zhang et al. (1998) confirmed variation among PI 294994 plants for control of resistance and the authors suggested that PI 294994 be subdivided into four sub accessions. Liu et al. (2001, 2002) found at least three resistance genes in PI 294994, which they identified as Dn5, Dn8, and Dn9. Thus, Dn1, Dn2, Dn4, Dn5, Dn6, Dn8, and Dn9 were reported to occur in PI 294994 RWA resistant plants (Saidi and Quick, 1996; Liu et al., 2001). Liu et al. (2001) concluded that Dn1, Dn2, and Dn5 must either be allelic at the same locus or tightly linked to each other and to the microsatellite marker Xgwm111 near the centromere on chromosome arm 7DS.

Saidi and Quick (1996) identified Dn6 in PI 243781. In crosses of PI 243781 with PI 137739 (Dn1), PI 262660 (Dn2), and PI 372129 (Dn4), the F2 segregated in 15:1 ratios, suggesting that Dn6 is nonallelic to these three genes. Liu et al. (2001) proposed that Dnx (PI 220127) is located close to the centromere on 7DS, 1.52 cM from microsatellite marker Xgwm111 and closely linked to, but different from, Dn1, Dn2, and Dn5. Liu et al. (2002) showed that Dn6 is also linked to microsatellite markers Xgwm44 and Xgwm111 on 7DS, which suggests that it is either allelic to, or tightly linked to Dn1, Dn2, Dn5, and Dnx. The resistance gene Dn8 was identified in near-isogenic wheat lines derived from PI 294994, the source of Dn5 (Liu et al., op cit) and showed close linkage to microsatellite marker Xgwm635 which maps near the telomere on chromosome arm 7DS.

The aims of the present study were (i) to determine (using physical mapping) if Dn5 is indeed located on 7DL as was reported by Du Toit et al. (1995); and (ii) To confirm the result through genetic mapping employing a DH mapping population, in which a single resistance gene (believed to be Dn5) segregated.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Confirmation of the Chromosome Arm Location of Dn5
A near-isogenic line, 92RL28, (pedigree = PI 294994/5*‘Palmiet’) carrying a single dominant RWA resistance gene, was used as source of Dn5. It was also the source material used to designate Dn5 (Marais and Du Toit, 1993) and allocate it to 7DL (Du Toit et al., 1995). The Small Grain Institute (Private Bag X29, Bethlehem 9700, South Africa) independently used PI 294994 to develop a number of near-isogenic lines in the cultivar Palmiet. Their derivatives, collectively referred to as Palmiet-Dn5 (as it was assumed at the time that PI 294994 contained only Dn5), were acquired by USA-based breeding programs. As PI 2942994 is now believed to be heterogeneous for Dn genes (Saidi and Quick, 1996; Zhang et al., 1998) it implies that the Small Grain Institute lines did not necessarily all contain the same gene or Dn5. Genetic studies done in the USA were apparently primarily based on the latter material. 92RL28 was crossed as male parent with (i) a CS plant ditelosomic for chromosome arm 7DS and (ii) a segregate from the cross 89M88 (‘Indis’//‘Inia 66’ monosomic 7D/CS DT7DLMT7DS/3/Inia 66) that was monotelodisomic for 7DL. Monotelodisomic F1 plants (2n = 41 + t7D) were selected from each cross and were test crossed with the RWA susceptible CS nullisomic 7D.

Derivation of Tester Lines Ditelosomic for 7DS
Nine monotelosomic (2n = 40 + t7DS) TF1 plants were selected and selfed. Ten TF2 seeds from each were germinated to identify ditelosomic plants (2n = 40 + 2t7DS). Microsatellite analysis with known 7DS markers was used to verify the telosome in each family. Twenty-five TF3 progeny of each ditelosomic family and the controls CS and PI 294994 were then tested for resistance to RWA (Du Toit, 1987).

Derivation of Tester Lines Monotelosomic for 7DL
Twenty-three monotelosomic TF1 plants (2n = 40 + t7DL) were selected and microsatellite analyses were done on each to confirm the telosome. Twenty-five TF2 progeny of each plus two controls, CS and PI 294994, were screened for RWA resistance. Du Toit et al. (1995) reported that Dn5 occurs on 7DL and is not linked with the centromere. If this was correct, then some of the testcross progeny with 7DL should have acquired Dn5 through crossover and would have segregated for resistance. Conversely, no 7DS testcross progenies should have been resistant.

Verification of the Telosomes
Four microsatellite markers, Xgwm111 and Xgwm44 (specific for chromosome arm 7DS) and Xgdm150 and Xgwm437 (that map to chromosome arm 7DL) (Röder et al., 1998; Pestsova et al., 2000; Liu et al., 2001) were used to verify the telosomes. The markers were applied to five controls, PI 294994, 92RL28, CS, CS nullisomic 7D, and CS ditelosomic 7DS as well as each ditelosomic 7DS TF2 and monotelosomic 7DL TF1 plant. The Ep-D1 (endopeptidase) locus was used as additional 7DL marker. Endopeptidase analyses were done as described by Marais and Marais (1990).

Genetic Mapping of a RWA Resistance Gene in PI 294994
To map a single dominant gene in PI 294994 believed to be Dn5, a DH mapping population was derived from the F1 of the cross: PI 294994/CS using the maize pollination protocol of Pienaar et al. (1997). Seedling RWA resistance reaction was determined for each DH line employing five to 10 seedlings.

Microsatellite markers Xgwm37, Xgwm428, Xgwm437 (Röder et al., 1998), Xgdm46, Xgdm67 (Pestsova et al., 2000), Xwmc94, Xwmc157 (Gupta et al., 2002), Xbarc26, Xbarc76, and Xbarc172 (Ward et al., 2003), that map to 7DL were selected to screen the DH population. Two markers specific for chromosome arm 7DS, Xgwm44 and Xgwm111 (Röder et al., 1998; Liu et al., 2001), were included to deduce the approximate position of the centromere. The markers were first tested on six controls (PI 294994, 92RL28, CS, CS nullisomic 7D, CS ditelosomic 7DS, and a monotelosomic W1378 plant (the family was derived in the first part of the study from a monotelosomic 7DL plant that expressed Dn5) to verify their chromosome arm locations and to confirm that they were polymorphic in the parents. All individuals in the mapping population were then characterized. Chi-square tests were performed for each marker locus to determine if the segregation ratio conformed to 1:1. Mapmaker (Lander et al., 1987) was used to determine the locus order and distances between markers. A LOD threshold of 3.0 and maximum recombination frequency of 0.40 were used to determine linkage groups. The Kosambi function was selected to correct recombination frequencies.

DNA Extraction and Analyses
Genomic DNA was extracted according to Doyle and Doyle (1990). DNA concentrations were determined on 0.8% agarose gels run in 1 x TBE running buffer stained with ethidium bromide (EtBr). One microliter of DNA, 4 µL double distilled water, and 10 µL Ficoll Orange G loading dye were loaded together with two lambda DNA concentration standards (0.1 and 0.3 µg µL–1) and run for 1 h at 70 V. The DNA bands were visualized under UV light and the concentrations estimated by comparing band intensity to the lambda concentration standards. Microsatellite amplification and high-resolution size separation was done as described by Groenewald et al. (2003). The gels were silver-stained according to Beidler et al. (1982).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Physical Mapping of Dn5
The 23 F2 families derived from F1 monotelosomic 7DL plants included 18 nonsegregating, susceptible families and five families that segregated for resistance. The five families that segregated for resistance obviously resulted from recombination between 7DL and 7D of 92RL28 and represent one of two recombinant classes expected. Initially, 54 testcross F1 were screened, making it possible to derive a rough estimate of the distance between the centromere and Dn5 (19 cM). Thus, Dn5 appeared to be linked to the centromere. Du Toit et al. (1995) found that Dn5 was not linked to the centromere (distance = 59.3 cM) while their data showed strong deviant segregation of Dn5, which probably influenced the estimate. The nine ditelosomic 7DS families were all susceptible to RWA.

The authenticity of the telosomes was confirmed using known chromosome 7D markers. Gwm437 primers amplified a fragment of 112 bp in CS and a fragment of 105 bp in PI 294994, while gwm150 primers amplified a fragment of 117 bp in both CS and PI 294994. Both markers map to chromosome arm 7DL and were only amplified in 7DL monotelosomic plants. Microsatellite gwm111 primers amplified polymorphic fragments of 215 and 209 bp in PI 294994 and CS, respectively. The Xgwm111 locus could only be detected in 7DS ditelosomic plants, which is in agreement with Liu et al. (2001) and Somers et al. (2004), who mapped the locus on 7DS. Gwm44 specific primers amplified fragments of 180 and 185 bp in PI 294994 and CS, respectively, and only produced amplicons in plants having 7DS. Results for Xgwm150 and Xgwm44 are shown in Fig. 1 and 2, respectively.


Figure 1
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  		Fig. 1. Amplification products obtained using DNA of ditelosomic 7DS (lanes marked S) and monotelosomic 7DL plants (lanes marked L) in conjunction with microsatellite marker Xgwm150.

 

Figure 2
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  		Fig. 2. DNA fragments amplified using primers of microsatellite marker Xgwm44, specific for chromosome arm 7DS, in ditelosomic 7DS (lanes marked S) and monotelosomic 7DL (lanes marked L) plants.

 
When the endopeptidase bands produced by the TF2 progeny of each 7DL monotelosomic family were determined, it was found that the Ep-D1a allele which occurs in CS was present in 16 of the TF2 families, but absent in the remaining six families which possessed the Ep-D1e allele that occurs in PI 294994 (Marais et al., 1998). The variation in Ep-D1 alleles among the 7DL TF2 progeny may be explained by the fact that this locus was not selected for during backcrossing to produce the near isogenic line 92RL28. This may have resulted in co-transfer of Ep-D1e and Dn5 in a proportion of plants. The ditelosomic 7DS families did not produce any Ep-D1 products. The data therefore confirmed that Dn5 is located on chromosome arm 7DL as was proposed by Du Toit et al. (1995). In view of this result, the Palmiet-Dn5 source used by Liu et al. (2001) should be treated as Palmiet-Dn (unknown). Mistaken identity of the latter gene could also explain at least some of the inconsistencies surrounding earlier studies with Dn5, particularly tests of allelism with the other group 7 Dn loci.

Mapping of an Unidentified Russian Wheat Aphid Resistance Gene
Two hundred and four excised embryos derived from the cross: PI 294994/CS//maize, developed into plantlets, of which 94 survived the transfer to soil and subsequent colchicine treatment. Root tip chromosome counts were done to confirm that all were haploid. The 12 microsatellite markers and the Ep-D1 locus produced polymorphic bands that could be scored unambiguously. The reported location (Materials and Methods) of each microsatellite marker could be confirmed using the aneuploid stocks. A single dominant RWA resistance gene segregated in the DH population. There had been no genetic evidence of the presence of more than one RWA resistance gene in the PI 294994 plants used at the time, and it was assumed that the source contained only Dn5. However, results obtained by Elsidaig and Zwer (1993), Dong and Quick (1995), Saidi and Quick (1996), and Liu et al. (2001) suggested that PI 294994 is a heterogeneous source of Dn-genes. The Dn-gene that segregates in the DH population is therefore unconfirmed, and will be referred to as Dn. The mapped loci, segregation ratios observed and probabilities that they conform to a 1:1 ratio are shown in Fig. 3 . The computer software organized the markers in two linkage groups, since the recombination frequency between Xbarc157 and Xgwm428 exceeded 40%. All distances were calculated with a LOD score higher than 3.0 except between Xbarc157 and Xgwm428 where a LOD score was not determined. The marker order of the first linkage group generally corresponded with the 7D map given by the ‘Komugi’ database (http://www.shigen.nig.ac.jp/wheat/komugi/top/top.jsp; accessed February 2005; verified 4 Oct. 2005) except for markers Xgwm428 and Xwmc157 that were reversed. When comparing the marker order of the second linkage group to the Komugi map it was found that markers Xgwm37 and Xbarc 76 were also switched around. The genetic distances between markers did not always correspond well with those of the Komugi map, and these differences may relate to the sizes of the mapping populations used, variation in parental genetic background, and segregation distortion. The centromere is located between markers Xgwm111 and Xbarc26 (Liu et al., 2001).


Figure 3
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  		Fig. 3. (a) A linkage map constructed using Mapmaker. The approximate position of the centromere is indicated. The ratio (percentages) of the PI 294994 vs. CS alleles and the probability of conformance to 50:50 segregation are also given. (b) A map of the same region from the ‘Komugi’ database.

 
According to the linkage map (Fig. 3), Dn is located on the short arm of chromosome 7D and is loosely linked to microsatellite markers Xgwm111 (43 cM) and Xgwm44 (31 cM). Liu et al. (2001) obtained a similar result in a genetic mapping experiment with a PI 294994 derived RWA resistance gene in Palmiet-Dn5. However, the physical map data obtained in the present study, using the source material, clearly showed that Dn5 is located on 7DL. Thus, Dn is either a gene other than Dn5 or the linkage map data are inaccurate. Inconsistent results have been obtained in various earlier studies of the six Dn genes assigned to chromosome 7D. At least some of the Dn genes may be allelic (Saidi and Quick, 1996; Liu et al., 2001, 2002) although symbols were originally assigned on the basis of data that suggested distinct loci. Similarly, different studies assigned Dn2 and Dn5 to opposite chromosome arms. Segregation studies differed regarding the number of loci involved in PI 137739 and PI 294994. Since most of the conclusions were based on segregation ratios, it is possible that preferential survival of specific gene combinations may have resulted in erroneous conclusions. Marais et al. (1998) and Du Toit et al. (1995) reported non-Mendelian segregation of Dn5 and in the present study evidence of segregation distortion was also found. The unpredictable nature of these variations may have consequences for genetic studies. From Fig. 3 it would appear that two areas could have had distorted segregation, that is, Dn as well as an area around Xbarc26. Furthermore, the two regions had effects with opposite direction that may have magnified the total distortion. For Dn, the PI 294994 region had preferential transmission; for Xbarc26, the PI 294994 region showed impaired transmission.

A more detailed analysis of the results obtained with a subset of markers in areas with apparent normal transmission (Xgwm44 and Xgdm67) and the two areas with affected transmission (Dn and Xbarc26) was therefore done. Simpler symbols were assigned to the four loci and the appropriate map data summarized in Table 1. Gametes 3 and 4 should have occurred in approximately equal numbers; however, there were four times as many Rc gametes (Dn from PI 294994; Xbarc26–7D from CS) as there were rC (dn from CS; Xbarc26–7D from PI 294994) gametes. Considering all the recombinant gametes there were 20 Rc gametes but only three rC. The parental gametes, RC and rc, appeared to segregate normally and amounted to 35 and 34, respectively. The recombinant classes deviated significantly from 1:1 segregation ({chi}2 = 12.6, P = 0.0004) and suggested that interaction of the R and c chromosome regions resulted in differential viability of gametes. This effect may have affected calculations to determine the linear order of the four genes. The two genes in the middle could have been A and C, A and G, or C and G. To decide which, it is necessary to consider the gamete pairs that could have been produced by triple crossovers, that is, 5 and 6, 9 and 10, or 15 and 16. Normally, triple crossover products are expected to be very rare and should be those gametes occurring at the lowest frequency. However, for the three candidate triple crossover classes, very similar numbers of gametes were found, respectively, 3, 3, and 1, raising doubts as to which is the correct group to use. Furthermore, the two groups containing three members each included the preferentially transmitted Rc gene combination, which may have resulted in inflated numbers. If the three pairs of gametes are considered as possible triple recombinants, three possible gene orders can be deduced as is shown in Table 1. On the basis of the physical map data, the sequence based on recombinants 5 and 6 is clearly wrong. Assuming that 15 and 16 are the triple recombinants, the gene order is RACG (Dn–Xgwm44–Xbarc26–Xgdm67) with Dn on 7DS as suggested by the Mapmaker results of Fig. 3. However, if 9 and 10 are taken as the triple recombinant group, the gene order becomes RCGA (Dn–Xbarc26–Xgdm67–Xgwm44), which would suggest that Dn occurs on 7DL and is probably Dn5. This possibility can of course be investigated through telosomic analysis of DHs carrying Dn. It would therefore appear that the DH population developed in this study is not suited for mapping of the genes in question.


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  		Table 1. Summary of mapping data for Dn (R/r), Xgwm44 (A/a), Xbarc26 (C/c), and Xgdm67 (G/g). Capital letters indicate the PI 294994 allele whereas small letters indicate the CS derived allele of a locus.

 
If Dn is in fact located on 7DS, then there may be three or more Dn genes or clusters on 7D, namely Dn5 on 7DL, a proximal 7DS cluster (which could include Dn1, Dn2, Dn6, and Dnx) and a 7DS distal locus, Dn8. Conversely, if Dn is Dn5 on 7DL, then it is possible that some or all of the closely linked/allelic loci (Liu et al., 2001, 2002) in reality also occur on 7DL.

Apart from gametocidal effects and chance, another possible explanation for nonconformance of the DH data set to Mendelian segregation may simply be that specific genotypic combinations had a lower chance of being successful when pollinated with maize, raised in tissue culture and subsequently treated with colchicine. However, the fact that numerous earlier studies involving diverse crosses (and research groups) suggested irregular and inconsistent segregation of Dn5 and other centromere-linked chromosome 7D Dn genes, would suggest that the area surrounding the 7D centromere may be problematic in terms of genetic analysis. It is therefore also possible that polymorphism for microchromosomal rearrangements could cause irregular transmission of meiotic products in certain crosses. Whatever the cause, the present DH mapping data were clearly skewed and conclusions drawn regarding the affected genes will need to be confirmed using physical mapping.

Received for publication March 2, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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