Cao W

Fedak G

McCartney C

Savard ME

Somers DJ

Voldeng H

Xue A

English

Sydney eScholarship

 1Effectiveness of marker assisted selection for resistance to Fusarium head blight in a wheat backcross breeding program  Cao W1, Fedak G1, Somers DJ2, McCartney C2, Xue A1, Savard ME1 and Voldeng H1 1Eastern Cereal and Oilseed Research Center, Agriculture and Agri-Food Canada, Ottawa K1A 0C6; 2Cereal Research Center, Agriculture and Agri-food Canada, Winnipeg R3T 2M9  ABSTRACT  The objective of this study was to determine the effectiveness of MAS relative to conventional visual selection (CVS) to augment resistance to FHB in a spring wheat backcross breeding program.  BW301, a line susceptible to FHB, was crossed in 2002 with HC374, a line resistant to FHB and the F1 backcrossed to BW301. A MAS population (MAS BC2F5) has been developed through F2-derived method, while two conventional visual selection populations (CVS BC1F6 and BC2F5) have been developed though single seed descent. Seven lines with all three FHB QTLs were selected from the MAS population, while the top 10 resistant lines were selected from CVS BC1 and CVS BC2 populations, respectively, based on FHB symptoms in the FHB nursery of 2005. The 27 lines, plus two parents, were further evaluated for FHB resistance in a four replicate field experiment in 2006 and 2007. A combined analysis from two years data showed that the means for FHB incidence,  severity and index and deoxynivalenol (DON) content for the MAS population were 39.2%, 27.3%, 11.2% and 4.6 ppm, for the CVS BC1 population the values  were 43.0%, 30.8%, 14.4% and 6.1 ppm and; for the  CVS BC2 population the values  50.6%, 41.2%, 21.6% and 9.1 ppm.  Marker profiles showed that the lines in the CVS BC1 population carried from 0 to 2 QTLs whereas none of the lines in CVS BC2 population carried the FHB resistance QTLs. The results confirmed that MAS is more effective than CVS for improvement of resistance to FHB in this wheat backcross breeding program and indicated that the more backcrosses, the greater the risk of loss of resistance genes in backcross breeding if the CVS is used. The results also suggested that minor genes have an important role for resistance to FHB of wheat.  INTRODUTION  Marker-assisted selection (MAS) is a cost-effective tool in plant breeding, especially for quantitative traits that are associated with low heritability and inconsistent environmental effects. Since the cost to evaluate these quantitative traits at multiple locations is high, selection gain could be maximized through the use of MAS. Yousef and Juvik (2001) compared phenotypic and MAS selection for quantitative traits such as seedling emergence, kernel sucrose concentration, kernel tenderness and hedonic rating in sweet corn. They concluded that MAS used in traditional breeding programs can expedite selection progress and be cost-effective. Recently, Ender et al.. (2008) used MAS for the improvement of white mold resistance in common bean. The results indicated that MAS effectively enhanced selection for resistance to white mold.    Fusarium head blight (FHB) is an important wheat disease, caused by Fusarium graminearum.  It not only reduces wheat yield, but also degrades wheat grain quality, plus causes deposition of deoxynivalenol (DON) in the grain, which makes grains unsuitable for food and feed. FHB symptoms are not consistent across locations and years. This makes selection difficult in wheat breeding programs. MAS might make selection more effective when DNA markers are tightly linked to the gene for the resistance to FHB. Somers et al (2003) identified QTLs for FHB resistance, using SSR markers, on chromosomes 2DS, 3BS and 5AS in a population of 91 doubled haploid lines derived from the cross Wuhan-1/Nuybay. The QTLs on chromosomes 2DS and 5AS control accumulation of DON in wheat grain. Those on chromosomes 3BS and 5AS control the development of FHB on wheat spikes. The objective of this study was to determine the effectiveness of MAS relative to conventional visual selection (CVS) for resistance to FHB in a spring wheat backcross breeding program.    MATERIALS AND MATHODS  Plant materials and experimental design  BW 301, an elite breeding line but susceptible to FHB, was crossed with HC 374, a line resistant to FHB. The latter, derived from the cross Wuhan-1/Nuybay, has three QTLs for resistance to FHB situated on chromosomes 2D, 3BS and 5D. The F1 plants were backcrossed to BW 301.   In the BC1F1, these molecular markers were used to screen for resistance to FHB.  Only plants with all three markers were selected.  At the same time, other BC1F1 plants were selected based on visual assessment of FHB symptoms following point inoculation in the greenhouse.  In both instances, the selected resistant plants were again backcrossed with BW 301 to produce BC2F1 populations.  Again, the two selection methods were applied in their respective BC2F1 populations.  Selected FHB resistant plants were advanced to the F2 in each population, except that single seed descent (SSD) was used for the CVS population.  The MAS population consisted of 26 BC2F3 families and the CVS populations consisted of 40 BC1F4 and 67 BC2F3- SSD families. These three populations were evaluated for reaction to FHB in the field in 2004 and  2 2005. The top 7 lines from MAS population, the top 10 lines from BC1 population and the top 10 lines from BC2 population were selected, based on level of resistance. Seed of these 27 lines plus the two parents HC374 and BW301 were increased in a greenhouse for FHB evaluation in a four replication field experiment in 2006 and 2007. The nursery was inoculated at the tillering stage (a month after planting) with corn and barley kernels that were infected with three isolates of F. graminearum.  Molecular marker analysis  The DNA of each accession and progeny line was extracted from leaf tissue of single plants using the DNeasy Plant DNA extraction kit (Qiagen, Mississauga, ON). Three SSR molecular markers:  gwm608, cfd233 and cfd73 on chromosomes 2D, wmc705, gwm293 and gwm304 on 5A and gwm493 and gwm533 on 3B, were used in this study for the marker-assisted selection. Method of PCR analysis was as described by Somers  et al. (2003).  RESULTS AND DISCUSSION Fusarium head blight developed very well in both years. FHB symptoms in the nursery were more severe in 2007 than in 2006, but DON levels were lower in 2007 than in 2006 (Table 1). In 2006, the means for FHB incidence, severity and index and DON content for the MAS population were 32.5%, 27.1% 9.2% and 6.7 ppm, for the  CVS-BC1 population, 35.4%, 30.6%, 11.6% and 9.0 ppm and for CVS-BC2 population, 41.1%, 37.4%, 15.7% and 13.1 ppm, respectively. The symptoms and DON contents in MAS population were much lower than those in CVS BC1 and BC2 populations.  Similar findings were obtained in 2007 (Table 1). These results support the expectation that MAS is more effective than CVS for improvement of resistance to FHB. Combined DON analysis from the data of 2006 and 2007 showed five lines with the lowest DON content, four from MAS population and one from CVS BC1 population. None of these lines with three markers from the MAS population was significantly different from the line of the CVS BC1 population on the basis of DON content (Table 2).  Among the MAS population, all lines have three QTLs. However, three lines have significantly higher level of DON content than the resistant parent, HC 374 with three QTLs (data not shown). This suggests that minor genes might have an important role for resistance to FHB. These observations can be explained by the fact that the three QTLs under study provide Type II resistance, where as DON accumulation is controlled by different QTL (Somers et a., 2003)   Marker analysis indicated that in CVS BC1 population, there are two lines with one QTL, one line with two QTLs and the others without any QTL, while in the CVS BC2 population there were no lines with QTL (data not shown). The line, BC1-60 with two QTLs, had as high level of resistance as other lines with three QTLs including resistant parent, HC 374, in terms of FHB index and DON content (Table 2). These findings support the suggestion that minor genes have a significant effect for resistance to FHB.    Table 1. Means of MAS, CVS-BC1 and CVS-BC2 populations for FHB incidence, severity and index and DON content in the FHB field nursery in 2006 and 2007.  Year       Population # of Lines Incidence Severity  Index  DON        (%)   (%)     (%)  (ppm) ______________________________________________________________________________________ 2006 MAS  7  32.5  27.1  9.2  6.7  CVS-BC1 10  35.4  30.6  11.6  9.0  CVS-BC2 10  41.1  37.4  15.7  13.1  BW301  Parent   55.0       50.0  30.0  10.6   HC 374  Parent  11.3  7.5  0.9  5.4   2007 MAS  7  43.6  25.9  12.0  2.1   CVS-BC1 10  50.6  31.1  17.3  3.2  CVS-BC2 10  61.0  46.1  28.7  5.4  BW301  Parent  80.0  53.8   43.0  6.0   HC 374  Parent  5.0  5.0  0.3  0.8    3Table 2. Mean of FHB index and DON contents of 5 most resistant lines, plus resistant parent HC374, from the FHB filed nursery in 2006 and 2007.  Lines  Selection methods Index (%)  DON (ppm)  # of QTLs ______________________________________________________________________________________ HC 374  Parent   0.59  3.10    3  2-2614  MAS   7.15  3.51    3 2-2683  MAS   9.38  4.86    3 2-2991  MAS   8.88  3.64    3 2-2934  MAS   7.31  3.43    3 BC1-60  CVS   6.69  4.33    2 LSD  α = 0.05   7.43  1.79  Backcross is a useful means for a breeder to transfer traits of interest into a target cultivar. The traits are limited to be controlled by major genes and less environment-affected.  Usually, it is necessary to have three backcrosses for recovery of the recurrent parent. FHB resistance in bread wheat is controlled by at least three major genes (Somers et al., 2003). Symptoms of FHB are inconsistent not only in the fields, but also in the greenhouse. For example, Roblin, a susceptible check, sometimes showed resistance to FHB with 10% of infection in the greenhouse. At current study, most of lines in BC1 population lost three QTLs resistant to FHB and all lines in the BC2 population completely lost resistant QTLs. It suggested that the more backcrosses, the greater risk of loss of resistance genes in backcross breeding if the CVS is used. The current finding is supported by Davies et al. (2006) that MAS is more advantageous for transferring high grain protein content in a backcrossing program.  This study supported the effectiveness of MAS to select for resistance to FHB of wheat in the backcross breeding program. The use of MAS for FHB resistance should become more routine in wheat breeding programs since MAS can be performed in the absence of the pathogen and in early generations. However, Davies et al (2006) compared marker-assisted and phenotypic selection for high grain protein content in spring wheat. They found that the use of phenotypic selection was as effective as MAS in selection for high grain protein content genotypes and more effective in some environments.  Agronomic traits and most disease resistance are controlled by multiple genes including major and minor genes. During the development of DNA markers for the traits, only major genes are identified associated with the traits. MAS only focuses on the major genes for the traits of interest that are associated with DNA markers and ignores other minor genes contributing to the target traits, while phenotypic selection can select for the major QTLs and minor genes contributing the target traits in a reasonable size of population.  Therefore, MAS is an alternative selection tool but cannot replace the conventional selection methodology in breeding programs.     REFERENCES  Davies, J., Berzonsky,W. A. and G.D. Leach 2006. A comparison of marker-assisted and phenotypic selection for high grain protein conternt in spring wheat. Euphytica 152: 117-134.  Ender, M., K. Terpstra and J. D. Kelly. 2008. Marker-assisted selection for white mold resistance in common bean. Mol. Breeding. 2: 149-157. Somers, D.J., G. Fedak and M. Savard. 2003. Molecular mapping of novel genes controlling Fusarium head blight resistance and deoxynivalenol accumulation in spring wheat.  Genome 46: 555-564. Yousef, G.G. and A. Juvik. 2001. Comparison on phenotypic and marker-assisted selection for quantitative traits in sweet corn. Crop Sci. 41:645-655.  4  

Effectiveness of marker assisted selection for resistance to Fusarium head blight in a wheat backcross breeding program

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Effectiveness of marker assisted selection for resistance to Fusarium head blight in a wheat backcross breeding program

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