ZHENGZHOU, CHINA
Henan Agricultural University
Identification and molecular tagging of two complementary dominant resistance genes to Maize Dwarf Mosaic Virus --Wu, J-Y, Ding, JQ, Du, Y-X, Chen, W-C Maize dwarf mosaic is one of the most devastating and widespread viral diseases in the world. It has become economically important since the late 1980s. The breeding and cultivation of resistant varieties are the basic and most important ways to prevent the yield losses caused by the pathogen. The resistant materials and their inheritance play an important role in the efficiency of resistant breeding. Many reports showed that both major genes and polygenes were involved in the resistance to the virus. It is basic work in resistant breeding to identify new resistant materials and resistance genes. A new elite inbred line, Siyi, from the maize hybrid 78698, was identified that is resistant to maize dwarf mosaic virus strain B at an early and an adult stage. The genetic analysis and mapping of these genes were done by microsatellite markers.

The parents, F1, F2 and backcrosses from the combination Siyi x Mo17 were planted in years 2000 and 2001. Inbred line Siyi is resistant to the virus, while inbred line Mo17 is susceptible to the virus. Resistant plants and susceptible ones of the F2 progeny are easily distinguished, which can be used to map the resistance genes.

DNA extraction was performed on the youngest leaves by the SDS procedure. The sequences of SSR primer sets came from Maize DB. DNA of parents, F1, and F2 plants was analysed by PCR. The main results are as follows:

1) Resistance to maize dwarf virus was investigated. Both parents, Siyi and Mo17, differed significantly in resistance to MDMV over two years (Table 1). Siyi expressed complete resistance at the adult stage. Thirty-one plants in year 2000 and 60 plants in year 2001 of Siyi x Mo17 were symptomless. The (Siyi x Mo17) x Siyi plants in year 2001 were resistant. The progenies of (Siyi x Mo17) x Mo17 segregated into a 1:3 ratio in both year 2000 and 2001, while F2 progenies of (Siyi x Mo17) segregated into a 9:7 ratio in year 2001. The genetic model, the two complementary dominant gene model, was found in the resistant x susceptible combination in two years.

Table 1. Genetic analysis of resistance to maize dwarf mosaic virus.
 
Year Materials No. of plants No. of resistant plants No. of susceptible plants Theoretical ratio Chi-square test
Summer, 2000 Siyi 19 19      
  Mo17 30   30    
  Siyi x Mo17 31 31      
  (Siyi x Mo17)BC2 66 12 44 1:3 0.214
Summer, 2001 Siyi 50 50      
  Mo17 50   50    
  Siyi x Mo17 60 60      
  (Siyi x Mo17)BC1 150 150      
  (Siyi x Mo17)BC2 193 43 150 1:3 0.106
  (Siyi x Mo17)F2 344 190 154 9:7 0.627
x20.05, 1=3.84

2) Microsatellite primers were screened. Based on the genetic analysis, 87 pairs of microsatellite primers distributed randomly on 10 chromosomes were selected (Table 2) to screen parents and different plants from F2 progenies. Only 4 pairs of microsatellite primers on chromosome 3 and 8 pairs of microsatellite primers on chromosome 6 were able to identify the polymorphic fragments which amplified in parents and different plants from F2 progenies. Two genes, one on chromosome 3, the other on chromosome 6, were identified. The genetic analysis on phenotype was confirmed by the molecular analysis. Only 2 pairs of microsatellite primers, phi029 on chromosome 3 and phi126 on chromosome 6 link tightly with the two resistance genes.

Table 2. Microsatellites screened.
 
Chr. No. of SSR Chr. No. of SSR
1 8 6 16
2 7 7 8
3 10 8 7
4 8 9 10
5 7 10 6

Table 3. Microsatellites with polymorphisms between resistant and susceptible parents.
 
Microsatellite Bin Microsatellite Bin
phi029** 3.04 UMC1023 6.00
mmc0132 3.04 bnlg161 6.00
bnlg1019 3.04 bnlg2191 6.01
bnlg1628 3.04 bnlg1867 6.01
phi126** 6.00 bnlg1433 6.01
UMC1002 6.00 UMC1018 6.01
** Indicate the microsatellite primers linked tightly with the new resistance gene

3) Molecular tagging of the two resistance genes to maize dwarf mosaic virus showed the primers, phi029 and phi126, linked tightly with the two resistance genes, and were amplified successfully on both parents and 100 individuals selected from 344 individuals of the F2 progeny, The linkage distance between phi029 and the resistance gene on chromosome 3 was 14.5 cM, and the distance between phi126 and the other resistance gene was 7.2 cM, which confirmed the genetic analysis.
 
 


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