| Maize rough dwarf disease (MRDD) is a kind of virus disease widely distributed in the world and causes great losses in grain yield. The pathogen has been identified as rice black-streaked dwarf virus in China. In recent years, maize rough dwarf disease has been popular and made a great damage to the economy in our country. Disease resistance breeding could potentially reduce grain losses with minimal efforts by growers, in an environmentally safe, cost-effective manner. Chinese breeders have selected a couple of maize inbred lines resistant to black-streaked dwarf virus such as 90110 with their endless work, but the majority of elite selflines and commercial hybrids remain sensitive to MRDD. Making full use of the existing resistant germplasm resources, by molecular marker-assisted selection techniques, to breed disease-resistant new varieties has been an important objective.Dr. Wang fei has identified three loci conferring resistance to MRDD in 90110 through bulk segregant analysis (BSA). The three loci were separately located on chromosome 6,7 and 8 and denoted as Mrddl, Mrdd2 and Mrdd3, respectively. The Mrddl locus was approximately mapped to a 4.5 cM region between the SSR markers umc1656 and bnlg2191. The Mrdd2 locus was approximately mapped to a 11.1 cM region between the SSR markers umc1401 and umc1666. The Mrdd3 locus was approximately mapped to a 5.8 cM region between the SSR markers bnlg1823 and umc1268. In this present study, we try to transfer the three loci from selfline 90110 to the susceptible maize inbred lines through multi-generation backcrossing and marker-assisted selection, the latter lines include Ye478, Zheng58, Chang7-2, DH4866,9801,8723, L189, L62,917,5003,65232,502, etc, each of which acts as a receptor and the reincarnation of parent. In the process, only those plants harbouring the target genes were accepted, and the background selection could considerably shorten the breeding process.During the foreground selection, accepted plants from the first backcross generation (BC1F1), the second backcross generation (BC2F1), the third backcross generation (BC3F1), as well as parental lines,90110 and the recurrent parents were identified for MRDD resistance under the same conditions in the field. Plants of 90110 line did not show any the disease occurrence, while Ye478, Chang 7-2, Zheng58, DH4866,8723,9801, etc had MRDD plants with different ratio; the majority of plants selected by molecular marker-assisted selection showed resistance to MRDD, except few individuals. It was concluded that the resistance genes to MRDD had been transferred into the accepted plants.The accepted plants in the foreground selection were evaluated for the agronomic traits of plants, and were compared with their recurrent parents. The plants with relatively high similarity were selected as the parents for next backcross generations that were selected again by marker-assisted selection. After three generations of backcrossing and selection, the accepted plants were semblable to their recurrent parents, but a couple of characters such as leaf length, leaf width, plant height, number of tassel branches showed some differences compared with the recurrent parents.Based on the Ye478×90110 genetic map, fifty SSR markers coveraging 10 chromosomes were chosen for genome-wide background selection. In theory, the share of recurrent parent genome usually reaches 93.75% in plants after three generations of backcrossing, but the average of recurrent parent-genome content didn't actually reach the theoretical value in the accepted plants. One explanation is that the resistant genes locate on three chromosomes, so the linkage drag is very serious. Compared with BC2F1 plants, the genetic background of BC3F1 plants is more similar, therefore, introduction of the genes located on 3 chromosomes usually need backcrossing for 3 generations and simultaneously selected by marker-assisted selection to produce the near-isogenic lines of recurrent parents. In this study, marker-assisted selection was used for maize rough dwarf disease resistance breeding and produced 11 inbred lines that were close to the near-isogenic lines of elite selflines. After another one generation selfcrossing and selection, a number of excellent elite selflines with high resistance to MRDD will be bred. Moreover, this study set up the technology system of breeding maize resistant selflines to MRDD, and displayed a successful case of transferring several genes from donor material to elite lines by marker-assisted seclection breeding, which would promote the development of maize breeding.
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