Genotypical Identification Of Important Breeding Parents On Two QTL Intervals Of Resistance To Rice Sheath Blight

The objective of this research is to establish a good method in order to evaluate the genotype of important breeding parents in two QTL intervals of resistance against rice sheath blight and to know which QTL could be available in resistance improvement for a given parent. This is a foundation of utilizing some resistance QTLs against rice sheath blight in molecular breeding.Backcross along with marker-assisted selection (MAS) is a good method in genotype evaluation. In this experiment, qSB-9 and qSB-11 is two resistance QTLs against rice sheath blight and on chromosome 9 and chromosome 11, respectively. The relatively resistant indica rice cultivars, Teqing, possessed the other resistant allele, qSB-9Tq, while the resistant allele as qSB-11^Le underlying the qSB-11 came from a relatively susceptible japonica rice cultivar, ^Lemont. With Teqing or ^Lemont as non-recurrent parent and different japonica rice cultivars or indica rice cultivars as recurrent parent respectively in this research, we constructed F1 populations and BC1F1 populations. From many backcross populations, based on molecular marker polymorphism in tagged intervals (qSB-9 or qSB-11), we selected many objective individual plants. On summer of 2006, we planted parents, F1 population and backcross population on the experiment field in the College. According to the marker genotypes in the intervals of qSB-9 or qSB-11, two kinds of plants with homozygous genotypes of recurrent parent alleles (qSB-9^RP or qSB-11^RP) and heterozygous genotypes (qSB-9H or qSB-11H) were selected from each BC₁F₁ population before transplanting to constitute two sub-populations. From late tillering stage to primary shooting stage, all experimental plants were inoculated with short woody toothpicks incubated with Rhizoctonia solani, and the disease score of each plant was made about 30 days after heading stage using an improved'0-9'rating scale based on that of Rush. Significance of difference between average disease ratings in two sub-groups of each backcross was estimated. Hereby, we determined genotype of special breeding parents in each of the two QTL intervals of resistance to rice sheath blight. Results indicated that:(1) The development of sheath blight disease in 21 breeding parents was different from highly susceptible to moderate resistance. In general, severity of the disease in japonica rice cultivars are more serious than in indica rice cultivars, and it is urgent to improve their resistance against sheath blight.(2) Disease ratings of F1 populations between breeding parents and Teqing or ^Lemont were often less than corresponding breeding parents and their resistances were strengthened in some degree. It may be because different parents possessed different QTL of resistance to sheath blight and these QTLs in subsequent generation cause nonallelic interaction (additive effect and interaction effect of nonallelic QTL) or allelic interaction (dominant effect).(3) On the whole, in populations of backcross between Teqing and japonica rice cultivars (recurrent parent, ^RP), scores of sheath blight rating (SBR) in the sub-population with heterozygous genotype (qSB-9H) in target interval were markedly or highly markedly less than SBR of the relative sub-population with homozygous genotypes (qSB-9^RP). It was shown that all japonica rice cultivars in this experiment possess the unfavorable allele at the corresponding locus of the qSB-9^Tq. And so, the qSB-9^Tq of Teqing could be utilized to enhance the resistance of these japonica rice cultivars.(4) We detected populations of backcross between ^Lemont and indica rice cultivars (^RP) in 2005 and 2006, respectively. The results in two years was alike and showed that the scores of SBR in the sub-population with heterozygous genotypes (qSB-11H) in target interval were less than in the relative sub-populations of homozygous genotypes (qSB-11^RP), with only one exception of Shengyiu 2 in 2005. But the difference between two sub-populations in a backcross may be significant or highly significant in a year, but not in another year.(5) Further research found that, in the level of near-isogenic lines, about 1.0 SBR score of difference could be expected between Teqing (genotype: qSB-9^TqqSB-9^Tq, a resistant allele) and its susceptible near-isogenic line (genotype: qSB-9^LeqSB-9^Le), and the same to ^Lemont. However, qSB-9^Tq is considered as a dominant gene because highly significant difference of disease scores existed between heterozygous genotypes (genotype: qSB-9^TqqSB-9^Le) and susceptible genotypes with a difference level of around 1 score, while qSB-11^Le is considered as a additive gene and significant difference of disease scores existed between heterozygous genotypes (qSB-11^LeqSB-11^Tq) and susceptible genotypes (qSB-11^Tq qSB-11^Tq) with a difference level of around 0.5 score. This may be the major cause of efficiency difference in genotype identification of breeding parents in the two resistance QTLs.(6) We discussed the factors influenced detection efficiency of genotype identification of important breeding parents in objective QTLs. Factors such as gene's effect and action model for a given QTL, and the genetic distance of markers flanking the QTL, and generation number of backcross all influenced the efficiency. And we brought forth possible measures in order to enhance the reliability of the detection method. This experiment gave a base to improve methodology of genotypical identification of given breeding parents in objective QTL interval using backcross method.