gene is responsible for both leaf and neck blast resistances and located on chromosome 6, and its resistance allele is derived from Gumei 2. In this study, the same population was used to analyze the effect of the chromosomal segments harboring the blast resistance genes on yield traits. The genes on chromosome 6 and 12 were tentatively designated as Pi25(t) and Pi24(t), respectively.
In 1996 and 1998, nine plants of each of the148 RILs were grown in the paddy field at China National Rice Research Institute, Hangzhou, China. The middle five plants were harvested at maturity. Grain yield per plant (GYD), number of panicles per plant (NP), number of filled grains per panicle (NFGP), total number of spikelets per panicle (TNSP), spikelet fertility (SF), 1000-grain weight (TGWT) and panicle length (PL) were measured. Mean values over two years were used for QTL analysis.
A linkage map consisting of 173 DNA markers was constructed. QTL were determined by QTLMAPPER 1.0 of mixed linear model (Wang et al. 1999), using a threshold of P<0.005. Stepwise regression analysis was used to select important markers and marker pairs. Background genetic variation control mode A was employed in which background variation due to main and interaction effects of important markers was controlled.
A total of 26 QTLs were detected (Table 1), and 4 of these were located on chromosomes 6 and 12. While the 2 QTLs on chromosome 12 were apart from the interval in which Pi24(t) was located, the 2 QTL on chromosome 6 were located in similar position in which Pi25(t) was located (Fig. 1). QTL qNFGP-6 detected with an LOD score of 5.8 explained 7.3% of the phenotypic variation and had an additive effect of 6.0. QTL qSF-6 detected with an LOD score of 7.2 explained 18.4% of the phenotypic variation and had an effect of 3.5%. In both cases, the Gumei 2 allele acted to reduce the trait value. Since the resistance allele at Pi25(t) was derived from Gumei 2, negative association between Pi25(t) and QTL for yield traits was evident. Fine mapping was underway to resolve whether the genetic drag observed was resulted from pleiotropism of the same gene or from close linkage of different genes.
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