An RIL population consisting of 304 lines was constructed from the indica cross Zhong 156/Gumei 2. Two genes conferring complete resistance to blast with isolate 92-183 (ZC15) were mapped (Zhuang et al., 1997, Zhuang et al. 2002). One of the genes, Pi-25 (t) on chromosome 6, conferred resistance to both leaf blast and neck blast. The other gene, Pi-24 (t) on chromosome 12, conferred resistance to leaf blast only. Seventy-four RILs that did not possess the resistance allele of Pi-25 (t) based on flanking markers and were susceptible to isolate 92-183 for neck blast, were selected for mapping genes conditioning partial resistance to neck blast with 92-183. The heading dates of these lines were similar.

Both isolates 99-30-1(ZB1) and Ca89 (lineage 4 in the Philippines) were compatible to Zhong 156 and incompatible to Gumei 2 for leaf blast. One hundred and sixteen RILs and one hundred and forty-six RILs susceptible to isolates 99-30-1 and Ca89 respectively, were chosen to analyze partial resistance to leaf blast.

Diseased leaf area (DLA), lesion size (LS) and lesion number (LN) were measured as parameters of partial resistance to leaf blast. Lesion length (LL) and the conidium amount (CA) were recorded as parameters for QTL mapping of neck blast resistance.

As shown in Fig. 1, 62 QTLs scattered in 28 intervals of the 12 chromosomes were detected using LOD >= 2.4, of which 11 QTLs showed main effects only, 47 QTLs showed epistatic effects only and four showed both main and epistatic effects. Fifty-one QTLs showing epistatic effects were involved in 28 pairs of significant interactions between QTLs. General contributions of QTLs showing epistatic effects of each parameter ranged from 16.08% to 51.70%, while those of QTLs showing main effects of each parameter ranged from 5.04% to 42.2%. The general contributions of QTLs showing main effects of most parameters were smaller than that of QTLs showing epistatic effects, confirming the importance of epistasis as the genetic basis for complex traits. The general contributions of the main plus epistatic effects of all QTLs detected for the two parameters for neck blast reached 74.0% and 74.6% of trait variance, respectively, which obviously represented a major part of the genetic basis controlling partial resistance at reproductive stage. In nine out of the 28 intervals, QTLs were detected for parameters of both leaf and neck blast resistance, indicating that the genetic bases for partial resistances were related but quite different for different isolates and at different growth stages. For the seven out of 15 QTLs with main effects, resistance alleles were from the susceptible parent.

A great number of genes with complete or partial resistance to blast in rice have been mapped so far, which included Pi1(t), Pi2(t), Pi4(t), Pi5(t), Pi6(t), Pi9(t), Pi10(t) and Pi11(t) (Causse et al.,1994), Pi5(t) and Pi7(t) (Wang et al., 1994), Pia, Pib, Pik, Pit, Pita, Pi12(t), Pi17(t), Pi18(t), Pi19(t), Pi20(t), Pi23(t), Pi62(t) and Pi157(t) (Nagato and Yoshomura, 1998), Pitq1, Pitq5, Pi-tq6 and Pilm2 (Tabien et al., 2000), the recessive gene pi21(t) (Fukuoka et al., 2001), the QTLs for partial resistance to leaf blast (Wang et al., 1994) and QTLs for partial

resistance to neck blast (Bagali et al., 2000). The comparison of locations of the major genes and QTLs mapped in this population with reported resistance loci for complete and partial resistance established the association of blast resistance loci in rice (Fig. 1). Clusters of blast resistance genes are more obvious in the rice genome. The coexistence of major genes and QTLs also provides evidence to support the hypothesis that QTLs and major genes were different alleles of a same locus and qualitative mutant alleles that affected quantitative traits represented one extreme in a spectrum of alleles (Robertson 1985).

This work was funded by Chinese 863 program (2001AA211081), Asian Rice Biotechnology Network and the Rockefeller Foundation International Rice Biotechnology Program.

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