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
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
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