Genetic and pathological studies of blast resistance based on the differential system have not been fully elucidated in the tropics due to the absence of common differential varieties (DVs). The previous DVs (Yamada et al. 1976; Kiyosawa et al. 1981; Tsunematsu et al. 2000) have japonica type genetic backgrounds and are not so applicable to tropical conditions. Several of the DVs have been found to have additional genes against the blast isolates from the Philippines (Kiyosawa 1981; Inukai 1994). Near-isogenic lines (NILs) carrying single resistance genes in an indica type genetic background would be the most appropriate DVs for understanding the mechanism of resistance to the blast pathogen and to develop the differential system for use in the tropics. Mackill and Bonman (1992) developed four NILs with the genetic background of an indica type rice, CO39, and targeted Pi1, Piz-5, Pi3, and Pita but the numbers and kinds of covered resistance genes were insufficient to constitute a differential system.

NILs with CO39 genetic background were developed through a recurrent cycle of backcrossing and selection with the 20 donor varieties targeting 14 kinds of resistance gene - Pib, Pik-s, Pik, Pik-h, Pik-m, Pik-p, Pi1, Pi7, Pish, Pita, Pita-2, Piz-5, Piz-t, and Pi5 (t). CO39 possess a resistance gene, Pia, in its genetic background (Tsunematsu et al. 2000). Backcrossing and selection from self-pollinated populations were carried out six and eleven times, respectively, in each combination by the second season of 2003. A total of 21 NILs were developed, and the resistance of each was confirmed by the analysis against 20 standard blast isolates from the Philippines. Based on the differential system (Yanoria et al. 2000; Tsunematsu et al. 2000), Pia was found to be incompatible with isolates B90002 and C923-49. All NILs were resistant to isolates B90002 and C923-49. This confirmed that Pia is in the genetic background of each NIL. Reaction patterns were the same for NILs having the Pik alleles (Pik, Pik-h, Pik-m, and Pik-p) and for those containing Pi1 and Pi7 (t), genes known to be allelic with the Pik alleles (Inukai 1994). These multi-allelic genes were not differentiated by the differential system due to the inability of the isolates to differentiate each allele. NILs carrying the Pish gene (such as IRBLsh-Ku/CO, IRBLsh-S/CO, IRBLsh-B/CO and Ribs-Fu/CO from four different donors) showed moderate resistance to 20 isolates, except for two isolates M64-1-3-9-1 and IK81-3 that showed moderate susceptibility. Moderate reaction was typical of the Pish gene against almost all isolates from the Philippines (Yanoria et al. 2000). Each NIL was designated as an IRBL line, and distinguished from other NILs by a nomenclature based on the resistance gene, the first one or two letters of the donor variety, and the recurrent parent, CO 39 (Table 1).

Six morphological traits (culm length, panicle length, panicle number, spikelet fertility,

days to heading, and 100-seed weight) were comparable with those of CO39, but the heading date of one NIL, IRBLz5-CA, was significantly later than that of CO39 (data are not shown).

To evaluate the genetic backgrounds and introgression of resistance genes from donor varieties, the graphical genotypes of the NILs were constructed using 127 simple sequence repeats (SSR) markers. This analysis established that the NILs had the same genomic structure as that of CO 39, with a few variants of 66.9 to 76.3% of the indica type segments and 7.9 to 18.1% of the japonica type. These variations might be derived from the cross of donor variety and selection process. Four NILs for Pish and four NILs for Pik alleles revealed that japonica type segments were introgressed into the middle region of the long arm on chromosome 1 and the terminal region of the long arm on chromosome 11 where each gene has been mapped, respectively. Donor segments of target resistance gene have not yet been detected in the other NILs. Additional analyses are needed to define the introgressions using DNA markers that link tightly or can detect directly the target gene. These materials will be used in pathogenic differentiation and as sources of resistance for indica type rice breeding.

Acknowledgment

This study was carried out under the IRRI-Japan Collaborative Research Project (Phase III and IV) donated from Ministry of Agriculture, Forestry, and Fisheries, and Ministry of Foreign Affairs of Japan.

References

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