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A Major Quantitative Trait Locus for Rice Yellow Mottle Virus Resistance Maps to a Cluster of Blast Resistance Genes on Chromosome 12

First, second, and third authors: ORSTOM-LRGAPT, BP 5045, 34032 Montpellier Cedex 1, France; fourth author: IER/CIRAD, BP 183, Sikasso, Mali; fifth author: LPRC/CIRAD, BP 5035, 34032 Montpellier Cedex 1, France; sixth author: IRRI, PO Box 933, Manila 1099, Philippines; seventh author: Department of Plant Breeding and Biometry, Cornell University, Ithaca, NY 14853; and eighth author: CIRAD, BP 5035, 34032 Montpellier Cedex 1, France

Two doubled-haploid rice populations, IR64/Azucena and IRAT177/ Apura, were used to identify markers linked to rice yellow mottle virus (RYMV) resistance using core restriction fragment length polymorphism (RFLP) maps. Resistance was measured by mechanical inoculation of 19-day-old seedlings followed by assessment of virus content by enzyme-linked immunosorbent assay tests 15 days after inoculation. IR64/Azucena and IRAT177/Apura populations, 72 and 43 lines, respectively, were evaluated, and resistance was found to be polygenic. Resistance was expressed as a slower virus multiplication, low symptom expression, and limited yield loss when assessed at the field level. Bulked segregant analysis using the IR64/Azucena population identified a single random amplified polymorphic DNA marker that mapped on chromosome 12 and corresponded to a major quantitative trait locus (QTL) evidenced by interval mapping. When pooling RFLP data, integrated mapping of this chromosome revealed that the QTL was common to the two populations and corresponded to a small chromosomal segment known to contain a cluster of major blast resistance genes. This region of the genome also reflected the differentiation observed at the RFLP level between the subspecies indica and japonica of Oryza sativa. This is consistent with the observation that most sources of RYMV resistance used in rice breeding are found in upland rice varieties that typically belong to the japonica subspecies.