A MAJOR QUANTITATIVE TRAIT LOCUS FOR RICE YELLOW MOTTLE VIRUS-RESISTANCE MAPS TO A CLUSTER OF BLAST RESISTANCE GENES ON CHROMOSOME-12

Two doubled-haploid rice populations, IR64/Azucena and IRAT177/ Apura, were used to identify markers linked to rice yellow mottle virus (RYM V) resistance using core restriction fragment length polymorphism (RFL P) maps. Resistance was measured by mechanical inoculation of 19-day-o ld 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 evaluat ed, 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 analysi s using the IR64/Azucena population identified a single random amplifi ed polymorphic DNA marker that mapped on chromosome 12 and corresponde d to a major quantitative trait locus (QTL) evidenced by interval mapp ing. When pooling RFLP data, integrated mapping of this chromosome rev ealed that the QTL was common to the two populations and corresponded to a small chromosomal segment known to contain a cluster of major bla st resistance genes. This region of the genome also reflected the diff erentiation observed at the RFLP level between the subspecies indica a nd japonica of Oryza sativa. This is consistent with the observation t hat most sources of RYMV resistance used in rice breeding are found in upland rice varieties that typically belong to the japonica subspecie s.