Genetic basis and mapping of the resistance to Rice yellow mottle virus. III. Analysis of QTL efficiency in introgressed progenies confirmed the hypothesis of complementary epistasis between two resistance QTLs

Our previous studies have hypothesised that a complementary epistasis betwe en a QTL located on chromosome 12 and a QTL located on chromosome 7 was one of the major genetic factors controlling partial resistance to Rice yellow mottle virus (RYMV). We report research undertaken to verify this hypothes is and to introgress the resistant allele of these two QTLs from an upland resistant japonica variety, Azucena. into a lowland susceptible indica vari ety IR64. Three cycles of molecular marker-assisted back cross breeding wer e performed using RFLP and microsatellite markers. Resistance to RYMV was e valuated in F-2 and F-3 offspring of the BC1 and BC2 generations. Marker-as sisted introgression (MAI) was very efficient: in the selected BC3 progeny the proportion of the recipient genome was close to 95% for the ten non-car rier chromosomes, and the length of the donor chromosome segment surroundin g the two QTLs was less than 20 cM. The relevancy of the complementary epis tasis genetic model proposed previously was confirmed experimentally: in BC 1 and BC2 generations only F-3 lines having the allele of the resistant par ent on QTL(12) and QTL(7) show partial resistance to RYMV. Comparison of ou r experimental process of MAI with the recommendations of analytic and simu lation studies pointed out the methodological flexibility of MAI. Our resul ts also confirmed the widely admitted, but rarely verified, assumption that QTL-alleles detected in segregating populations could be treated as units of Mendelian inheritance and that the incorporation of these alleles into e lite lines would result in an enhanced performance. The next step will be t he design of tools for the routine use of molecular markers in breeding for partial resistance to RYMV and the development of material for the analysi s of resistance mechanisms and the structure of a virus resistance gene in rice.