Genetic basis and mapping of the resistance to rice yellow mottle virus. II. Evidence of a complementary epistasis between two QTLs

The genetic basis of resistance to rice yellow mottle virus (RYMV) was stud ied in a doubled-haploid (DH) population derived from a cross between the v ery susceptible indica variety 'IR64' and the resistant upland japonica var iety Azucena. As a quantitative trait locus (QTL) involved in virus content estimated with an ELISA test has been previously identified on chromosome 12, we performed a wide search for interactions between this QTL and the re st of the genome, and between this QTL and morphological traits segregating in the population. Multiple regression with all identified genetic factors was used to validate the interactions. Significant epistasis accounting fo r a major part of the total genetic variation was observed. A complementary epistasis between the QTL located on chromosome 12 and a QTL located on ch romosome 7 could be the major genetic factor controlling the virus content. Resistance was also affected by a morphology-dependent mechanism since til lering was interfering with the resistance mechanism conditioned by the epi stasis between the two QTLs. Marker-assisted backcross breeding was develop ed to introgress the QTLs of chromosome 7 and chromosome 12 in the suscepti ble 'IR64' genetic background. First results confirmed that if both QTLs do not segregate in a backcross-derived F-2 population, then the QTL of chrom osome 12 cannot explain differences in virus content. A near-isogenic line (NIL) approach is currently being developed to confirm the proposed genetic model of resistance to RYMV.