Molecular Breeding for Insect Resistance - Rice a Case Study

Brown planthopper (BPH) is a serious insect pest of rice and causes significant damage to rice crop annually in Asian countries. Many elite rice cultivars are becoming susceptible to new BPH populations in recent years. A novel resistance gene, Bph18 derived from a distantly related wild species, Oryza australiensis was incorporated into a BPH susceptible elite japonica cultivar, Junam by marker-assisted backcross breeding. Advanced backcross breeding lines (ABL) were evaluated for desirable agronomic and grain quality traits, and breeding lines with BPH resistance were selected by BPH bioassays in the greenhouse and foreground selection of the Bph18 gene. Of the 26 selected ABL, four lines showed agronomic traits similar to those of the recurrent parent, with strong resistance to BPH. Genotyping of the four ABL with genome-wide SSR markers revealed genetic conversion of the genotypes closely resembling the genotype of Junam. A progressive reduction of donor chromosome segments in ABL was 12.3% in the BC₂ to 9.4%, 8.4% and 5.3% in BC₃, BC₄ and BC₅ generations, respectively. ABL retained small sizes of the donor chromosome segments on chromosomes 1, 2, 8, 10, 11 and 12 but the genomes of ABL2, ABL3 and ABL4 were homo-sequential to the recurrent parent on chromosomes 3, 4, 5, 6, 7 and 9 without donor chromosome segment introgression. However, a 1320kb O. australiensis-derived chromosome segment in ABL containing the Bph18 gene was consistently maintained in ABL irrespective of advances in backcross generations. BPH resistant elite breeding lines with desirable agronomic traits, high yield potential and high eating quality comparable with the recurrent parent led to the development of a new variety “Anmi” by marker-assisted backcross breeding without linkage drag.