RFLP mapping of a brown planthopper (Nilaparvata lugens Stal) resistance gene bph2 of indica rice introgressed into a japonica breeding line 'Norin-PL4'

A rice gene bph2 for resistance against brown planthopper (BPH), Nilaparvata lugens St (a) over circle l, was reported to be recessive and either allelic or closely linked to a dominant BPH resistance gene, Bph1. bph2 was introgressed from an indica resistance donor variety, 'IR1154-243', into a japonica breeding line, 'Norin-PL4'. A segregation analysis of BPH resistance in F-2 and F-3 progenies from a cross of a japonica susceptible variety, 'Tsukushibare', and 'Norin-PL4', however, showed that the resistance gene in 'Norin-PL4' behaved as a dominant gene. Genotyping of 'Norin-PL4' using 99 RFLP markers covering all 12 rice chromosomes showed that 'Norin-PL4' possessed a large segment of chromosome 12 introgressed from 'IR1154-243'. Six RFLP markers on the introgressed segment was cosegregated with BPH resistance and bph2 was mapped at 3.5 cM from the closest RFLP marker, G2140. The position of bph2 on the standard 'Nipponbare'/'Kasalath' map was at a considerable distance (about 30 cM) from that of Bph1 previously mapped using a different population. Despite this, no susceptible recombinants were obtained in a large number of F3 progeny from crosses between two Bph1 carrier lines and 'Norin-PL4'. Problems of dominance/recessiveness and no recombinations between the two loci were discussed.