Detection of a brown planthopper resistance gene bph4 at the same chromosomal position of Bph3 using two different genetic backgrounds of rice

The brown planthopper (BPH) resistance gene bph4 has previously been assigned on the short arm of rice chromosome 6. However, the map position of the gene could not be determined. To detect the bph4 locus, 15 polymorphic simple sequence repeat (SSR) markers covering genetic distance of 0.0–63.4 cM on chromosome 6 were used to survey 15 BPH resistant (R) and susceptible (S) individuals from each of the 95 and 78 F2 populations derived from crosses of TN1/Babawee and Babawee/KDML105, respectively. One SSR marker, RM586, was associated with the R and S from the F2 populations. Additional markers surrounding the RM586 locus were examined to define the location of bph4. From the genetic linkage map and QTL analysis of 95 and 78 F2 individuals, the bph4 locus was mapped at the same chromosomal region of Bph3 between two flanking markers RM589 and RM586. Markers linked to the resistance gene explained 58.8–70.1% of the phenotypic variations and can be used for marker-assisted selection in BPH-resistant breeding programs. In addition, our experiment provides evidence that a recessive gene could behave as a dominant gene under different genetic backgrounds.