Abnormal heading in hybrid rice production has caused great economic loss in recent years, but the genetic basis of this phenomenon remains elusive. In this study, we developed four testcross populations using 38 introgression lines （ILs） from Shuhui 527 （SH527）/Fuhui 838 （FH838）//SH527 population as male parents and four male sterile lines （MSLs; namely 11-32A, Xieqingzao A, Gang 46A and Jin 23A） as female parents. Progeny testing allowed us to identify 55 abnormal heading combinations in Hefei, but had late heading date in Hangzhou and Guangzhou of China. By one- and two-way analysis of variance, a total of 21 QTLs and 31 pairs of epistatic QTLs associated with photosensitivity were identified in the four populations, respectively. Genotypic analysis showed that the IL parent of most abnormal heading combinations showed some introgressions at markers RM331 and RM3395 on chromosome 8 （strongly associated with the known genes OsHAP3H/DTH8/Ghd8/LHD1） of donor FH838 alleles, and these two markers were also identified as affecting photosensitivity. The observation that the recipient parent （SH527）, donor parent （FH838）, their testcross combinations with four MSLs, and the IL parents of abnormal heading combinations had normal heading date in Hefei suggested that OsHAP3H/DTH8/Ghd8/LHD1 showed no independent regulation on abnormal heading in the abnormal heading combinations. It is noteworthy that complex epistasis among RM331 or RM3395 with other loci, including dominant x additive, additive x dominant, and dominant x dominant epistases, were identified only in the four testcross populations of the current study, but not in the SH527/FH838//SH527 population suggesting the cause of abnormal heading in abnormal heading combinations in Hefei and delayed heading in Hangzhou and Guangzhou.
