Abstract
The variation of heading time in the hybrids of the cross between a Japanese rice variety Koshihikari and a U.S.A. rice variety Saturn was genetically analyzed by use of close linkage with the gene for resistance to blast disease. The experiments were conducted in the central part of Japan at 350' North Latitude. Saturn and Koshihikari headed 95 and 98 days after sowing, respectively, and their F1 hybrids headed 9 days later than Koshihikari. The F2 population showed a bimodal distribution and transgressive segregation for earlier and later heading (Fig. 1). The majority of the BC1F1[S](P1/P2/P2) plants headed as early as the recurrent parent Saturn with some plants heading later, while the BC1F1[K](P1/P2/P1) showed relatively larger variations and the convergence was not remarkable even by backcrossing with Koshihikari. The plants of the three hybrid populations were divided into early and late groups on the 101 st day that separated the heading time of the F1 from that of the parents. These variations showed a good fit to the segregation ratios that were attributed to three gene pairs (Table 1).
