Thermo-sensitive action of an earliness gene Ef-x in rice, Oryza sativa L.

An extremely early line of rice, T65·ER-20, was bred after 8 times of backcrossings, using two early heading lines, Kokusyokuto-2 harboring several unknown earliness genes besides the earliness gene Ef-1 as a donor parent of earliness genes, and an isogenic line of Taichung 65 (T65) harboring the Ef-1 gene (T65·ER-1) as a recurrent parent. Since extremely early and early individuals segregated in a 3:1 ratio in the B8F2 generation and in a 1:1 ratio in the B9F1, T65·ER-20 harbors a new dominant earliness gene, Ef-x, besides the Ef-1 gene. T65·ER-21 having the new earliness gene was also bred. Segregation mode for heading time in F2 plants showing disgenic segregation for two earliness genes, Ef-1 and Ef-x, was different between experiments conducted in the natural field and in the green house field. Extremely early, early and late individuals segregated in a 9:3:4 ratio in the former and in a 9:6:1 ratio in the latter. Then, the Ef-x gene was asserted to show an allele specific interaction to genes on the Ef-1 locus, and an action for earliness by itself was accelerated in the high temperature condition. A screening test surveying a chromosome carrying the Ef-x gene by the use of 40 RT-lines implied that it located on the second chromosome.