A near isogenic line T65-LH1 was developed from an upland rice variety R300 (Thailand) through eight successive backcrossing with Thaichung 65 (T65) as a recurrent parent. Previously unpublished study indicated T65-LH1 may harbor a recessive lateness gene which was likely located on the sixth chromosome. The present study was aimed at further identifying the lateness gene of this newly bred line by linkage study and allelism test. Linkage study was conducted using seven kinds of interchange homozygotes involved with the sixth chromosome. Individual interchange homozygous lines were crossed with T65-LH1, then, each F 1 population was backcrossed to T65-LH1. Segregations for heading times in all B 1 F 1 populations showed early and late types fitted to the 1 : 1 expected ratio, suggesting that T65-LH1 carried a recessive lateness gene. Chi-square values for independence between the present gene and four breakpoints, 2-6, 6-7b, 6-7c and 6-10 were insignificant. However, those to three breakpoints, 4-6, 6-7 and 6-8 were significant at 1 % level and the respective recombination values were estimated as 3.2 %, 4.3 % and 3.4 %. This suggested the gene under study was located on the sixth chromosome. Subsequently, allelism test between the present gene and other three lateness genes, ef2(t), ef3(t) and ef4(t) was carried out. Crosses of T65-LH1 and three testers having each of those lateness genes were made. Segregations for heading times in F 2 populations of those crosses fitted to the 9 : 3 : 3 : 1, 12 : 3 : 1 and 9 : 3 : 3 : 1 expected ratios. It suggested the present gene was independent from the lateness genes, ef2(t), ef3(t) and ef4(t). In conclusion, the lateness gene harbored by T65-LH1 was a new gene and designated as ef5. It was carried by the sixth chromosome.

A near isogenic line, T65-LH7 bred from a rice variety, Ketan Nangka by five times of successive backcrossing with Taichung 65 (T65) as recurrent parent was found to carry a recessive lateness gene tentatively designated as ef6(t). The present study was performed to investigate the allelic relationships between ef6(t) and other heading time genes, Ef1, Efx, ef2(t), ef3(t), ef4(t) and ef5 by allelism test and to locate the chromosomal location of ef6(t) by trisomic analysis. In allelism test, six testers carrying each of heading time genes, Ef1, Efx, ef2(t), ef3(t), ef4(t) and ef5 were used. Those testers were the near isogenic lines of T65. T65-LH7 was crossed with respective testers. Heading times in F 2 and/or B 1 F 1 plants were examined. All F 2 and/or B 1 F 1 populations derived from those crosses exhibited digenic segregations, respectively. These results suggested that ef6(t) was independent of Ef1, Efx, ef2(t), ef3(t), ef4(t) and ef5. Subsequently, trisomic analysis of ef6(t) was performed using seven Triplo lines having extra chromosomes, 2, 4, 5, 7, 9, 10 and 12. These Triplo lines were the near isogenic lines of T65. They were used as maternal parent to cross with T65-LH7. Heading times in F 2 plants obtained from self-pollination of F 1 plants were observed. Among F 2 plants examined only those derived from a cross between Triplo-7 and T65-LH7 showed a typical trisomic segregation manner, suggesting that ef6(t) was located on chromosome 7. Consequently, the nomenclature of the present gene should be designated as ef6.

A near isogenic line T65-LH2 bred from IR8 by five times of successive backcrossing with Taichung 65 (T65) as recurrent parent was found to harbor a lateness gene ef4(t). In the present study, we investigated the allelic relationship between ef4(t) and two earliness genes, Efx and Ef1, followed by linkage analysis to reveal the chromosomal location of ef4(t). T65-LH2 was crossed with two testers, T65-ER21 and T65-ER1 harboring the earliness genes, Efx and Ef1, respectively. F 1 plants of the two crosses were raised to obtain F 2 seeds. Moreover, the F 1 plants were backcrossed with T65-LH2 as to obtain the B 1 F 1 seeds. Heading time in F 2 and B 1 F 1 plants as well as parental lines were examined. F 2 and B 1 F 1 plants of the crosses between T65-LH2 and T65-ER21 showed monogenic segregation while F 2 and B 1 F 1 plants obtained from the cross between T65-LH2 and T65-ER1 exhibited digenic segregation. Thus, ef4(t) is allelic to Efx on chromosome 3 and independent of Ef1. Subsequently, linkage analysis of ef4(t) was carried out using three marker genes on chromosome 3, Hg, dl and bc1. T65-LH2 was crossed with the isogenic lines of T65 carrying those marker genes. Heading time and morphological characters in F 2 plants and parental lines were observed. Results showed that ef4(t) was linked to Hg and dl with recombination values estimated at 33.2% and 16.8%, respectively, but segregated independently of bc1.

The lateness gene ef2(t), formerly designated as lf-1, was induced by X-ray irradiation applied to a rice variety Taichung 65. The present study was performed to investigate the allelic relationship between ef2(t) and two earliness genes, Efx and Ef1, and the chromosomal location through trisomic analysis. Rice line T65-ef2(t) carrying gene ef2(t) was crossed with two tester lines, T65-ER-21 and T65-ER-1 harboring the earliness genes, Efx and Ef1, respectively and heading time in the F 2 plants was examined. F 2 plants of the cross T65-ef2(t)/T65-ER-21 showed a trimodal distribution: T65-ER-21 and T65 type, new type and T65-ef2(t) type. The F 2 segregation fitted to a 12 : 3 : 1 expected ratio. On the other hand, the F 2 plants of the cross T65-ef2(t)/T65-ER-1 exhibited a tetramodal distribution: T65-ER-1 type, T65 type, new type and T65-ef2(t) type, the segregation fitting to a 9 : 3 : 3 : 1 expected ratio. These results suggested that ef2(t) was non-allelic to both Efx and Ef1. Trisomic analysis was performed using seven Triplo lines carrying extra chromosomes 2, 4, 5, 7, 9, 10 and 12, respectively. The trisomics were used as maternal parent for crossing with T65-ef2(t). Heading time in the F 2 plants obtained from self-pollination of F 1 plants (trisomics) was examined. F 2 population derived from a cross between T65-ef2(t) and Triplo 9 showed a typical trisomic segregation pattern, suggesting that ef2(t) was located on chromosome 9.