Tsai and Oka (1965, 1968) identified Ef1, a dominant gene for early-heading. It accelerates floral initiation by about 10 days in the genetic background of T65 (Taichung 65 from Taiwan) carrying the recessive allele, ef1. The different Ef1 alleles were considered to be isoalleles, and the near-isogenic lines of T65 with these Ef1 alleles were established (Tsai 1976). The early heading mutant of T65, T65(0)E^x ((0)E^x), with Ef1-x gene was obtained by X-ray irradiation (Tsai 1971). The (0)E^x line was backcrossed to T65 five times and (5)E^x line was obtained. The backcrossing seemed to weaken the effects of the earliness relative to the other Ef1 lines tested. Thus, the backcrosses were further increased to nine times to purify the Ef1-x gene, and (9)E^x was obtained (Tsai 1988).

In the first crop season, the line (0)E^x headed 12 days earlier and was 8 cm shorter, (9)E^x line was 7 days earlier and 14 cm shorter in height than that of T65. The Ef1-x gene seemed to have reduced the earliness in heading and had the pleiotropic effect in reducing plant height as a result of additional backcrosses to eliminate the part of induced Ef1-x gene block. Table 1 shows that the Ef1 lines had no significant difference in heading days and in most of the characters compared among additional backcrossed lines. T65 was crossed with (0)E^x and (9)E^x lines. The degree of dominance for the former and the latter was estimated to be -0.57 and -1.17, respectively (Falconer 1961). The plant height showed complete dominance in (9)E^x line, emphasizing the pleiotropic effects on height

for the earliness gene carried in line (9)E^x to reduce height. The estimated effective factors for heading time are 1.81 by using a hybrid between (0)E^x and (9)E^x lines . It suggested that x-ray irradiation might have induced ef1 domain to show earliness behavior at least two effective factors or equivalent units. Investigation of the F₂ population of a cross, (9)E^x x (0)E^x, revealed that their heritability of heading days was approximately 48.4% (V_F2>V_E). Three offtypes out of 762 plants of the F₂ were produced and they headed much earlier than T65 and later than E^x-lines. Frequency of offtype estimated was to be 0.39%. F₃ lines of these offtypes showed a 3 early-type: 1 late-type (T65-like) segregation. The F₄ families of these F₃ lines were divided into 1 early-lines: 2 segregating-lines: 1 late-lines groups. Seven F₅ late fixed lines were further crossed with T65 and proved, by allelism test, that these late lines had the ef1 alleles of T65. The Ef1-x locus may be a complex structure, and the three offtypes may have been heterozygous for this gene. The progeny plants raised would be recombined from the heterozygous genotype.

It may be assumed that the offtypes have resulted from the certain internal mechanism. The Ef1-x locus represents a region expressing the early heading and can be divided into several subloci within the locus. Assuming that line (0)E^x has a sublocus, E'₁, and (9)E^x has another sublocus, E'₂, within the Ef1-x locus. The E'₁ tightly linked with E'₂. The homozygote recombined between subloci, E'₁ and E'₂, in F₃ showed the ef1-phenotype (T65-like). Based on this consideration, recombination values between subloci, E'₁ and E'₂, were estimated to be 0.42% from the F₂ data by the maximum likelihood technique. The difference of length in target chromosome segment between two subloci in two lines were somewhat large. This could result from the intralocus recombination within the Ef1-x locus. It was proposed that isogenic lines can be derived from gene mutation in a pure line (Stam and Zeven 1981). But it seems that more repeating backcrosses are needed to purify the target gene.

References

Falconer, D.S., 1960. Introduction to quantitative genetics. Olive and Boyd, London, UK. p. 365.

Stam, P. and A.C. Zeven, 1981. The theoretical propotion of the donor genome in near-isogenic lines of selffertilizers bred by backcrossing. Euphytica 30: 227-238.

Tsai, K.H., 1971. Comparison between radiation-indueed and isogenic early-maturing lines of a rice variety, Taichung 65. SABRAO Newsl. 2: 91-102.

Tsai, K.H., 1976. Studies on earliness genes in rice, with special reference to analysis of isoalleles at the E-locus. Jpn. J. Genet. 51: 115-128.

Tsai, K.H., 1988. Occurrence of weak plants in backcrossed progeny of Taichung 65 and their genetic behavior. RGN 5: 90-92.

Tsai, K.H., and H.I. Oka, 1965. Genetic studies of yielding capacity and adaptability in crop plants. 1. Characters of isogenic lines in rice. Bot. Bull. Acad. Sinica 6: 19-31.

Tsai, K.H. and H.I. Oka, 1968. Ditto, 3. Further observations on the effects of an earliness gene, E, in the genetic background of a rice variety, Taichung 65. Ibid. 9: 75-88.

Wright, S., 1934. The results of crosses between inbred strains of guinea pigs, differing in number of digits. Genetics 19: 537-551.