Near-isogenic lines of Taichung 65 (abbr. T65 from Taiwan) carrying several different dominant Ef1-alleles differing in heading advancing effect were obtained by repeated backcrosses. These Ef1-lines were about 10 days earlier than that of T65 carrying the recessive allele, ef1 (Tsai 1976; Tsai and Oka 1965, 1968). The F₂ populations between isolines with these Ef1-alleles produced an amount of genetic variance (V_F2>V_E) indicating their difference in effect on heading time. When the isolines with different Ef1-alleles were compared with enough replications, their difference in heading time were recognized to be significant statistically. This indicated that their linkage drags are not identical even though their Ef1-alleles are similar. They may be regarded as isoalleles (Tsai 1976).

When backcrosses were further increased the heading advancing effect of Ef1-a (derived from Tatung-tsailai) was reduced while that of Ef1-b (Bozu 5) was increased significantly (Tsai 1976). A sib line of (7)E^a, (7)E^a1, showed significant earlier heading than that of (7)E^a and was suggested to have an allele Ef1-a1. Its heading advancing effect was also reduced with additional backcrosses (Tsai 1973). The author has obtained another strain of Taichung 65 named T65-R and its early heading isoline, T65(R)E^R, donated by Dr. S. Sato of University of Ryukyus. Both T65-T (Taichung, Taiwan) and T65-R lines have an ef1-allele on chromosome 10 (Sato 1976), and they differ significantly in heading time, showing that the latter heads a few days earlier than the former (Tsai 1986). The induced earliness mutants, (O)E^x and (O)E^gamma, with alleles, Ef1-x and Ef1-gamma, both decreased heading-advancing effect due to repeated backcrosses. These experimental results suggested that the Ef1-locus had a compound structure which can be divided into some subloci and intralocus recombination existed (Tasi 1976). When the Ef1-isolines were intercrossed, a few ef1-like offtypes were produced occasionally. The frequencies of the occurrences of ef1-offtypes are shown in Table 1. These offtypes fixed were crossed with T65 all having the same morphological features. The heading date variations in each F₂ for allelism tests were examined. The results showed that the F₂ variations were larger than those of parental lines and produced genetic variance but distributed almost within the parental ranges suggesting the presence of ef1-isoalleles. It was considered that the ef1-alleles might be produced by the intralocus recombinatoin within the Ef1-locus. When some extent of the Ef1-allele are eliminated by the crossing over within the Ef1-locus, it will express the ef1-phenotype.

Based on this consideration, recombination values between any pair of subloci were sumarized from the published F₂ data as listed in Table 1 by the maximum likelyhood method. The estimates of distance between subloci were 0.23% to 0.80% unit, which suggested the general magnitude of the Ef1-locus. As isogenic lines, we refer to two or more lines differing from each other genetically at one (Allard 1960) or a few loci (Oka and Tsai 1978) only. So, offtypes or unusual segregation modes could happen occasionally

in heading behavior when an experiment of isolines is carried out because of the complex locus.

References

Allard, R.W., 1960. Principles of Plant Breeding. Wiley and Sons, New York. p 484.

Oka, H.I. and K.H. Tsai, 1978. The use of isogenic lines in breeding and genetic research: A review with special reference to experiments in rice. SABRAO J. 10: 130-142.

Sato, S., 1976. Linkage analysis of rice plant, by the use of reciprocal translocation lines. Bull. Col. Agric., Univ. of Ryukyus 23: 73-104. (in Japanese, with English summary)

Tsai, K.H., 1973. Comparison of intraallelic-structure related to the major earliness genes involving in isogenic and radiation-induced early-maturing lines of a rice variety, Taichung 65. J. Agr. Associ. of China New Series 84: 23-47. (in Chinese, English summary)

Tsai, K.H., 1986. Possible genic differences between two Taichung 65 strains, one preserved at Taichung and the other from Ryukus. RGN 3: 75.

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.

Tasi, 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.

Tasi, 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.