Shigetoshi Sato, Ichiro Sakamoto and Satoshi Nakasone
College of Agriculture, University of the Ryukyus, Nakagami-gun, Okinawa-ken, 903-01 Japan
An earliness gene controlling the duration of vegetative period, E or Ef-1, was detected first by Tsai and Oka (1966). The presence of an earliness gene on Nishimura's 7th chromosome was suggested by Sato and Shinjyo (1983). In order to confirm the location of E on chromosome 7, it was crossed with an interchange homozygote, T3-7, which was obtained from Kokushokuto-2 (a native cultivar of Hokkaido) after gamma-ray treatment (Sato et al. 1975; Sato and Shinjyo 1983). The F1 was successively backcrossed 9 times with Taichung 65 (T65), and plants showing early-heading and semi-sterility were always selected for further backcrosses. B7F1 to B9F1 progenies were observed to examine the linkage relations between heading time and fertility. These segregated into 353 early-heading and 414 late-heading plants (approximately 1:1 ratio). The early types were heterozygous for an earliness gene tentatively designated as Ef-2, and the F2 progenies of early-type plants selected from B5F1 to B7F1 segregated into 932 early : 324 late and approximated a 3:1 ratio. Each of the populations also segregated into 1 fertile : 1 semi-sterile. The chi-square value for independence between earliness and semi-sterility was significant at the 0.1% level. Recombination values of 8.1% and 7.5% between them were obtained from the BnF1 and BnF2 data, respectively. Their weighted mean was 7.8%.
A fertile and early-heading plant obtained from the B5F1
progeny was backcrossed 5 more times, and an early-heading line, T65.Ef2
was selected. To determine the chromosomal location of Ef-2, it was
crossed with several translocation lines involving chromosome 7 and with another
early line having Ef-1. Lines used for crosses were as follows:
a)T65.Ef1b-an early-heading isogenic line of T65 with Ef-1 derived from Bozu 5, backcrossed with T65, 20 times by Tsai (1973).
b) RT7-11.T65-an isogenic 7-11 interchange homozygote line of T65 with an earliness gene derived from an unknown variety exposed to the atomic bomb at Nagasaki, and backcrossed with T65, 9 times.
c)RT2-7a.T65, RT6-7.T65, RT7-8a.T65, RT7-8b.T65, and RT7-9.T65-all being isogenic lines of T65 with respective interchanges and the recessive allele ef-1. These were backcrossed with T65 more than 8 times (cf. RGN 1, p. 49-51).
Three-way crosses, (T65.Ef1bxT65.Ef2)xT65 and (RT7- 11.T65xT65.Ef2)xT65, produced progeny plants whose heading dates were distributed in a narrow range of the early-heading parents, and showed no trace of segregation. This indicated that the early-heading lines had the same genes for earliness, so that Ef-1 and Ef-2 were allelic. The 1:1 segregation of ferile and semi-sterile plants was also observed in the latter cross.
To determine the linkage relations between Ef-1 and the breakpoints, three-way crosses such as (RT-linexT65.Ef2)xT65 and (RT7-11.T65xT65)xT65 were studied. Chi-square values for independence between earliness and semi-sterility were significant in all crosses at the 0.1% level. The recombination values between Ef-1 and the breakpoints were estimated to be 28.5% with 2-7a, 3.0% with 6-7, 25.9% with 7-8a, 11.9% with 7-8b, 23.3% with 7-9, and 2.2% with 7-11.
The locations of three genes, pgl (pale green leaf), fgl (faded green leaf), and Rf-1 (fertility-restoring gene), and six of the breakpoints (except for 2- 7a) in the above stocks on chromosome 7 have been determined earlier (Sato and Shinjyo 1985). The recombination values between fgl and breakpoints, 7-11, 6- 7, and 3-7 were 4.3%, 10.4% and 16.1%, respectively, while those between Ef-1 and these three breakpoints were 2.2%, 3.0% and 7.8%, respectively. A comparison of these values suggests that Ef-1 is located between breakpoints 7-11 and 6-7. Thus, three genes and the breakpoints may be arranged on chromosome 7 as follows: pgl-7-8b-Rf-1-fgl-7-11-Ef-1-6-7-3-7-7-9-7-8a.
Tsai (1973) once considered that Ef-1 belonged to linkage group VIII (the 9th chromosome), as it was linked with la (lazy habit). However, the linkage was weak, recombination value being 38%. Thus the linkage detected was spurious.
Sato, S., T. Kinoshita and M.E. Takahashi, 1975. Linkage analysis of rice plant by the use of reciprocal translocation lines induced from linkage testers. Genetical studies on rice plant, 62. Memoir Fac. Agr., Hokkaido Univ. 9: 193-199 (Jap. with Eng. summary).
Sato, S., and C. Shinjyo, 1983. An experiment of genetic study on heading time in rice, Oryza sativa L. Bull. Col. Agr., Univ. Ryukyus 30: 145-153 (Jap. with Eng. summary).
Sato, S., and C. Shinjyo, 1985. Construction of a new linkage group corresponding to the Nishimura's seventh chromosome in rice, Oryza sativa L. Bull. Col. Agr., Univ. Ryukyus 32 (in press).
Tsai, K.H., 1973. Comparison of intra-allelic structure related to the major earliness genes involved in isogenic and radiation-induced early-maturing lines of a rice variety, Taichung 65. J. Agr. Assoc. China, N.S. 84: 23-47 (Chinese with Eng. summary).
Tsai, K.H. and H.I. Oka, 1966. Analysis of genes controlling heading time in Taichung 65 and other rice varieties. Bot. Bull. Acad. Sinica 7(2):54-70