15. Chromosomal location of fertility-restoring gene Rf-2

College of Agriculture, University of Ryukyus, Nishihara, Okinawa, 903-01 Japan

A cytoplasm causing male sterility and a fertility-restoring gene were first discovered in an Indica variety, Chinsurah-boro II (Shinjyo and Omura 1966; Shinjyo 1969, 1975). The cytoplasm (designated as cms-B) and the restoring gene (Rf-1) were introduced into Taichung 65 (T65) by recurrent backcrosses to establish an isogenic line having both, called BT. Since then, different male-sterile cytoplasms and restoring genes were found in wild rice, O. rufipogon, and were each transferred to T65 (Rutger and Shinjyo 1980). Most of the cytosterilities were restored by alleles at the Rf-1 locus. The locus was found to be located on chromosome 10 (Shinjyo 1975).

On the other hand, Watanabe et al. (1968) found a male-sterile cytoplasm (cms-L) in a Burmese cultivar, Lead rice, and introduced it into a Japanese cultivar Fujisaka 5. They did not isolate its restoring gene from Lead rice, but found it in a Japanese cultivar, Fukuyama.

We looked into the effects of restoring genes, Rf-1 and Fukuyama's on the male sterility due to (cms-B) and (cms-L). First, the Fukuyama's restorer and (cms-L) were transferred to T65 by recurrent backcrosses to obtain a near-isogenic line, (cms-L)F-T65. Its reciprocal crosses with line BT were made in order to examine the interrelation between the two restoring genes. Denoting the restoring gene of Fukuyama by F, the F`1` plants have genotype Rf-1/rf-1 F/f, and the pollen genotypes would be Rf-1.F, rf-1.F, Rf-1.f and rf-1.f, occurring in a 1:1:1:1 ratio if Rf-1 and F are not linked.

The reciprocal F`1` plants were found to have 73% to 76% pollen fertilities. In principle, the restoring gene overcomes cytoplasmic interruption of pollen development and its action may be assumed to be gametophytic (determined by gametic genotype). Then, the about 75% pollen fertility observed suggests that Rf-1 and F are similarly effective for both (cms-B) and (cms-L) at least in their heterozygotes, and that they are recombined in hybrid without restriction.

Secondly, the F`1` plants from reciprocal crosses between (cms-L)F-T65 and line BT were pollinated by T65 which has no restoring genes, and the B`1`F`1` plants were examined for pollen fertility. The reciprocal F`1` plants both showed segregation of the B`1`F`1` plants into sterile (O%), semi-fertile (around 50%) and nearly fertile (around 75 %) classes in a 1 : 2: 1 ratio (Table 1). Presumably, the sterile plants have no restoring genes (rf-1 / rf-1 f/f); the semi-sterile plants would have either (Rf-1/rf-1 f/f) or (rf-1/rf-1 F/f) which are expected to have a 50% pollen fertility; the nearly fertile plants would have Rf-1/rf-1 F/f which would have a 75% pollen fertility as mentioned above.

     
Table 1.  Distribution of pollen fertility among plants derived from
three-way crosses of (cms-L)F-T65, BT and T65
_______________________________________________________________________________
Cross combination 0% ... 25 30 35 40 45 50 55 60 65 70 75 80 85 Total Chi2  
                                                                      (1:2:1)
_______________________________________________________________________________
(cms-L)F-T65/BT   105     9 22 28 18 33 73 22  7 26 28 27  2     400    1.4
//T65                     |_________________|  |___________|
                              205                     90
BT/(cms-L)F-T65   163        4 11 26 60    74 23  6 24 78 31 14  666    2.0
//T65                        |_________________|  |___________|
                                    350                153
_______________________________________________________________________________

The chromosomal location of Rf-2 was examined by using primary trisomic lines developed by Iwata and Omura (1984), by conducting three-way crosses (cms-L)Ms-T65//trisomic plant/(cms-L)F-T65, where (cms-L)Ms-T65 is a cyto- sterile segregant from (cms-L)F-T65xT65. For chromosomes 4 to 12, all the crosses gave 1 fertile: 1 sterile segregation in seed fertility, indicating that Rf-2 is not involved in those chromosomes. For chromosomes 1 to 3, this method cannot be used because of low pollen fertilities of the trisomic lines. Then, linkage tester lines with 37 recessive markers located on the three chromosomes were mobilized, and were used in crosses as: (cms-L)F-T65/linkage tester// linkage tester.

Observations of the progeny plants showed that Rf-2 was located on chromosome 2. One of the linkage testers carrying d-32 (dwarfing) and gh-3 (gold hull) disclosed the location of Rf-2 as follows:

               Rf-2-(15.8%)-d-32-(36.3%)-gh-3
               ___________(43.2%)___________

Iwata, N. and T. Omura, 1984. Studies on the trisomics in rice plants, VI. An accomplishment of a trisomic series in japonica rice plants. Jpn. J. Genet. 59: 199-204.

Rutger, J. N. and C. Shinjyo, 1980. Male sterility in rice and its potential use in breeding. In Innovative approaches to rice breeding. p. 53-66. IRRI, Manila.

Shinjyo, C., 1969. Cytoplasmic-genetic male sterility in cultivated rice, II. The inheritance of male sterility. Jpn. J. Genet. 44: 149-156.

Shinjyo, C., 1975. Genetical studies of cytoplasmic male sterility and fertility restoration in rice, Oryza sativa L. Sci. Bull. Coil. Agr., Univ. Ryukyus 22: 1-57.

Shinjyo, C. and T. Omura, 1966. Cytoplasmic-genetic male sterility in cultivated rice, I. Fertilities of F`1`, F`2` and offsprings obtained from their mutual reciprocal backcrosses, and segregation of completely male sterile plants. Japan. J. Breed. 16 (Suppl. 1): 179-180. (in Japanese)

Watanabe, Y., S. Sakaguchi and M. Kudo, 1968. On the male sterile plant possessing the cytoplasm of Burmese variety, Lead rice. Japan. J. Breed. 18 (Suppi. 2): 77-78. (in Japanese)