15. A gene accelerating the heading of late lines with ef-1 alleles

Kuo-Hai TSAI

Department of Agronomy, National Chung-Hsing University, Taichung, Taiwan 40227, R.O.C.

Taichung 65 (abbr. T65) is a well-known Ponlai (Japonica) rice variety of Taiwan, selected from a cross between two Japanese varieties, Kameji and Shinriki, in 1923 (Iso 1957). It is adapted to the first (January to June) and second (July to October) crop-seasons in Taiwan. It also has a wide regional adaptability, and has been found adaptive in many tropical and subtropical countries. By 1947, T65 was extended to Ryukyu Islands covering most of the rice fields (Iso 1947).

The author has obtained a strain named T65-R and its early-flowering isoline, T65(R)Ef-1 having an Ef-1 allele, through the kindness of Dr. S. Sato of the University of Ryukyus (Sato et al. 1988). In Taichung, we have long used another strain, T65-T, and its early isoline T65(20)Ef-1a having an allele Ef-1a derived from Tatung-tsailai. This isoline, about 12 days earlier than T65-T, was established after 20 backcrosses; it is designated as (T)Ef-1a in this paper. The two strains, T65-T and T65-R, both having an ef-1 allele on chromosome 10, differ significantly in heading time. T65-R is a few days earlier than T65-T (Tsai 1986, 1992; Table 1).

To investigate the genic difference between T65-T and T65-R, crosses were made between the two lines and their respective early-heading derivatives, as shown in Table 2. All the F1 plants headed a few days earlier than the early parent, showing the dominance of genes for earliness. The F2 population of the cross between T65-T and T65-R (cross 1, Table 2) showed a range of heading dates within the parental limits. This suggests that the two T65 lines differ in ef-1 allele, as has been inferred previously (Tsai 1992). The allele in T65-R is shown as ef-1R in this paper. However, the R- and T-type segregants were not distinctly classifiable in their F2 population because of continuous variation. The F2 populations from crosses T65-Rx(T)Ef-1a and T65-Rx(R)Ef-1 produced T-type plants which were not present in the parental lines, with a frequency of about 1/16 (crosses 2 and 4, Table 2). This suggests that two independent genes are involved in these crosses.


  Table 1. Days to heading of strain T65-T (taken as standard) and other
lines (shown by difference from T65-T)
================================================================
Line              Genotype              Mean days to heading
                  assumed               1st crop    2nd crop
================================================================
T65-T              ac ef-1              0 (=126.9)  0 (=82.4)
T65-R              Ac ef-1R              - 7.7       - 4.2
T65(T)Ef-1a        ac Ef-1a              -11.9       -10.0
T65(R)Ef-1         ac Ef-1               -13.6       -11.2
LSD 5%                                     0.53        1.26
    1%                                     0.71        1.68
================================================================



  Table 2.  F2 segregation patterns of days to heading
===============================================================================
Cross                 F1 genotype        Ef-1   T65-R    T65-T     NP   RE   X2  
                      assumed                                   
===============================================================================
1) T65-RxT65-T       Ac/ac ef-1R/ef-1               13======24     154  3:1   7.3**
                                                  (continuous)         15:1  22.9**
                                           (pooled)
2) T65-Rx(T)Ef-1a    Ac/ac Ef-1A/ef-1R   ===157===           13     170 15:1   0.6
3) T65-Tx(T)Ef-1a    ac/ac Ef-1a/ef-1    117                 51     168  3:1   2.6
4) T65-Rx(R)Ef-1    Ac/ac ef-1R/Ef-1     117      40         15     172 12:3:1 4.7
5) T65-Tx(R)Ef-1    ac/ac ef-1/Ef-1      147                 38     205  3:1   1.2
===============================================================================
** Significant at 1% level.

Such segregation pattern was not found in crosses between T65-T and lines with Ef-1 (crosses 3 and 5, Table 2). In those crosses, early-heading plants of Ef-1 type and late-heading plants of T65-T type segregated in a 3:1 ratio. To account for these F2 patterns, it may be assumed that T65-R has gene Ac-ef-1 (abbr. Ac) which promotes the heading of ef-1 carriers for a few days, in addition to ef-1R. The genotypes assumed for the parental lines are given in Table 1, and those for the crosses tested are given in Table 2.

As mentioned, the F2 Population of T65-RxT65-T showed a continuous variation of heading time (cross 1, Table 2). In T65-Rx(T)Ef-1a (cross 2), the Ef-1 type and T65-R type plants also showed an overlapping variation. When they were pooled, the F2 pattern fitted a 15 (Ef-1 & R types): 1 (T type) ratio. In T65-Rx(R)Ef-1 (cross 4), the Ef-1 type included Ac Ef-1 and ac Ef-1 plants, as Ac does not affect the expression of gene Ef-1. The occurrence of T-type segregants suggests the genotype of (R)Ef-1 be ac Ef-1 (Table 1). The F2 ratio was considered to be 12 (Ef-1 type): 3 (R type): 1 (T type). In T65-Tx(T)Ef-1a (cross 3) and T65-Tx(R)Ef-1 (cross 5), the F2 ratios fitted 3 (Ef-1): 1 (ef-1), reflecting the dominance of Ef-1 over ef-1.

These experimental results are explained by assuming that line T65-R has genes ef-1R and Ac, and line T65-T has ef-1 and ac.

References

Iso, E., 1947. Some considerations on the regional adaptability of rice varieties with special reference to the recent development in the yield per unit area in Taiwan. Bull. Taiwan Agr. Res. Inst. 4: 1-108.

Iso, E., 1957. Lecture on Rice Culture. Department of Economics, Ryukyu District Government. 491 pp (in Japanese).

Sato, S., I. Sakamoto, K. Shirakawa and S. Nakasone, 1988. Chromosomal location of an earliness gene Ef1 of rice, Oryza sativa L. Japan. J. Breed. 38: 385-396.

Tsai, K.H., 1986. Possible genic differences between two T65 strains, one preserved at Taichung and the other from Ryukyu. RGN 3: 75-76.

Tsai, K.H., 1992. A T65-like line obtained from Kameji X Shinriki cross. RGN 9: 71-73.