14. Evolutionary significance of chromosome 7 in an annual type of wild rice

        M. Eigluchi1 and Y. Sano2
        1) National Institute of Genetics, Mishima. 411 Japan
        2) Plant Breeding Institute, Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan

     In order to look into the genetic divergence between wild and cultivated rice, a segment of chromosome 7 was examined after introducing it into a cultivated form from a common wild rice by successive backcrossings. An annual type of wild rice from India
(W107) was crossed to a near-isogenic line of Taichung 65 (Japonica type from Taiwan) with g (long empty glume) and lg (liguleless) which was used as the reccurent parent (T65glg) in back-crosses. W107 like most of the common wild rice has red pericarps (Rc) and short empty glumes (g+) and T65glg has white pericarps and long empty glumes. The two dominant genes which are located on chromosome 7 were used to transfer the chromosome segment with Re and g+. The genetic divergence due the segment was studied since weedy forms frequently show red pericarps even after introgression, suggesting the adaptive significance of the segment under natural conditions.
    Two other dominant genes were detected to be located on the segment in question. One was responsible for short stature only in the vegetative phase (tentatively called Ssv) while no difference in culm length was observed at the flowering stage. Dominant dwarfism expressed at the vegetative phase has not been reported so far. The other gene was responsible for late heading at Mishima (tentatively called Lh) while plants with Lh(t) headed earlier than T65glg under a short-day treatment. This indicates that the dominant gene may be a photosensitivity gene. Ratios of 3:1 were repeatedly observed from BC3 F2 to BC5 F2 showing that short stature and late heading, both are controlled by single dominant genes in the genetic background of cultivated rice. The allelism tests between Lh(t) and other known photosensitivity genes remain to be studied.
    The segregation in B5 F2 showed that Ssv(t) and Lh(t) were linked to both g and Rc (Table 1 ). The linkage relations were estimated as shown in Fig. 1. Distorted segregations were detected for Rc and Lh(t), showing significant values in c2 (Table 1). Although the mechanism for the distortion is unknown, the frequency of phenotypes with Rc decreased most severely in the B5 F2 than the expected value (60.8%, significant at 1%) as shown in Table 1. Linkage relations among the 4 loci, g, Ssv(t), Rc and Lh(t) on
chromosome 7 in the BC5 F2 between T65glg and W107
Gene combination

(A)           (B)

 No. of phenotypes Recombination value (sd) X2

(df=3)
AB Ab aB ab Total
Ssv(t) g Obs. 84 5 10 31 130 0.17(0.04) 6.0 ns
Exp. 87.4 10.1 10.1 22.4
Ssv(t) Rc Obs. 68 21 11 30 130 0.29(0.05) 16.6**
Exp. 81.4 16.1 16.1 16.4
Lh(t) Rc Obs. 75 10 4 41 130 0.13(0.03) 15.8**
Exp. 89.6 7.9 7.9 24.6
Ssv(t) Lh(t) Obs. 70 19 15 26 130 0.34(0.05) 11.5**
Exp. 79.2 18.3 18.3 14.2
Rc g Obs. 66 13 28 23 130 0.40(0.06) 17.8**
Exp. 76.7 20.8 20.8 11.7 6.0 ns
Lh(t) g Obs. 67 18 27 18 130 0.46(0.06) 6.0*
g 74.5 23.0 23.0 9.5

Note; The genotype of F1 was g+ Ssv(t) Rc Lh(t)/g Ssv(t)+ rcLh(t)+.
ns shows non-significance.
* and ** show significance at 5% and 1%, respecitvely. Fig. 2, suggesting that the introduced segment is quickly eliminated when backcrossed
        with T65glg.