22. Possible wide distribution of a photosensitivity gene, se-pat (t), in wild and cultivated rice

L.V. DUNG and Y. SANO
Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan

Recent studies are focused on dissection of QTLs into major loci by using molecular markers and it was pointed out that quantitative traits such as heading date are generally controlled by numerous genes with large as well as small effects (Yano et al. 1996). Out of several QTLs for heading date, a major QTL seemed to be the same as Se-1 which was suggested to play a role in photosensitivity (Yokoo et al. 1980). To elucidate the complex nature of photeperiod sensitivity we need to compare the related regions among different accessions of rice. An approach is to introduce an alien segment harboring natural variants into an insensitive line and compare their effects in the same genetic background. Especially, this is needed for comparisons of genes with minor effects when their genic interactions show a unique effect (Tsai and Oka 1965).

Recently, we reported that a recessive gene for photoperiod sensitivity, se-pat (t), is present between wx and C on chromosome 6 (Dung and Sano 1996). The gene was first detected in Patpaku cultivar of Indica rice after introducing a chromosomal segment by backcrossing, se-pat (t) acted additively with another dominant photoperiod sensitivity gene, Se-1, on chromosome 6, suggesting that the chromosomal region is responsible for its polygenic nature. In addition, plants homozygous for se-pat (t) showed no heading until 170 days after germination under any shortday treatments (9h, 10h and 12h photoperiods) in growth chambers (37°C, about 20,000 lux). Although examinations of the detailed photoperiodic responses are underway, the present report shows a possibility of its wide distribution among wild and cultivated rice strains.

The materials used in this study were two of Indica type, three of O. rufipogon and one of O. longistaminata (Table 1). All the strains except for Pehkuh were photoperiod sensitive. Since se-pat (t) is tightly linked with the wx locus, Wx alleles carried by alien lines were introduced into the recurrent parent of T65wx which is homozygous for wx and is almost insensitive to photoperiod. After several backcrosses, segregation for heading date was observed in selfed populations from the heterozygotes for wx.

A 3 early: 1 late ratio of segregation suggested an involvement of a recessive gene in all the populations observed, although heading date of the recessive segregants varied depending on the donor parents (Table 1). The segregation patterns for wx showed that the recessive genes all were tightly linked to wx. The recombination values with wx also varied from 0.06 to 0.19 depending on the donor parents. Although allelism tests are needed, the present results suggested that se-pat (t) might be widely distributed among wild and cultivated rice strains. The possibility of its wide distribution might show the important role for adjustment of floral initiation in response to photoperiods in tropical areas. (Gene symbol: Old system)

References

Dung, L.V. and Y. Sano, 1996. Dissection of a gene complex responsible for photoperiod sensitivity. RGN 13:72-73.

Tsai, 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.

Yano, M., H. Yoshiaki, Y. Kuboki, S.Y. Lin, Y. Nagamura, N. Kurata, T. Sasaki, and Y. Minobe, 1996. QTL analysis as an aid to tagging genes that control heading time in rice. In Rice Genetics III. IRRI, Manila. pp. 650-656.

Yokoo, M., F. Kikuchi, A. Nakane and H. Fujimaki, 1980. Genetic analyses of heading time in rice through its close linkage with blast resistance. Bull. Nat. Inst. Agri. Sci., Series D32: 95-126.