Cryptic differences in the Sel alleles as revealed by curvilinear regression analysis under controlled conditions

20. Cryptic differences in the Sel alleles as revealed by curvilinear regression analysis under controlled conditions Y. ITOH and Y. SANO. Plant Breeding Laboratory, Graduate School of Agriculture, Hokkaido University, Sapporo, 060-8589 Japan Photoperiod sensitivity is a major factor determining the days to heading in rice. A number of genes with major and minor effects seem to be involved in the quantitative trait which cause a continuous variation in flowering time. Among the photoperiod sensitivity genes identified, Sel (= Liii) was proposed to play a significant role for determining the duration of growth among rice cultivars (Yokoo and Kikuchi 1982). To explain differences in the degree of photoperiod sensitivity, a series of alleles at the Sei locus were proposed, however, it is not easy to ascertain whether or not the effects associated with the specific chromosome regions are due to the action of single gene or clusters of genes. By comparing the near isogenic lines (NILs), modifiers or additional photoperiod sensitivity genes were found to be loosely linked with Sei (Sano 1992, Dung et al. 1998), indicating that Sei behaved as different alleles when modifiers were loosely linked. We examined here two Sel alleles which were introduced from photopenod sensitive lines into a practically insensitive line by backcrossing. After removing loosely linked genes, the two NILs showed no significant difference in heading time under natural conditions (Table 1). Nevertheless, the NILs showed different responses to photoperiods under controlled conditions as mentioned bellow. Materials used were the two NILs (BC7 generation) with Se] derived from Patpaku (Indica Type) or 0. rufipogon (W593 from Malaysia). The recurrent parent, T65wx (Japonica type) was also examined for comparison. Under natural daylength at Mishima (35°N) and Sapporo (43°N), the two NILs (T65LF3 and T65LF5) headed about a month later than T65wx and no significant difference was detected in heading time between the NILs at the both locations. Photoperiod responses to heading were examined in growth chambers (30°C, about 20, 000 lux). After 4—5 pre-germinated seeds were transplanted in a plastic pot, the plants were subjected to 9-h, 10-h, 12-h and 14-h daylengths until 170 days after germination. Curvilinear regression equations were prepared from the data to compare their photo- period responses as reported by Chandraratna (1954). As shown in Fig. 1 and Table 1, days to heading (Y) were expressed as a function of daylength (X). All the curvilinear regression equations obtained were significant at 1% level. When the equation is Y = a0 + a1X + a2X2, the minimum duration of vegetative growth (the basic vegetative growth, BVG) could be estimated from 4a2 a0 - a12 which is the number of days to heading at the optimum daylength (- a1/2 a2). In addition, d2Y/d2X = 2a2 gives an estimate of the degree of photopenod sensitivity. The estimated equations revealed that T65LF5 had smaller values in the optimum daylength as well as in the BVG than the recurrent parent while T65LF3 had a smaller value only in the BVG. This caused a difference between T65LF3 and T65LF5 that only T65LF5 headed under the 14-h daylength (Fig. 1). In other words, the optimum daylength of T65LF5 was similar to that of T65wx, however, the BVG in T65LF5 was similar to that of T65LF3. Although it is uncertain why the two NILs showed no difference under natural daylengths, the present results suggest that Sel alleles (or complex loci) might change the optimum daylength and the BVG simultaneously. References Chandraratna, M. F., 1954. Photoperiod response in rice (Oryza sativa L.). I. Effects on inflorescence initiation and emergence. New Phytologist 53: 397-405. Dung, L.V., T. Inukai andY. Sano, 1998. Dissection of major QTL for photopctiod sensitivity in rice: its association with a gene expressed In an age dependent manner. Theor. Appi. Genet. 97:714-720. Sano, Y., 1992. Genetic comparison of chromosome 6 between wild and cultivated rice. Jpn. J. Breed. 42: 56 1-572. Yokoo, M. and F. Kikuchi, 1982. The monogenic control of basic vegetative phase and photoperiod-sensitive phase in rice. Jpn. J. Breed. 32: 1-8.

20. Cryptic differences in the Sel alleles as revealed by curvilinear regression analysis

under controlled conditions

Y. ITOH and Y. SANO.

Plant Breeding Laboratory, Graduate School of Agriculture, Hokkaido University,
Sapporo, 060-8589 Japan

Photoperiod sensitivity is a major factor determining the days to heading in rice. A number of genes with major and minor effects seem to be involved in the quantitative trait which cause a continuous variation in flowering time. Among the photoperiod sensitivity genes identified, Sel (= Liii) was proposed to play a significant role for determining the duration of growth among rice cultivars (Yokoo and Kikuchi 1982). To explain differences in the degree of photoperiod sensitivity, a series of alleles at the Sei locus were proposed, however, it is not easy to ascertain whether or not the effects associated with the specific chromosome regions are due to the action of single gene or clusters of genes. By comparing the near isogenic lines (NILs), modifiers or additional photoperiod sensitivity genes were found to be loosely linked with Sei (Sano 1992, Dung et al. 1998), indicating that Sei behaved as different alleles when modifiers were loosely linked. We examined here two Sel alleles which were introduced from photopenod sensitive lines into a practically insensitive line by backcrossing. After removing loosely linked genes, the two NILs showed no significant difference in heading time under natural conditions (Table 1). Nevertheless, the NILs showed different responses to photoperiods under controlled conditions as mentioned bellow.

Materials used were the two NILs (BC7 generation) with Se] derived from Patpaku (Indica Type) or 0. rufipogon (W593 from Malaysia). The recurrent parent, T65wx (Japonica type) was also examined for comparison. Under natural daylength at Mishima (35°N) and Sapporo (43°N), the two NILs (T65LF3 and T65LF5) headed about a month later than T65wx and no significant difference was detected in heading time between the NILs at the both locations. Photoperiod responses to heading were examined in growth chambers (30°C, about 20, 000 lux). After 4—5 pre-germinated seeds were transplanted in a plastic pot, the plants were subjected to 9-h, 10-h, 12-h and 14-h daylengths until 170 days after germination.

Curvilinear regression equations were prepared from the data to compare their photo- period responses as reported by Chandraratna (1954). As shown in Fig. 1 and Table 1, days to heading (Y) were expressed as a function of daylength (X). All the curvilinear regression equations obtained were significant at 1% level. When the equation is Y = a0 + a1X + a2X2, the minimum duration of vegetative growth (the basic vegetative growth, BVG) could be estimated from 4a2 a0 - a12 which is the number of days to heading at the optimum daylength (- a1/2 a2). In addition, d2Y/d2X = 2a2 gives an estimate of the degree of photopenod sensitivity. The estimated equations revealed that T65LF5 had smaller values in the optimum daylength as well as in the BVG than the recurrent parent while T65LF3 had a smaller value only in the BVG. This caused a difference between T65LF3 and T65LF5 that only T65LF5 headed under the 14-h daylength (Fig. 1). In other words, the optimum daylength of T65LF5 was similar to that of T65wx, however, the BVG in T65LF5 was similar to that of T65LF3. Although it is uncertain why the two NILs showed no difference under natural daylengths, the present results suggest that Sel alleles (or complex loci) might change the optimum daylength and the BVG simultaneously.

References

Chandraratna, M. F., 1954. Photoperiod response in rice (Oryza sativa L.). I. Effects on inflorescence initiation and emergence. New Phytologist 53: 397-405.
Dung, L.V., T. Inukai andY. Sano, 1998. Dissection of major QTL for photopctiod sensitivity in rice: its association with a gene expressed In an age dependent manner. Theor. Appi. Genet. 97:714-720.

Sano, Y., 1992. Genetic comparison of chromosome 6 between wild and cultivated rice. Jpn. J. Breed. 42:

56 1-572.
Yokoo, M. and F. Kikuchi, 1982. The monogenic control of basic vegetative phase and photoperiod-sensitive phase in rice. Jpn. J. Breed. 32: 1-8.