9. A dominant suppressor for the photoperiod sensitivity gene, Se/1, detected in a photoperiod-insensitive cultivar of Indica type

Yoshio SANO

National Institute of Genetics, Mishima, 411 Japan

A gene for photoperiod sensitivity, Se, has been well documented and seems to play a role in differential photoperiodic responses of rice cultivars (Yokoo and Kikuchi 1977; Yokoo et al. 1982). Regarding the genetic elements which suppress the expression of Se\1\, a semidwarf Indica cultivar (1-geo-tse) appeared to have a recessive inhibitor, i-Se\1\ (Chang and Vergara 1969). In this report, evidence is presented showing that a photoperiod-insensitive cultivar of Indica type (Peiku from Taiwan, Acc1O8) carries a dominant suppressor for the Se\1\ gene.

The materials used were Acc108 and four near-isogenic lines having different genes in the genetic background of Acc108 or of T65 (Taichung 65), which are listed in Table 1 with their days to heading at Mishima. Acc108 is photoperiod- insensitive and comes to heading about 5 days earlier than T65wx (an isogenic line of T65 with gene wx) which seems to be homozygous for se\1\. Acc108S\1\(g) carries a small segment of chromosome containing gene S\1\ for gamete elimination (due to allelic interaction, Sano et al. 1979), which was introduced from a strain of O. glaberrima (W025). This line was c/c (no apiculus coloration) similar to Acc108. Loci +/wx, S\1\S\1\a, C/c, and Se\1\/se\1\ are all linked and located in the given order on Chromosome 6. The presence of c in Acc108S\1\(g) suggests that the Se/1/se/1 locus in this line is not included in the segment introduced from W025 by backcrosses and belongs to the original genotype of Acc1O8 (cf. Sano 1987).

Table 1. Days to heading in parental lines and F1 plants recorded at Mishima 

================================================================
Line or cross       Days to heading  Difference Genotype assumed
                                     from T65wx
================================================================
T65wx                111.4+-1.46          0   wxS\1\a CBrse\1\
Acc1O8               106.5+-1.51         -4.8 +S\1\acSe\1\ Su-Se\1\
Acc1O8S/1(g)         106.7+-1.15         -4.7 +(S\1\)cSe\1\  Su-Se\1\
T65Se/1(g)           130.9+-1.60         19.5 +S\1\a CBr(Se\1\)
T65S/1               130.2+-1.42         18.8 +(S\1\ c Se\1\)
F1 plants,
 T65wx X Acc1O8      108.3+-0.58         -3.1
  T65wxXAcclO8S,(g)  109.1+-1.12         -2.3
================================================================
Enclosure in parentheses shows alien genes introduced by backcrosses. Gametes with S\1\a are eliminated when the plant has S\1\ (Sano et al. 1979).

Table 2. Segregation for heading time observed in backcross generation from (T65Se\1\ X Acc108) F2 (early) X T65Se\1\(g)

================================================================
                No. of plants observed
Generationa   ===========================        X2 from 3:1
              Earlyb   Latec    Total
================================================================
B1F2            27        7       34             0.35(P>0.5)
B2F2            71       12       83             4.92(P>0.025)
================================================================
a-Genotype assumed for B1F1 and B2F1: Se\1\/Se\1\ Su-Se\1\/su-Se\1\.
b-Heading time being similar to that of Acc1O8.
c-Heading time being similar to that of T65Se\1\(g).
It has been found that crossing over is strongly restricted around the S\1\ gene (Sano 1990). The S\1\ gene in ACC108S\1\(g) was introduced into T65 by 7 backcrosses. The resultant line, T65S, is expected to carry a relatively large segment containing S\1\ on account of its recombination restriction. ACC108S\1\(g) appeared to have se\1\/se\1\ as its heading time was the same as of Acc1O8 and slightly earlier than that of T65 or T65wx (Table 1). However, the near-isogenic line of T65 carrying S\1\ derived from Acc108S\1\(g) headed nearly at the same time as T65Se\1\(g), a line carrying Se\1\ introduced from W025 by 11 backcrosses (Table 1). The linkage relations with wx and S\1\ also confirmed that this line had gene Se\1\.

These experimental results suggest that both Acc108 and Acc108S\1\(g) possess Se\1\, which is not expressed because these plants have a suppressor for Se\1\ symbolized Su-Se\1\(t) tentatively. Lines T65S\1\ and T65Se\1\(g) have Se\1\ but no such suppressor, so that their heading is delayed. To introduce the supposed suppressor into T65 by backcrosses, T65Se\1\(g) was used as the recurrent parent and was crossed with Acc108, to obtain homozygotes for Se\1\ segregating for the suppressor.

The F1 plants from T65Se\1\(g) X Acc108 were partly seed-fertile (24.7%), and the F2 population showed a continuous variation in heading time as there were many segregating genes. One of early-flowering segregants was selected and backcrossed with T65Se\1\(g). Then, a B1F2 line with no infertile segregant was obtained. Although successive backcrosses are still under way, B1F2 and B2F2 data showed that Acc108 has at least one dominant suppressor for Se\1\, proving the above assumption of a suppressing gene (Table 2). The segregation pattern observed in B2F2 suggested that the homozygote for Su-Se\1\(t) flowered a few days earlier than the heterozygote which segregated. These plants were also tested under shortdays. The result showed that plants homozygous for Se/1 and Su-Se\(t) were photoperiod-insensitive.

A dominant suppressor for Se\1\ has not been reported so far. It is known that crosses between early-flowering cultivars sometimes produce late-flowering F1 photoperiod-sensitive. Poonyarit et al. (1987, 1989) showed that gene Se\3\(t) which shortened critical daylength interacted with Se\1\ for photoperiod sensitivity. Cai et al. (1987) suggested that three dominant complementary genes controlled late flowering due to photoperiodic response. The dominant suppressor for Se\1\ detected in the present work may be useful controlling heading time in the breeding of hybrid rice.

References

Cai, C. M., W. M. Li and Y. C. Zhou, 1987. Complementary genes controlling photoperiod sensitivity in hybrid rice. RGN 4: 90-92.

Chang, T. T. and B. S. Vergara, 1969. Component analysis of duration from seeding to heading in rice by the basic vegetative phase and the photoperiod-sensitive phase. Euphytica 18: 79-91.

Poonyarit, M., D. J. Mackill and B. S. Vergara, 1987. Two genes affecting photoperiod sensitivity in rice. RGN 4: 87-89.

Poonyarit, M., D. J. Mackill and B. S. Vergara, 1989. Genetics of photoperiod sensitivity and critical daylength in rice. Crop. Sci. 29: 647-652.

Sano, Y., 1987. Oryza glaberrima and 0. sativa carry wx, Se-1, A and Rc at the same chromosomal locations. RGN 5: 66-67.

Sano, Y., 1990. The genic nature of gamete eliminator in rice. Genetics 125: 183-191.

Sano, Y., Y. E. Chu and H. I. Oka, 1979. Genetic studies of speciation in cultivated rice species, 1. Genic analysis for the F1 sterility between Oryza sativa and 0. glaberrima. Jpn. J. Genet. 54: 121-132.

Yokoo, M. and F. Kikuchi, 1977. Multiple allelism of the locus controlling heading time in rice, detected using the close linkage with blast-resistance. Jpn. J. Breed. 27: 123-130.

Yokoo, M., K. Toriyama and F. Kikuchi, 1982. Responses of heading conferring Lm alleles of rice to seasonal changes of natural daylength. Jpn. J. Breed. 32: 378-384.