15. Dissection of a gene complex responsible for photoperiod sensitivity

L.V. duno and Y. sano Faculty of Agriculture, Hokkaido University, Sapporo, 060 Japan

We compared genetic differentiation for photosensitivity on chromosome 6 between an Indica (Patpaku, 868) and a Japonica (Taichung 65 with wx, T65wx) types. An alien segment of chromosome was introduced from Patpaku into T65wx by successive backcrossing (Kobayashi and Sano, in this issue). The near-isogenic line (868A) was established from BC8 and seemed to carry at least a segment from Wx^a (non-glutinous) to Se1 (photosensitivity) of chromosome 6 based on the segregation pattern in the F2 of 868A x T65wx as described below. The heading time of 283 F2 plants were determined by growing them at Mishima and they were divisible into 3 classes (early, intermediate and late), regarding to heading time (Table 2). An incomplete dominant gene controlling photosensitivity seemed to be linked to wx and C (chromogen for anthocyanin) with a recombination value of 0.31 ± 0.03 and 0.19 ± 0.06, respectively.The gene is then considered to be identical to Se1 judging from the linkage intensities.

It was, however, noticed from the Fz that the frequency of segregants homozygous for Se1 was significantly higher than expected although wx and C showed no such a tendency and that plants showing very late heading time (around 20 October at Mishima) were observed suggesting a transgressive segregation. In addition, the significant reduction in the number of heterozygotes for Se1 is an indication of that unknown mechanisms rather than certation cause the distortion in heading time. These findings prompted us to explore more genes on the segment since infertilities which often cause distorted segregation were not detected earlier in this case. From the selfed progeny (F4 generation), 5 different types of true-breeding lines in respect to heading time were obtained (Table 1 ). Heading of Type I was as early as that of T65wx while heading of the other types (II to V) were significantly late. Since number of days to heading was reduced by short-day treatments, the genes introduced from Patpaku controls

Table 1. Differences in heading time and produced leaf number in the near-isogenic lines in comparison with the reccurent parent (T65wx)

Traits

Near-isogenic lines

868A

Type I

Type II

Type III

Type IV

TypeV

Heading time Leaf number

39.3* 3.5*

0.3ns -0.5ns'

17.0* 0.8*

27.8* 0.5 ns

36.8* 1.0*

62.3* 1.8*

Note: No. of days to heading from germination and no. of leaves on the main culm in T65wx are 116.5 and 15.5, respectively. Plants were seeded on 1 st May. ns- shows no significance. * shows significance at 1%.

I

Research Notes 73

photosensitivity. The 5 different lines suggest that at least 3 genes have to be involved on the introduced segment. Based on segregations in crosses among them, we determined the genotypes of Type I to Type IV.

All f2 plants from a cross of T65wx x Type I headed as early as T65wx, indicating that their genotypes are the same for photosensitivity. Among the remaining 4 types, crosses were made in 6 possible combinations. F2 segregations from the two crosses (Type I x Type IV and Type II x Type III) showed a complicated pattern while the other 4 crosses gave a monogenic segregation (Table 2). A ratio of 3:1 observed in Type I x Type II f2 indicated that Type II has a recessive gene for photoperiod sensitivity while a ratio of 1:2:1 in Type I x Type III F2 indicated that Type III has an incomplete dominant gene for photosensitivity. The former gene was estimated to be linked to the two markers, wx and C, with recombination values of 0.14 ± 0.03 and 0,13± 0.04, respectively. The gene seemed to be located between wx and C since the recombination value between the two markers was estimated to be 0.18x0.03. The gene was tentatively designated se-pat(t), while the latter was regarded as Se1 from its location. No f2 segregant heading as early as T65wx was observed in crosses of Type II x Type IV and Type III x Type IV, showing that Type IV has the both genes for photosensitivity. Therefore, Se1 is expected to segregate in Type II x Type IV F^2 and se-pat(t) is expected

Table 2. Segregation patterns for heading time in crosses among the near-isogenic lines of T65 wx

carrying different genes from Patpak

Cross

Heading time

No. of

Ratio

X2

August 18202224262830

September Octorber 13579 11131517 192123252729 1 3 5 7<

Plants

observed

868Ax T65wx

1 310 91414 6

611 3 218433318 14322 81411 2

283

1:2:1

17.64*

(65)

(117)

(101)

IxII

624403225 6

/ 333 812

3 1

167

3:1

1.92"-

(133)

(34)

IxIII

38544

1 2 819 9

1 311 6

84

1:2:1

0.57ns

(24)

(40)

(20)

IIxIV

3 714 8 7

1 5253015 9984

145

1:2:1

1.55ns

(40)

(75) (30)

lllxlV

172960118 12448 8

311

3:1

0.09ns

(231) (80)

0.09ns

Note: Italic numbers indicate grouping of segregants. Number of segregants in each group is shown in

parenthesis. Number of days to heading of the parental lines are shown in Table 1. * shows significance at 1 %. ns- shows non-significance.

74 Rice Genetics Newsletter Vol. 13

in Type III x Type IV F2, which was confirmed by the present data (Table 2). Thus, the two genes seemed to act additively to enhance photosensitivity.

Although genie analysis is still underway regarding to Type V, it gave a 3:1 ratio in a cross with Type IV. This suggests that another recessive gene might be involved, however, it might be carried by T65wx since 868A has the same heading time as that of Type IV. A recessive gene carried by T65wx seems to enhance the photosensitivity caused by se-pat(t) and Se1 which were brought from Patpaku. The present observations confirmed that a gene complex on chromosome 6 is able to produce a range of variation in heading time by recombining genes on the segment after hybridization. Polygenic traits are apparently controlled by the interaction of numerous genes whose effects are essentially interchangeable and are small relative to environmental sources of variation. The present results give an example that loosely linked genes on the segment have a potentiality to make heading time of the hybrid population adjusted to different localities through the reconstruction of the genie content. (Gene symbol: Old system)

References

Kobayashi, S. and Y. Sano, 1996. Three genes involved in the unidirectional cross-incompatibility system

observed between a sativa-rufipogon hybrid on chromosome 6. RGN (this issue).