Marker Genotypes for Parents of Japonica Hybrid Rice with High Combining Ability of Yield Traits

ACTA AGRONOMICA SINICA Volume 36, Issue 8, July 2010 Online English edition of the Chinese language journal Cite this article as: Acta Agron Sin, 2010, 36(8): 1270–1279.

RESEARCH PAPER

Marker Genotypes for Parents of Japonica Hybrid Rice with High Combining Ability of Yield Traits LIANG Kui1,**, HUANG Dian-Cheng1,2,**, ZHAO Kai-Ming1, Phuong-Tung NGUYEN1,3, XIE Hui1,4, MA Wen-Xia1, and HONG De-Lin1,* 1

State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing 210095, China

2

Cotton Research Institute, Chinese Academy of Agricultural Sciences, Anyang 455000, China

3

Agricultural Science Institute of Northern Central Vietnam, Nghe An-Vinh, Vietnam

4

Tianjin Subcentre of China National Hybrid Rice Research and Development Center, Tianjin 300457, China

Abstract: Scarcity of restorer line with high combining ability is a crucial restraint in the extension of japonica hybrid rice (Oryza sativa L.). This study aimed to screen simple sequence repeat (SSR) marker genotypes that can efficiently reveal high combining ability of hybrid parents. A total of 115 candidate markers were amplified in 6 BT-type cytoplasmic male sterile (CMS) lines and 12 restorer lines. Daily yield per plant (DYP), number of productive panicle per plant (PP), total number of spikelet per panicle (TSP), number of filled spikelet per panicle (FSP), and thousand-grain weight (TGW) were investigated in the 18 parents and 72 F1s derived from the crosses under an NCII design. Twenty marker genotypes were significantly associated with combining abilities (CA) of the parents. Among them, 12 marker genotypes were related to CA for more than one trait simultaneously, and the remaining 8 marker genotypes were only related to CA for one trait. The effect direction of marker genotype on combining ability of multiple traits was either positive or negative. RM152-165/170 was a marker genotype with the largest combining ability of PP and DYP, which increased DYP and PP in F1 by 20.6% and 12.7%, respectively. The marker genotypes with positive effect on increasing yield can be applied directly in the combining ability improvement of japonica rice restorer lines. Keywords: japonica rice; combining ability; yield and yield components; SSR marker genotype

The development of japonica hybrid rice (Oryza sativa L.) in China has a history over three decades. However, the progress is not as conspicuous as that in indica hybrid rice. At present, the planting area of japonica hybrid rice accounts for 3% of the total national japonica rice area, which is 8.28 million hectares annually [1]. The planting area of indica hybrid rice accounts for 78% of the total national indica rice area, which is 22.72 million hectares annually in China. Therefore, japonica hybrid rice has a great potential in extension. Compared with japonica hybrid rice, pure line cultivars of japonica rice bring a great challenge due to its better quality and equivalently high yield. Nevertheless, the crucial restraint for enlarging the growing area of japonica hybrid rice is scarcity of restorer line with high combining

ability. As a result, the competitive heterosis of japonica hybrid is not conspicuous in yield. In fact, such situation results from the breeding of restorer lines. Many cytoplasmic male sterile (CMS) lines have been developed since the first introduction of a CMS line with Boro II cytoplasm (BT type) in 1972. However, no effective restorers for these CMS lines could be found in japonica cultivars unless introducing restoring genes from indica restorers through crossing japonica cultivars with indica restorers and backcrossing with the japonica cultivars [2, 3]. To date, japonica restorers used in production are all bred from the progenies of indica × japonica crosses. The restoring ability was always the focus during development of japonica restorers, resulting in neglect of combining ability. High general combining ability (GCA)

Received: 1 February 2010; Accepted: 19 April 2010. * Corresponding author. E-mail: [email protected] ** Contributed equally to this article. Copyright © 2010, Crop Science Society of China and Institute of Crop Sciences, Chinese Academy of Agricultural Sciences. Published by Elsevier BV. All rights reserved. Chinese edition available online at http://www.chinacrops.org/zwxb/ DOI: 10.1016/S1875-2780(09)60064-X

LIANG Kui et al. / Acta Agronomica Sinica, 2010, 36(8): 1270–1279

is required for restorer lines to develop japonica hybrid combinations with strong vigor [4], and breeding japonica restorers with high GCA is the only way to enhance competitive heterosis of japonica hybrids. Previous results of combining ability analyses for restorer lines are difficult to guide the practice of combining ability improvement directly, although they can be used in evaluating the level of combining ability and proposing pairing mode of heterosis utilization [5–11]. Marker-assisted selection is an effective technique in target trait improvement in rice [12–14] based on the obtainment of molecular markers tightly linked with target genes. Combining ability is a characteristic that differs from morphological trait. It must be estimated using the measured values of corresponding trait in the F1 population. Therefore, molecular markers for combining ability should been screened in diallel-cross experiments. Liu et al. [15–18] screened marker genotypes for combining ability in yield and grain quality, and the elite marker genotypes were effective in improving the yield-related combining ability of Minghui 63, a famous indica restorer. Similar research has not been reported in japonica rice. In this study, to improve the combining abilities of japonica restorers, we tested 5 yield-related traits of 6 CMS lines and 12 restorer lines using simple sequence repeat (SSR) markers, and screened elite marker genotypes for combining ability.

sterilized by immersing in 1000× solution of 50% carbendazim for 24 h and incubated in water for another 24 h. The F1 seeds were hulled prior to the treatment for better germination. All seeds were germinated in petri dishes from 10 to 18 May, and then grown on seedling bed at a density of 3 cm × 3 cm. On 13 June, the seedlings were transplanted into paddy rice field at a density of 17 cm × 20 cm. The field experiment was arranged in a randomized complete block design with 3 replicates. Each plot had 4 rows with 8 hills per row and single seedling per hill. Traditional practice was followed for the field managements.

1

1.3

1.1

Materials and methods Plant materials and field planting

The 6 CMS lines, Wu 3A, 863A, 6427A, 9522A, Liuqianxin A, and Wuqiang A, were all of BT-type. The 12 restorer lines were 3726R-21, Wuyujing R-39, 6427R-16, 157TR-68, 4016LHR-69, Xiushui 04R, C418, 77302-1, C-Bao, R254, Xiangqing, and Ninghui 8. These CMS and restorer lines were all foundation parents of hybrids used in production or typical lines used in breeding researches. Under North Carolina II (NCII) design, 72 F1 hybrids were obtained after crossing CMS lines and restorer lines. All lines and hybrids were planted in a paddy rice field at the Jiangpu Experimental Station of Nanjing Agricultural University, Nanjing, Jiangsu Province, China. In 2007, the 6 CMS lines and their maintainer lines and the 12 restorer lines were sown on 7 May and transplanted on 10 June. At flowering stage, we made the crosses as designed. At maturity, F1 seeds of the 72 crosses were harvested. Because very few (less than 100) F1 seeds were obtained in 16 hybrids, these crosses were repeated in Lingshui County, Hainan Province, China in winter of 2007. In the repeated experiment, seeds were sown on 3 December 2007 and 5 January 2008. On 8 May 2008, seeds of all parents and F1 hybrids were sown at the Jiangpu Experimental Station. Before germination, seeds were

1.2

Trait investigation

At maturity stage, number of productive panicle per plant (PP) was investigated in 10 plants from the central rows of each plot. The productive panicle was defined as a panicle with more than 5 filled spikelets. Total number of spikelet per panicle (TSP) and filled spikelet number per panicle (FSP) were investigated based on main stems (the highest one) of 5 plants. After the seeds were naturally dried under the sunlight, thousand-grain weight (TGW) was measured with 3 replicates. Yield per plant (YP) was calculated with yield per plot (edging rows excluded) and the number of plants harvested. Daily average yield per plant (DYP) was the ratio of average YP to days from sowing to maturity. SSR primers and PCR amplification

Genomic DNA was extracted from leaves of seedlings at tillering stage in 2007 using SDS method [19, 20]. These DNA samples were amplified with 115 pairs of SSR primers (synthesized in the GenScript Corporation, Nanjing, China) including 62 pairs associated with grain yield and its components (Table 1) and 53 pairs selected randomly from the collection of our laboratory. The sequences of all primers were obtained from the rice genome database at http://www.gramene.org. Primers were). The 10 μL PCR reaction mixture was composed of 1 μL of 10× buffer, 1 µL of template DNA (20 ng μL−1), 0.6 µL of MgCl2 (25 mmol L−1) each 0.7 µL of forward and reverse primers (2 pmol μL−1), 0.2 µL of dNTPs (2.5 mmol L−1), 0.1 µL of Taq DNA polymerase (5 U μL−1), and 5.7 μL of double-distilled water. The DNA amplification was carried out in a PTC-100 Peltier Thermal Cycler (MJ Research Inc., USA) under the condition of 1 cycle of predenaturating for 3 min at 95°C, 35 cycles of denaturating for 30 s at 95°C, annealing for 30 s at 55°C, and enlongation for 1 min at 72°C. The last cycle was followed by a final incubation for 5 min at 72°C. PCR products were maintained at 4°C. The DNA amplification products were separated in 8.0% PAGE and visualized after silver staining. The fragment sizes of amplification bands were estimated with the standard size markers (100 bp ladder).

LIANG Kui et al. / Acta Agronomica Sinica, 2010, 36(8): 1270–1279

Table 1 Chr. Marker

Sixty-two pairs of SSR primers linked with QTLs for yield and yield components of rice

Trait YP [21, 22], PP [23], FSP [22],

Chr. Marker

Trait

Chr. Marker

Trait

PP [23]

7

RM10

TSP [22]

RM85

TSP [26, 33], TGW [24]

7

RM11*

YP [27, 30], PP [27, 30], TSP [22],

RM142

YP [27], PP [27], TSP [27] YP [22], TSP [22]

7

RM234

YP [33], TSP [28], TGW [23]

RM280

FSP [23], TSP [23]

7

RM478

YP [33]

4

RM303

FSP [23], TSP [23, 25, 26]

7

RM542

YP [21], FSP [21]

YP [21, 24], PP [21, 23, 27], FSP [23, 28],

4

RM317

TSP [25]

8

RM137

YP [33]

TSP [23, 25, 29], TGW [27]

5

RM163

YP [33], PP [22]

8

RM210

PP [22], TSP [33]

1

RM1

3

RM570*

TSP [23]

3

1

RM104

YP [24], FSP [24], TSP [24]

4

1

RM212

YP [24], FSP [24], TSP [24]

4

RM252

1

RM23*

YP [24], FSP [23], TSP [23]

4

1

RM246

TSP [23, 25, 26], TGW [23]

1

RM259

TGW [27]

1

RM428

TSP [25]

5

RM164

PP [25], FSP [25]

8

RM223

PP [22], TSP [23]

1

RM5*

YP [27], TGW [27]

6

RM19715*

YP [31], FSP [31], TSP [31], TGW [31]

8

RM331

YP [23], FSP [23], TSP [23]

2

RM207

PP [29], FSP [29], TSP [29]

6

RM19784

YP [31], FSP [31]

8

RM5556

YP [23], FSP [23], TSP [23]

2

RM208*

YP [30], PP [29], FSP [29, 31],

6

RM204

FSP [23, 31], TSP [31]

8

RM72

YP [33], TSP [27],

TSP [27, 29, 31], TGW [27]

6

RM225

TSP [31]

10 RM228

YP [33], TGW [24]

2

RM221

YP [24], PP [24], TSP [24]

6

RM253

PP [27], FSP [31], TGW [27]

10 RM258*

YP [33]

2

RM263

YP [21, 24], PP [24], FSP [22, 24],

6

RM276

YP [30, 31], FSP [31], TSP [25, 27]

11 RM167

PP [22], TSP [25]

TSP [24], TGW [25]

6

RM314

PP [27], TSP [31], TGW [27]

11 RM224

PP [26], TGW [27, 32]

RM340*

FSP [25], TSP [25]

11 RM4B

YP [22], FSP [22], TSP [22] PP [30], TSP [27], FSP [33]

2

RM290

TGW [32]

6

2

RM341*

TGW [32]

6

RM345*

PP [27], FSP [25], TSP [25]

12 RM235

2

RM475*

YP [32], TGW [25]

6

RM402

YP [31], FSP [31], TGW [31]

12 RM247*

TGW [25]

3

RM148*

YP [33], TSP [33], TGW [24]

6

RM510

YP [31], FSP [31], TSP [31]

12 RM270

PP [30], TSP [27]

3

RM16

YP [22], PP [23], TSP [22]

6

RM549

TSP [31], TGW [31]

12 RM277

TGW [24]

3

RM227

YP [33], TSP [26]

6

RM587

YP [31]

12 RM309

YP [33]

3

RM569

TSP [27]

Polymorphic markers among the 18 parents are in bold. Markers related to combining ability are marked with asterisks (*). YP: Yield per plant; PP: Number of productive panicle per plant; FSP: Number of filled spikelet per panicle; TSP: Total number of spikelet per panicle; TGW: Thousand-grain weight.

1.4

Analysis of phenotypic combining ability in parents

Parental combining abilities based on phenotypic traits were analyzed using the p × q mating model described by Mo [34, 35]. The integrative evaluation of parents was carried out using the following method: first, calculating the GCA effect and effect variance of specific combining ability (SCA) in each parent; second, ranking the parents according to their GCA effects and variances of SCA effect [36]. For the GCA effects of trait investigated and the variance of SCA effect, the ranking order was from large to small. For example, the largest effect or variance was ranked as the first, and the smallest effect or variance was ranked as the last one. Finally, the total rank for all traits investigated was used as an indicator for integrative evaluation of parent. 1.5

Marker genotypes for high combining ability of parents

Assumed that 2 bands (100 p and 200 bp) were amplified on locus RM1 in 18 the parents. The marker genotype of an F1 hybrid on RM1 is regarded as a homozygote if the parents had identical banding pattern on this locus; otherwise, the marker genotype of the F1 hybrid is regarded as a heterozygote on this locus. Thus, the total 72 combinations derived from the crosses

between the 6 CMS lines and the 12 restorers can categorized into homozygous and heterozygous groups on this locus. The homozygous group is composed of 2 marker banding patterns (RM1-100 or RM1-200), and the heterozygous group only contains a single banding pattern (RM1-100/200). The phenotypic average of the heterozygous group is then compared with that of the homozygous group according to t-test. If there is a positive and significant (P < 0.05 or P < 0.01) difference between the heterozygous group and homozygous groups, RM1-100/200 is accepted as a marker genotype that increases combining ability. If the difference is negative and significant, RM1-100/200 is regarded as a marker genotype that decreases combining ability. Either of them may be a marker genotype of elite combing ability subject to breeding target. If the favorable trait is represented by a large phenotypic value, the marker genotype with positive effect on combing ability should be selected in the breeding process. If there are 3 or more amplification bands on a locus among the 18 parents, 2 bands will be considered each time, and the effect of all kinds of the heterozygous genotype will be calculated separately. The calculations were carried out using the self-developed program CAScreen 1.0 written by MATLAB language.

LIANG Kui et al. / Acta Agronomica Sinica, 2010, 36(8): 1270–1279

2

Results

2.1 GCA analysis of CMS and restorer lines in yield-related traits The result of analysis of variance (calculated in Microsoft Excel) showed that there were significant differences (P < 0.05) among the 72 combinations in the 5 yield-related traits investigated. Thus, GCA effects of these traits were estimated and compared. In the CMS lines the largest GCA effects were in BT-Liuqianxin A for DYP and PP, 863A for TSP and FSP, and 9522A for TGW; in the restorer lines, the largest GCA effects were in 157TR-68 for DYP, 3726R-21 for PP, 77302-1 for TSP and FSP, and Xiushui 04R for TGW. The integrative evaluation based on total ranks of GCA effect and SCA variance showed that 863A was the best CMS line, followed by Wu 3A and Liuqianxin A; and 157TR-68 was the best restorer line, followed by R254, C418, and Ninghui 8 (Table 2). 2.2 Marker genotypes for elite combining ability of parents in yield-related traits Among the 115 pairs of SSR primers, 59 pairs amplified polymorphic bands in the 18 parents including 35 pairs of primers associated with yield and its components (Table 1). A total of 153 alleles were detected on these polymorphic loci in the 18 parents. There were 2–5 alleles on each locus with an average of 2.6. Forty-two marker genotypes were significantly related to combining ability in the 5 traits, of which 25 were

marker genotypes for elite combining ability (Table 3). Among the 7 marker genotypes significantly related to GCA in DYP, 4 were marker genotypes for elite combining ability with the GCA effect ranging from 11.1% to 20.6% (Table 3). The largest effect of DYP was detected with marker genotype RM152-165/170. The RM152-165 allele was carried by Liuqianxin A and 157TR-68 and the RM152-170 allele was carried by Wu 3A, 863A, 6427A, 9522A, Wuqiang A, 3726R-21, Wuyujing R-39, 6427R-16, 4016LHR-69, Xiushui 04R, 77302-1, C-Bao, R254, Xiangqing, and Ninghui 8. However, 3 marker genotypes had negative GCA effect in DYP, of which RM148-145/150 had an effect to reduce DYP of F1 by 13.3% (Table 3). Parents with the RM148-145 allele were 3726R-21, Wuyujing R-39, 6427R-16, C418, 77302-1, C-Bao, R254, Xiangqing, and parents with the RM148-150 allele were 157TR-68, 4016LHR-69, Xiushui 04R, Ninghui 8, Wu 3A, 863A, 6427A, 9522A, Liuqianxin A, and Wuqiang A. In TSP, FSP, TGW, and PP traits, there were 10, 7, 2, and 2 positive marker genotypes for GCA effect, respectively. Marker genotypes with the largest effects were RM341-150/180 in TSP and FSP, RM5-110/115 in TGW, and RM152-165/170 in PP (Table 3). 2.3 Marker genotypes affecting GCA effects of multiple traits simultaneously Twelve marker genotypes were found in relation to GCA effects for more than one trait (Table 4), and the remaining

Table 2 GCA effect and their significance test of 5 traits and integrative evaluation in 18 parents Parent

Yield-related traits DYP (g)

PP

GCA

TSP

FSP −2.88

TGW (g)

SCA variance

Total rank Sequence

Total rank

Sequence

CMS lines Wu 3A 863A

0.001

−0.35**

−0.99

−0.010*

−0.64**

10.25**

6427A

−0.001

−0.22

9522A

−0.001

−0.77**

Liuqianxin A Wuqiang A

4.06 −0.60

0.03

15

1

23

4

8.00**

0.03

15

1

10

1

6.21**

−0.66**

16

2

22

3

−3.32

1.58**

17

3

20

2

0.030**

1.38**

−1.80

−3.65

−0.85**

18

4

10

1

−0.020**

0.60**

−10.92**

−4.35

−0.13

24

5

20

2

−0.76

Restorer lines 3726R-21

−0.010

1.20**

−22.98**

−19.94**

Wuyujing R-39

−0.020*

1.11**

−59.86**

−41.58**

6427R-16

−0.030**

0.73**

−19.30**

−8.09*

−0.22

31

157TR-68

0.080**

0.23

11.80**

19.08**

−0.98

16

4016LHR-69

−0.030**

−0.45**

2.22

−5.05

−1.11

43

10

29

4

Xiushui04 R

0.030**

0.43**

−27.20**

−20.81**

2.21**

36

7

41

7

C418

0.001

−0.96**

19.02**

7.04*

1.64**

22

2

31

5

77302-1

0.001

−0.42*

53.18**

33.23**

28

3

21

2

C-Bao

−0.020**

−0.27

1.43

41

9

28

3

R254

−0.020**

−0.69**

12.07**

4.63

−0.40*

33

5

17

1

Xiangqing

−0.010

−0.48**

18.48**

17.15**

−0.99

35

6

32

6

−0.44**

11.13**

17.68**

22

2

41

7

Ninghui 8

0.030**

−3.34

0.51**

−1.73 1.17**

0.67**

39

8

49

9

44

11

42

8

4

28

3

1

31

5

YP: Yield per plant; PP: Number of productive panicle per plant; FSP: Number of filled spikelet per panicle; TSP: Total number of spikelet per panicle; TGW: Thousand-grain weight. * P < 0.05; ** P < 0.01.

LIANG Kui et al. / Acta Agronomica Sinica, 2010, 36(8): 1270–1279

Table 3

Marker genotypes significantly related to combining ability of hybrid rice parents in yield and its components

Marker genotype

Heterozygous group Number of F1 cross

Average of F1s

Homozygous group Number of F1 cross

Average of F1s

Percentage of

t-value

increment (%)

(t0.01 = 2.64)

Daily yield per plant RM152-165/170

15

0.33

57

0.27

20.6

5.02

RM570-207/257

12

0.32

60

0.28

16.8

3.55 3.08

RM19715-160/170

16

0.31

56

0.28

13.3

RM23-150/160

30

0.30

42

0.27

11.2

3.00

RM148-145/150

48

0.27

24

0.31

−13.3

−4.13

RM5639-150/160

32

0.27

40

0.30

−10.2

−3.02

RM7120-170/180

36

0.27

36

0.30

−9.1

−2.67

Total number of spikelet per plant RM341-150/180

48

229.11

24

180.61

26.7

8.92

RM1211-150/160

36

233.89

36

191.99

21.8

7.51

RM11-131/147

42

229.73

30

189.43

21.3

6.85

RM340-120/171

36

232.40

36

193.48

20.1

6.63

RM7120-170/180

36

230.67

36

195.21

18.2

5.74

RM475-200/250

35

230.77

37

196.07

17.7

5.56 4.64

RM3-120/150

24

234.43

48

202.20

15.9

RM23-150/160

30

226.72

42

203.10

11.6

3.35

RM345-160/172

30

225.43

42

204.02

10.5

2.99

RM208-185/175

21

228.02

51

206.73

10.3

2.72

RM208-175/180

17

192.04

55

219.40

−12.5

−3.34

RM475-220/250

12

183.17

60

218.89

−16.3

−3.92 7.63

Number of filled spikelet per plant RM1211-150/160

36

195.47

36

162.53

20.3

RM341-150/180

48

190.30

24

156.39

21.7

7.23

RM340-120/171

36

191.68

36

166.32

15.3

5.08

RM475-200/250

35

191.66

37

167.02

14.8

4.88

RM11-131/147

42

189.19

30

164.73

14.9

4.75

RM7120-170/180

36

187.94

36

170.06

10.6

3.29

RM3-120/150

24

191.49

48

172.75

10.9

3.24

RM247-160/172

20

167.12

52

183.57

−9.0

−2.65

RM208-175/180

17

165.48

55

183.18

−9.7

−2.71

RM475-220/250

12

158.10

60

183.18

−13.7

−3.46

Thousand-grain weight RM5-110/115

15

26.40

57

24.52

7.7

4.42

RM258-142/165

18

25.85

54

24.60

5.1

2.93

RM340-120/171

36

24.31

36

25.50

−4.7

−3.26

RM3-120/150

24

23.85

48

25.44

−6.2

−4.29

RM152-165/170

15

10.34

57

9.17

12.7

3.60

RM208-175/180

17

10.13

55

9.19

10.2

2.95

RM208-185/175

21

8.81

51

9.66

−8.9

−2.88

RM1211-150/160

36

8.96

36

9.87

−9.3

−3.47

RM340-120/171

36

8.95

36

9.87

−9.3

−3.47

RM7120-170/180

36

8.87

36

9.96

−10.9

−4.25 −4.98

Number of productive panicle per plant

RM341-150/180

48

8.98

24

10.28

−12.7

RM11-131/147

42

8.89

30

10.15

−12.5

−5.12

RM475-200/250

35

8.60

37

10.18

−15.5

−7.33

8 marker genotypes were only related to GCA effect for one trait. The effect direction of marker genotypes for PP was opposite to that for TSP and FSP. For instance, RM340-120/171 and RM7120-170/180 had negative effects

on PP but positive effects on TSP and FSP. Similarly, the GCA effect for TGW was opposite to that for TSP or FSP as revealed by marker genotypes. The effect directions of PP and DYP were consistent according to marker genotypes, such as

LIANG Kui et al. / Acta Agronomica Sinica, 2010, 36(8): 1270–1279

Table 4

Marker genotypes related to combining abilities of multiple traits in parents Increasing percents of yield-related traits in F1 (%)

Marker genotype

Number of productive panicle Total number of spikelet per Number of filled spikelet per plant

panicle

per plant

Thousand-grain weight

Daily yield per plant

For 4 traits RM340-120/171

−9.3

20.1

15.3

RM7120-170/180

−10.9

18.2

10.5

−4.7 −9.1

For 3 traits RM11-131/147

−12.5

21.3

14.9

RM208-175/180

10.2

−12.5

−9.7

15.9

10.8

−12.7

26.9

21.7

RM3-120/150 RM341-150/180 RM1211-150/160

−9.3

21.8

20.3

RM475-200/250

−15.5

17.7

14.8

−6.2

For 2 traits RM152-165/170

12.7

RM208-185/175

−8.9

20.6 10.3

RM23-150/160

11.6

RM475-220/250

−16.3

RM7120-170/180 and RM152-165/170. Although the marker genotype RM7120-170/180 had positive effects to increase TSP and FSP, the yield loss caused by the reductions of PP could not be retrieved, leading to the decrease of DYP ultimately (Table 4). This result suggests that improvement of PP in F1 hybrid is of most importance in japonica hybrid rice breeding aiming at high yield.

3

Discussion

The marker genotypes detected can be directly used when a single marker genotypes controls GCA of one trait only. However, when a single marker genotype controls GCAs of several traits simultaneously, the effect directions of the marker genotype on different traits should be considered. For example, marker genotype RM152-165/170 had positive effects on GCAs for PP and DYP, and the both traits can thus be improved simultaneously under the selection of this marker genotype. However, marker genotype RM208-185/175 had opposite directions on GCAs for PP (negative) and TSP (positive), resulting in the alternative of improving GCA for PP or TSP using this marker genotype. In the application of marker genotype for parental GCA improvement, we should pyramid marker genotypes that contribute positively to several target traits in one variety or line. As a result, this variety or line may carry elite alleles for GCAs on several target traits. In this study, 863A is the best CMS line, and 157TR-68, R254, C418, and Ninghui 8 are superior restorer lines based on integrative evaluation. These lines have good potentials in breeding of japonica hybrid rice. Hybrid “86-You-8” is an elite japonica combination authorized and extended in Jiangsu

11.2 −13.7

Province, China [37], which is derived from the cross between 863A and Ninghui 8. To further enhance the combining ability for yield-related traits of Ninghui 8 as the restorer of 863A, several alleles can be introduced from 157TR-68 to improve the combining abilities for PP, DYP, TSP, and TGW, such as of RM152-170 for PP and DYP, RM23-160 for TSP and DYP, RM570-257 for DYP, RM345-172 for TSP, and RM258-165 for TGW. Nevertheless, the improvement of combining ability in a restorer through introducing elite alleles must be on the basis of a representative CMS line with high combining ability (as the tester), and vice versa. In this study, GCA was calculated based on the difference of phenotypic averages between “heterozygous group” and “homozygous group” on a single locus divided by marker genotypes in a set of p × q crosses. The GAC values obtained refer to the combining abilities between 2 types of parents, which are determined by marker genotypes. We call it marker genotypic combining ability (MGCA). MGCA is applicable based on the premises that 1) the data of NC II combinations must be perfect; and 2) when the t-test showed significant difference (P < 0.01) between the “homozygous group” and the “heterozygous group” for average trait values, the number of combinations with homozygous loci must be close to that with heterozygous loci.

4

Conclusions

Twenty SSR marker genotypes were found to be significantly related to elite combining ability for grain yield and its component traits. The marker genotypes with increasing effects in F1s can be directly used to improve combining ability of restorer lines in japonica hybrid rice

LIANG Kui et al. / Acta Agronomica Sinica, 2010, 36(8): 1270–1279

through marker-assisted selection.

Acknowledgments This study was supported by the Program of Introducing Talents of Discipline to University of China (B08025), the Program of Introducing International Super Agricultural Science and Technology of China (2006-G8[4]-31-1), the key project of Basic Platform for Science and Technology Research, Ministry of Education (505005), and the National High Technology Research and Development Program of China (2010AA101301).

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