Combining Ability Analysis on Yield Traits of Recombinant Inbred Lines in Hybrid Rice

Yingjiang CAO, Shumei YOU, Jiakui ZHENG, Kaifeng JIANG, Tao ZHANG, Li YANG, Qianhua YANG, Xianqi WAN, Jing LUO, Zhaoxiang LI, Lei GAO

Rice and Sorghum Research Institute, Sichuan Academy of Agricultural Sciences /Luzhou Branch of National Rice Improvement Center/Key Laboratory of Southwest Rice Biology and Genetic Breeding, Ministry of Agriculture, Deyang 618000, China

As one of the most important grain crops around the world,rice is widely planted in 30%of the total area for food crops and about 2/3 population feeds on it in China[1].At present, improving yield per unit area plays a crucial role in effectively alleviating the conflict between the decreasing cultivated area caused by industry construction, natural damage,ecological restoration, and the increasing demand for rice due to population growth. Optimum cross combinations can be obtained through combining ability analysis of various culti vars[2],and the contributions of parental characters to F1generation can be determined by genetic analysis[3-6]. During the growth period, male parent often contributes more to the traits of F1than female parent, but in such traits,kernel length, kernel width and 1 000-grain weight, female parent has more contribution rate. Several important and widely planted materials from which many cultivars were derived were found in breeding practice, such as Minghui 63[7-10]. By 2010, a total of 34 cultivars derived from Minghui 63 as male parent had been approved by Provincial or National Crop Approval Committee,including four approved byNational Crop Approval Committee. In addition, 543 new restorer lines were bred using Minghui 63 as the core parent, and then 922 combinations derived from the 543 new restorer lines have already been examined and approved by Provincial or National Crop Approval Committee[10]. Then the questions were whether these materials contain high combining ability genes, and how to isolate them. To find the answers, Qi et al.[11], from Huazhong Agricultural University,constructed some combinations using Zhong 3 and HB 522, and then discovered 56 QTLs determining the general combining ability of maize agronomic traits and 21 QTLs controlling the specific combining ability in these combinations. Qu et al.[12]by crossing the chromosome segments substitution lines derived from 97 B and 9311, with three two-line GMS lines Hua 893S, Hua 88S and Aipei 64S, obtained a diallel cross population, and found that using the population could locate the QTLs controlling the combining ability of main agronomic traits of rice. In the present study, a permanent F2population was first obtained using a three-line restorer Luhui 8258 which had high combining ability as core parent, and then diallel cross scheme was made by testcrossing three-line CMS lines with the individuals of the population, to analyze the combining ability of main agronomic traits of rice, aiming at providing reference for discovering and locating gene loci for high combining ability.

Materials and Methods

Materials

A populatio n of 140 recombinant inbred lines at F8generation were obtained after seven successive generations of self-pollination using single seed descent (SSD)method from the F2hybrids of three-line restorers Luhui 8258×Yanghui 34. NC Ⅱdesign was adopted to conduct diallel cross through crossing three different types of cyto-plasmic male sterile (CMS)lines (Gang 46A,Ⅱ-32A and Lu 98A)with the 140 inbred lines obtained above and their parents Luhui 8258 and Yanghui 34,then the seeds of 426 F1materials were obtained and tested later. Eight cultivars, which were bred from Luhui 8258 with high combining ability, had already been examined and approved by Provincial or National Crop Approval Committee until March of 2014(Table 1).

Table 1 Eight cultivars derived from the restorer line Luhui 8258

Methods

Randomized block design was adopted to conduct field experiments at Deyang Base of Rice and Sorghum Research Institute, Sichuan Academy of Agricultural Sciences and Suining Base of Dayin County, Suining City of Sichuan Province in 2012.Seeds were sown on March 25, and the seedlings were transplanted on May 15 at Deyang Base. At Suining Base seeds were sown on April 1, and the seedlings were transplanted on May 15.Three repetitions were set for each treatment. The rows were 33.3 cm long each and spaced 19.8 cm.The 21 plants for each material were planted in three rows, with 7 plants in each row,under conventional field management pest control.

Determination index

Three plants those grew well and were similar in size were selected from every plot at maturity to measure the yield per plant, effective panicle number per plant, seed setting rate, filled grain number per panicle and 1 000-grain weight of every material. In detail, the rice yield per plant was calculated by dividing the total seed weight of three plants by three. The effective panicle number per plant was calculated by dividing the number of panicles with more than five filled grains of three plants by three.Seed setting rate was equal to the total number of filled grains divided by the total number of glumous flowers. Filled grain number per panicle was equal to the total number of filled grains of three plants divided by the total number of effective panicles of the three plants. 1 000-grain weight was equal to the weight 1 500 filled grains (500 from each plant)multiplied by two thirds.

Analysis on combining ability

According to the studies of Liu et al.[13]and Mo[14],combining ability effect was estimated by the fixed model(model 1), and variance components and some related genetic parameters were estimated by the random model(model 2).

Normality test on 142 rice materials was performed using D’Agostino method[15]. Under the principle of D’Agostino test,the critical value of Y for a population of normal distribution increases as samples number increasing. According to the percentile points of statistic Y in D’Agostino test, the critical value of Y is -2.000 if n=150 and the test level is 0.05. Whether the combining ability of the five traits fit a normal distribution was judged depending on their Y values(Y≤-2.000).

Data analysis

Excel 2007 was used for dataprocessing.DPS 7.05 was adopted for combining ability analysis and normal distribution test.

Table 2 Variance analysis on five yield traits at Deyang and Suining bases

Table 3 Genotype variance of combining ability and heritability of the 5 traits

Results and Analysis

Variances of yield traits and combing ability

The variance analysis on the five yield traits of 426 combinations revealed that extremely significant differences in the yield traits existed among these combinations at Deyang and Suining bases, indicating that the combinations were genetically different(Table 2). Further study on the variances of yield traits of sterile lines,restorer lines and sterile lines× restorer lines at Deyang and Suining bases indicated that these traits were affected by additive and non-additive effects of genes based on their extremely significant difference.

Genotype variance and heritability of combining ability of yield traits

Except for yield per plant at Deyang Base, the genotype variance for the general combining ability of every yield trait ranged from 73.65% to 100%, which demonstrated that the additive effects of parental genes weredominant during the formation of F1hybrids(Table 3).Judging from genotype variance of general combing ability,the contribution rates of the male parent in effective panicle number per plant and filled grain number per panicle at two sites were higher than 50%,indicating that the two traits were mainly controlled by the male parent.However, the female parent’s genes contributed more than male parent’genes to the seed setting rate of F1hybrids, because the contribution rate of the female parent was above 50%at two sites.

Table 4 Normality test on general combining ability of five yield traits

The results also revealed that 1 000-grain weight was little affected by environmental factors and thus could be stably inherited to later generations,as its broad and narrow heritability were both higher than 70% at two sites. In contrary, the other traits with low heritability could be easily affected by environment(Table 3).

Distribution of general combining ability of yield traits

Based on the value and variation range of combining ability, the 142 materials were divided into 13 groups.As shown from Fig.1 to Fig.5, the maximum distribution frequency of general combing ability of the yield traits ranged from 20 to 29. Meanwhile, at Deyang Base the yield per plant had the largest combining ability,while at Suining Base the effective panicle number per plant had the largest combining ability, and their class mid-values were 1.44 and -1.98 respectively.Only 20.42%of all tested materials, namely 29 lines varied in this range.

The ratios of recombinant inbred lines with GCA positive effect at each site were calculated and described as follows.In detail,47.18% (67 lines)of the recombinant inbred lines at Deyang Base and 52.11%(74 lines)of the recombinant inbred lines at Suining Base showed GCA positive effect in yield per plant (Fig.1); 46.48% (66 lines) of the recombinant inbred lines at Deyang Base and 47.18%(67 lines)of the recombinant inbred lines at Suining Base showed GCA positive effect in panicle number per plant (Fig.2); 43.66% (62 lines) of the recombinant inbred lines at Deyang Base and 52.11% (74 lines) of the recombinant inbred lines at Suining Base showed GCA positive effect in panicle number per plant (Fig.3); 50.70% (72 lines) of the recombinant inbred lines at Deyang Base and 52.82%(75 lines)of the recombinant inbred lines at Suining Base showed GCA positive effect in filled grain number per plant (Fig.4);49.30% (70 lines) of the recombinant inbred lines at Deyang Base and 59.43% (78 lines) of the recombinant inbred lines at Suining Base showed GCA positive effect in 1 000-grain weight(Fig.5).

Normality test on the general combining ability of five yield traits in permanent F2 population

As shown in Table 4, Y values of the five traits was between-4.896 and-3.691 at two sites, lower than the critical value-2.000,indicating that the general combining ability of five traits had a normal distribution in permanent F2population.

Correlation coefficient of combining ability of yield traits

The correlation coefficient of the general combining ability of the five traits at the two bases ranged from-0.65 to 0.46 (Table 5). Twelve pairs of the traits at the two bases were positively correlated in general combining ability, among which, four pairs at Deyang and five pairs at Suining shared extremely significant positive correlation. However, the general combining ability had negative correlations in eight pairs of the traits at the two bases, among which four pairs at Deyang and two pairs at Suining had extremely significant negative correlation.

Table 5 Correlation coefficient among the general combining ability of five traits

Conclusion and Discussion

In 1980s,Zhou et al.[18]and Huang et al.[19]started breeding based on the theory of combining ability which was proposed by Sprague[16]and Griffing[17].In the present paper, the recombinant inbred lines were bred from Luhui 8258 and Yanghui 34 at Deyang and Suining bases to analyze the combining ability of rice. The results proved that significant or very significant correlation existed among the variances of general and specific combining abilities of five yield traits, revealing that the five traits were controlled by both additive and non-additive effects of genes. Meanwhile, the general combining ability variance of every parental trait was larger than their specific combining ability variance, indicating that their additive effects, not non-additive effects played a dominant role during the trait formation in their later generations.

In general, broad heritability reflects the effects of genetic variation and environment change.In this study,the broad and narrow heritability of four yield traits were below 50%, except that the broad and narrow heritability of 1 000-grain weight were both higher than 70%at two sites,suggesting all the traits except 1 000-grain weight, were vulnerable to environment impact, which agreed well with the findings of Jiang’s et al[6].

Correlation analysis proved that the combining abilities of yield traits(except 1 000-grain weight) and yield had an extremely significant positive correlation,with correlation coefficients ranging from 0.28 to 0.46. The results suggested that selecting parental materials with suitable traits is critical tobreeding target combinations with high combining ability.

Along with the development of molecular biotechnology, much progress has been made in locating the genes controlling target traits of crops by using molecular makers. In recent years, the combing ability of crops was even considered as a trait in some studies[11-12].This study proved that the general combining ability of five yield traits showed a normal distribution in recombinant inbred lines,right in line with inheritance of quantitative traits. So, the genes controlling rice general combining ability can be targeted by QTL mapping.

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