A Study on Combining Ability and Heritability of Agronomic Trait of Indica Hybrid Rice

PENG Jian, ZHU Yi-xiang, ZHONG Xu-cheng, ZHOU Xiao-ping, TANG Xiao-mei, LIU Jun, YU Jiang-hui

1. Hunan Academy of Agricultural and Forest science, Changde 415000, PRC;

2. Key Laboratory of Agro-ecological Processes in Subtropical Region, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, PRC

Abstract In present study, under North Carolina design II, 5 sterile lines and 2 restorer lines were analyzed by crossing test so as to study the combining ability, habitability and correlation of each major agronomic characters. The results showed that plant height, panicle length, filled grains per plant, grains per plant, 1 000-grain weight, yield per day, yield per plant by parent gene additive effect, seed set of parent gene additive and the additive effect work together, additive effect played a relatively important role, effective tillers per plant by additive genes of the parents and the additive effect had more consistent effects on offspring traits. General combining ability effect analysis showed that the better parents were P1A and R102. The special combining ability of plant height and panicle length in hybrid rice combination was mainly correlated with the general combining ability of restorer line, whereas not with the general combining ability of parent. For the other 7 traits, if the general combining ability of both parents was higher, the special combining ability of hybrid F1 combinations was also higher. The best combination of special combining ability of 9 traits in hybrid rice was P2A/R102, followed by P1A/R102.

Key words Hybrid rice; Heterosis; Combining ability; Heritability

1. Introduction

Rice is one of the three most important food crops in our country, and more than 65% of the population in the country feeds on rice as the staple food, causing high consumption demand. However, due to the large population, the shortage of land resources, and the long-term uncontrollable trend of population increase and land decrease in our country, increasing rice yield is the basic way to ensure our country’s food security[1]. One of the most important reasons for the significant increase in rice yield is the wide application of heterosis[2-6]. The key to selection of the hybrid rice strong advantage combination is to cultivate the sterile line and restorer line, because both fertility stability and combining ability of sterile line and restoring ability and combining ability of restorer line greatly affect the advantage of heterosis combination[7]. The exertion of heterosis is affected by the combining ability of the parents, and the combining ability of yield and related traits is an important indicator for screening a combination of strong heterosis. Therefore, it is of great significance to explore the combining ability and heritability of hybrid rice parents[8]. Studies have shown that the optimal hybrid rice combination can be screened out by analyzing the combining ability of different lines or varieties[9]; the contribution rate of male and female traits to hybrid F1traits can be determined through genetic analysis[2,10]. For the controversial genetic basis of heterosis, and different conclusions are obtained in different genetic designs and different crop groups[11]. Some studies believe that yield per plant of hybrid rice and its constituent factors are affected by both additive and non-additive effects of genes, with additive effects being the mainstay, while the additive effects of restorer line have higher contribution rate for its yield per plant and effective tillers per plant, total grains per ear, seed set and single panicle weight[8]; some studies also showed that the inheritance of height, effective ear, main panicle length, total grains per ear, seed set, 1 000-grain weight and yield per plant of hybrid rice plant is affected by the parental gene additive and dominant effects, and the gene additive effect plays a major role, and general combining ability of restorer line plays a decisive role for effective panicle and yield per plant in addition to the special combining ability[12]; MA G Het al.[13], however, showed that the genetic variation of effective tillers per plant is mainly affected by the non-additive effects of genes, and the genetic variation of yield per plant, total grains per ear, and filled grains per ear, 1 000-grain weight, panicle length and plant height are mainly affected by gene additive effects. In this study, the general combining ability, special combining ability and heritability of yield and related traits of hybrid F1of 5 sterile lines and 2 restorer lines of indica rice were studied to evaluate the utility value of sterile line and restorer line, screen out the combination of strong heterosis, and provide a theoretical basis for parent breeding and combination selection.

2. Materials and Methods

2.1. Test materials

Choose 5 sterile lines: P1A, P2A, P3A, P4A, Quan 9311A, and 2 restorer lines R101, R102; among them, Quan 9311A was provided by Sichuan Quanyin Seed Industry Co., Ltd., and other materials were all newly selected parents of this research group; the 10 hybrid rice combinations prepared were all one-season late rice, with C two optimal 343 as the control (CK).

2.2. Experimental design and analysis methods

The above-mentioned parents were selected according to the p×q incomplete diallel cross design, and a total of 10 hybrid rice combinations were prepared and planted in the experimental field of the Rice Institute of Changde Academy of Agriculture and Forestry Sciences in 2019. The experiment adopted a random block design, the plant was sowed on May 25th, and transplanted on June 20th, with planting row spacing of 16.5 cm × 26.4 cm, 2 seedlings per hole and 340 pockets per combination. At the maturity stage, 20 plants were selected from each plot to determine the number of effective ears, and 3 plants that had grown uniformly for the test were selected. Plant height, panicle length, panicle number per plant, filled grains per plant, grains per plant, 1 000-grain weight, yield per plant were checked. The beginning and maturity periods were recorded.

Seed set (%) = Filled grains per plant/Grains per plant×100%

Yield per day (kg) = Theoretical yield per mu/Full growth period

DPS v14.1 software was used to calculate the combining ability parameters of data, and further calculate the special combining ability variance of each parent. Combining ability adopted genetic variance, general heritability and narrow heritability parameters.

3. Results and Analysis

3.1. The phenotype value and combining ability analysis of the main agronomic traits

It can be seen from Table 1 that the differences between the 9 agronomic traits did not reach a significant level, indicating that the phenotypes of these traits were relatively stable. Further analysis of combining ability showed that plant height, average panicle length, effective tillers per plant, grains per plant, seed set, and 1 000-grain weight between combinations reached extremely significant levels (P<0.01), indicating the obvious genetic differences of these traits; the general combining ability (GCA) and plant height between the restorer lines reached extremely significant differences, and the panicle length, filled grains per plant and grains per plant reached significant differences (P<0.05), and the effective panicle and seed set of the special combining ability (SCA) of the set also reached a significant level, indicating that the heterosis of plant height and panicle length was controlled by the gene additive effect of the male parent, and the heterosis of the effective tillers per plant and seed set was controlled by the combined effect of non-additive and nonadditive effects of the gene; GCA between sterile lines had no significant difference in other traits except for the 1 000-grain weight that was extremely significant, indicating that 1 000-grain weight was controlled by the additive effect of the sterile line gene.

3.2. Analysis of population genetic parameters

It can be seen from Table 2 that the GCA gene variance Vg accounts for more than 80% of plant height, panicle length, filled grains per plant, grains per plant, 1 000-grain weight, yield per day, and yield per plant, indicating that among these traits, the additive effect of parent gene played a major role in offspring traits; while the seed set accounts for 59.97%, indicating that the additive effect and non-additive effect of parent gene worked together, and the additive effect played a relatively important role; effective tillers per plant accounted for 49.39%, indicating that the additive effect and non-additive effect of parent gene on offspring traits were more consistent. Among the traits of plant height, panicle length, effective tillers per plant, grains per plant, yield per day, yield per plant, the contribution to the variance of GCA genotype by restorer line (Vg2) was greater than that by sterile line (Vg1), indicating that restorer line played a major role in the formation of these traits of hybrid F1, especially for plant height, panicle length, and effective panicle traits. Therefore, the choice of hybrid rice was the combination of large ear, multiple panicle or tall stalk, and the choice of restorer line was the key. In the formation of seed set, 1 000-grain weight traits, sterile line (Vg1) genotype GCA had a greater contribution to the variance, and played a decisive role in the formation of F1. Therefore, hybrid rice selected the combination with a high seed set and 1 000-grain weight, and the choice of sterile line was the key.

Heritability reflects the ability of parents to inherit traits to offspring. From the variance of broad heritability (HB) and narrow heritability (HN) (Table 2), the heritability of HBand HNof plant height, panicle length, and 1 000-grain weight were all higherthan 80%, indicating the strong ability of parents to pass on to offspring; HBof effective tillers per plant and seed set are all more than 80%, indicating that genetic inheritance played a major role in hybrid combinations; HBand HNof filled grains, grains per plant, yield per day, yield per plant were all smaller, the variance of SCA gene was also smaller but the variance of GCA gene was larger, indicating that the stability of these 4 traits in offspring inheritance was poor, and the gene additive genetic effect was larger.

Table 1 Variance analysis of the phenotypic values and combining ability of main agronomic traits

Table 2 Genetic parameter estimation in major characters

3.3. Parent’s general combining ability effect analysis

It can be seen from Table 3 that the ability GCA between different traits of the same sterile line and different sterile lines of the same trait were both positive and negative, and their effect values were also quite different. Among them, except that grains per plant and 1 000-grain weight were the negative effects, the other 7 traits in P1A were all positive effects, and, especially, yield per day and yield per plant had higher effect values, indicating that P1A had better general combining ability; P2A’s plant height, effective ear, filled grains, and seed set were all positive effects, and the effect value was high, hence for the application of the sterile line, these four traits were mainly considered; P3A’s effective tillers per plant, grains per plant were all positive effect value, and it was the highest among the 5 sterile lines, indicating that the GCA of these two traits was better; the highest GCA positive effect of P4A was grains per plant, followed by panicle length and filled grains per plant; the highest GCA positive effect of Quan 9311A was 1 000-grain weight and seed set. Compared with the two restorer lines, the GCA of R102 was better, and both were positive effects, with better application prospects.

3.4. Analysis of special combining ability effects of main agronomic traits

The magnitude of the non-additive effect of the hybrid combination can be known from the special combining ability effect value, which provides a theoretical basis for the breeding or selection of excellent combinations. According to Table 4, the effect size of SCA was significantly different among different combinations and different traits, and different traits of the same combination and different combinations of the same parent showed different SCAs. From the perspective of parental GCA, the SCA of plant height and panicle length was mainly related to restorer line. If the parent GCA was larger, the combination can produce a larger SCA, such as the combination with R102 as the parent; if theGCA of the male parent was small, the SCA of the hybrid rice combination was not large, such as the combination with R101 as the male parent; while the GCA of the female parent had little correlation with the SCA of the hybrid combination. The other 7 traits were analyzed from the parental GCA combination. If the parental GCA was higher and the combination was M×H or H×H, the SCA of the hybrid F1was also higher; but the lower parental GCA can also produce high SCA, such as the effective panicle and yield per plant of Quan 9311A/R101, the yield per plant of P3A/R101 and P4A/R101; the GCA of both parents was a combination of H×L or M×L, and the changes of SCA with different combinations and different traits were more complicated, indicating no correlation between GCA and SCA. The best combination of SCA of 9 traits of hybrid rice was P2A/R102 (all positive effects), followed by P1A/R102, except for 1 000-grain weight, all others were positive effects; those with the positive yield advantage SCA were P1A/R102, Quan 9311A/R101, P4A/R101, P2A/R102, P3A/R101, respectively; it indicated that the yield traits of these combinations were greatly affected by non-additive effects of genes.

Table 3 Effects of general combining ability of agronomic characteristics in parents

Table 4 The SCA of the trait and the relationship between SCA and parental GCA

4. Discussion

The GCA is determined by the additive effect of the parental genes and can be stably passed on to the next generation; while the SCA is determined by the parental non-additive effect, without the hereditary. Therefore, the combining ability of agronomic trait of sterile line and restorer line of hybrid rice is studied to acknowledge the role of each trait of the parent material in the inheritance of the offspring, which has important guiding significance for the breeding of hybrid rice combination or the selection of parent material. The results of this study showed that the SCA of plant height and panicle length was mainly related to the restorer line, and a larger male parent GCA could produce a larger SCA combination; while for 7 traits of filled grains per plant, grains per plant, seed set, panicle per plant, 1 000-grain weight, yield per plant and yield per day, higher female parent and male parent GCA could produce a higher special combining ability for the hybrid F1; for other types of parental GCA, the changes in SCA of F1were more complicated, without correlation between GCA and SCA. This was not exactly the same as the previous research, but the previous research conclusions were not completely consistent. Different test results could be obtained from different materials and different test environments[4,7,13-15]. Although the results of research on the relationship between GCA and SCA were different, combining ability could reflect the prospects of germplasm resources in breeding applications[16]. In general, the changes in the GCA of hybrid rice parents were relatively complicated. The general combining ability of the same parent or different parental traits in different hybrid F1combinations was different, and the general combining ability of the parents could not completely determine the special combining ability of hybrid combination.

This study showed that the three traits of plant height, panicle length, and 1 000-grain weight had high broad and narrow heritability. The formation of plant height and panicle length was mainly controlled by restorer line, and the formation of 1 000-grain weight was mainly controlled by sterile line, hence, for the breeding of hybrid rice F1, the agronomic traits of its parents should be considered. The relatively high broad heritability of effective panicle and seed sets indicated that the choice of parents was important, however, the genetic contribution of male parent to effective tillers per plant was greater, and the genetic contribution of female parent to seed set was greater; the heritability of the other four traits was weaker and may be more affected by environmental variance. Therefore, in hybrid rice breeding, selecting parents with both higher general combining ability and higher special combining ability was the key to obtaining the combination of strong heterosis.