Assessment of heterotic potential and combining ability of immortal restorer lines derived from an elite rice hybrid, KRH-2, for the development of superior rice hybrids

Present investigation was carried out to assess the heterotic potential and combining ability of immortal restorer lines [consisting of two recombinant inbred lines (RILs) and two doubled haploid lines (DHLs)] developed from an elite rice hybrid, KRH-2 by crossing them with three popular WA-CMS lines, IR58025A, CRMS32A and APMS6A through line × tester analysis. The doubled haploid line 1 (DHL-1) was observed to be a good general combiner for total grain yield per plant (YLD) and other yield component traits and among the CMS lines, IR58025A was observed to be the best combiner as it showed positive significant values for the traits viz., total grain yield per plant, panicle length and spikelet fertility. Higher preponderance of the variance associated with specific combining ability (SCA) as compared to general combining ability (GCA) variance was observed for most of the traits indicated the predominant role of non-additive gene action in the expression of the traits. Out of twelve novel crosses between the immortal restorer lines derived from KRH-2 and the WA-CMS lines, 66.66% (eight crosses) showed significant and desirable SCA effects for the traits viz., total grain yield per plant, days to fifty percent flowering, plant height, flag leaf length, flag leaf width, number of filled grains per panicle and spikelet fertility. Two crosses IR58025A/RIL-24 and CRMS32A/RIL-24 were observed to be the most promising cross combinations showing standard heterosis of >50% for YLD trait (as compared with KRH-2) with higher prevalence of GCA and SCA, respectively. Heterotic yield advantage of IR58025A/RIL-24 and CRMS32A/RIL-24 was 77.05% and 54.74%, respectively over KRH-2 and these can be utilized for developing commercial hybrids. The present study also indicates the potentiality of RILs in providing useful parental lines for developing heterotic hybrids which are hard to get from outside sources in the new intellectual property regime.


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yielding RIL-24, were observed to be heterotic than KRH-2 and these four restorers were 149 further selected for test crossing with other CMS lines.

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Test crossing of selected restorers with popular WA-CMS lines 152 In wet season of 2018, four selected parental lines viz., DHL-1 (D 7 ), DHL-2 (D 7 ), RIL-1 (F 8 ) 153 and RIL-24 (F 8 ) were test crossed with three popular CMS lines viz., IR58025A, CRMS32A 154 and APMS6A to assess the general combining ability (GCA) of parents and specific 155 combining ability (SCA) of crosses using line × tester mating design [18].  248 Genetic parameters such as variance due to general combining ability (GCA), specific 249 combining ability (SCA), GCA variance ratio, additive genetic variance, dominance genetic 250 variance, degree of dominance and broad sense-narrow-sense heritability were estimated 251 among the hybrids. It was observed that for all traits under study, the variance due to SCA 252 (σ 2 sca) was higher than GCA (σ 2 gca). Also, the dominance genetic variance (δ 2 D) was larger 253 than the additive genetic variance (δ 2 A) for all the traits. These results are supported with 254 GCA variance ratio (σ 2 gca / σ 2 sca) being less than 1 for all traits and degree of dominance  Table).  Table).The coefficient of variation (CV %) values were observed 267 to be <20% for most of the traits except NPT, TNGPP, NFGPP and YLD. The agro-268 morphological performance of the novel hybrids, parents along with standard hybrid checks 269 is presented in    Table). The details of heterosis of the cross 290 combinations with respect to YLD trait is presented in S7 Table. 291

Estimation of genetic variances and heritability in novel crosses
For NPT trait, novel hybrids from all crosses were observed to have more number of checks whose heterosis was in the range of 9.99% (SH over PA6444) to 31.87% (MPH).

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Combining Ability analysis 313 The Analysis of Variance of the parental lines and checks using L×T mating design is 314 presented in Table 2. The variances due to parents were significant for the following traits:    (S10 Table,  were observed to have a higher prevalence of crosses SCA.

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The hybrid derived from the cross, APMS6A/DHL-2 (S10 Table,  IR58025A/DHL-2 (S10 Table,    The primary objective of heterosis breeding programs in crops is to identify best performing 422 hybrids for commercialization along with identifying genetically diverse and better parental 423 lines which could be utilized for the development of promising hybrids in future crosses [32].

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Selection of best performing hybrids for yield and other desired characters relies on 425 undertaking trials in multiple environments followed by a rigorous statistical analysis [33].

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If the potential of a parental line to combine well in a particular cross combination is 554 observed then they are supposed to have a good SCA [33]. The SCA estimates of cross combinations and GCA effects of the parents are described