Identification of main effects, epistatic effects and their environmental interactions of QTLs for yield traits in rice

Drought Yield QTL (qDTY) with Consistent Effects on Morphological and Agronomical Traits of Two Populations of New Rice (Oryza sativa) Lines

Drought has been a major limiting factor for rice production. Drought yield QTLs (qDTYs; QTLs = quantitative trait loci) were pyramided into MRQ74 and MR219 to produce drought tolerant lines. In this study, new drought tolerant MRQ74 and MR219 pyramided lines (PLs) were evaluated under drought stress (RS) and non-stress (NS) conditions to evaluate the effects of different qDTYs combinations on morphological and agronomical traits. MRQ74 PLs having qDTY12.1 possessed the best root length (RL) under both RS and NS but the effect was only significant for MR219 PLs under RS. Some qDTYs combinations also found to have consistent effect on the same trait of both populations. PLs with only qDTY12.1 showed the highest grain yield (GY) under RS in both populations which means qDTY12.1 controlled RL and caused higher GY under drought condition. The interaction of major-effect qDTY12.1 with qDTY2.2 also shows significant effect on leaf rolling (LR) of both PL populations. These qDTYs proved to be beneficial in improving traits related to drought tolerance. Selected PLs with qDTY12.1 combinations also found to have better RL and root weight (RW) under RS. Improvement of morphological and agronomical traits led to higher GY of PLs. Therefore, qDTY12.1 either is present singly or in combination with other qDTYs was the best qDTY due to its consistent effect on morphological and agronomical traits and GY across populations under RS and NS.

Intervention of molecular breeding in water saving rice production system: aerobic rice

The aerobic rice system/methods developed to tackle shortage of water, is a sustainable method to enhance rice productivity. Approximately 50% of irrigation water could be saved using this system in contrast to lowland rice cultivation. The crop can be directly seeded or transplanted in dry soil in this system rather than irrigated system of rice production. Here in this review we had tried to present all the important development made in regards to aerobic rice. Many QTLs responsible for aerobic traits in rice that have been mapped already are enlisted here. Brief comparisons of aerobic rice and conventional rice, further improvements made in aerobic rice have also been discussed.

Cruz MT, Kumar A. Association Mapping of Yield and Yield-related Traits Under Reproductive Stage Drought Stress in Rice (Oryza sativa L.)

BACKGROUND

The identification and introgression of major-effect QTLs for grain yield under drought are some of the best and well-proven approaches for improving the drought tolerance of rice varieties. In the present study, we characterized Malaysian rice germplasm for yield and yield-related traits and identified significant trait marker associations by structured association mapping.

RESULTS

The drought screening was successful in screening germplasm with a yield reduction of up to 60% and heritability for grain yield under drought was up to 78%. There was a wider phenotypic and molecular diversity within the panel, indicating the suitability of the population for quantitative trait loci (QTL) mapping. Structure analyses clearly grouped the accessions into three subgroups with admixtures. Linkage disequilibrium (LD) analysis revealed that LD decreased with an increase in distance between marker pairs and the LD decay varied from 5-20 cM. The Mixed Linear model-based structured association mapping identified 80 marker trait associations (MTA) for grain yield (GY), plant height (PH) and days to flowering (DTF). Seven MTA were identified for GY under drought stress, four of these MTA were consistently identified in at least two of the three analyses. Most of these MTA identified were on chromosomes 2, 5, 10, 11 and 12, and their phenotypic variance (PV) varied from 5% to 19%. The in silico analysis of drought QTL regions revealed the association of several drought-responsive genes conferring drought tolerance. The major-effect QTLs are useful in marker-assisted QTL pyramiding to improve drought tolerance.

CONCLUSION

The results have clearly shown that structured association mapping is one of the feasible options to identify major-effect QTLs for drought tolerance-related traits in rice.

Association Mapping of Yield and Yield-related Traits Under Reproductive Stage Drought Stress in Rice (Oryza sativa L.)

BACKGROUND

The identification and introgression of major-effect QTLs for grain yield under drought are some of the best and well-proven approaches for improving the drought tolerance of rice varieties. In the present study, we characterized Malaysian rice germplasm for yield and yield-related traits and identified significant trait marker associations by structured association mapping.

RESULTS

The drought screening was successful in screening germplasm with a yield reduction of up to 60% and heritability for grain yield under drought was up to 78%. There was a wider phenotypic and molecular diversity within the panel, indicating the suitability of the population for quantitative trait loci (QTL) mapping. Structure analyses clearly grouped the accessions into three subgroups with admixtures. Linkage disequilibrium (LD) analysis revealed that LD decreased with an increase in distance between marker pairs and the LD decay varied from 5-20 cM. The Mixed Linear model-based structured association mapping identified 80 marker trait associations (MTA) for grain yield (GY), plant height (PH) and days to flowering (DTF). Seven MTA were identified for GY under drought stress, four of these MTA were consistently identified in at least two of the three analyses. Most of these MTA identified were on chromosomes 2, 5, 10, 11 and 12, and their phenotypic variance (PV) varied from 5% to 19%. The in silico analysis of drought QTL regions revealed the association of several drought-responsive genes conferring drought tolerance. The major-effect QTLs are useful in marker-assisted QTL pyramiding to improve drought tolerance.

CONCLUSION

The results have clearly shown that structured association mapping is one of the feasible options to identify major-effect QTLs for drought tolerance-related traits in rice.

Marker assisted pyramiding of drought yield QTLs into a popular Malaysian rice cultivar, MR219

Background

Three drought yield QTLs, qDTY_2.2, qDTY_3.1, and qDTY_12.1 with consistent effect on grain yield under reproductive stage drought stress were pyramided through marker assisted breeding with the objective of improving the grain yield of the elite Malaysian rice cultivar MR219 under reproductive stage drought stress. Foreground selection using QTL specific markers, recombinant selection using flanking markers, and background selection were performed. BC₁F₃-derived lines with different combinations of qDTY_2.2, qDTY_3.1, and qDTY_12.1 were evaluated under both reproductive stage drought stress and non-stress during the dry seasons of 2013 and 2014 at IRRI.

Results

The grain yield reductions in the stress trials compared to non-stress trials ranged from 79 to 93 %. In the stress trials, delay in days to flowering and reduction in plant height were observed. In both seasons, MR219 did not produce any yield under stress, however it produced a yield of 5917 kg ha⁻¹ during the 2013 dry season and 8319 kg ha⁻¹ during the 2014 dry season under non-stress. Selected introgressed lines gave a yield advantage of 903 to 2500 kg ha⁻¹ over MR219 under reproductive stage drought stress and a yield of more than 6900 kg ha⁻¹ under non-stress during the 2014 dry season. Among lines with single qDTY, lines carrying qDTY_2.2 provided a higher yield advantage under reproductive stage drought stress in the MR219 background. Two-qDTY combinations (qDTY_3.1+qDTY_2.2 and qDTY_3.1+qDTY_12.1) performed better than lines with three qDTY combinations, indicating the absence of positive interactions between the three qDTYs.

Conclusion

We successfully developed drought-tolerant MR219 pyramided lines with a yield advantage of more than 1500 kg ha⁻¹. Differential yield advantages of different combinations of the qDTYs indicate a differential synergistic relationship among qDTYs. This is the first report on the successful effect of qDTYs in increasing the yield under drought in genetic backgrounds other than those in which the qDTYs were earlier identified.

Detection and characterization of epistasis between QTLs on plant height in rice using single segment substitution lines

Hua-jing-xian 74 and its 12 single segment substitution lines (SSSLs) in rice were used as crossing parents to construct a half diallel crossing population. A total number of 91 materials were grown under three planting densities. By analysis of average plant height (PH) over all environments 10 SSSLs were detected with significant additives and 6 SSSLs with significant dominances. These SSSLs were further tested under different densities respectively, indicating that some of single locus effects were sensitive to densities and the conditions under the density of 16.7 cm × 16.7 cm maybe inhibited the expressing of these PH QTLs. Qualitative and quantitative analyses of each four participating genotypes indicated that digenic interactions among these QTLs were prevalent. Of 66 tested interactions, about 42.4% were epistatic (P < 5%). Although some QTLs hadn't single locus effects, they were possible to form digenic interactions. A significant finding was that the detected epistases were mostly negative. Additionally, these epistases were also found being sensitive to planting densities, the conditions under the density of 10 cm × 16.7 cm perhaps promoted the expressing of epistatic interactions among PH QTLs.

Comparative Genetics of Seed Size Traits in Divergent Cereal Lineages Represented by Sorghum (Panicoidae) and Rice (Oryzoidae)

Seed size is closely related to fitness of wild plants, and its modification has been a key recurring element in domestication of seed/grain crops. In sorghum, a genomic and morphological model for panicoid cereals, a rich history of research into the genetics of seed size is reflected by a total of 13 likelihood intervals determined by conventional QTL (linkage) mapping in 11 nonoverlapping regions of the genome. To complement QTL data and investigate whether the discovery of seed size QTL is approaching “saturation,” we compared QTL data to GWAS for seed mass, seed length, and seed width studied in 354 accessions from a sorghum association panel (SAP) that have been genotyped at 265,487 SNPs. We identified nine independent GWAS-based “hotspots” for seed size associations. Targeted resequencing near four association peaks with the most notable linkage disequilibrium provides further support of the role(s) of these regions in the genetic control of sorghum seed size and identifies two candidate causal variants with nonsynonymous mutations. Of nine GWAS hotspots in sorghum, seven have significant correspondence with rice QTL intervals and known genes for components of seed size on orthologous chromosomes. Identifying intersections between positional and association genetic data are a potentially powerful means to mitigate constraints associated with each approach, and nonrandom correspondence of sorghum (panicoid) GWAS signals to rice (oryzoid) QTL adds a new dimension to the ability to leverage genetic data about this important trait across divergent plants.

A comparison of two methods to estimate additive-by-additive interaction of QTL effects by a simulation study

Additive-by-additive epistasis plays an important role in the genetic architecture of complex traits. The parameter connected with the additive-by-additive interaction can influence decisions concerning usefulness of the breeding material for the generation of new genotypes with characteristics improved over the parental forms. This study presents comparisons of two estimation methods of additive-by-additive interactions of QTL effects by the Monte Carlo simulation study. In the first method we assume that we observed only the plant phenotype, while in the second method we have additional information from the molecular marker observations. The obtained results show that the additive-by-additive interaction effect calculated on the basis of the marker observations is always smaller than the total additive-by-additive interaction effect obtained from phenotypic observations only. The lack of influence of the distance between markers and the number of linkage groups on the estimation of effects of additive-by-additive epistasis interaction genes by the two methods shows that both these methods may be used for different genetic maps and for different plant species.

Molecular evaluation of genetic diversity and association studies in rice (Oryza sativa L.)

In the present study, we tested rice genotypes that included un(der)exploited landraces of Tamil Nadu along with indica and japonica test cultivars to ascertain their genetic diversity structure. Highly polymorphic microsatellite markers were used for generating marker segregation data. A novel measure, allele discrimination index, was used to determine subpopulation differentiation power of each marker. Phenotypic data were collected for yield and component traits. Pattern of molecular differentiation separated indica and japonica genotypes; indica genotypes had two subpopulations within. Landraces were found to have indica genome, but formed a separate subgroup with low linkage disequilibrium. The landraces further separated into distinct group in both hierarchical clustering analysis using neighbour-joining method as well as in the model based population structure analysis. Japonica and the remaining indica cultivars formed two other distinct groups. Linkage disequilibrium observed in the whole population was considerably reduced in subpopulations. Low linkage disequilibrium of landforms suggests their narrow adaptation in local geographical niche. Many population specific alleles could be identified particularly for japonica cultivars and landraces. Association analysis revealed nine marker-trait associations with three agronomic traits, of which 67% were previously reported. Although the testing landraces together with known cultivars had permitted genomewide association mapping, the experiment offers scope to study more landraces collected from the entire geographical region for drawing more reliable information.