Analysis of Allelic Imbalance in Rice Hybrids Under Water Stress and Association of Asymmetrically Expressed Genes with Drought-Response QTLs.
BACKGROUND
Information on the effect of stress on the allele-specific expression (ASE) profile of rice hybrids is limited. More so, the association of allelically imbalanced genes to important traits is yet to be understood. Here we assessed allelic imbalance (AI) in the heterozygote state of rice under non- and water-stress treatments and determined association of asymmetrically expressed genes with grain yield (GY) under drought stress by in-silico co-localization analysis and selective genotyping. The genotypes IR64, Apo and their F1 hybrid (IR64 × Apo) were grown under normal and water-limiting conditions. We sequenced the total RNA transcripts for all genotypes then reconstructed the two chromosomes in the heterozygote.


RESULTS
We are able to estimate the transcript abundance of and the differential expression (DE) between the two parent-specific alleles in the rice hybrids. The magnitude and direction of AI are classified into two categories: (1) symmetrical or biallelic and (2) asymmetrical. The latter can be further classified as either IR64- or Apo-favoring gene. Analysis showed that in the hybrids grown under non-stress conditions, 179 and 183 favor Apo- and IR64-specific alleles, respectively. Hence, the number of IR64- and Apo-favoring genes is relatively equal. Under water-stress conditions, 179 and 255 favor Apo- and IR64-specific alleles, respectively, indicating that the number of allelically imbalanced genes is skewed towards IR64. This is nearly 40-60 % preference for Apo and IR64 alleles, respectively, to the hybrid transcriptome. We also observed genes which exhibit allele preference switching when exposed to water-stress conditions. Results of in-silico co-localization procedure and selective genotyping of Apo/IR64 F3:5 progenies revealed significant association of several asymmetrically expressed genes with GY under drought stress conditions.


CONCLUSIONS
Our data suggest that water stress skews AI on a genome-wide scale towards the IR64 allele, the cross-specific maternal allele. Several asymmetrically expressed genes are strongly associated with GY under drought stress which may shed hints that genes associated with important traits are allelically imbalanced. Our approach of integrating hybrid expression analysis and QTL mapping analysis may be an efficient strategy for shortlisting candidate genes for gene discovery.
Genetics and Biotechnology Division, International Rice Research Institute (IRRI), Los Baños, Laguna, Philippines. nelzo.ereful@niab.com.|The John Bingham Laboratory, National Institute of Agricultural Botany (NIAB), Huntingdon Road, Cambridge, CB3 0LE, UK. nelzo.ereful@niab.com.
Nelzo C
http://orcid.org/0000-0003-4460-743X
NC
Ereful
Department of Agronomy, National Taiwan University (NTU), Taipei City, 100, Taiwan.
Li-Yu
LY
Liu
Department of Agronomy, National Taiwan University (NTU), Taipei City, 100, Taiwan.
Eric
E
Tsai
Department of Agronomy, National Taiwan University (NTU), Taipei City, 100, Taiwan.
Shu-Min
SM
Kao
Genetics and Biotechnology Division, International Rice Research Institute (IRRI), Los Baños, Laguna, Philippines.
Shalabh
S
Dixit
Genetics and Biotechnology Division, International Rice Research Institute (IRRI), Los Baños, Laguna, Philippines.
Ramil
R
Mauleon
Genetics and Biotechnology Division, International Rice Research Institute (IRRI), Los Baños, Laguna, Philippines.|Crop Science Cluster, College of Agriculture, University of the Philippines, Los Baños, Laguna, 4031, Philippines.
Katrina
K
Malabanan
Genetics and Biotechnology Division, International Rice Research Institute (IRRI), Los Baños, Laguna, Philippines.|Texas A &M, Department of Soil and Crop Sciences 2474 TAMU, College Station, TX, 77843-2474, USA.
Michael
M
Thomson
Institute of Plant Breeding, University of the Philippines, Los Baños, Laguna, Philippines.
Antonio
A
Laurena
The John Bingham Laboratory, National Institute of Agricultural Botany (NIAB), Huntingdon Road, Cambridge, CB3 0LE, UK.
David
D
Lee
The John Bingham Laboratory, National Institute of Agricultural Botany (NIAB), Huntingdon Road, Cambridge, CB3 0LE, UK.
Ian
I
Mackay
The John Bingham Laboratory, National Institute of Agricultural Botany (NIAB), Huntingdon Road, Cambridge, CB3 0LE, UK.
Andy
A
Greenland
SRUC, Peter Wilson Building, West Mains Road, Edinburgh, EH9 3JG, UK.
Wayne
W
Powell
Genetics and Biotechnology Division, International Rice Research Institute (IRRI), Los Baños, Laguna, Philippines.
Hei
H
Leung
Allele-specific expression (ASE); Allelic imbalance (AI); Co-localization analysis; Drought; Quantitative trait loci (QTL); RNA-seq; Rice (Oryza sativa L.); Selective genotyping
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