The Transcriptome Shock In An Interspecific F1 Triploid Hybrid Of Oryza And The Hybrid Weakness Of One Interspecific Hybrid Is Nitrogen-dependent

Interspecific hybridization often induces immediate and saltational changes in gene expression,a phenomenon collectively termed “transcriptome shock”.The extent and pattern of shock-induced expression changes are often highly idiosyncratic,and hence entails additional investigations.Hybrid weakness,a phenomenon opposite to heterosis,refers to inferior growth and development in a hybrid relative to its pure-line parents.Little attention has been paid to the phenomenological or mechanistic aspect of hybrid weakness,probably due to its rare occurrence.Here,we produced two sets of interspecific F1 triploid hybrid plants,Hybrid 1was obtained from crossing between Oryza sativa,ssp.japonica,cv.Nipponbare(2n= 2x = 24,genome AA)and the tetraploid form of O.punctata(2n=4x=48,genome BBCC),and Hybrid 2 was obtained from crossing between Oryza sativa,ssp.japonica,cv.Nipponbare(2n = 2x = 24,genome AA)and the tetraploid wild rice species,O.alta(2n=4x=48,genome CCDD).We conducted RNA-seq transcriptome profiling of the F1 Hybrid 1 and their exact parental plants.We analyzed both homeolog expression bias and overall gene expression level difference in the hybrids relative to the in silico “hybrids”(parental mixtures).We also investigated the phenotypic and physiological differences between the F1 Hybrid 2 and their parents under normal and nitrogen-limiting conditions.We quantified the expression levels of21 key genes involved in three important pathways and assayed expression partitioning of parental alleles for eight genes in the F1 Hybrid 2.We found that approximately 16%(2,541)of the 16,112 expressed genes in leaf tissue of the F1 Hybrid 1 showed nonadditive expression,which were specifically enriched in photosynthesis-related pathways.Interestingly,changes in the maternal homeolog expression,including non-stochastic silencing,were the major causes for altered homeolog expression partitioning in the F1 Hybrid 1.Our findings have provided further insights into the transcriptome response to interspecific hybridization and heterosis.In addition,we reported that the F1 Hybrid 2 showed weakness in several phenotypic traits.Nine of the 21 studied genes showed contrasted expression levels between hybrids and parents(MPVs)under normal vs.N-limiting conditions.Interestingly,under N-limiting conditions,the overtly enhanced partitioning of maternal allele expression in the hybrids for eight assayed genes echo their attenuatedhybrid weakness in phenotypes.Our observations suggest that both overall expression level and differential partitioning of parental alleles of critical genes contribute to condition-specific hybrid weakness.