The transcriptional landscape of polyploid wheat
Résumé
The coordinated expression of highly related homoeologous genes in polyploid species underlies the phenotypes of many of the world's major crops. Here we combine extensive gene expression datasets to produce a comprehensive, genome-wide analysis of homoeolog expression patterns in hexaploid bread wheat. Bias in homoeolog expression varies between tissues, with ~30% of wheat homoeologs showing nonbalanced expression. We found expression asymmetries along wheat chromosomes, with homoeologs showing the largest inter-tissue, inter-cultivar, and coding sequence variation, most often located in high-recombination distal ends of chromosomes. These transcriptionally dynamic genes potentially represent the first steps toward neo- or subfunctionalization of wheat homoeologs. Coexpression networks reveal extensive coordination of homoeologs throughout development and, alongside a detailed expression atlas, provide a framework to target candidate genes underpinning agronomic traits in wheat.nonbalanced expression patterns, with higher or lower expression from a single homoeolog with respect to the other two. These differences between homoeologs were associated with epigenetic changes affecting DNA methylation and histone modifications. Although nonbalanced homoeolog expression could be partially predicted by expression in diploid ancestors, large changes in relative homoeolog expression were observed owing to polyploidization. Our results suggest that the transposable elements in promoters relate more closely to the variation in the relative expression of homoeologs across tissues than to a ubiquitous effect across all tissues. We found that homoeologs with the highest inter-tissue variation had promoters with more frequent transposable element insertions and more varied cis-regulatory elements than homoeologs that were stable across tissues. We also identified expression asymmetry along wheat chromosomes. Homoeologs with the largest inter-tissue, inter-cultivar, and coding sequence variation were most often located in the highly recombinogenic distal ends of chromosomes. These transcriptionally dynamic homoeologs are under more relaxed selection pressure, potentially representing the first steps toward functional innovation through neo- or subfunctionalization. We generated tissue- and stress-specific coexpression networks that reveal extensive coordination of homoeolog expression throughout development. These networks, alongside detailed gene expression atlases (www.wheat-expression.com and http://bar.utoronto.ca), lay the groundwork to identify candidate genes influencing agronomic traits in wheat. CONCLUSION This study provides detailed insights into the transcriptional landscape of bread wheat, an evolutionarily young polyploid. Our work shows that homoeolog expression patterns in bread wheat have been shaped by polyploidy and are associated with both epigenetic modifications and variation in transposable elements within promoters of homoeologous genes. The extensive datasets and analyses presented here provide a framework that can help researchers and breeders develop strategies to improve crops by manipulating individual or multiple homoeologs to modulate trait responses.