Mapping of Illumina reads to the P. trichocarpa genome. (A). Distribution of Illumina GAII RNA-Seq reads along annotated poplar annotated genomic features (P. trichocarpa v. 2.2). (B). Summary of the average expression level across 20 individuals. (High: FPKM>40, Medium: FPKM >5-40, Low: FPKM 0–5) (C). Expression level of exon, intergenic, intron. Mean read density values for exons, introns, and intergenic regions were computed in units of fragments per kilobase of exon/intron/intergenic (FPKM) model.