The F2 backcross population was generated from a cross between an Eucalyptus grandis × E. urophylla F1 interspecific hybrid parent (GUSAP1, Sappi Forest Research, South Africa) and an unrelated E. grandis parent (GSAP2). At 3 years old, immature xylem tissue was harvested from 127 individuals. Total RNA was isolated and used for RNA-Seq expression profiling (30 million; Illumina PE50, BGI Hong Kong).