library_construction_protocol: Total RNA was extracted with TRIZOL reagent (Life Technologies, Cat. No. 15596), followed by on column extraction with the Aurum Total RNA mini kit (Life Science Research – Bio-Rad Cat, No. 732-6820). Concentrations were measured by the NanoDrop technology (Thermo Fisher Scientific) and adjusted to 300 ng/ml. RNA quality was assessed by the Bioanalyzer RNA 6000 Nano assay (Agilent, Cat. No. 5067-1511) and found to be highly intact for all samples (RNA Integrity Number of 9.9 to 10). Library preparation was by the VIB Nucleomics Core facility (Leuven, Belgium), using the Illumina TruSeq stranded mRNA prep kit (Illumina Inc., San Diego, CA, USA). Samples were analyzed by the Illumina NextSeq 500 High 75 Sequencing System, yielding single-end 75-bp reads.