library_construction_protocol: Gradients were fractionated using a Biocomp Piston Gradient Fractionator. Samples were resuspended in Trizol and Drosophila S2 cells were added as an internal spike-in control. RNA was harvested using the Direct-zol RNA Miniprep Kit (Zymo Research) following manufacturer's protocols. RNA-seq libraries were prepared using the Illumina TruSeq RNA Sample Prep v2 Kit following manufacturer's protocols.