Single-cell RNA-seq of tailbuds from control, Pbx1/Pbx2 compound and Pbx1/Pbx2 double-knockout E8.5/E9.0 embryos.

In vertebrates, body axis elongation is fuelled by bipotent neuromesodermal progenitors (NMPs), which support the development of both spinal cord and paraxial mesoderm (PM). NMPs reside in the caudal lateral epiblast and tailbud, from where they sustain axial elongation. HOX transcription factors have been historically implicated in axial elongation, with their sequential activation playing a fundamental role in timing PM development. PBX1 and PBX2 are obligate anterior HOX cofactors, and therefore they represent prominent candidates for controlling the distinct response to individual HOX factors. To pinpoint the role of PBX proteins in the development of pre-somitic mesoderm, single-cell RNA sequencing (scRNA-seq) was performed on cells isolated from the tailbuds of control, Pbx1/Pbx2 compound (Pbx1/2-com) and Pbx1/Pbx2 double-mutant (Pbx1/2-DKO) embryos at embryonic days 8.5 and 9.0. Single cells from at least three embryos per genotype and stage were FACS-sorted into 384-well capture plates, and scRNA-seq was performed using MARS-seq (Jaitin D.A. et al., Science, 343, 776-779 (2014)). As a spike-in internal control for batch-effect correction, 71 EpiSCs cultivated in vitro were sorted into each plate, together with 311 cells from embryonic tailbuds. Two wells were left empty in each plate, as a no-cell control during data analysis.