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Figure 3—figure supplement 1. (A) Direct visualization of long-range spread of Nodal ligand was carried out using a modified sensor cell assay in host embryos producing Vg1-HaloTag and Squint in the YSL. Sensor cells from a wild-type donor embryo injected with 110 pg oep mRNA were transplanted to the animal pole of host embryos injected with vg1-halotag and squint mRNAs in the YSL. Sensor cells producing extra Oep were used to enhance the sensitivity of the assay, akin to previous morphotrap studies (Stapornwongkul et al., 2020; Almuedo-Castillo et al., 2018; Harmansa et al., 2017; Harmansa et al., 2015). The experiment compared wild-type hosts (left), MZoep hosts (middle), or MZoep hosts injected with 110 pg oep mRNA at the one-cell stage (right). HALO-tagged ligands were labeled by staining with Janelia Fluor 646 Halo ligand. Sensor cells were transplanted to the animal pole of labeled host embryos after 1 hr of staining (approximately sphere stage). Loss of oep results in increased accumulation of tagged ligand at the animal pole. (B) Quantification of ligand accumulation in sensor cells. Sensors were manually segmented and average Janelia Fluor 646 channel pixel intensities were calculated. Each point represents a single sensor cell. Violin plots depict the median (white circle) and 25–75 percentile ranges (grey line). Asterisks denote statistical significance of median comparisons between indicated samples (Wilcoxon rank-sum test).