FGF and BMP pathway genes are expressed in segmentally iterated domains during segmentation of germ layers in a tardigrade

BackgroundA small number of signaling pathways regulates development in most animals, yet we do not know where these pathways are deployed in embryos of many animal phyla. Filling such gaps can contribute to understanding how diverse body shapes arise from differential deployment of conserved signaling pathways. Here, we examined where conserved pathways are deployed in tardigrades, a panarthropod phylum with miniaturized, segmented bodies. ResultsWe used in situ mRNA detection in the tardigrade Hypsibius exemplaris to reveal expression patterns of FGF and BMP signaling pathway components during body segmentation and early mesoderm development. Among the patterns examined, we found an FGF ligand and receptor expressed near each other in segmentally iterated regions of ectoderm and endomesoderm, respectively. We also found a BMP ligand and antagonist expressed in dorsoventrally-restricted patterns in the lateral ectoderm. ConclusionsThe detected patterns suggested specific hypotheses for further research: possible FGF signaling between ectoderm and endomesoderm, and possible roles of BMP signaling in dorsal-ventral patterning of lateral ectoderm. We compared our results with published expression patterns for FGF and BMP pathways across panarthropods, to contribute to previous hypotheses for how the development of segments and mesoderm may have evolved in the emergence of this clade of diversly-shaped animals. Bullet PointsO_LIDouble mRNA detection in tardigrades revealed expression patterns of FGF and BMP pathway genes C_LIO_LIfgf8 was detected primarily in ectoderm but absent from internal germ layers, and C_LIO_LIfgfrl1 was detected in all layers and enriched in internal layers C_LIO_LIdpp was detected more dorsally than sog in lateral ectoderm C_LIO_LIfgf8 and dpp gene expression exhibit different segmentally iterated domains in ectoderm C_LIO_LIResults suggest specific hypotheses for roles for FGF signaling and BMP signaling in tardigrade development C_LI