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

A 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.

We 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.

The 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.