Heikes, K. L. | Search bioRxiv

bioRxiv2024

Expression patterns of FGF and BMP pathway genes in the tardigrade Hypsibius exemplaris

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

developmental biology↗

bioRxiv2023

The Embryonic Origin of Primordial Germ Cells in the Tardigrade Hypsibius exemplaris

Primordial germ cells (PGCs) give rise to gametes - cells necessary for the propagation and fertility of diverse organisms. Current understanding of PGC development is limited to the small number of organisms whose PGCs have been identified and studied. Expanding the field to include little-studied taxa and emerging model organisms is important to understand the full breadth of the evolution of PGC development. In the phylum Tardigrada, no early cell lineages have been identified to date using molecular markers. This includes the PGC lineage. Here, we describe PGC development in the model tardigrade Hypsibius exemplaris. The four earliest-internalizing cells (EICs) exhibit PGC-like behavior and nuclear morphology. The location of the EICs is enriched for mRNAs of conserved PGC markers wiwi1 (water bear piwi 1) and vasa. At early stages, both wiwi1 and vasa mRNAs are detectable uniformly in embryos, which suggests that these mRNAs do not serve as localized determinants for PGC specification. Only later are wiwi1 and vasa enriched in the EICs. Finally, we traced the cells that give rise to the four PGCs. Our results reveal the embryonic origin of the PGCs of H. exemplaris and provide the first molecular characterization of an early cell lineage in the tardigrade phylum. We anticipate that these observations will serve as a basis for characterizing the mechanisms of PGC development in this animal.

developmental biology↗