Background: During gastrulation, endoderm and mesoderm are specified from a bipotential precursor (endomesoderm) that is argued to be homologous across bilaterians. Spiralians also generate mesoderm from ectodermal precursors (ectomesoderm), which arises near the blastopore. While a conserved gene regulatory network controls specification of endomesoderm in deuterostomes and ecdysozoans, little is known about genes controlling specification or behavior of either source of spiralian mesoderm or the digestive tract. Results: Using the mollusc Crepidula, we examined conserved regulatory factors and compared their expression to fate maps to score expression in the germ layers, blastopore lip, and digestive tract. Many genes were expressed in both ecto-and endomesoderm, but only five were expressed in ectomesoderm exclusively. The latter may contribute to epithelial-to-mesenchymal transition seen in ectomesoderm. Conclusions: We present the first comparison of genes expressed during spiralian gastrulation in the context of high-resolution fate maps. We found variation of genes expressed in the blastopore lip, mouth, and cells that will form the anus. Shared expression of many genes in both mesodermal sources suggests that components of the conserved endomesoderm program were either co-opted for ectomesoderm formation or that ecto-and endomesoderm are derived from a common mesodermal precursor that became subdivided into distinct domains during evolution.
Since the discovery that the TGF-b signalling molecule Nodal and its downstream effector Pitx have a parallel role in establishing asymmetry between molluscs and deuterostomes the debate over the degree to which this signalling pathway is conserved across the Bilateria as a whole has been ongoing. Further taxon sampling is critical to understand the evolution and divergence of this signalling pathway in animals. Using genome and transcriptome mining we confirmed the presence of nodal and Pitx in a range of additional animal taxa for which their presence has not yet been described. In situ hybridization was used to show the embryonic expression of these genes in brachiopods and planarians. We show that both nodal and Pitx genes are broadly conserved across the Spiralia, and nodal likely appeared in the Bilaterian stem lineage after the divergence of the Acoelomorpha. Furthermore, both nodal and Pitx mRNA appears to be expressed in an asymmetric fashion in the brachiopod Terebratalia transversa. No evidence for the presence of a Lefty ortholog could be found in the non-deuterostome genomic resources examined. Nodal expression is asymmetric in a number of spiralian lineages, indicating a possible ancestral role of the Nodal/ Pitx cascade in the establishment of asymmetries across the Bilateria.
During cleavage, different cellular processes cause the zygote to become partitioned into a set of cells with a specific spatial arrangement. These processes include the orientation of cell division according to: an animal-vegetal gradient; the main axis (Hertwig's rule) of the cell; and the contact areas between cells or the perpendicularity between consecutive cell divisions (Sachs' rule). Cell adhesion and cortical rotation have also been proposed to be involved in spiral cleavage. We use a computational model of cell and tissue biomechanics to account for the different existing hypotheses about how the specific spatial arrangement of cells in spiral cleavage arises during development. Cell polarization by an animal-vegetal gradient, a bias to perpendicularity between consecutive cell divisions (Sachs' rule), cortical rotation and cell adhesion, when combined, reproduce the spiral cleavage, whereas other combinations of processes cannot. Specifically, cortical rotation is necessary at the 8-cell stage to direct all micromeres in the same direction. By varying the relative strength of these processes, we reproduce the spatial arrangement of cells in the blastulae of seven different invertebrate species.
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