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Front Genet
2016 Jan 01;7:16. doi: 10.3389/fgene.2016.00016.
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Robustness and Accuracy in Sea Urchin Developmental Gene Regulatory Networks.
Ben-Tabou de-Leon S
.
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Developmental gene regulatory networks robustly control the timely activation of regulatory and differentiation genes. The structure of these networks underlies their capacity to buffer intrinsic and extrinsic noise and maintain embryonic morphology. Here I illustrate how the use of specific architectures by the sea urchin developmental regulatory networks enables the robust control of cell fate decisions. The Wnt-βcatenin signaling pathway patterns the primary embryonic axis while the BMP signaling pathway patterns the secondary embryonic axis in the sea urchin embryo and across bilateria. Interestingly, in the sea urchin in both cases, the signaling pathway that defines the axis controls directly the expression of a set of downstream regulatory genes. I propose that this direct activation of a set of regulatory genes enables a uniform regulatory response and a clear cut cell fate decision in the endoderm and in the dorsal ectoderm. The specification of the mesodermal pigment cell lineage is activated by Delta signaling that initiates a triple positive feedback loop that locks down the pigment specification state. I propose that the use of compound positive feedback circuitry provides the endodermal cells enough time to turn off mesodermal genes and ensures correct mesoderm vs. endoderm fate decision. Thus, I argue that understanding the control properties of repeatedly used regulatory architectures illuminates their role in embryogenesis and provides possible explanations to their resistance to evolutionary change.
Figure 1. Sea urchin embryonic development and endoderm specification. Developmental time is described in hours post fertilization according the developmental rate of the purple sea urchin, S. purpuratus. (A) Sea urchin endomesoderm cell lineage diagram. Color key is described in the figure. (B) βcatenin nuclearization pattern, dark green indicates high concentration, light green low. (C) Spatio-temporal expression profiles of endodermal control genes. (D) Partial endodermal GRN model depicting Tcf/βcatenin-Tcf/Groucho switch and regulatory interactions within the endodermal genes. (E) Spatio-temporal expression of the Delta ligand. (F) spatio-temporal expression of non-skeletogenic mesodermal genes. (G) GRN model of the triple positive feedback loop that Delta reception activates in the non-skeletogenic cells.
Figure 2. Sea urchin dorsal-ventral patterning. (A) Sea urchin lineage diagram showing ventral (yellow) and dorsal (light green) ectoderm. (B) Partial model on Dosrsal-Ventral patterning in the sea urchin depicting key regulatory processes in the ectoderm.
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