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Figure 6


Fig. 6. Evolutionary modifications in echinoderm skeletogenesis. Only embryonic and larval stages are illustrated. Green cells indicate non-skeletogenic mesoderm, red cells indicate skeletogenic mesoderm, and heavy black lines represent skeletal rods. (A) The ancestral echinoderm exhibited indirect development and had an adult skeleton. The embryo had an ancestral program of mesoderm specification but lacked a skeleton. (B) In the ancestral echinoid, the adult program of biomineralization was imported into the late embryo. This pattern of skeletogenesis is still seen in modern cidaroid sea urchins. (C) In modern euechinoids, a second heterochronic change occurred, shifting the skeletogenic program into the early embryo. This change was associated with the invention of micromeres and an early-ingressing, skeletogenic mesenchyme (PMCs). It required the establishment of new regulatory links between the ancestral skeletogenic GRN and an even more ancient system of early patterning mediated by β-catenin. It was also associated with the invention of a PMC-derived signal that suppresses the skeletogenic potential of NSM cells (white arrows).





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