Evolution of the micromere gene network in sand dollars

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It is considered that, in echinoids, indirect development is the ancestral mode, and direct development has evolved independently in several lineages. To examine modifications of molecular mechanisms that drive the evolution of direct development, I analyzed expression patterns of the micromere specification genes in two sand dollars with distinct developmental modes. Between the two species, early expression patterns of the regulatory genes were largely conserved. After the mid blastula stage, however, a unique domain emerged in the direct developer, where either delta or hesC is not expressed. Since this region gives rise to mesenchyme cells which will form coelomic compartments, this modification may promote formation of adult structures in the direct developer.