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Title: Lessons from a transcription factor: Alx1 provides insights into gene regulatory networks, cellular reprogramming, and cell type evolution
The skeleton-forming cells of sea urchins and other echinoderms have been studied by developmental biologists as models of cell specification and morphogenesis for many decades. The gene regulatory network (GRN) deployed in the embryonic skeletogenic cells of euechinoid sea urchins is one of the best understood in any developing animal. Recent comparative studies have leveraged the information contained in this GRN, bringing renewed attention to the diverse patterns of skeletogenesis within the phylum and the evolutionary basis for this diversity. The homeodomain-containing transcription factor, Alx1, was originally shown to be a core component of the skeletogenic GRN of the sea urchin embryo. Alx1 has since been found to be key regulator of skeletal cell identity throughout the phylum. As such, Alx1 is currently serving as a lens through which multiple developmental processes are being investigated. These include not only GRN organization and evolution, but also cell reprogramming, cell type evolution, and the gene regulatory control of morphogenesis. This review summarizes our current state of knowledge concerning Alx1 and highlights the insights it is yielding into these important developmental and evolutionary processes.
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Current topics in developmental biology
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National Science Foundation
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  1. Abstract

    Understanding how changes in developmental gene expression alter morphogenesis is a fundamental problem in development and evolution. A promising approach to address this problem is to compare the developmental transcriptomes between related species. The echinoderm phylum consists of several model species that have significantly contributed to the understanding of gene regulation and evolution. Particularly, the regulatory networks of the sea star,Patiria miniata(P.miniata), have been extensively studied, however developmental transcriptomes for this species were lacking. Here we generated developmental transcriptomes ofP.miniataand compared these with those of two sea urchins species. We demonstrate that the conservation of gene expression depends on gene function, cell type and evolutionary distance. With increasing evolutionary distance the interspecies correlations in gene expression decreases. The reduction is more severe in the correlations between morphologically equivalent stages (diagonal elements) than in the correlation between morphologically distinct stages (off-diagonal elements). This could reflect a decrease in the morphological constraints compared to other constraints that shape gene expression at large evolutionary divergence. Within this trend, the interspecies correlations of developmental control genes maintain their diagonality at large evolutionary distance, and peak at the onset of gastrulation, supporting the hourglass model of phylotypic stage conservation.

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