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Perry, KJ, Lyons DC, Truchado-Garcia M, Fischer AHL, Helfrich LW, Johansson KB, Diamond JC, Grande C, Henry JQ.  2015.  Deployment of regulatory genes during gastrulation and germ layer specification in a model spiralian mollusc Crepidula. Developmental Dynamics. 244:1215-1248.   10.1002/dvdy.24308   AbstractWebsite

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.: 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. : 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. Developmental Dynamics 244:1215-1248, 2015. (c) 2015 Wiley Periodicals, Inc.

Lyons, DC, Henry JQ.  2014.  Ins and outs of Spiralian gastrulation. International Journal of Developmental Biology. 58:413-428.   10.1387/ijdb.140151dl   AbstractWebsite

Gastrulation is a critical stage of metazoan development during which endodermal and mesodermal tissues are internalized, and morphogenesis transforms the early embryo into each animal's unique body-plan. While gastrulation has been studied extensively in classic model systems such as flies, worms, and vertebrates, less is known about gastrulation at a mechanistic level in other taxa. Surprisingly, one particularly neglected group constitutes a major branch of animals: the Spiralia. A unique feature of spiralian development is that taxa with diverse adult body-plans, such as annelids, molluscs, nemerteans and platyhelminths all share a highly stereotyped suite of characters during embryogenesis called spiral cleavage.The spiral cleavage program makes it possible to compare distantly related embryos using not only morphological features, and gene expression patterns, but also homologous cell lineages. Having all three criteria available for comparison is especially critical for understanding the evolution of a complex process like gastrulation.Thus studying gastrulation in spiralians is likely to lead to novel insights about the evolution of body-plans, and the evolution of morphogenesis itself. Here we review relevant literature about gastrulation in spiralians and frame questions for future studies. We describe the internalization of the endoderm, endomesoderm and ectomesoderm; where known, we review data on the cellular and molecular control of those processes. We also discuss several morphogenetic events that are tied to gastrulation including: axial elongation, origins of the mouth and anus, and the fate of the blastopore. Since spiral cleavage is ancestral for a major branch of bilaterians, understanding gastrulation in spiralians will contribute more broadly to ongoing debates about animal body-plan divergence, such as: the origin of the through-gut, the emergence of indirect versus direct development, and the evolution of gene-regulatory networks that specify endomesoderm. We emphasize the fact that spiralian gastrulation provides the unique opportunity to connect well-defined embryonic cell lineages to variation in cell fate and cell behavior, making it an exceptional case study for evo-devo.