To resolve these debates, it is necessary to compare gastrulation in multiple species, in the context of a solid phylogenetic framework. Debate remains about how this diversity in blastopore fate arose, whether the differences have any influence on body-plan divergence, and even if the blastopore can be properly homologized between distantly related animals. Depending on the species, the blastopore has been reported to become the mouth (protostomy), the anus (deuterostomy), both mouth and anus (amphistomy), or neither (i.e., the blastopore closes). Deuterostomy appears to be ancestral for the Ambulacaria and Chordata, but among the groups traditionally thought to belong to a protostome branch-the Ecdysozoa and Spiralia (which includes the Lophotrochozoa)-the fate of the blastopore is more complex and variable. Although the terms deuterostome and protostome are still in our vernacular, they have lost much of their phylogenetic significance because cladistic analyses based on molecular characters (which are independent of morphology) show that protostomy and deuterostomy are not a reliable diagnostic character for major clades of bilaterians. The fate of the blastopore was traditionally used as a character for building taxonomic trees, separating protostomes (in which the blastopore becomes the opening to the mouth) from deuterostomes (in which the mouth forms separately from the blastopore, and the latter often becomes the anus) (reviewed in ). , and references therein).įor example, several evolutionary theories concern the site of gastrulation, a transient embryonic location where endoderm and mesoderm are internalized, called the blastopore. Because the basic body-plan is often established by the end of gastrulation, theories explaining body-plan evolution often suggest that body-plan divergence was driven in part by modifications to gastrulation events themselves (e.g. Germ layer segregation occurs when ectodermal, endodermal, and mesodermal cells adopt distinct gene regulatory states morphogenetic events internalize endodermal and mesodermal cell types. Gastrulation accomplishes several tasks that are critical for metazoan development, most importantly the segregation of germ layers and re-arrangement of cells to form the basic body-plan. The anus forms days later, as a secondary opening within the 2d 2 clone, and not from the classically described “anal cells”, which we identify as the 3c 221 and 3d 221 cells.

Progeny of 2a-2c and 3a-3d make the mouth and foregut, and the blastopore becomes the opening to the mouth. During this process, several of these cells, as well as the 2d clone, become displaced posteriorly, away from the blastopore. Posteriorly, cells derived from 3c 2 and 3d 2 undergo a form of convergence and extension that involves zippering of cells and their intercalation across the ventral midline. These cells make a novel spiralian germ layer, the ectomesoderm. Anteriorly, cells derived from 3a 2 and 3b 2 undergo a unique epithelial-to-mesenchymal transition involving proliferation and a collective movement of cells into the archenteron. As the blastopore narrows, the micromeres’ progeny exhibit lineage-specific behaviors that result in certain sublineages leaving the lip’s edge. Initially, descendants of the second and third quartet micromeres (2a–2d, 3a–3d) occupy a portion of the blastopore lip. Crepidula gastrulation occurs by epiboly: the first through third quartet micromeres form an epithelial animal cap that expands to cover vegetal endomesodermal precursors.

Blastopore that becomes mouth