While gastrulation is unique to metazoans, the site at which it occurs is not uniform for all metazoans. Non-bilaterians and bilaterians diverged from an urbilaterian common ancestor, and with that divergence came a change in the site of gastrulation. In observed non-bilaterian taxa, such as Cnidaria and Ctenophora, endoderm specification and archenteron formation occur at the animal pole, allowing for gastrulation to occur here. On the other hand, bilaterian taxa experience this process at the vegetal pole. It is not only the difference in the site of gastrulation that is of interest, but it is that the site of gastrulation is completely opposite in polarity. Bilaterian organisms have evolved to develop extremely complex functions, and it is believed that the switch in polarity from the divergence from non-bilaterians may have an important role in their complex development. The cnidarian, Nematostella vectensis, has been shown to utilize the Wnt/Planar Cell Polarity pathway protein Strabismus for bottle cell formation and initial gut invagination, which is localized to the animal pole and allows for gastrulation to occur. This protein is of interest to determine if it possesses a function in the gastrulation of bilaterian organisms. It is hypothesized that if Strabismus is present in bilaterian taxa, it will be localized to the vegetal pole and function in the initial invagination for gastrulation to occur. Determining the molecular mechanism for gastrulation in both non-bilaterian and bilaterian taxa is important in understanding the evolution of the polarity of metazoan embryos.
