Diversification of epithelial adherens junctions with independent reductive changes in cadherin form: identification of potential molecular synapomorphies among bilaterians

The adherens junction (AJ) is the most universal junction found in bilaterian epithelia and may represent one of the earliest types of cell-cell junctions. The adhesion molecules responsible for forming AJs are the classic cadherins (referred to simply as cadherins), whose extracellular domain organization displays marked variety among species examined so far. In this study, we attempted to reconstruct the evolution of cadherin by analyzing new data from several arthropods (two insects, one noninsect hexapod, three crustaceans, and one chelicerate) and previously published sequences for Drosophila melanogaster and other animals. The results of comparative analyses using the BLAST tool and immunohistochemical analyses revealed that the extracellular domain organizations of a decapod, an isopod, a spider, and a starfish cadherin, which are present at AJs in the embryonic epithelia are homologous. Independent reductive changes from the ancestral state were evident in the epithelia of hexapods+branchiopod, vertebrates+urochordates, and a cephalochordate. The form of cadherins in hexapods is more closely related to that of a branchiopod than to that of malacostracan crustaceans, and one of those of vertebrates is more closely related to that of urochordates than to that of a cephalochordate. Although the sampling of taxa is limited at this stage of research, we hypothesize that the reductive events in cadherin structure related to AJ formation in the epithelia may possess information about bilaterian relationships as molecular synapomorphies.