Cleavage in Nemertoderma westbladi (Nemertodermatida) and its phylogenetic significance

摘要

Recent phylogenetic analyses of ribosomal and protein coding nuclear genes place the marine worms within the Nemertodermatida as one of the oldest lineages among the bilaterian animals. We studied the early embryonic cleavage in Nemertoderma westbladi to provide the first account of nemertodermatid early development. Live embryos were studied with interference microscopy and fixed embryos were either sectioned or studied with confocal laser scanning microscopy. Initially the divisions in the embryo are radial, but then micromeres are shifted clockwise generating a spiral pattern. The four-cell stage is characterized by duets of macromeres and micromeres and thus resembles the duet cleavage reported from members of the Acoela. However, subsequent stages differ from the acoel duet pattern and also from quartet spiral cleavage. The optimization of the cleavage pattern on current phylogenetic hypotheses with Nemertodermatida and Acoela as early bilaterian branches is discussed.