The functional and phylogenetic significance of dinoflagellate eyespots

The eyespots or stigmas from five species of dinoflagellates fall into three distinct categories: independent eyespot which is not membrane bound; independent eyespot which is surrounded by three membranes; eyespot situated at the periphery of a chloroplast. In all cases the eyespot is situated behind the longitudinal groove or sulcus and there is a strand of microtubules between the eyespot and the cell covering or theca. In two cases the strand has been clearly shown to originate near the base of the longitudinal flagellum, which is the one passing over the eyespot and is also responsible for directional movement of the cell. The microtubular strand is presumed to play a part in the transmission of directional stimulation from eyespot to flagellum and a hypothesis is advanced to explain how this may be brought about. Phylogenetically, the structure of the various types of eyespots would link these dinoflagellates with euglenids and chrysophytes , and the diversity found in the dinoflagellates is probably a reflection of the diverse origin of chloroplasts in this group.