2019 Volume 31 Issue 1 Pages 3-9
A variety of eukaryotic phytoflagellates belonging to green algae, chromalveolates, and excavates exhibit three-dimensional phototaxis, movements directed toward (positive) or away from (negative) the light source along a light vector. Most phototactic algae possess a characteristic structure called ‘eyespot’ (also called ‘stigma’) in the chloroplast close to the flagella or the plane of flagellar beat. The eyespot is a cluster of lipid globules containing carotenoids and is mostly located close to the actual photoreceptor for photomovements (phototaxis and photophobic responses). In most algae, the eyespot has been believed to function as a reflective device providing a periodic illumination contrast for photoreceptors in combination with helical swimming patterns. Eyespots considerably vary in shape, size, and position among algal groups, and seem to have been independently acquired by those algae. Euglenophyta, photosynthetic euglenoids belonging to supergroup excavates, also possesses the eyespot and exhibits negative and positive phototaxis. Unlike other algal eyespots, eyespot globules of Euglenophyta are arranged irregularly in the cytoplasm and seemed to lack reflective properties. The eyespot function and its significance in the phototaxis of Euglenophyta have not been fully proved and have still been controversial. Here we summarize studies on the structure of eyespots and its function in the phototaxis of phytoflagellates focusing on green algae and Euglenophyta, one of the best studied algae in the eyespot and photomovement researches. In addition, we introduce our ongoing researches aiming at the elucidation of the eyespot function in the phototaxis of Euglena gracilis, one of the best studied Euglenophyta.
