It is proposed that the chloroplasts of Euglena have arisen from the progressive reduction of endosymbiotic green algae. The theory is supported by the presence of a third membrane around the chloroplasts of Euglena which is not endoplasmic reticulum (ER) in origin and may be derived from the plasmalemma of the original symbiont. In addition, Euglena is the only organism in which chloroplast loss can be induced experimentally. Dinoflagellate chloroplasts are also surrounded by three membranes, and it is proposed that they too evolved from symbiotic eucaryotic algae.
Annals New York Academy of Sciences plasm, rather than in the chloroplast, and the photoreceptive pigment is found not in the cell membrane, as is believed to be the case in green algae,l* *' but in a small crystalline body located near the base of the long flagellum. Unlike green algae, the chromosomes of Euglena remain condensed throughout interphase and the nucleolus does not disperse during mitosis, but simply elongates and constricts in two.
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