Thus, even a single cell which contains both photo- and shading pigments, as found in some flatworms and algae, fits this definition. In spite of such enormous diversity, an eye can be simply defined as a photoreceptor shielded on one side by pigment which allows the detection of a source of light and its direction. However, those studies are restricted to eumetazoan animals.ĭiverse types of photoreceptive organs have been observed at every level of structural design and ontogenetic origin. To examine Darwin’s hypothesis, the evolutionary transition of eye form has been studied in various systems, e.g., the pit eyes of worms, and mollusks, and the complex camera-type eyes of vertebrates. He was quite convinced of his hypothesis that even such a perfect and complex eye could be formed by natural selection. Charles Darwin called eyes an example of “organs of extreme perfection and complication”. The evolution of the eye has been of special interest in discussing the origin of complex organs. Taken together, we suggest that the ocelloid is an intracellular camera-type eye, which might be originated from endosymbiotic origin. The rhodopsin gene identified is most closely related to bacterial rhodopsins. Identifying a rhodopsin gene fragment in Erythropsidinium ESTs that is expressed in the retinal body by in situ hybridization, we also show that ocelloids are actually light sensitive photoreceptors. Here, we show that the morphology of the retinal body changed depending on different illumination conditions and the hyalosome manifests the refractile nature. To determine if the ocelloid is functionally photoreceptive, we investigated the warnowiid dinoflagellate Erythropsidinium.
The ocelloid has been an evolutionary enigma because of its remarkable resemblance to the multicellular camera-type eye. It contains retina- and lens-like structures called the retinal body and the hyalosome. The ocelloid is an extraordinary eyespot organelle found only in the dinoflagellate family Warnowiaceae.
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