狹口鏈渦蟲的負趨光行為

Abstract

狹口鏈渦蟲（Stenostomum grande）是一種非寄生性的扁形動物，棲息於淡水環境。其身體結構十分簡單，且身上無明顯色素，且不具眼點。在大部分的動物當中，有色素的眼點可以用來感測光的方向，因此可以猜測缺乏眼點的狹口鏈渦蟲應該不具有方向性視覺。更令人驚訝的是，在鏈渦蟲的轉錄體資料庫中，並沒有找到動物界最常用來作為感光蛋白的視蛋白(opsin)及隱花色素(cryptochrome)，這代表鏈渦蟲可能甚至沒有感光能力。然而在鏈渦蟲的行為實驗中，我們發現鏈渦蟲具有負趨光的行為反應，且這個行為反應具有光波長的專一性。對於短波長的可見光(波長454 nm的藍光及514 nm的綠光)，傾向往光線來向的相反方向進行移動來躲避，且該反應的方向明確，並非透過隨機移動的方式抵達暗處，對長波長的光(波長594 nm的黃光及629 nm的紅光)則沒有特別的反應，即使被高強度的紅光雷射(650 nm)照射，也不會進行迴避。由於鏈渦蟲缺乏動物界常見的感光蛋白，但卻可對特定波長的光線產生行為反應，因此未來對鏈渦蟲感光機制的研究，將可進一步探索動物界裡感光機制的多樣性及演化可塑性。

Stenostomum grande is a free-living freshwater flatworm that has simple anatomy. It lacks pigmentation and has no apparent eyespot. An eyespot with a pigment screen is instrumental for sensing the direction of light in many animal species. Therefore, one might predict that S. grande cannot sense the directionality of light. Furthermore, transcripts encoding opsin and cryptochrome, the commonly used photosensory molecules in the animal kingdom, are missing from the transcriptome of S. grande. This would suggest that S. grande may not even have the ability to sense the light. However, we discovered that S. grande could respond to light and exhibit a negative phototaxis through behavioral experiments. Furthermore, the phototactic response of S. grande is spectrum-sensitive. S. grande exhibited negative phototactic behavior toward blue and green light (wavelength: 454 nm and 514 nm), and these worms would move to the dark side directly. However, S. grande is irresponsive to orange and red light (wavelength: 594 nm and 629 nm), even if the worms are exposed to a red laser ray with high intensity (650 nm). Given that this flatworm lacks the conventional photosensory molecules and yet exhibits a defined phototactic response, it is of great interest to further characterize the novel photosensory mechanism in this flatworm. Future studies of Stenostomum may shed light on the diversity and evolutionary plasticity of photosensory mechanisms in the animal kingdom.