黃裳鳳蝶複眼光感受器的多樣化與光譜拮抗作用

Abstract

視覺是大部分動物用以感知外界環境的重要生理機能，分辨環境中的顏色訊號對於動物的生存扮演不可或缺的角色。為提升色彩辨識能力，蝴蝶的視覺系統已發展出不同策略以增加光譜解析力，例如窄化光感受器的光譜感度、改變光譜感度峰值或增加光感受器的種類。黃裳鳳蝶透過光感受器光譜感度的修飾，應有助於種間辨識。為瞭解黃裳鳳蝶的彩色視覺，本研究以神經電生理之細胞內記錄技術量測黃裳鳳蝶複眼光譜感度。結果顯示黃裳鳳蝶具有九種光感受器，分別是對波長 360 nm、390 nm、440 nm、510 nm、540 nm、550 nm、580 nm、600 nm 和 620 nm 最敏感的光感受器。與視色素光譜吸收理論值比對後，顯示部分光感受器的光譜感度與預測值並不完全相符，光譜感度曲線的推移及光感受器種類的多樣化應與色素的過濾作用有關。已知位在小眼頂端的螢光色素和圍繞在視官柱體周圍的屏蔽色素可修飾蝴蝶光感受器的光譜感度，扮演光譜過濾的角色。除了光學的過濾作用，光譜感度量測的同時亦發現 negative-going 的反應，顯示在黃裳鳳蝶複眼光感受器中對不同波長的刺激有拮抗電位的交互作用。為瞭解 negative-going 反應的來源，本研究分別以產生不同反應極性的波長進行光刺激，以比較刺激後光感受器產生反應所需的時間差異。結果顯示 negative-going 反應所需時間較短，推測此抑制電位可能源自光感受器本身的細胞反應，而非間接地來自胞外電流的回饋。黃裳鳳蝶在初階視覺系統的光感受器即存在光譜拮抗作用，可能有助於強化彩色視覺，但此課題仍有待更深入的研究。

Vision is one of the important physiological functions to perceive surrounding information for most animals, and discriminating colors is essential for a visual system to seeing color. For a better discrimination, several strategies have been developed to enhance spectral resolution in the visual system of butterfly, such as narrowing spectral sensitivity, shifting the peak sensitivity, or diversifying spectral types of photoreceptor. Troides butterflies are presumably tuned the properties of the photoreceptor sensitivity spectra for intra-species recognition. Electrophysiological recording in the compound eye of Troides aeacus formosanus showed that there are 9 spectral types of photoreceptor with peak sensitivities at 360 nm, 390 nm, 440 nm, 510 nm, 540 nm, 550 nm, 580 nm, 600 nm, and 620 nm. The spectral sensitivities of some photoreceptors do not match to those predicted spectra of visual pigments, when fitting the theoretical pigment absorption curves to the recorded spectral sensitivities. The spectral tuning and diversification appear to the optical interaction of the fluorescing and perirhabdomal pigments, which are known to function as spectral filters in the compound eyes of some butterfly species. In addition to the optical filtering effects, negative-going responses were observed when gave light stimulation with the specific wavelengths, indicating that there are strong antagonistic interactions in the Troides retina. To explore the origin of the negative-going responses, the time-to-peaks of temporal impulse responses elicited by brief flashes with the specific wavelengths were compared. The result indicates that the inhibitory current could not be attributed to the extracellular field current but the direct current flow within the photoreceptor itself. The spectral opponency at the level of photoreceptor could subserve the Troides color vision, however further study on this issue is needed. The variety of spectral receptor types caused by the pigments filtering effects and electrical interactions in the Troides eye suggest that the Troides butterfly has a good ability on color discrimination.