草蜥屬視蛋白基因的序列與表現

Abstract

光譜調節（spectral tuning）可由感光細胞中視蛋白序列突變所導致的結構改變，使最敏感的波長在光譜上往長波或短波位移。位移造成的視覺敏感度差異，除了反映光環境的感官適應外，也同時在性擇上幫助尋覓適存度最佳的配偶。分布於東亞的草蜥屬（Takydromus）物種在短時間內演化出多樣的雌雄色型型式，包含在同一支系中演化出具雌雄雙色型與缺乏雌雄雙色型的物種。其中，季節性雌雄雙色型的翠斑草蜥雌蜥會以雄蜥體側綠色斑點的飽和程度做為擇偶依據，因此我推測物種間的色型差異可能與視覺敏感度差異有關。本研究第一部分增幅並定序來自臺灣的8種草蜥之視蛋白基因片段，由此設計特定基因引子，透過RACE增幅並定序來自草蜥視網膜的全長視蛋白cDNA。經序列比對後，發現所有物種皆表現感應紫外光（SWS1）、藍光（SWS2）、綠光（Rh2）和紅光（LWS）的四種視蛋白基因。光譜調節位點上，在任一視蛋白基因、任一點位上都沒有變異，顯示草蜥屬物種間的視蛋白光敏感度相仿，與多樣的雌雄色型並沒有關聯。研究第二部分則利用偵測視蛋白表現量的改變，了解翠斑草蜥在繁殖季與非繁殖間視覺與婚姻色的關聯。除了比對採樣自繁殖季和非繁殖季的雄蜥、雌蜥和幼雄蜥視蛋白表現量外，亦在繁殖季前期進行睪固酮的操控性實驗，比對實驗組和控制組雄蜥間的表現量差異。實驗結果顯示，雌蜥在繁殖季有顯著較高於非繁殖季的LWS表現量；雄蜥則有較高的Rh2表現量。幼雄蜥、非繁殖季雄蜥、控制組雄蜥與睪固酮操作雄蜥正好呈現Rh2由低漸高的比重表現量趨勢。經反射光譜量測後，我推測Rh2視蛋白的表現量比重上升，可能使繁殖季雄蜥對綠色（婚姻色）更加敏感，並增加對色彩的辨識能力。雌蜥繁殖季LWS的表現量上升，則可能提升對明暗度的敏感度，有助選擇最佳的配偶。最後，無論LWS或Rh2的表現量，均在繁殖季才有性別上的顯著差異，暗示婚姻色與季節性起伏的視蛋白表現量具有演化上的關聯性。本篇研究為首次在陸生脊椎動物上，記錄性激素如何影響物種內的視覺差異的研究，亦提供感官驅動性擇演化的佐證。Spectral tuning is an evolutionary adaptation by changing structures of opsins in photorecptor cells. In addition to the adaptation to different optical environments, it also plays a critical role in sexual selection to choose mates with optimal fitness. Takydromus lizards in eastern Asia have evolved multiple mate-choice patterns, involving species with and without sexual dichromatism in the same clade. I deduced that the color variation among Takydromus species may associate to their visual sensitivity. The first part of my study aimed to clone the four classes of cone opsin for the full-length expressed cDNA from retinas of chosen Takydromus species with different dichromatic patterns. All speciesexpress 4 opsins that are sensitive to ultraviolet, blue, green and red light wavelength. However, no mutation was detected at any tuning site among species. Therefore, in the second part, I evaluated the relative expression of cone opsins in breeding and nonbreeding seasons from male, female and juvenile T. viridipunctatus, whose males display prominent nuptial coloration along the body side during the breeding seasons. Our results suggest that males possess increased proportion of Rh2 and decreased LWS expression during the breeding seasons; while females show the reversed pattern. Same expression pattern can be found comparing male lizards with artificial testosterone treatment to those without in the breeding seasons. Integrated with predicted spectral sensitivity and nuptial color reflectance, seasonal modification of opsin gene expression has a close link to sex hormone, which can enhance visual sensitivity toward greenish nuptial coloration in males. Females, on the other hand, do not directly raise color discrimination ability or greenish light sensitivity, however, they adjust achromatic sensitivity toward brightness and possibly benefit in mate choice.