鯉科魚類視覺基因的適應演化

Adaptive Evolution of Visual Genes in Cyprinids

王豐寓(Feng-Yu Wang)

指導教授 ： 曾晴賢

國立清華大學/生命科學院/生命科學系/博士(2008年)

摘要

鯉科是淡水魚類最大的一群，絕大多數都是體色樸素的種類，但少數的亞科、屬具有鮮豔的體色。其中魚丹亞科中一些特化的魚種其體色鮮豔並具有婚姻色，適合用來研究視覺基因與體色的相關性。此外鯉科魚類棲息的環境差異很大，從清澈的小溪到混濁湖泊都可以發現其蹤跡，因此，鯉科魚類也是用來研究棲地光環境與視覺基因相關性的好材料。本研究主要是利用鯉科魚類棲地與體色的多樣性來測試兩個假說：一，魚類的視覺能力與視覺基因是否與體色上的差異具有相關性；二，鯉科魚類的視覺能力及其相對應的視覺基因是否會受到棲地光環境的影響而產生適應演化的現象。
我們定序了魚丹亞科裡的長鰭鱲(Opsariichthys evolans )、粗首鱲(Opsariichthys pachycephalus )與馬口魚(Candidia barbatus )三種體色獨特且鮮豔的台灣特有鯉科魚類的視覺基因（分別是吸收UV光的SWS1、藍光的SWS2、綠光的Rh2、紅光的LWS以及感應明暗視覺的rhodopsin），並且測量這三種魚的視網膜吸收光譜與體表反射光譜。結果發現，鯉科魚類的體色跟視覺基因有很高的相關性，各魚種之體色上在藍色的反射光譜有明顯的差異；而在視覺能力方面，各魚種在藍色與綠色視覺基因的最大吸收光波則有不同程度的光譜位移。這三種鮮豔的鯉科魚類體色的差異反應了視覺能力的不同，而這樣的差異是由視覺基因序列的改變、視覺基因表現的不同與基因序列改變所造成。
我們並定序了其他四種體色樸素且生活習性截然不同的鯉科魚類（台灣石□Acrossocheilus paradoxus、台灣鏟頷魚Onychostoma barbatula、羅漢魚Pseudorasbora parva以及唇□Hemibarbus labeo）的視覺基因，同時測量其視網膜吸收光譜。結果發現，棲地光環境的改變影響了感應明暗、藍光、綠光與紅光的視覺基因之表現，並在這些基因的λmax上發現明顯的光譜位移。生活在清澈水域的鯉科魚類使用較短波長的視覺基因，而在混濁水域的種類則使用較長波長的視覺基因。這樣的差異，在rhodopsin、Rh2 與 LWS 等視覺基因中可能是因為使用不同的chromophore或者累積了基因序列上比較次要的改變所造成的，而在 SWS1 與 SWS2則是由基因序列改變所造成的。由以上的結果，我們認為體色與棲地光環境的差別影響了鯉科魚類的視覺能力，而鯉科魚類視覺基因也因為這樣的差異而產生了適應演化的現象。

關鍵字

視覺基因 ； 鯉科 ； 適應演化 ； 最大吸收光波長 ； 光譜位移

並列摘要

Cyprinidae is the largest freshwater fish family, and their habitats are diverse from clear to turbid water. Several specific genera from this group exhibit unique nuptial coloration, although most of them are drab in their coloration. This study was aimed to test two hypotheses. First, differences in nuptial coloration among colorful cyprinids could reflect differences in color vision and opsin gene sequences. Second, photic environments of habitats could affect the visual abilities and the opsin genes of cyprinids. To test the first hypothesis, genes encoding the visual pigments of three colorful cyprinids (Opsariichthys evolans, Opsariichthys pachycephalus and Candidia barbatus) were cloned and sequenced, the λmax of cone photoreceptor absorption spectra and the reflectance spectra of their body coloration were measured. It was found that the differentiation of spectral sensitivities and unique nuptial coloration of the colorful cyprinids might have evolved through sexual selection. The results also indicated that the spectral shift among colorful cyprinids could result from differential expression of opsin genes and amino acid substitutions. At the same time, other four dull-colored cyprinid (Acrossocheilus paradoxus, Hemibarbus labeo, Pseudorasbora parva and Onychostoma barbatula) lived in different habitats were tested to test the second hypothesis. The photic environments of habitats where the cyprinids lived could affect their visual abilities in blue-, green-, and red-sensitive photoreceptors. The cyprinids inhabited clear water use photoreceptors with shorter wavelength opsins, yet the species lived in turbid water possess photoreceptors with longer wavelength opsins. The alternative chromophore usage and accumulation of the complex interactive substitutions could dominate the obvious spectral differences of rhodopsin, Rh2, and LWS between the cyprinids inhabited two distinct photic environments. The amino acid substitutions may be responsible for differences in the absorbance of SWS1 and SWS2. These results support our hypotheses that nuptial coloration and photic environment of habitats could be tied to visual sensitivities and opsin genes.

並列關鍵字

spectral shift ； λmax ； nuptial coloration ； opsin gene ； photic environment

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