摘要
復(fù)眼是昆蟲的主要視覺器官,在導(dǎo)航、覓食和避敵等關(guān)鍵行為中起核心作用,其獨(dú)特的微觀結(jié)構(gòu)和視覺轉(zhuǎn)導(dǎo)機(jī)制對(duì)昆蟲的生態(tài)適應(yīng)性極為重要。本文重點(diǎn)聚焦復(fù)眼的微觀結(jié)構(gòu)、研究方法、視覺行為和信號(hào)轉(zhuǎn)導(dǎo)機(jī)制。微觀結(jié)構(gòu)方面,本文描述了昆蟲復(fù)眼的基本構(gòu)造,涉及小眼的精細(xì)構(gòu)造和視網(wǎng)膜細(xì)胞的功能分化,探討了它們?nèi)绾螀f(xié)同工作以實(shí)現(xiàn)高效的視覺信息處理。研究方法部分重點(diǎn)介紹了高分辨率顯微成像和基因編輯等新技術(shù)在復(fù)眼研究中的應(yīng)用。視覺行為部分強(qiáng)調(diào)了復(fù)眼在昆蟲晝夜節(jié)律調(diào)控和空間導(dǎo)航中的關(guān)鍵作用。最后,本文詳細(xì)闡述了視覺信號(hào)轉(zhuǎn)導(dǎo)的分子機(jī)制。重點(diǎn)分析了從光感受到信號(hào)轉(zhuǎn)導(dǎo)的完整過程,包括視蛋白的光活化、G蛋白偶聯(lián)信號(hào)放大、離子通道調(diào)控等關(guān)鍵步驟。特別強(qiáng)調(diào)了新發(fā)現(xiàn)的信號(hào)調(diào)控分子和反饋機(jī)制在視覺信號(hào)精確轉(zhuǎn)導(dǎo)中的重要作用。昆蟲復(fù)眼的研究促進(jìn)多種技術(shù)的開發(fā),包括農(nóng)業(yè)害蟲的誘捕策略、基因靶向技術(shù)以及害蟲監(jiān)測(cè)和生態(tài)管理方法等。
關(guān)鍵詞
復(fù)眼結(jié)構(gòu); 視覺行為; 技術(shù)與方法; 視覺轉(zhuǎn)導(dǎo)機(jī)制
中圖分類號(hào):
Q 964
文獻(xiàn)標(biāo)識(shí)碼: A
DOI: 10.16688/j.zwbh.2024361
Structure and function of insect compound eyes and research approaches
ZHANG Maosen1, CHEN Junjie1, LUO Qiaozhi1, WANG Hanzhang1, CHEN Wanbin1,3, LI Yuyan1,SHEN Zhongjian1, ZHANG Lisheng1,2*
(1. Key Laboratory of Natural Enemy Insects, Ministry of Agriculture and Rural Affairs, State Key Laboratory
for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural
Sciences, Beijing 100193, China; 2. Key Laboratory of Animal Biosafety Risk Prevention and Control (North),
Ministry of Agriculture and Rural Affairs, Shanghai Veterinary Research Institute, Chinese Academy of
Agricultural Sciences, Shanghai 200241, China; 3. Institute of Plant Protection, Gansu Academy
of Agricultural Sciences, Lanzhou 730070, China)
Abstract
The compound eye is the primary visual organ in insects, playing a crucial role in essential behaviors such as navigation, foraging, and predator avoidance. Its unique microstructure and visual signal transduction pathways are essential for ecological adaptation. This review summarizes recent advances in four major aspects of compound eye research: microstructure, research methodologies, visual behaviors, and signal transduction mechanisms. The structural section outlines the fundamental architecture of compound eyes, including the intricate organization of ommatidia and the specialized functions of retinal cells, highlighting their synergistic role in efficient visual processing. The section on research techniques emphasizes the use of cuttingedge tools such as highresolution microscopy and gene editing technologies. The discussion on visual behaviors focuses on the compound eyes’s involvement in circadian rhythm regulation and spatial orientation. Finally, the signal transduction section elaborates on the molecular mechanisms underlying visual information transmission from photoreception,
including photoactivation of visual proteins, amplification of G proteincoupled signal
and ion channel regulation, with particular emphasis on newly identified regulatory molecules and feedback loops that ensure precise signal fidelity. Research on insect compound eyes supports the development of novel pest control technologies, including lightbased trapping strategies, genetargeting techniques, and advanced pest monitoring and ecological management tools.
Key words
compound eye structure; visual behavior; research methods; visual signal transduction
昆蟲具有機(jī)械力感受器、化學(xué)感受器、溫濕度感受器和視覺感受器等多種感受器。昆蟲利用這些感受器對(duì)外界環(huán)境變化做出精確反應(yīng),以適應(yīng)不同的生存需求[14]。其中,作為重要的視覺感受器,復(fù)眼不僅涉及昆蟲的晝夜活動(dòng)節(jié)律、導(dǎo)航與定位、光譜敏感等行為,在其成功適應(yīng)多變環(huán)境過程中也具有關(guān)鍵作用[5]。
昆蟲復(fù)眼大多由數(shù)千個(gè)小眼組成,每個(gè)小眼都包含角膜、晶錐和視網(wǎng)膜細(xì)胞等結(jié)構(gòu)。這些小眼共同作用,使昆蟲能夠處理復(fù)雜的視覺信息,如運(yùn)動(dòng)檢測(cè)和形狀識(shí)別[6]。昆蟲復(fù)眼的視覺轉(zhuǎn)導(dǎo)機(jī)制涉及光信號(hào)的接收、轉(zhuǎn)換以及處理等后續(xù)步驟[7]。其中,視網(wǎng)膜細(xì)胞中的光感受器負(fù)責(zé)將光信號(hào)轉(zhuǎn)換,并啟動(dòng)視覺信號(hào)的傳遞。換句話說,感光細(xì)胞內(nèi)的視蛋白分子在吸收光子后會(huì)引發(fā)一系列分子和電化學(xué)變化,將光信號(hào)轉(zhuǎn)換為電信號(hào)。隨后這些信號(hào)通過神經(jīng)網(wǎng)絡(luò)傳遞到昆蟲的大腦,形成視覺感知[8]。這一復(fù)雜過程不僅涉及光感受和信號(hào)轉(zhuǎn)換,還包括信號(hào)的放大、細(xì)化和傳遞,構(gòu)成了昆蟲行為控制的神經(jīng)生物學(xué)基礎(chǔ)。
昆蟲通過復(fù)眼執(zhí)行多種視覺行為,包括根據(jù)晝夜節(jié)律的變化進(jìn)行自我調(diào)整、導(dǎo)航與定位,以及對(duì)不同光譜作出反應(yīng)。研究復(fù)眼的方法和技術(shù)主要包括顯微鏡技術(shù)、分子生物學(xué)方法和遺傳學(xué)技術(shù)等。這些技術(shù)的應(yīng)用和發(fā)展極大地推動(dòng)了對(duì)復(fù)眼結(jié)構(gòu)和功能的深入認(rèn)知。昆蟲復(fù)眼的研究有助于多種技術(shù)的開發(fā),包括農(nóng)業(yè)害蟲的誘捕策略、基因靶向技術(shù),以及害蟲監(jiān)測(cè)和生態(tài)管理方法等。
了解昆蟲的感光機(jī)制,進(jìn)而明確視覺信號(hào)在昆蟲行為中的作用,不僅可為農(nóng)業(yè)害蟲誘捕裝置的研發(fā)提供方向,也可為制定行為調(diào)控策略奠定理論基礎(chǔ)。例如在白紋伊蚊Aedes albopictus中,Opsin1基因的沉默顯著影響了其避光行為,從而減少了該蟲在光照區(qū)域的活動(dòng)和叮咬[9]。亞洲玉米螟Ostrinia furnacalis的光趨向行為與3個(gè)關(guān)鍵視蛋白基因OfLW、OfBL和OfUV的表達(dá)密切相關(guān),降低這些基因的表達(dá)量能顯著降低成蟲對(duì)綠光、藍(lán)光和紫外光的光趨向效率,表明它們?cè)谡{(diào)控害蟲光趨向行為中發(fā)揮著重要作用[10]。同時(shí)昆蟲復(fù)眼的研究有助于開發(fā)新型殺蟲劑,例如植物源農(nóng)藥印楝素(azadirachtin)可通過誘導(dǎo)靶向凋亡途徑,從而顯著延緩果蠅Drosophila melanogaster復(fù)眼的形態(tài)發(fā)育進(jìn)程[11]。
因此,本文旨在從復(fù)眼的結(jié)構(gòu)、視覺行為、相關(guān)研究方法和技術(shù)以及視覺信號(hào)傳導(dǎo)機(jī)制等方面進(jìn)行系統(tǒng)全面總結(jié),以期為害蟲綜合治理及相關(guān)研究的開展提供參考。
1 昆蟲復(fù)眼結(jié)構(gòu)
昆蟲復(fù)眼是視覺系統(tǒng)的核心組件和主要光感受器,其復(fù)雜的結(jié)構(gòu)直接決定了昆蟲的視覺敏感度[6, 1216]。以蓮草直胸跳甲Agasicles hygrophila為例,其復(fù)眼具有并列型結(jié)構(gòu),每個(gè)視母細(xì)胞由角膜、晶狀體和8個(gè)具視小桿的視網(wǎng)膜細(xì)胞組成。角膜覆蓋了整個(gè)復(fù)眼表面,晶狀體分為4瓣,8個(gè)視網(wǎng)膜細(xì)胞環(huán)繞同一軸心的視桿排列成圈。復(fù)眼的外部形態(tài)和內(nèi)部微結(jié)構(gòu)上雌雄個(gè)體間無顯著差異[17]。復(fù)眼表面的形狀影響昆蟲對(duì)不同環(huán)境條件的適應(yīng)性。Hayashi等在研究果蠅D.melanogaster復(fù)眼表型時(shí)發(fā)現(xiàn),六邊形和四邊形的鑲嵌排列形式是復(fù)眼發(fā)育過程中視母細(xì)胞位置變化的結(jié)果,由視母細(xì)胞的規(guī)則分布和均勻生長共同決定[18]。研究人員分別在杜鵑三節(jié)葉蜂Arge similis和2種象甲科昆蟲中觀測(cè)到典型的蜂窩狀六邊形復(fù)眼表面[1920],對(duì)比發(fā)現(xiàn),這2種象甲科昆蟲的復(fù)眼參數(shù)有顯著差異。
昆蟲復(fù)眼由密集排列的數(shù)以千計(jì)的小眼組成,能從多個(gè)角度同時(shí)捕捉視覺信息,這些小眼共同構(gòu)成了昆蟲視覺系統(tǒng)的核心[2123]。這些小眼的表面呈半球形或六面體結(jié)構(gòu),集結(jié)成曲面。每個(gè)小眼都由一組視網(wǎng)膜細(xì)胞組成,包括光檢測(cè)感光細(xì)胞和晶狀體分泌細(xì)胞。晶狀體分泌細(xì)胞產(chǎn)生位于光感受器頂部的典型六邊形小面晶狀體,復(fù)眼表面由這些六邊形小面晶狀體的準(zhǔn)晶體陣列構(gòu)成[6](圖1)。復(fù)眼中的高密度小眼為環(huán)境中導(dǎo)航和覓食的昆蟲提供了廣闊的視野[1920]。此外,昆蟲視覺系統(tǒng)可以整合來自許多小眼的信息,顯著提高視覺的整體性能。多個(gè)小眼的信息在昆蟲大腦中被綜合和處理,形成一個(gè)統(tǒng)一且連貫的視覺圖像[6, 23]。
色素細(xì)胞,亦稱為含黑色素細(xì)胞,分布在小眼間,在吸收、減少反射、防止光線散射和提升圖像清晰度等方面發(fā)揮著的重要作用[2426]。小眼中色素細(xì)胞分布不均勻,依特定環(huán)境光暗變化模式排列,以優(yōu)化其功能[2728]。例如,在橘小實(shí)蠅Bactrocera dorsalis等昆蟲復(fù)眼中,在不同單色光下色素細(xì)胞內(nèi)的色素顆粒會(huì)縱向移動(dòng)。在白光、綠光和黃光下,這些顆粒沿小眼縱軸均勻分布,而在紫光、藍(lán)光和紅光下則主要集中在網(wǎng)狀細(xì)胞的遠(yuǎn)端和角膜的近端[29]。色素顆粒的分布受到多種因素的影響,包括生物體的種類、復(fù)眼類型和光環(huán)境條件。例如,在黑帶食蚜蠅Episyrphus balteatus中,色素顆粒的分布受光暗適應(yīng)性條件影響,以助其適應(yīng)不同光暗條件下的飛行行為[30]。
復(fù)眼的獨(dú)特結(jié)構(gòu)提供了昆蟲廣泛的視野和對(duì)運(yùn)動(dòng)的高度敏感性,使其能迅速響應(yīng)環(huán)境變化,進(jìn)而影響昆蟲的生存和繁衍。
2 復(fù)眼結(jié)構(gòu)的研究方法與技術(shù)
2.1 顯微鏡技術(shù)
昆蟲復(fù)眼研究中,顯微鏡是不可或缺的工具,主要包含光學(xué)顯微鏡、掃描電子顯微鏡(scanning electron microscope,SEM)、透射電子顯微鏡(transmission electron microscope,TEM)和共聚焦顯微鏡。這些顯微鏡及其技術(shù)應(yīng)用為研究昆蟲復(fù)眼結(jié)構(gòu)奠定了基礎(chǔ)。SEM和TEM在研究昆蟲外部形態(tài)和超微結(jié)構(gòu)方面發(fā)揮了關(guān)鍵作用,特別是在檢測(cè)昆蟲感受環(huán)境中生物和非生物刺激的感覺器官方面[3233]。
SEM是一種高級(jí)電子光學(xué)儀器,通過使用電子束掃描樣品表面,借助樣品與電子束相互作用所產(chǎn)生的二次電子來生成圖像。其主要特點(diǎn)包括高分辨率、大景深和立體感強(qiáng),是觀察昆蟲表面形態(tài)與結(jié)構(gòu)的理想工具[33]。通過SEM,能夠詳細(xì)了解昆蟲復(fù)眼表面的透鏡排列方式、形狀、大小和間隔等參數(shù),以精確計(jì)算復(fù)眼透鏡的數(shù)量、形狀比例和面積占比等,還可深入了解小眼的組成元件,揭示小眼的精確組織和形態(tài)[3436]。例如,運(yùn)用SEM、納米壓痕技術(shù)及共聚焦激光掃描顯微鏡研究沙漠蝗蟲Schistocerca gregaria角膜角質(zhì)層的微觀結(jié)構(gòu)、成分和特性,發(fā)現(xiàn)其呈完全螺旋結(jié)構(gòu)以及含較高比例的彈性蛋白,該結(jié)構(gòu)與成分組合使得角膜角質(zhì)層能更有效傳遞光線,提高昆蟲視覺效能,這可能是有助于優(yōu)化光線傳輸?shù)倪m應(yīng)性演化[37]。
TEM是一種利用電子束透射和散射觀察樣品內(nèi)部結(jié)構(gòu)的高分辨率顯微鏡,其空間分辨率遠(yuǎn)超傳統(tǒng)光學(xué)顯微鏡,適用于深入分析昆蟲復(fù)眼的細(xì)胞和組織結(jié)構(gòu)[38]。利用TEM觀察感光細(xì)胞內(nèi)色素體和光感受器蛋白的分布與結(jié)構(gòu),能夠獲得感光細(xì)胞內(nèi)部結(jié)構(gòu)的高分辨率圖像,使研究者能夠揭示其如何對(duì)特定波長的光作出反應(yīng)并轉(zhuǎn)換為電信號(hào),以便神經(jīng)系統(tǒng)進(jìn)一步處理[39]。昆蟲復(fù)眼的顏色產(chǎn)生機(jī)制亦與復(fù)眼發(fā)育過程有關(guān)。研究顯示,昆蟲復(fù)眼的形態(tài)和功能多樣性依賴于一系列保守基因和發(fā)育過程[40],其中借助TEM觀察相關(guān)關(guān)鍵支持細(xì)胞(如Semper細(xì)胞)結(jié)構(gòu)變化及其在復(fù)眼發(fā)育中的作用,亦有助于理解復(fù)眼如何形成特定的顏色感知能力。
SEM提供的外部結(jié)構(gòu)信息有助于解釋光信號(hào)在復(fù)眼中如何被收集和初步處理,TEM觀察細(xì)胞內(nèi)部結(jié)構(gòu)的變化,可以了解光信號(hào)的感知和轉(zhuǎn)化過程,兩者聯(lián)合使用有助于更全面地描述昆蟲復(fù)眼的復(fù)雜視覺系統(tǒng)。利用光學(xué)顯微鏡、SEM和TEM研究溝眶象Eucryptorrhynchus scrobiculatus和臭椿溝眶象E.brandti的復(fù)眼外部形態(tài)和內(nèi)部超微結(jié)構(gòu)。結(jié)果顯示,2種昆蟲的復(fù)眼均呈腎形排列,各含約550個(gè)小眼,且兩者擁有不同的視覺器官結(jié)構(gòu),以感知外部環(huán)境的視覺信息[19]。這些研究定量比較了2個(gè)物種在復(fù)眼透鏡數(shù)量、排列,視網(wǎng)膜細(xì)胞數(shù)目、類型,微絨毛結(jié)構(gòu)等的差異,提示這些結(jié)構(gòu)特征與物種的生活習(xí)性和視覺行為有關(guān),為比較不同物種的復(fù)眼結(jié)構(gòu)提供了詳細(xì)依據(jù)。
同步輻射X射線相襯顯微斷層掃描技術(shù)(SRXPCT)彌補(bǔ)了SEM和TEM在揭示內(nèi)部結(jié)構(gòu)信息方面的不足,通過虛擬切片和三維體積渲染技術(shù),展現(xiàn)了昆蟲復(fù)眼詳細(xì)的內(nèi)部結(jié)構(gòu)視圖。通過該技術(shù)研究4種鞘翅目昆蟲的復(fù)眼結(jié)構(gòu),經(jīng)其頭部虛擬切片和3D體積渲染,測(cè)得角膜厚度、小眼數(shù)量等參數(shù),這些參數(shù)將有助于深度解析其內(nèi)部解剖結(jié)構(gòu)[36]。此外,多種顯微鏡技術(shù)的聯(lián)合使用為昆蟲復(fù)眼的結(jié)構(gòu)和功能研究提供了多角度線索,增進(jìn)了對(duì)該復(fù)雜視覺系統(tǒng)的綜合理解。
2.2 分子生物學(xué)方法
隨著分子生物學(xué)的迅速發(fā)展,如RNA干擾(RNAi)、CRISPRCas9基因編輯、多組學(xué)分析等現(xiàn)代技術(shù)在昆蟲研究中的應(yīng)用比例已顯著增加。特別是RNAi,不僅在昆蟲基因功能研究中發(fā)揮著重要作用,也在害蟲治理和天敵昆蟲防控等領(lǐng)域中展現(xiàn)其價(jià)值[4144]。昆蟲復(fù)眼研究中,應(yīng)用RNAi技術(shù),發(fā)現(xiàn)果蠅復(fù)眼發(fā)育中Semper細(xì)胞(SCs)內(nèi)cut基因的關(guān)鍵作用,如晶狀體破壞、光學(xué)缺陷及光感受器形態(tài)異常,表明cut基因?qū)S持復(fù)眼的正常結(jié)構(gòu)至關(guān)重要[40]。此外,使用RNAi技術(shù)模擬的先天性靜止性夜盲癥(CSNB)果蠅模型展示了mtt基因的功能及autophagy基因在眼原基細(xì)胞生存和眼發(fā)育中的重要性[4546]。Chen等通過RNAi技術(shù)降低赤擬谷盜Tribolium castaneum差異基因的表達(dá),揭示了復(fù)眼發(fā)育中的關(guān)鍵基因。TcasCPR15為該蟲主要的晶狀體角質(zhì)層蛋白(CP)基因[47]。
研究發(fā)現(xiàn),PR結(jié)構(gòu)域蛋白13(PRDM13)基因在果蠅中過表達(dá)會(huì)導(dǎo)致嚴(yán)重的視網(wǎng)膜發(fā)育異常,與人類的相關(guān)病理表型相似[48]。果蠅視覺形態(tài)基因Om(1E)的突變體表現(xiàn)為復(fù)眼擴(kuò)大的特殊表型,這是由逆轉(zhuǎn)錄轉(zhuǎn)座子插入導(dǎo)致基因過表達(dá)引起的[49]。這種突變體保持了小眼的規(guī)則排列,與其他導(dǎo)致小眼減少和排列紊亂的Om突變形成鮮明對(duì)比[49]。
復(fù)眼的形成是一個(gè)復(fù)雜過程,涉及多個(gè)關(guān)鍵基因的協(xié)同作用。Liu等[50]和Weasner等[51]分別針對(duì)不同的昆蟲物種開展了cardinal基因家族和eya基因的克隆鑒定、表達(dá)調(diào)控、功能解析等工作,以揭示這些關(guān)鍵基因的調(diào)控和協(xié)同作用。
3 昆蟲復(fù)眼視覺行為
3.1 晝夜節(jié)律
昆蟲復(fù)眼高度發(fā)達(dá)的光感應(yīng)能力使其能精確調(diào)節(jié)其生物鐘,適應(yīng)晝夜周期的變化。夜行性昆蟲的復(fù)眼經(jīng)過適應(yīng)性演化,小眼的尺寸顯著增大和數(shù)量顯著增加,使它們能在昏暗環(huán)境中更有效捕捉光線,優(yōu)化了低光照條件下的視覺性能[52]。對(duì)長期生活在地下環(huán)境的Trechiama kuznetsovi的研究發(fā)現(xiàn),其復(fù)眼結(jié)構(gòu)已退化,但仍保留特化的感光細(xì)胞,表明即使完全無光的環(huán)境下,昆蟲仍持有基本的感光能力,對(duì)光信號(hào)具有一定程度的響應(yīng)[53]。生活在水下的日本端毛龍虱Agabus japonicus,通過增大視桿的橫截面積增強(qiáng)其光敏感性,以適應(yīng)低光照環(huán)境[54]。以上研究表明,對(duì)于晝夜變化和特定生態(tài)位光照條件復(fù)眼均發(fā)生了適應(yīng)性演化。
生物鐘夾帶機(jī)制(circadian clock entrainment)是指昆蟲通過視覺系統(tǒng)感知光信號(hào),實(shí)現(xiàn)內(nèi)部生物鐘與外界晝夜節(jié)律同步的機(jī)制。該機(jī)制根據(jù)外界光照變化調(diào)整昆蟲內(nèi)部生理活動(dòng),確保生物鐘與自然環(huán)境保持同步。例如,果蠅通過視覺系統(tǒng)與晝夜光感受器協(xié)同作用調(diào)整其生理狀態(tài)應(yīng)對(duì)外界環(huán)境的變化[55]。在調(diào)節(jié)昆蟲晝夜節(jié)律時(shí),對(duì)藍(lán)光高度敏感的光感受器隱花色素蛋白(cryptochrome,Cry)發(fā)揮著核心作用。Cry在果蠅晝夜節(jié)律神經(jīng)元中廣泛表達(dá),在復(fù)眼和外周組織中發(fā)揮作用[5658],特別在光誘導(dǎo)的生物鐘重置過程中起著關(guān)鍵作用[5960]。當(dāng)Cry在藍(lán)光下激活時(shí),可快速促進(jìn)特定晝夜節(jié)律神經(jīng)元的電活動(dòng),觸發(fā)對(duì)特定光譜反應(yīng)的行為調(diào)整,如通過促進(jìn)timeless (Tim)蛋白的光敏感降解來重新校準(zhǔn)晝夜時(shí)鐘[61]。
晝夜變化直接影響復(fù)眼的光感應(yīng)機(jī)制,調(diào)節(jié)昆蟲的交配和覓食行為。昆蟲的交配行為與其感知光的能力密切相關(guān),該感知能力受到晝夜變化的影響。例如,Euglossa dilemma的交配行為受視覺和嗅覺線索的調(diào)節(jié),且感知這些線索的方式在晝夜間有所不同,特別是其負(fù)責(zé)視覺和嗅覺的大腦中心表現(xiàn)出性別二態(tài)性,表明在與交配至關(guān)重要的感覺系統(tǒng)上雄性和雌性的投入不同。雄性的復(fù)眼更大,視覺大腦區(qū)域也更大,可能表明其交配行為更多依賴視覺輸入[62]。同樣地,覓食行為亦受到復(fù)眼的光感應(yīng)機(jī)制和晝夜變化的顯著影響。針對(duì)Psyttalia concolor的覓食行為研究揭示,該蜂主要依靠嗅覺線索定位宿主,且其宿主定位效率受到一天中不同時(shí)間的影響,進(jìn)而影響不同時(shí)間段復(fù)眼感知此線索的方式[63]。
3.2 導(dǎo)航行為
昆蟲復(fù)眼具有對(duì)偏振光敏感的獨(dú)特光學(xué)特性,這是它們進(jìn)行精確空間導(dǎo)航的關(guān)鍵。處理偏振光信息的大腦區(qū)域主要包括視葉的薄板、髓質(zhì)和小葉,以及中央大腦的前視神經(jīng)結(jié)節(jié)、外側(cè)副葉和中央復(fù)合體。在視葉內(nèi),通過神經(jīng)元的匯聚和對(duì)立作用機(jī)制,昆蟲對(duì)偏振光的敏感性和對(duì)比度感知增強(qiáng)。前視神經(jīng)結(jié)節(jié)則整合偏振光信號(hào)與天空色彩對(duì)比度信息。特別是,結(jié)節(jié)神經(jīng)元能夠同時(shí)響應(yīng)天空的紫外線/綠色的對(duì)比度和偏振光的電矢量方向,這種機(jī)制可以補(bǔ)償全天不同時(shí)段太陽高度變化導(dǎo)致的天體偏振模式變化[64]。研究人員基于太陽方位角的復(fù)眼感受模型及其相應(yīng)的神經(jīng)回路模型發(fā)現(xiàn),蝴蝶在一天中不同時(shí)間點(diǎn)的飛行軌跡均呈現(xiàn)向西南方向的一致性行為[65]。蜻蜓、蝴蝶等昆蟲的小眼中含有對(duì)偏振光高度敏感的感光細(xì)胞,這顯著提高了它們的定向能力[66]。另一個(gè)例子是澳大利亞的擬廣布弓背蟻Camponotus consobrinus雄性個(gè)體,它們擁有數(shù)量更多但尺寸更小的小眼,同時(shí)具備對(duì)偏振光敏感的特殊受體,這賦予了它們對(duì)偏振光的感知能力[67]。
蜜蜂是少數(shù)具有高度發(fā)達(dá)社會(huì)結(jié)構(gòu)、豐富行為技能、精細(xì)導(dǎo)航系統(tǒng)和復(fù)雜通信系統(tǒng)的昆蟲之一,能夠快速且準(zhǔn)確地識(shí)別形狀和導(dǎo)航路線[6869]。蜜蜂的復(fù)眼由約4 500個(gè)小眼構(gòu)成[70],每個(gè)小眼含有1個(gè)小透鏡和9個(gè)光感受器。根據(jù)所含光敏色素的光譜敏感性,這些光感受器可分為紫外光敏感、藍(lán)光敏感和綠光敏感三類[7173]。這種結(jié)構(gòu)使蜜蜂能夠迅速且準(zhǔn)確地學(xué)習(xí)和識(shí)別顏色、形狀以及導(dǎo)航路線。此外,蜜蜂復(fù)眼在背側(cè)邊緣區(qū)域有特化結(jié)構(gòu),專門用于感知天空中的偏振光,輔助方向?qū)Ш剑?4]。蜜蜂等社會(huì)性昆蟲復(fù)眼的空間定位能力,清晰展示了它們?nèi)绾卫铆h(huán)境地標(biāo)和太陽位置進(jìn)行導(dǎo)航,從而完成復(fù)雜任務(wù)。
4 復(fù)眼視覺信息轉(zhuǎn)導(dǎo)機(jī)制
感覺信息的處理涉及專門的神經(jīng)元,它們將外部信號(hào)轉(zhuǎn)換成電信號(hào),這些信號(hào)隨后被傳入大腦處理中心。在視網(wǎng)膜中,感光神經(jīng)元可吸收光子,并通過光轉(zhuǎn)導(dǎo)過程引發(fā)細(xì)胞膜離子滲透性的分級(jí)變化。感光細(xì)胞具有高靈敏度、快速的響應(yīng)動(dòng)力學(xué)和寬廣的動(dòng)態(tài)響應(yīng)范圍等優(yōu)良特性,使其能在各種光強(qiáng)度下有效地檢測(cè)信號(hào)[75]。
4.1 光感受和信號(hào)轉(zhuǎn)換
光感受過程開始于感光細(xì)胞內(nèi)視覺色素分子對(duì)光子的吸收,這一過程引起色素分子的構(gòu)型變化,構(gòu)成了視覺信號(hào)轉(zhuǎn)換的基礎(chǔ)。以黑腹果蠅D.melanogaster為例,其復(fù)眼的每個(gè)小眼包含R1至R8等8種感光細(xì)胞(圖2)。R1~R6細(xì)胞主要負(fù)責(zé)寬帶對(duì)比度和運(yùn)動(dòng)檢測(cè),表達(dá)視蛋白R(shí)h1,并將信號(hào)傳遞至薄板;而R7和R8細(xì)胞則主要參與色覺處理,將信號(hào)傳遞至髓質(zhì)[76]。R7細(xì)胞對(duì)藍(lán)光至綠光波段具有復(fù)雜的響應(yīng)能力[77],而R8細(xì)胞在色彩辨識(shí)[7880]和晝夜節(jié)律調(diào)節(jié)[8183]中發(fā)揮關(guān)鍵作用。值得注意的是,R8細(xì)胞能夠調(diào)節(jié)組胺和乙酰膽堿的釋放,顯示出多樣的視覺功能。這一現(xiàn)象揭示了視覺信號(hào)處理的初級(jí)階段即涉及信號(hào)的分離,表明了一種獨(dú)特的視覺信號(hào)傳遞方式[84]。
視蛋白是動(dòng)物中主要的視覺色素分子[85],由7個(gè)跨膜G蛋白偶聯(lián)受體(GPCR)組成,并結(jié)合了維生素A衍生分子,稱為發(fā)色團(tuán)(chromophore),在脊椎動(dòng)物中為11順式視黃醛,果蠅中為3羥基11順式視黃醛[86]。發(fā)色團(tuán)中保守的賴氨酸殘基與視蛋白結(jié)合,形成功能性的視紫紅質(zhì)視覺色素分子。光使發(fā)色團(tuán)從11順式構(gòu)象轉(zhuǎn)變?yōu)槿词?,激活視紫紅質(zhì),啟動(dòng)G蛋白介導(dǎo)的光轉(zhuǎn)導(dǎo)級(jí)聯(lián),這一過程導(dǎo)致離子交換,將光信息放大為細(xì)胞電信號(hào)[81]。根據(jù)視蛋白中所含光敏色素的類型及其在細(xì)胞中的表達(dá)和功能,視蛋白主要分為纖毛視蛋白(copsin)、桿狀視蛋白(ropsin)和視網(wǎng)膜RGR/Go視蛋白(RGR/Goopsins)[8889]。纖毛視蛋白主要在脊椎動(dòng)物的視網(wǎng)膜中表達(dá),與11順式視黃醛(11cisretinal)結(jié)合。它也與G蛋白偶聯(lián)受體的信號(hào)轉(zhuǎn)導(dǎo)相關(guān),主要參與視覺的形成。桿狀視蛋白主要在無脊椎動(dòng)物的復(fù)眼和某些脊椎動(dòng)物的視網(wǎng)膜中表達(dá),與11順式3羥基視黃醛(11cis3hydroxyretinal)結(jié)合。桿狀視蛋白與三聚體G蛋白的Gq亞型偶聯(lián),并通過激活磷脂酶C催化磷脂酰肌醇二磷酸(PIP2)的水解來啟動(dòng)信號(hào)轉(zhuǎn)導(dǎo),導(dǎo)致瞬時(shí)受體電位(transient receptor potential,TRP)
通道開啟。RGR/Goopsins是一類與Go型G蛋白偶聯(lián)的視蛋白,包括RGR、視蛋白和神經(jīng)視蛋白[9091],該類視蛋白在光感應(yīng)和信號(hào)轉(zhuǎn)導(dǎo)中發(fā)揮作用,但其具體功能和信號(hào)轉(zhuǎn)導(dǎo)機(jī)制與纖毛視蛋白和桿狀視蛋白不同。同時(shí),不同視蛋白的基因在不同生長發(fā)育階段和不同組織呈現(xiàn)表達(dá)特異性,是行為、生態(tài)和適應(yīng)的基礎(chǔ)[92]。
當(dāng)視蛋白等色素分子經(jīng)過光激活后,細(xì)胞內(nèi)的信號(hào)轉(zhuǎn)導(dǎo)過程不僅局限于色素分子自身的構(gòu)型變化,還觸發(fā)一系列精細(xì)調(diào)控的分子事件,其核心是離子通道狀態(tài)的改變,以致細(xì)胞膜電位的調(diào)整。離子通道的開啟或關(guān)閉是細(xì)胞響應(yīng)外界刺激的基本方式之一,視覺信號(hào)的處理同樣遵循這一規(guī)則。激活的視紫紅質(zhì)或變視紫紅質(zhì)將激活Gq家族的異三聚體G蛋白,從而激活由norpA基因編碼的磷脂酶C[93]。其中,G蛋白的激活引起第二信使系統(tǒng)的變化,例如cGMP水平下降,直接影響離子通道的開閉狀態(tài)[94]。光感受過程中,尤其在視網(wǎng)膜的感光細(xì)胞中,cGMP控制的離子通道在無光照時(shí)保持開放狀態(tài),允許鈉離子流入細(xì)胞內(nèi)。當(dāng)光激活視蛋白時(shí),cGMP水平降低導(dǎo)致這些通道關(guān)閉,減少了鈉離子流入,進(jìn)而引起細(xì)胞膜電位的變化[95]。
細(xì)胞電位變化是信號(hào)傳遞的關(guān)鍵環(huán)節(jié),它將光信號(hào)轉(zhuǎn)換成電信號(hào),使得信號(hào)經(jīng)視網(wǎng)膜神經(jīng)元傳遞到大腦。在這一轉(zhuǎn)換過程中,各類型的離子通道參與了信號(hào)的不同調(diào)節(jié)階段,例如鈣離子通道在調(diào)節(jié)神經(jīng)遞質(zhì)釋放中發(fā)揮重要作用,影響神經(jīng)信號(hào)的傳遞效率和精確度。此外,由視蛋白激活引發(fā)的離子通道狀態(tài)變化及其對(duì)細(xì)胞膜電位的影響,不僅構(gòu)成光感受和視覺信號(hào)轉(zhuǎn)換的基礎(chǔ),也是視覺系統(tǒng)能夠?qū)獯碳た焖俜磻?yīng)的原因之一。借助這種機(jī)制,生物能在極短的時(shí)間內(nèi)感知光的存在、變化和移動(dòng),從而實(shí)現(xiàn)復(fù)雜環(huán)境中的導(dǎo)航和物體識(shí)別。
4.2 光信號(hào)的處理與傳遞
從感光細(xì)胞產(chǎn)生的神經(jīng)沖動(dòng)通過神經(jīng)突觸傳遞到其他神經(jīng)元,此過程涉及復(fù)雜的化學(xué)遞質(zhì)的釋放和接收機(jī)制。感光細(xì)胞末端的突觸傳遞過程包括幾個(gè)關(guān)鍵步驟:1)去極化和鈣離子的流入;2)神經(jīng)遞質(zhì)的釋放;3)神經(jīng)遞質(zhì)的接收。
典型的突觸傳遞始于光敏蛋白復(fù)合體在昆蟲感光細(xì)胞微絨毛中對(duì)光信號(hào)的感知。這一復(fù)合體吸收光子后發(fā)生構(gòu)型變化,引發(fā)細(xì)胞膜去極化。這一初步反應(yīng)主要由TRP通道和瞬時(shí)受體電位類通道(TRPlike,TRPL)的活化引起,最終導(dǎo)致細(xì)胞內(nèi)鈣離子濃度上升。對(duì)于日行性昆蟲如果蠅,TRP和TRPL通道對(duì)光信號(hào)的快速響應(yīng),增加了細(xì)胞內(nèi)鈣離子濃度,促進(jìn)視覺信號(hào)的放大和傳遞。這些通道的有效活化是日行性昆蟲適應(yīng)明亮環(huán)境并進(jìn)行快速視覺處理的關(guān)鍵[96]。鈣離子的增加,進(jìn)一步激活電壓依賴性鈣通道,如Shab通道(KV2家族鉀通道),該通道的開放對(duì)于光信號(hào)的放大和傳遞至關(guān)重要。對(duì)夜行性昆蟲如美洲大蠊Periplaneta americana,視覺轉(zhuǎn)導(dǎo)機(jī)制展現(xiàn)出獨(dú)特特征[97]:TRPL通道依然負(fù)責(zé)光信號(hào)的初步接收,但是EAG通道(鉀通道,KV10家族)而非Shab通道扮演著調(diào)節(jié)細(xì)胞后期去極化和信號(hào)傳遞的關(guān)鍵角色[98],特別是EAG通道的光依賴性抑制(lightdependent inhibition,LDI)特性,可能是通過調(diào)節(jié)內(nèi)部鈣離子儲(chǔ)量的釋放來實(shí)現(xiàn)。這種機(jī)制反映了夜行性昆蟲適應(yīng)低光照環(huán)境的復(fù)雜調(diào)控策略。
當(dāng)光信號(hào)導(dǎo)致感光細(xì)胞去極化,進(jìn)而使電壓依賴性鈣通道開啟時(shí),鈣離子流入直接觸發(fā)了神經(jīng)遞質(zhì)的釋放。這些鈣離子迅速與突觸前末端的突觸小泡胞吐復(fù)合體相互作用,推動(dòng)含有神經(jīng)遞質(zhì)的突觸小泡向前膜移動(dòng)并與之融合。此融合過程使得神經(jīng)遞質(zhì)被釋放到突觸間隙中,并與突觸后膜上的受體結(jié)合,從而完成信號(hào)傳遞[99100]。此過程不僅傳遞了感光細(xì)胞接收的原始光信號(hào),還通過神經(jīng)遞質(zhì)的量化釋放增強(qiáng)了信號(hào)的強(qiáng)度與效果,在視覺信號(hào)放大中起到關(guān)鍵作用[101]。多種神經(jīng)遞質(zhì)參與調(diào)控昆蟲的光信號(hào)傳遞,例如組胺[84,102104]、乙酰膽堿[105]、章魚胺[106]等。其中組胺的作用受到廣泛關(guān)注,組胺主要由R8型感光細(xì)胞釋放,這些細(xì)胞負(fù)責(zé)介導(dǎo)視覺信號(hào)的傳遞[107]。研究顯示,R8細(xì)胞通過釋放組胺,激活下游視神經(jīng)節(jié)細(xì)胞,從而參與視覺信息的處理[103]。此外,組胺的釋放可能也受到其他神經(jīng)遞質(zhì)的調(diào)控,這種協(xié)同效應(yīng)對(duì)視覺信號(hào)的放大和細(xì)化可能起到關(guān)鍵作用[84]。雖然組胺在視覺轉(zhuǎn)導(dǎo)中的作用已被認(rèn)可,其具體的作用機(jī)制仍有爭議。有研究提出,組胺可能通過激活特定的離子通道,影響細(xì)胞的興奮性,從而調(diào)節(jié)視覺信號(hào)的傳遞效率[108]。而其他研究則認(rèn)為組胺可能通過影響細(xì)胞間的突觸連接,改變視覺信息的傳遞路徑[109]。這些不同的觀點(diǎn)表明,組胺在昆蟲視覺系統(tǒng)中的作用可能更加復(fù)雜。此外,在蜜蜂對(duì)光刺激的神經(jīng)反應(yīng)的研究中,章魚胺和酪胺被證明
具有相反效應(yīng):章魚胺增強(qiáng)視覺神經(jīng)元對(duì)光刺激的反應(yīng)敏感性,而酪胺則降低這種敏感性。這種拮抗性調(diào)節(jié)機(jī)制有助于蜜蜂適應(yīng)不同光照環(huán)境[106]。在一些昆蟲中,乙酰膽堿作為另一種神經(jīng)遞質(zhì),通過與組胺的協(xié)同作用參與視覺信號(hào)的放大和細(xì)化,從而提高視覺信息的傳遞效率和準(zhǔn)確性[85]。神經(jīng)遞質(zhì)之間的相互調(diào)節(jié)和協(xié)同作用顯示了昆蟲視覺系統(tǒng)在信號(hào)放大和信息處理方面的高適應(yīng)性和效率,凸顯了昆蟲視覺轉(zhuǎn)導(dǎo)機(jī)制的復(fù)雜性及其生物學(xué)重要性。這些遞質(zhì)具體如何調(diào)節(jié)視覺信號(hào)的傳遞及其相互之間的具體交互作用,仍有待進(jìn)一步的研究探索。
昆蟲感光細(xì)胞產(chǎn)生的電信號(hào)經(jīng)過一系列的處理與傳遞,最終形成視覺。這一復(fù)雜過程涉及從復(fù)眼到中樞多個(gè)層級(jí)的神經(jīng)網(wǎng)絡(luò)。綜上所述,昆蟲視覺系統(tǒng)通過多級(jí)網(wǎng)絡(luò)處理和傳遞信號(hào),形成一個(gè)復(fù)雜的信息處理流水線,以實(shí)現(xiàn)對(duì)環(huán)境中運(yùn)動(dòng)、方向和色彩的感知,指導(dǎo)其視覺導(dǎo)航行為。
5 討論與展望
本文綜述了復(fù)眼的基本結(jié)構(gòu)、視覺行為的多樣性、視覺轉(zhuǎn)導(dǎo)機(jī)制,以及目前研究中所使用的技術(shù)方法。昆蟲適應(yīng)環(huán)境變化過程中復(fù)眼起著關(guān)鍵作用,未來的研究應(yīng)進(jìn)一步探索復(fù)眼適應(yīng)性演化的分子機(jī)制。復(fù)眼的視覺轉(zhuǎn)導(dǎo)機(jī)制,包括光信號(hào)的接收、轉(zhuǎn)換和處理,細(xì)胞分子水平依然是研究的焦點(diǎn),特別是視蛋白的作用以及光信號(hào)在神經(jīng)網(wǎng)絡(luò)中的傳遞路徑。光學(xué)顯微鏡、分子生物學(xué)和遺傳學(xué)工具等研究方法與技術(shù)的進(jìn)步,已極大推動(dòng)了對(duì)復(fù)眼的深入研究。這些技術(shù)不僅揭示了復(fù)眼的微觀結(jié)構(gòu),而且視覺相關(guān)基因的功能也得到深入研究。新技術(shù)的應(yīng)用,例如基因編輯和神經(jīng)成像技術(shù),將為探索復(fù)眼的發(fā)育、功能和演化提供新視角。
目前尚有眾多需要解決的問題:1)分子與細(xì)胞機(jī)制。昆蟲是如何在進(jìn)化過程中通過分子機(jī)制調(diào)整復(fù)眼結(jié)構(gòu)以適應(yīng)不同光環(huán)境的?晝夜節(jié)律如何影響昆蟲復(fù)眼的發(fā)育和功能,尤其夜行性和晝行性昆蟲之間的區(qū)別?昆蟲復(fù)眼中視蛋白的多樣性與其對(duì)不同光譜、光強(qiáng)的敏感性是否相關(guān),這些視蛋白又是如何協(xié)同作用以實(shí)現(xiàn)顏色視覺的?2)神經(jīng)生物學(xué)與行為。在昆蟲從復(fù)眼到大腦的信號(hào)傳遞過程中,神經(jīng)網(wǎng)絡(luò)是如何組織的?這些網(wǎng)絡(luò)又是如何影響視覺信息的處理及行為輸出的?復(fù)眼的特定結(jié)構(gòu)特征是如何適應(yīng)其導(dǎo)航、覓食和避敵等視覺行為的?復(fù)眼是如何與其他感官系統(tǒng)(例如嗅覺、觸覺)整合,從而實(shí)現(xiàn)復(fù)雜的行為反應(yīng)的?3)進(jìn)化與多樣性。不同昆蟲類群和不同棲境的同一類群復(fù)眼在結(jié)構(gòu)和功能上有哪些異同,這些差異是如何在進(jìn)化過程中形成的?
昆蟲復(fù)眼的研究在農(nóng)業(yè)害蟲防治中具有重要意義,其意義主要體現(xiàn)在以下幾個(gè)方面:1)提高對(duì)農(nóng)業(yè)害蟲識(shí)別和監(jiān)測(cè)的準(zhǔn)確度。昆蟲復(fù)眼能感知物體的大小、形狀和顏色等。2)優(yōu)化害蟲誘捕裝置的設(shè)計(jì)。根據(jù)不同昆蟲對(duì)特定光波的趨向性,開發(fā)具有高專一性的誘捕裝置,以盡可能避免誤殺天敵昆蟲和有效減少化學(xué)農(nóng)藥的使用。3)發(fā)展基于視覺信號(hào)的害蟲防控技術(shù)。例如,模擬昆蟲視覺系統(tǒng),使用特定波長的光源吸引并滅殺害蟲。4)促進(jìn)生物防治技術(shù)的發(fā)展。通過研究捕食性天敵昆蟲的復(fù)眼結(jié)構(gòu)和光反應(yīng),可深入了解它們的生活習(xí)性和分布范圍,為生物防治提供科學(xué)依據(jù)。
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