总之,尽管这不是正常现象,但确实有许多哺乳动物因为SWS1视蛋白基因突变而丧失了SWS1感光色素。不同动物的失活突变是不同的,并且这种突变也可能发生在演化过程中的不同阶段,有的发生在物种分化时,有的发生在同源先祖上 (Levenson et al. 2006)。这种失活突变和动物的活动时间有一定相关性,主要出现在夜间活动的物种身上,但夜间活动的物种却不一定存在这种突变。这也是可以理解的,如果夜间活动和失活突变关系非常紧密,那么考虑到早期的哺乳动物经历了漫长的夜行阶段,那么SWS1基因很可能根本不会再出现在后来的哺乳动物身上。由于这种失活突变多发生在广泛分布的物种中,而不同地区的动物在系统发育和生活环境上通常很少有共同之处,因此SWS1视蛋白缺失产生的原因可能也是多种多样的。
(c) 哺乳动物感光色素的光谱定位 Spectral positioning of mammalian cone pigments
到目前为止,人们还没有用这些模型研究用能看到紫外线的啮齿动物(表1)。我们现在已经知道,紫外视觉在一些脊椎动物(尤其是鸟类)的视觉系统中有着非常重要的作用,一些线索表明对啮齿动物而言,紫外视觉可能也很重要。但对这个问题的研究还很少。一个假说认为,在适宜的光照下,新鲜尿液标记的紫外反射率较高,因此啮齿动物的紫外视觉主要是用来观察这些新鲜尿液标记 (Chavez et al. 2003)。而在唯一一项对哺乳动物紫外视觉功能的直接研究中,研究员发现觅食的小鼠并不依靠紫外光信号寻找食物 (Honkavaara et al. 2008)。人们依然不清楚紫外视觉有什么优势,也无法解释为什么哺乳动物中只有啮齿动物仍保留了紫外视觉。
(d) 视杆细胞的作用 Roles for rods
所有哺乳动物的视网膜上都存在视杆细胞和视锥细胞,但是不同的物种其两种细胞的含量有很大的差异。众所周知,这种差异通常和动物所适应的光环境有关,最近有很多对昼行和夜行动物视网膜中心区域的解剖实验和对两种细胞的定量研究都证实了这个观点 (Kaskan et al. 2005)。视网膜对环境压力的反应可能是很明显的。举个例子,尽管夜行大鼠的视网膜很小,但是其中视杆细胞的含量是昼行地松鼠的八倍,所有视觉细胞中只有约1%是视锥细胞,而松鼠视网膜上的视锥细胞则占了约86%。这两种动物身上至少还有两个视觉特性差异和此有关。一是松鼠体内传输颜色信号的神经纤维也比大鼠更多。二是由于信号的叠加可以改善信噪比(signal-to-noise ratio),因此局部视锥细胞密度增大也可以对更好地色觉成像提供帮助。这两个特性都显示了色觉能力的变化,而对这两种动物的色觉测试也证实了这一观点。大鼠和地松鼠都具有两种类型的视锥细胞,因此都可能拥有双色视觉,但是实验表明松鼠的辨色能力比大鼠更强 (Jacobs & Yolton 1971; Jacobs et al. 2001)。这类对比研究通常都专注于视锥细胞的数量和对光谱的敏感性,但是视杆细胞和视锥细胞的比例也应该和色觉高度相关。
(e) 灵长类色觉的独特之处 Primate colour vision is a special case
虽然三种(或更多)类型的感光色素在脊椎动物中很常见,但是在真兽类动物中,只有灵长类动物具有三种视锥细胞,而且这些视锥细胞的排布方式也很少见。而近年来进行的研究也极大地推进了我们对灵长类动物色觉分布和演化的理解 (Regan et al. 2001; Osorio et al. 2004; Jacobs 2007, 2008)。
最早的灵长类动物出现在80–90Ma (Bininda-Emonds et al. 2007; Springer & Murphy 2007)。通常认为,早期灵长类动物是夜行性的 (Martin & Ross 2005),尽管这个结论还很有争议 (e.g. Tan et al. 2005)。因此,和大多数真兽类动物一样,早期灵长类的视网膜上可能只具有SWS1和LWS感光色素,为二色视觉。随后,许多灵长类转为昼行性,它们对视觉更加依赖,这为感光色素和色觉的改变奠定了基础。哺乳动物的LWS基因位于X染色体上,而所有的现生狭鼻猴类(catarrhine,即旧大陆的猴、猿和人类)都具有两个首尾串联的LWS基因。这两个基因产生了两种感光色素(吸收峰分别为530和560 nm,以下称为M和L)。再加上SWS感光色素,这些动物体内就拥有了三种感光色素(图3a)。其中,M和L色素都显然来源于X染色体 (Nathans et al. 1986)。由于人类的这三种感光色素的排列通常和其他的类人猿、新大陆的阔鼻猴(platyrrhine monkeys)不同,而和狭鼻猴更接近,因此X染色体上的序列重复被认为是发生在狭鼻猴分化时,即30–40 Ma。
阔鼻猴中,有两个属和其他的动物不同。正如前文提到的,夜猴属(Aotus)具有非功能性的SWS1基因,因此这种动物只拥有一种视锥蛋白(图3c),缺乏传统意义上的色觉能力 (Jacobs et al. 1993b)。另一方面,吼猴属(Alouatta)又和狭鼻猴相似,它们的X染色体上拥有两段不同的视蛋白基因,能同时表达出S、M和L感光色素(图3a),因此是唯一一种普遍具有三色视觉的阔鼻猴类 (Araujo Jr. et al. 2008)。
视蛋白基因的变化可以改变感光色素的光谱吸收特性。至少,单个核苷酸的替换就足以让该基因编码出的感光色素吸收峰出现位移 (Merbs & Nathans 1993; Asenjo et al. 1994)。如果这个突变伴随着某种选择优势,那么种群中的感光色素基因就会出现多态性。这种情况可能就是某些原猴类和阔鼻猴类拥有如此高多态性的LWS基因的基础。但这样的演化也存在一个限制,即这个新等位基因带来的好处无法被群体中的所有成员平等共享。而如果这段新基因在染色体上重复出现,那么该物种所有的个体就都可以享受到新基因的好处,而这件事可能出现在狭鼻猴演化的早期。目前我们尚不清楚LWS基因的多态性是否是三色视觉演化过程中的必要条件,还是说多态性只是另一个独立的演化分支,和三色视觉没有必然联系。吼猴属的X染色体视蛋白基因序列被重复了,而这又是唯一一种具有三色视觉的阔鼻猴类动物,这也许可以说明,多态性确实是三色视觉的基础 (Dulai et al. 1999)。
没有任何证据表明其他非灵长类哺乳动物中也演化出了第三种感光色素。一个可能的原因是,新色素的形成需要适宜的视锥细胞比例,并且神经系统高度依赖于这个完善的视锥细胞系统提供多变、复杂的信息。许多哺乳动物的视网膜上都只含有少量的视锥细胞,在这种情况下,即使产生了新的色素,个体也很难从这种新的信号中取得生存上的优势,因此无法保留下来。即使动物的视网膜上已经有了足够多的视锥细胞,它们的神经系统也需要能够有效地对比新色素和原有色素传递的信息,这个新色素才是有益的。哺乳动物的视网膜上有两种神经系统用以对比视细胞传递的信息 (Martin 1998; Lee 2004)。一种涉及神经节细胞,接收来自S/UV和M/L光谱的信号输入,并将这种相互拮抗的信号反馈到视觉中枢,这也是辨色能力的生理基础 (Solomon & Lennie 2007)。另一种传递方式起源于所谓的小细胞系统(midget cell system),能将相互拮抗的M和L光谱信号传递给视觉中枢。虽然证据有限,但目前的研究发现第一种路径广泛存在于哺乳动物视网膜中,而第二种路径则为灵长类动物所特有。所有的三大类灵长类动物都拥有小细胞系统,包括那些只有一种类型M/L感光色素的物种,所以人们认为这个系统是在灵长类演化的早期进化出来的,可能是为了提高视觉能力,并成为了后续产生并保留新色素的神经基础 (Wa¨ssle 2004)。从这个角度看,非灵长类哺乳动物的视网膜中缺乏小细胞系统,这可能是限制其色觉发展的重要因素。但是这个结论并不绝对,最近有研究在小鼠的视网膜上改造出了第三类视锥细胞,并成功让小鼠获得了新的色觉,这说明就算缺乏小细胞系统,动物依然有可能获得三色视觉 (Jacobs et al. 2007)。
综上,灵长类动物为什么会演变出这种独特的色觉呢?长期以来,人们一直认为,因为许多灵长类是食果动物,而成熟的果实在颜色上会较为不同,所以这种由M/L色素支持的色觉演变和果实有关 (e.g. Regan et al. 2001)。近年来,人们用了许多计算模型来分析灵长类的M/L色素是否能够更有效地识别隐藏在树叶丛中的果实,答案是肯定的 (Osorio & Vorobyev 1996; Regan et al. 2001; Parraga et al. 2002)。模型的计算结果同样表明,灵长类的这种色觉还对其他一些行为有益,例如鉴别出可食用的树叶、或是分辨个体的皮肤颜色变化 (Dominy & Lucas 2001; Changizi et al. 2006)。到目前为止,仅依靠模型还是无法完全解释灵长类色觉演化的各个方面。
另一种研究色觉演化的方式则是直接通过研究行为来观察动物如何利用颜色信息。于是,拥有多态性的阔鼻猴类就成了这类研究的宝贵研究对象,它们多态性的基因能在自然选择中长期保持 (Boissinot et al. 1998; Surridge et al. 2003),并且种群中存在辨色差异的个体还能够有效地共同生存。对这些物种的研究可以解决一个问题,即有更多辨色能力的个体是否能在觅食中获利。在半自然条件下进行的实验表明,拥有三色视觉的个体相比只有二色视觉的个体,确实能够更有效地收集食物上的颜色信息 (Caine & Mundy 2000; Smith et al. 2003)。这样的结果暗示了三色视觉可能可以增加获取食物的效率,但其他类似的研究却让这个假设变得十分复杂。例如,在自然环境下进行的几组观察研究就并没有发现色觉差异和觅食效率之间的因果关系 (Dominy et al. 2003; Smith et al. 2003; Vogel et al. 2007)。最近对蜘蛛猴(Ateles)的果实采集效率研究发现,三色视觉和二色视觉的蜘蛛猴的效率并没有显著差异 (Hiramatsu et al. 2008)。这个实验特别关注在非常短的距离内(臂长范围内)进行的觅食,并且推测出影响觅食效率的物理特性并不是果实的颜色,而是亮度对比,三色视觉和二色视觉在判断亮度对比上并没有什么差别 (Hiramatsu et al. 2008)。需要注意的是,这种近距离觅食还可以利用除了视觉之外的其他环境信息 (e.g. Dominy et al. 2001)。
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