Davies Wayne L, Carvalho Livia S, Tay Boon-Hui, Brenner Sydney, Hunt David M, Venkatesh Byrappa
UCL Institute of Ophthalmology, London EC1V 9EL, United Kingdom.
Genome Res. 2009 Mar;19(3):415-26. doi: 10.1101/gr.084509.108. Epub 2009 Feb 4.
The cartilaginous fishes reside at the base of the gnathostome lineage as the oldest extant group of jawed vertebrates. Recently, the genome of the elephant shark, Callorhinchus milii, a chimaerid holocephalan, has been sequenced and therefore becomes the first cartilaginous fish to be analyzed in this way. The chimaeras have been largely neglected and very little is known about the visual systems of these fishes. By searching the elephant shark genome, we have identified gene fragments encoding a rod visual pigment, Rh1, and three cone visual pigments, the middle wavelength-sensitive or Rh2 pigment, and two isoforms of the long wavelength-sensitive or LWS pigment, LWS1 and LWS2, but no evidence for the two short wavelength-sensitive cone classes, SWS1 and SWS2. Expression of these genes in the retina was confirmed by RT-PCR. Full-length coding sequences were used for in vitro expression and gave the following peak absorbances: Rh1 496 nm, Rh2 442 nm, LWS1 499 nm, and LWS2 548 nm. Unusually, therefore, for a deep-sea fish, the elephant shark possesses cone pigments and the potential for trichromacy. Compared with other vertebrates, the elephant shark Rh2 and LWS1 pigments are the shortest wavelength-shifted pigments of their respective classes known to date. The mechanisms for this are discussed and we provide experimental evidence that the elephant shark LWS1 pigment uses a novel tuning mechanism to achieve the short wavelength shift to 499 nm, which inactivates the chloride-binding site. Our findings have important implications for the present knowledge of color vision evolution in early vertebrates.
软骨鱼类作为现存最古老的有颌脊椎动物类群,位于有颌类谱系的基部。最近,象鲨(Callorhinchus milii)的基因组已被测序,象鲨是一种银鲛目全头亚纲鱼类,因此成为首个以这种方式进行分析的软骨鱼类。银鲛在很大程度上被忽视了,人们对这些鱼类的视觉系统了解甚少。通过搜索象鲨基因组,我们鉴定出了编码一种视杆视觉色素Rh1以及三种视锥视觉色素的基因片段,即中波敏感或Rh2色素,以及长波敏感或LWS色素的两种异构体LWS1和LWS2,但没有发现两种短波敏感视锥类色素SWS1和SWS2的证据。通过逆转录聚合酶链反应(RT-PCR)证实了这些基因在视网膜中的表达。全长编码序列用于体外表达,得到了以下峰值吸光度:Rh1为496纳米,Rh2为442纳米,LWS1为499纳米,LWS2为548纳米。因此,不同寻常的是,作为一种深海鱼类,象鲨拥有视锥色素以及三色视觉的潜力。与其他脊椎动物相比,象鲨的Rh2和LWS1色素是迄今为止已知的各自类别中波长偏移最短的色素。我们讨论了其机制,并提供了实验证据,表明象鲨的LWS1色素使用一种新的调谐机制来实现向499纳米的短波长偏移,这使氯离子结合位点失活。我们的发现对目前早期脊椎动物色觉进化的认识具有重要意义。