使海洋浮游植物群落依光照条件在海水表层及深层之间转移的微小无脊椎动物幼虫,可通过双细胞眼点来探测光强度,与达尔文所假设的作为哺乳动物眼睛进化起源的“原始眼睛”(protoeyes)相似。
科学家对来自眼点的传感信息被翻译成运动的机制并不是很了解。Jékely等人研究了海洋蠕虫岩虫(Platynereis dumerilii)的幼虫,他们发现,眼点受到的光照,导致相邻纤毛通过胆碱能信号作用产生的节拍发生一个变化。计算机模型证实了趋光性所受到的这些局部效应的意义,并且预测,如果该生物采取一个螺旋形游动模式的话,其导航精确度将会提高。光传感与纤毛运动器以这种方式所进行的直接耦合,有可能是“原始眼睛”的一个功能,是动物眼睛演化过程中的一个重要里程碑。(生物谷Bioon.com)
生物谷推荐原始出处:
Nature 456, 395-399 (20 November 2008) | doi:10.1038/nature07590
Mechanism of phototaxis in marine zooplankton
Gáspár Jékely1,4, Julien Colombelli2, Harald Hausen3, Keren Guy1, Ernst Stelzer2, Fran?ois Nédélec2 & Detlev Arendt1
1 Developmental Biology Unit,
2 Cell Biology & Biophysics Unit, European Molecular Biology Laboratory, Heidelberg 69117, Germany
3 Institut für Biologie, Systematik und Evolution der Tiere, Freie Universit?t Berlin, Berlin 14195, Germany
4 Present address: Max Planck Institute for Developmental Biology, Tübingen 72076, Germany.
The simplest animal eyes are eyespots composed of two cells only: a photoreceptor and a shading pigment cell. They resemble Darwin's 'proto-eyes', considered to be the first eyes to appear in animal evolution1, 2, 3, 4. Eyespots cannot form images but enable the animal to sense the direction of light. They are characteristic for the zooplankton larvae of marine invertebrates and are thought to mediate larval swimming towards the light. Phototaxis of invertebrate larvae contributes to the vertical migration of marine plankton5, which is thought to represent the biggest biomass transport on Earth6, 7. Yet, despite its ecological and evolutionary importance, the mechanism by which eyespots regulate phototaxis is poorly understood. Here we show how simple eyespots in marine zooplankton mediate phototactic swimming, using the marine annelid Platynereis dumerilii as a model8. We find that the selective illumination of one eyespot changes the beating of adjacent cilia by direct cholinergic innervation resulting in locally reduced water flow. Computer simulations of larval swimming show that these local effects are sufficient to direct the helical swimming trajectories towards the light. The computer model also shows that axial rotation of the larval body is essential for phototaxis and that helical swimming increases the precision of navigation. These results provide, to our knowledge, the first mechanistic understanding of phototaxis in a marine zooplankton larva and show how simple eyespots regulate it. We propose that the underlying direct coupling of light sensing and ciliary locomotor control was a principal feature of the proto-eye and an important landmark in the evolution of animal eyes.