The eyespots of Platynereis larvae allow it to move toward light
In linkurl:2001,;http://www.ncbi.nlm.nih.gov/pubmed/11604122?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum Arendt's group discovered that the gene linkurl:Pax6,;http://www.the-scientist.com/article/display/19190/ which controls the development of eyes and other sensory organs in humans, was also active in rag-worm eyespots, confirming early traces of more sophisticated eye systems. In linkurl:2004,;http://www.ncbi.nlm.nih.gov/pubmed/15514158?ordinalpos=&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.SmartSearch&log$=citationsensor they identified two types of photoreceptor cells in the rag-worm -- the rhabdomeric photoreceptors in the eyespots and ciliary photoreceptors in the brain. In their latest set of experiments, the researchers used linkurl:immunohistochemistry;http://www.the-scientist.com/article/display/17060/ to trace the photoreceptor axons directly to neighboring ciliated cells in Platynereis larvae. "Essentially there is no nervous system in between" the photoreceptors and ciliated cells, Arendt said. "The axons contact ciliated cells in the neighborhood." Double whole-mount in situ hybridization revealed that photoreceptors and ciliated cells were communicating via the neurotransmitter acetylcholine. Blocking the acetylcholine receptor prevented the larvae's helical movement toward light without affecting the speed at which they swam, revealing the direct sensory-motor coupling between the photoreceptor and the cilia cells. To understand how light exposure influences the movement of cilia, they selectively illuminated one of the organism's two eyespots and recorded the force of cilia beats. The light activated five ciliated cells neighboring the eyespot, which resulted in a slowed beat with more force on one side of the larvae without affecting the cilia on the opposing side of the larvae, leading the larvae to make a helical turn. linkurl:John Spudich,;http://www.uth.tmc.edu/bmb/faculty/Spudich/Spudich.html a photobiologist at the University of Texas in Houston who was not involved in the study, told The Scientist that Arendt's "elegant study" identifies a photosensory mechanism that may be the "key evolutionary intermediate suggested by Charles Darwin's reflections on the evolution of human vision." Previous studies in Spudich's lab showed the unicellular algae Chlamydomonas also controls helical phototactic swimming by modulating asymmetric ciliary beating. The study also has ecological implications, Arendt said. "Phototaxis is of important component of marine life," he said. "If you want to understand anything in the ecosystem, including planet climate change, you need to start by understanding the mechanism by which [phototaxis is taking place]." Image courtesy of Arendt, D. and Jekely, G., EMBL This post has been updated from a previous post to correctly identify Russell Fernald of Stanford University. The Scientist regrets the error.