Saturday, April 11, 2009
Rhodopsin
On page 107, Carroll discusses the evolution of the detection of light, based on rhodopsin in terrestrial organisms versus organisms that live deep in the ocean. In the ocean-dwelling organisms, "blue-shifted" rhodopsin were found. Explain why this would be a selective advantage based on Carroll's brief explanation and other outside research. Also, discuss the research and experiments that were done to further understand the rhodopsin of deep-living creatures. Bring in other examples of experiments that were done on this topic and results that they found. Explain the differences found in terrestrial versus marine organisms, but then also explain the difference between deep-living and shallow-living marine organisms by using specific examples.
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A “blue-shifted rhodopsin” is a rhodopsin created from a gene that has certain mutations that makes the rhodopsin absorb wavelengths of light that are closer to the blue end of the the visible light spectrum. This shift towards the blue end allows an organism to absorb darker wavelengths of light. This is helpful for deep-sea dwelling because as we learned during the photosynthesis unit, that as light travels through more and more water it it is absorbed more and more so that only the low wavelengths of light penetrate the deep sea. Down in this region, organisms such as a coelacanth or the bottle-nosed dolphin need this specialized vision in order to see. This clearly makes blue shifted rhodopsin a selective advantage because it would aid species in survival by allowing easier escape from prey, and increased predatory skills. A key piece of evidence for the direct correlation between deep-sea dwelling creatures and their types of opsins is the experiment discussed on page 122. It does not involve rhodopsin, but rather the SWS opsin, which receives dim light also. The tree shows that throughout the splits of the genealogical tree of mammals, the only two groups that have lost their functional SWS opsin is the coelacanth and the ceteceans. Since ceteceans are lower down in the genealogical tree than the split for coelacanths, but their close relatives still have a functional SWS opsin means that the fossilization of the SWS opsin occurred independently in the coelacanth and the ceteceans. Independent convergence is one of the strongest pieces of evidence of the action of natural selection, or in this case, the relaxation of natural selection. Both the coelacanth and the ceteceans live in the deep sea and no longer need an SWS opsin because their need for vision has become very specialized to the very lowest wavelengths that are received by blue-shifted rhodopsin. For this reason, natural selection relaxed and when mutations occurred that impaired the SWS opsin, the species who had this mutation were unaffected in terms of survival. Because of relaxed selection, these mutations were “neutral selections”, so they were eventually retained by the coelacanth and the ceteceans.
ReplyDeleteThe main difference one would usually see between the rhodopsins of terrestrial and marine animals is a red shifted rhodopsin in terrestrial animals and a blue shifted rhodopsin in marine animals. Because terrestrial animals receive light after it has passed through the ozone, not as much light has been absorbed. In deep marine animals, the light has passed through both the ozone layer and many levels of water, so many of the wavelengths of light have already been absorbed. This situation can be superimposed on the difference between shallow water creatures and deep sea creatures. The deep sea creatures receive the light after it has passed through more water, and many of the wavelengths of light have already been absorbed. The shallow water, however, has not absorbed as any wavelengths.
http://en.wikipedia.org/wiki/Rhodopsin
http://www.online-vitamins-guide.com/deficiency/rhodopsin.htm
One such experiment that was done to better understand blue-shifted rhodopsins in deep-sea living creatures was that of the haloarchaeon Natronobacterium pharaonis. Haloarchaea are a form of extremophile that need high salt concentrations to survive. The rhodopsins in N. pharaonis, along with other archaea and bacteria, function in signal systems that react to environmental stimuli, which in this case is light. In these organisms, rhodopsins are phototactic receptors that serve as ion pumps. Ions are pumped across the membrane when light excites the chromophore on the rhodopsin. In this specific experiment, it was found that the rhodopsins not only functioned better at lower wavelengths, but also at a lower pH and lower temperatures. These would be ideal conditions for organisms that live at such great depths (1). Another experiment done on N. pharaonis showed how the sensory rhodopsins I and II are involved in a mechanism called phototaxis where changes in light intensity dictate an organism’s swimming patterns. In this specific organism, it was found that they responded better under light intensities that were shifted 70 to 80 nanometers in the blue direction. This proves the blue shift in deep-dwelling marine organisms (2).
ReplyDelete1. Igor Chizhov, Georg Schmies, Ralf Seidel, Jens R. Sydor, Beate Lüttenberg and Martin Engelhard. “The Photophobic Receptor from Natronobacterium pharaonis: Temperature and pH Dependencies of the Photocycle of Sensory Rhodopsin II”. Biophysical Journal, Volume 75, Issue 2, 999-1009, 1 August 1998. http://www.cell.co/biophysj m
2. Hartmut Luecke, Brigitte Schobert, Janos K Lanyi, Elena N Spudich, John L Spudich. "Crystal stucture of sensory rhodopsin II at 2.4 angstroms: Insights into color tuning and transducer interaction. " Science 293.5534 (2001): 1499-503. Platinum Periodicals. ProQuest. 14 Apr. 2009. http://www.proquest.com/