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ScienceWeek
CELL BIOLOGY: ON FAST RECOVERY IN VISUAL PIGMENTS
The following points are made by D. Osorio and D.-E. Nilsson (Current Biology 2004 14:R1051):
1) Visual photoreceptors measure light intensity by counting photons. This requires that receptors should transduce as large a proportion as possible of the incident light, and minimize noise caused by variations in responses to single photons or activity independent of light. To meet these requirements photoreceptors contain a visual pigment, rhodopsin, densely packed in membrane structures. Vertebrates have stacked membrane discs or lamellae, which form the outer segments of rods and cones, whereas in arthropods the photopigment is contained in microvilli that form rhabdoms.
2) Despite their basic similarities, the anatomy and physiology of photoreceptors varies substantially. For example, cone outer segments of terrestrial vertebrates are conical and short, ranging from 8-40 microns in length, whereas rod outer segments are longer, typically 30-90 microns, and cylindrical. Arthropods manage with a single type of receptor, and the length of the rhabdom is generally greater in diurnal than nocturnal species, and can therefore absorb a greater proportion of incident light. Some dragonfly rhabdoms are about 1 mm long, 50 times the length of most cone outer segments.
3) Physiologically, response speeds of the different types of receptor vary substantially. Whereas rod responses to a single photon or brief flash last several hundred milliseconds, light-adapted cone responses last about 50 milliseconds, and insect receptor responses last about 10 milliseconds [1]. This allows insects to see much more rapid flicker than vertebrates are able to detect. Recent work points to differences in the level of spontaneous photon-like events, or "dark light", as a key influence on photoreceptor design, and suggests that vertebrate phototransduction, and cones in particular, suffer a flaw which has been avoided by arthropods.
4) The faintest flash of light that humans can distinguish from total darkness contains five or fewer absorbed photons [2,3], suggesting that this is the number required to exceed spontaneous fluctuations in the patch of rods pooling signals to each retinal ganglion cell. Because spontaneous activity is indistinguishable perceptually from real light, it was hypothesized that the cause was activation of the photopigments themselves, for example by thermal isomerization of retinal chromophore from the 11-cis to the all-trans configuration, that is normally trigged by absorption of a photon [2,4].
5) This hypothesis predicts that dark noise, and hence absolute visual threshold, should rise with temperature, and this is the case in amphibians [5]. But it does not easily account for the fact that cones have a far higher absolute threshold than rods. The level of dark noise in human rods is equivalent to about 0.01 events per receptor per second; although estimates vary, the absolute threshold of cones suggests that the noise level is far higher, perhaps 3000 times noisier than in rods [2].
References (abridged):
1. Weckström, M. and Laughlin, S.B. (1995). Visual ecology and voltage-gated ion channels in insect photoreceptors. Trends Neurosci. 18, 17-21
2. Barlow, H.B. (1964). The physical limits of visual discrimination. In Photophysiology. Chapter 16. Giese, A.C. ed. (New York: Academic Press)
3. Aidely, D.J. (1998). The physiology of excitable cells. (4th edn.) (Cambridge University Press)
4. Barlow, H.B. (1957). Purkinje shift and retinal noise. Nature 179, 255-256
5. Ala-Laurila, P., Donner, K. and Koskelainen, A. (2004). Thermal activation and photoactivation of visual pigments. Biophysical J. 86, 3653-3662
Current Biology http://www.current-biology.com
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Related Material:
EVOLUTIONARY BIOLOGY: ON ANCIENT PHOTORECEPTORS
The following points are made by Thurston Lacalli (Nature 2004 432:454):
1) The image-forming eyes, simple eyes (ocelli), and other photoreceptor organs of animals are structurally diverse. But their photoreceptor cells are basically of two types only --either "ciliary" or "rhabdomeric", depending on whether they use cilia or arrays of microvilli for light reception. In a study of Platynereis, a marine segmented worm, Arendt et al(1) provided convincing evidence from gene-expression studies and sequence comparisons that the last common ancestor of bilaterally symmetric animals had both types. Their proposal for the functions the two performed, specifically the role of ciliary receptors in monitoring photoperiod, advances our understanding of the ancestral condition before the origin of divergent types of advanced image-forming eyes.
2) Our own eyes, like those of other vertebrates, have ciliary photoreceptors; so does the pineal "third eye", a structure that is buried in the brain and is involved in circadian rhythmicity, and which still, in lower vertebrates, functions directly as a photoreceptor. The various ocelli and image-forming eyes of invertebrates, in contrast, are rhabdomeric. This, for a while, provided a useful general rule that, along with embryological differences, distinguished between the two main groups of animals: protostomes (diverse worms, molluscs and arthropods) use rhabdomeric photoreceptors; deuterostomes (vertebrates and their kin) have ciliary ones.
3) The person most closely associated with the idea of a dichotomy was Richard Eakin (1910-1999), who carried out an extensive study of comparative eye structure using the then relatively new technique of electron microscopy(2). Exceptions to the rule do occur, and in some marine flatworm larvae even single ocelli can have both types of receptor cell(3). The small size and sporadic occurrence of such structures has discouraged any systematic study of their function, however, so they have remained little more than anomalies in an otherwise broadly accepted general pattern.
4) The advantage of molecular techniques, as applied to the problem by Arendt et al(1), is twofold: first, their ability to reveal gene-expression patterns in individual cells; second, the inferences one can make regarding function based on the known function of homologous genes (orthologues) in other animals. The results show that there are two forms of the gene for the photopigment opsin in Platynereis, one ciliary, previously unknown from protostomes, and one rhabdomeric.
5) In evolutionary terms, it is a long way from a simple ocellus, involving no more than a few cells, to the complexity of an optimally constructed image-forming eye. Evolution seems to have accomplished this transition piecemeal, by myriad small steps, each an adaptive improvement over what went before. A detailed accounting of the steps is as yet beyond us, but clarifying the nature of the ancestral condition is a useful beginning.(4,5)
References (abridged):
1. Arendt, D., Tessmar-Raible, K., Snyman, H., Dorresteijn, A. W. & Wittbrodt, J. Science 306, 869-871 (2004)
2. Eakin, R. M. in Visual Cells in Evolution (ed. Westfall, J. A.) 91-105 (Raven, New York, 1982)
3. Eakin, R. M. & Brandenberger, J. L. Science 211, 1189-1190 (1981)
4. Lacalli, T. C. Proc. R. Soc. Lond. B 212, 381-402 (1981)
5. Page, L. R. & Parries, S. C. J. Comp. Neurol. 418, 383-401 (2000)
Nature http://www.nature.com/nature
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VISION AND THE CAMBRIAN EXPLOSION
The following points are made by Andrew Parker (citation below):
1) Consider dividing geological time into two parts -- pre-vision and post-vision. The boundary separating these parts stands at 543 million years ago. Considering vision as the most powerful stimulus on Earth, the way the world functions today is the same way it functioned ten million years ago, 100 million years ago and 537 million years ago, after the Cambrian explosion. Similarly, the world was without vision 544 million years ago just as it was 600 million years ago. In the interval of life's history of these two parts, a light switch was turned on. For the second half it remained on, although during the first half it was always off.
2) We know that vision places major restrictions on the external forms of animals today, but before the Cambrian it could not have played such a role because eyes did not exist. Consequently light did not exist as a major stimulus in the behavioral system of animals. By vision I mean the ability to produce visual images, which can be achieved only by animals with eyes. Light is used to determine the direction of sun-light in numerous forms of simple animals. Testament to this are the algae found in the snow at the Burgess quarries in Canada, with their red eyespots but lack of vision. But these have nothing to do with vision. Indeed, some plants even possess simple light perceptors that regulate the shift from vegetative growth to floral development. But this form of light detection is not vision. Vision is the capacity to perceive and classify objects using light, or seeing.
3) The Precambrian was a time where only soft-bodied representatives of the multicelled animal phyla existed. Effectively light as a major stimulus is, or rather visual appearances are, removed from the Precambrian environment because the animals of that time did not possess eyes. Presumably Precambrian animals possessed chemical, sound and/or touch receptors. They may also have possessed simple light perceptors, like the algae in the Canadian snow, but nothing that could form an image. Light could be considered a very minor selection pressure in the Precambrian. It could not have had a direct effect on the evolution of multicelled animals (it could have had an indirect effect in that animals which fed on photosynthetic algae would have been restricted to sunlit zones).
4) Competition and predation would not have been major selective pressures in the Precambrian, but they were taking a foothold. The Ediacaran animals of the Precambrian were gradually developing brains. They were developing ways to pick up environmental cues, or news items, and process that information. They were also evolving the ability to chew, and were gradually developing a rudimentary form of rigidness in their limbs. Precambrian trace fossils or footprints suggest that legs could support bodies off the ground. But as in dark caves today, evolution in general would have been slow in the Precambrian, and may well have continued at a gradual pace had it not been for a single but monumental event.
5) This was an event that, in terms of body parts, would have seemed like any other evolutionary innovation, of which there have been many. But this event was different -- it changed the world forever on a scale not since witnessed. At the end of the Precambrian, while most phyla were evolving gradually, a serious transformation was taking place in the soft-bodied trilobites. A light sensitive patch was becoming more sophisticated. It was dividing into separate units. The nerves servicing each unit were becoming more numerous, and so too were the brain cells they serviced. These nerve and brain cells were either multiplying or being borrowed from the wiring and processing system of another sense. Then the outer covering of each unit began to swell and take on focusing properties. One day all this reached a crescendo -- a compound eye had formed.
Adapted from: Andrew Parker: In the Blink of an Eye. Perseus Publishing 2003, p.268
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Notes by ScienceWeek:
The geological period known as the Cambrian is the time frame from about 505 million years ago to 545 million years ago. Its most outstanding aspect is the rather sudden appearance of numerous invertebrate fossils, so numerous that some have termed it an explosion of evolutionary processes. Many of the life forms that existed during the Cambrian are long extinct, but their fossils are numerous, and through their fossils the various Cambrian species have been the subject of much study by paleobiologists.
The Cambrian explosion of life forms has been a long-standing puzzle for paleobiologists, and at present there is apparently no single generally accepted explanation. Among the ideas proposed have been, 1) that the explosion of new forms resulted from a sudden increase in atmospheric oxygen; 2) that the explosion is only apparent, and the Precambrian, the period previous to the Cambrian, lacks fossils because of heat and pressure associated with important geological changes; 3) that living forms evolved mostly in freshwater areas, and are therefore absent in Precambrian sediments, which are primarily marine; 4) that changes in the shape and extent of shorelines produced by continental drift dramatically transformed climate and environment; 5) that the previous evolution of DNA recombination and regulatory genes culminated in and sparked the diversity and anatomical complexity manifested in the explosion; 6) that an exponential increase of species could become significant only after attaining a threshold value at the start of the Cambrian; and, 7) that once multicellular organisms appeared, the intrinsic possibilities for variation increased enormously with a resultant explosion of evolved forms.
Unfortunately, there is no evidence to suggest a selection of one of these proposals, although some of them are less convincing than others. And of course the truth may be that more than one factor was involved. No matter the origin, the Cambrian explosion is apparently accepted by most paleobiologists as a real discontinuity, a period that saw the sudden emergence of dozens of new orders and phyla, including sponges, annelids, crustaceans, hemichordates, brachiopods, and mollusks.
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