Adaptive Evolution
Prog Retin Eye Res 1998 Oct;17(4):597-636
The eyes of deep-sea fish. I: Lens pigmentation, tapeta and visual pigments.
Douglas RH, Partridge JC, Marshall NJ
Department Optometry and Visual Science, City University, London, U.K.
Deep-sea fish, defined as those living below 200 m, inhabit a most unusual photic environment, being exposed to two sources of visible radiation; very dim downwelling sunlight and bioluminescence, both of which are, in most cases, maximal at wavelengths around 450-500 nm. This paper summarises the reflective properties of the ocular tapeta often found in these animals, the pigmentation of their lenses and the absorption characteristics of their visual pigments. Deep-sea tapeta usually appear blue to the human observer, reflecting mainly shortwave radiation. However, reflection in other parts of the spectrum is not uncommon and uneven tapetal distribution across the retina is widespread. Perhaps surprisingly, given the fact that they live in a photon limited environment, the lenses of some deep-sea teleosts are bright yellow, absorbing much of the shortwave part of the spectrum. Such lenses contain a variety of biochemically distinct pigments which most likely serve to enhance the visibility of bioluminescent signals. Of the 195 different visual pigments characterised by either detergent extract or microspectrophotometry in the retinae of deep-sea fishes, ca. 87% have peak absorbances within the range 468-494 nm. Modelling shows that this is most likely an adaptation for the detection of bioluminescence. Around 13% of deep-sea fish have retinae containing more than one visual pigment. Of these, we highlight three genera of stomiid dragonfishes, which uniquely produce far red bioluminescence from suborbital photophores. Using a combination of longwave-shifted visual pigments and in one species (Malacosteus niger) a chlorophyll-related photosensitizer, these fish have evolved extreme red sensitivity enabling them to see their own bioluminescence and giving them a private spectral waveband invisible to other inhabitants of the deep-ocean.
Annu Rev Genet 1997;31:315-36
Molecular genetic basis of adaptive selection: examples from color vision in vertebrates.
Yokoyama S
Department of Biology, Syracuse University, New York, USA. syokoyam@mailbox.syr.edu
A central unanswered question in phototransduction is how photosensitive molecules, visual pigments, regulate their absorption spectra. In nature, there exist various types of visual pigments that are adapted to diverse photic environments. To elucidate the molecular mechanisms involved in the adaptive selection of these pigments, we have to identify amino acid changes of pigments that are potentially important in changing the wavelength of maximal absorption (lambda max) and then determine the effects of these mutations on the shift in lambda max. The desired mutants can be constructed using site-directed mutagenesis, expressed in tissue culture cells, and the functional effect of virtually any such mutant can be rigorously determined. The availability of these cell/molecular methods makes vision an ideal model system in studying adaptive mechanisms at the molecular level. The identification of potentially important amino acid changes using evolutionary biological means is an indispensable step in elucidating the molecular mechanisms that underlie the spectral tuning of visual pigments.
Eye 1998;12 ( Pt 3b):531-40
The photoreceptor mosaic.
Ahnelt PK
Department of General and Comparative Physiology, Medical School, University of Vienna, Austria. peter.ahnelt@univie.ac.at
The organisation of the human photoreceptor mosaic reflects evolutionary strategies for optimising visual information under a wide range of stimulus conditions: (1) The rod population dominates (max. 170,000/mm2 at c. 30 degrees sup.) except for the central 2 degrees and along the ora serrata. (2) Density of cone inner/outer segments reaches up to 300,000 mm2 in the fovea. A bundle of c. 300-500 foveolar cones are further distinguished by having their synaptic terminals located within the capillary-free zone. Radial displacement (> 350 microns) of foveal cone terminals may result in the lesion of two sets of cone pathways by perifoveal laser treatment. Along the ora serrata peripheral cone density (c. 4000) rises within a small rim (1 degree) to up to 20,000, but may be considerably decreased by cystoid degenerations. For the L- and M-cone subpopulations ratios of 2:1 to 1:1 and random arrangement are suggested. (3) Blue-sensitive (S-) cones constitute a regular and independent submosaic of c. 7% across the periphery. An annular maximum (1000-5000/mm2) at c. 1 degree surrounds the foveola. There density decreases and irregular zones lacking S-cones result in tritan deficiencies.
Eye 1998;12 ( Pt 3b):541-7
Evolution of colour vision in vertebrates.
Bowmaker JK
Department of Visual Science, University College London, UK. j.bowmaker@ucl.ac.uk
The expression of five major families of visual pigments occurred early in vertebrae evolution, probably about 350-400 million years ago, before the separation of the major vertebrate classes. Phylogenetic analysis of opsin gene sequences suggests that the ancestral pigments were cone pigments, with rod pigments evolving last. Modern teleosts, reptiles and birds have genera that possess rods and four spectral classes of cone each representing one of the five visual pigment families. The complement of four spectrally distinct cone classes endows these species with the potential for tetrachromatic colour vision. In contrast, probably because of their nocturnal ancestry, mammals have rod-dominated retinas with colour vision reduced to a basic dichromatic system subserved by only two spectral classes of cone. It is only within primates, about 35 millions years ago, that mammals 're-evolved' a higher level of colour vision: trichromacy. This was achieved by a gene duplication within the longer-wave cone class to produce two spectrally distinct members of the same visual pigment family which, in conjunction with a short-wavelength pigment, provide the three spectral classes of cone necessary to subserve trichromacy.
Trends Neurosci 1990 Feb;13(2):55-64
From cornea to retinal image in invertebrate eyes.
Nilsson DE
Department of Zoology, University of Lund, Sweden.
The optical information processing that takes place in an eye involves a large variety of very different optical components that are put together to solve a task which, in its basic nature, does not differ much from one species to another. The principal task of an eye is to sort the incoming photons so that they excite specific sensory neurons depending on angle of incidence, wavelength and plane of polarization. Despite the apparent simplicity of the task, the solutions to it are often complex and the variation between species is enormous. The pinhole camera, the Keplerian and Galilean telescopes, the corner reflector, optical fibres, and interference filters, are all names of optical devices invented by man. It now appears that all of these devices, and many more, exist in various combinations in the optics of invertebrate eyes. The similarity between man's and nature's optical engineering has been useful in many ways. For the study of eyes, it has helped to understand biological design principles in unparalleled detail.