Rods or Cones?

JKH sorry at nospam.com
Sat Nov 18 19:35:05 EST 2000


After checking the literature, I found some information which might be
interesting to the discussion here:
1. Birds have photoreceptors which can detect wavelength in ultraviolet
range; it means they have even wider range of color perception than mammals.
2. Most vertebrates, including the "living fossil" ancient fish, have both
cone-type and rod-type photo pigments which are sensitive to different
wavelength of light.
3. From some phylogenetic evidence, cones actually appeared earlier in the
evolution than rods.

However, the presence of multiple photo pigments or cones in retina is only
necessary but not sufficient for color vision; there is a "knock-in" study
in mice that the engineered mice expressed human photo pigment in their
cones, which is necessary for trichromacy of color vision. These
artificially added photoreceptors are functional, expanding the range of
light sensitivity in these transgenic mice shown by electroretinogram, but
behavioral tests failed to show that this capacity is realized to their
ability of trichromacy. It is concluded that the wiring of photoreceptors is
important as well.


    Bowmaker JK.
      Evolution of colour vision in vertebrates.
    Eye, 1998, 12 ( Pt 3b):541-7.
Abstract: 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.






"Richard Norman" <rsnorman at mediaone.net> wrote in message
news:V%RQ5.5290$JU.220747 at typhoon.mw.mediaone.net...
> Actually, most mammals do not have cones and color vision at all.
> Most mammals are drab in color, and the color patterns are
> cryptic, for camouflage.  These include the carnivores (like dogs
> and cats) and the hooved grazing animals (cattle, sheep, pigs and
> horses).   Unlike brightly colored birds, reptiles, amphibians,
> and fish, they do not use color for species recognition or signaling.
>
>  Frugiverous (fruit eating) mammals including squirrels and
> primates usually have color vision. The plants reciprocate by signaling
> ripeness and availability with bright colors.
>
>
> "JKH" <sorry at nospam.com> wrote in message
> news:8v08ca$a61$1 at mark.ucdavis.edu...
> > When asking "why" questions, usually you can think about answers from an
> > evolutionary angle. For diurnal primate like us, the critical function
of
> > night vision is to detct predators or recognize surroundings of their
> nest,
> > and this function is critical for survival. Color perception is just a
> bonus
> > during the daytime that help you distinguish things like location and
> > edibility of food, etc. The color vision is a later product in the
> > evolution. I assume nocturnal animals have even higher rod/cone ratios.
> >
> > Jo
> >
> > "mike" <mike-anderson at nwu.edu> wrote in message
> > news:mike-anderson-1211002340380001 at dhcp083108.res-hall.nwu.edu...
> > > Cones in the retina are responsible for our color vision i believe.
> Rods
> > > allow us to perceive black and white.  Intuitively i would think that
we
> > > have much more cones than rods in our eyes, however, in many diagrams
i
> > > have seen there are more rods present, is this just a coincidence or
are
> > > there more rods than cones?
> > >
> > > Rods allow our night vision, however we see much better in daylight,
so
> if
> > > rods are more plentiful than cones do they also contribute to our
> > > processing visual information during the day?
> > >
> > > TIA
> > > -mike
> > >
> > > --
> > > mike
> > > When you decide that your life is a lie, its alright...
> > > http://www.mp3.com/mikeanderson
> >
> >
>
>







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