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BIOL 2400 Study Guide - Midterm Guide: Opsin, Gene Duplication, Crystallin


Department
Biology
Course Code
BIOL 2400
Professor
T.Ryan Gregory
Study Guide
Midterm

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EVOLUTION POST MIDTERM
Class 17
William Paley
(1743 - 1805)
Natural Theology (1802).
Watch has a matchmaker,
so eye has an eye-maker.
What is an eye?
A minimal eye?
• Some authors: At minimum, a light-sensitive cell
(photoreceptor) and a shield pigment cell (allows
light to enter from one direction alone).
• Other authors: At
minimum, an organ
capable of forming at
least a very basic image.
The diversity of eyes
• Eyes can be observed in about 1/3 of animal phyla.
Another 1/3 have light-sensing organs (but not eyes).
• The largest phyla all have eyes: about 95% of
named species.
• Eyes first appear in fossils from the Cambrian,
about 530 Mya (somewhat later for chordates).
• Several very different kinds of eyes exist,
sometimes in the same lineage (or even the same
organism!).
Important concepts
• Simple linear models show how small changes
could result in gradual improvement via functional
stages.
A comparison of modern species can show how
hypothetical intermediates could have been
functional, but this does not represent the actual
path of evolution.
• Most actual complex organ evolution does not
follow a single direct path
Phototransduction
• Series of chemical reactions that transduce the impact of a
photon to an electrical signal that can be transmitted by
neurons.
• Process begins with a change in the conformation of a
photopigment molecule in the membrane of a light-sensitive cell.
Photopigments
• Central component of
phototransduction pathway
(convert light to electrical signal).
• Two parts: 1) opsin protein, 2)
chromophore (retinal).
• Opsins are part of the very large
family of G protein-coupled
receptor proteins used in various
processes (e.g., vision, olfaction,
nervous system, immune system).
They predate the evolution of eyes.
• Retinal is a modified form of
vitamin A.
Rhabdomeric Ciliary
• Modified membrane (microvilli)
• r-opsin
• Protostomes (most invertebrates)
• Modified cilium
• c-opsin
• Deuterostomes (vertebrates and
related groups
Retinal ganglion cells

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• Do not serve a
visual function but
are functional for
circadian rhythms.
• Contain
melanopsin, which
is similar to r-opsin
(not c-opsin).
• Vestigial rhabdomeric
photoreceptors that took on a
new function
Opsin gene duplication
Lecture 24
All opsins (>1,000 are known) appear to descend from a
single ancestral opsin protein.
a) Ancestral opsin duplicates, diverges into r-opsin
(rhabdomeric photoreceptors) and c-opsin (ciliary
photoreceptors).
b) In vertebrates, c-opsin duplicates and diverges into
rhodopsin (rods, dim light) and photopsin (cones,
daylight).
c) In primates, cone opsins duplicate and diverge to
respond to alternate wavelengths, allowing colour vision.
Lens
• Serve a role in refracting light, but in vertebrates probably began
as a simple layer of transparent cells (role in development and
protection).
• Include high concentration of crystallin proteins – water soluble,
stackable, highly stable refractive molecules. May be cellular (e.g.,
vertebrates) or secreted (e.g., cephalopods).
The origins of crystallins
• Crystallins for lenses have been independently exapted from
different proteins in different lineages.
Based on True and Carroll (2002)
• Many derived from
small heat shock
proteins or
chaperonins.
Gene sharing
• In some cases, the crystallin protein is identical (i.e., is coded
by the same gene) to other proteins unrelated to vision. Some
serve both functions in the eye.
• Requires no change in the gene sequence, only when and
where it is expressed.
Graded refractive index lenses
Glass bead Fish lens
• Higher refractive index in
centre, lower near edges.
• Corrects spherical
aberration.
Corneas
• Initially served a protective role (as they still do in
aquatic animals).
• Contain crystallins which also show evidence of
gene sharing.
• In cephalopods, lens and corneal crystallins are the
same protein (modified glutathione S transferase).
• In mammals, corneal crystallins are derived from
aldehyde dehydrogenase class 3 enzyme (functions
in detoxification and may protect against UV
damage).
Exaptation of corneas on land
Piatigorsky (2007)
• In water, corneas have no
refractive function
(because water and eye
fluid have same refractive
index).
• On land, there is a
difference in index between the air and eye fluid.
• The cornea became more curved to enhance

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this novel function, it does up to 70% of the light
focusing in land vertebrates
Exaptation of lenses on land
• In water, only the lens can focuses
light.
• On land, similar lenses would be too
strong and would result in images
that were over focused.
• In land vertebrates, the lens has
been exapted in a role of
accommodation (focusing at different
distances) rather than serving the
primary focusing function.
• Flattened shape, and specialized
muscles allow to move the lens
(amphibians) or compress it (reptiles,
mammals, and birds).
Vertebrate eye evolution
1. G-protein coupled receptor proteins combine with retinal to
form photopigment. Simple light response. (As in nematode).
2. Duplication and divergence to produce ciliary and
rhabdomeric opsins and photoreceptors. Light sensing but no
vision. (As in non-vertebrate chordates).
3. Eye-field region of brain bulges out to be external to skull.
This then invaginates. Translucent layer of cells forms on top.
Can detect shadows or serve circadian function, but no vision.
(As in hagfish).
4. Duplication and divergence to form rod and cone cells. Iris.
Extraocular muscles. Simple lens from translucent cell layer.
Very simple image capability. (As in lamprey).
5. Further differentiation of photoreceptors. Processing
capability of retina increases. Effective iris. Intraocular
muscles. Refinement of lens (graded index). Strong image
forming ability. (As in fishes).
6. Movement onto land. Lens becomes elliptical and cornea
curves for focusing on land. (As in tetrapods).
7. Primates. Duplication and divergence of opsins to react to
different light wavelengths, evolution of colour vision.
Direct adaptation, exaptation, duplication
and divergence, collage, gene sharing.
The evolution of eyes
• Could proceed through a series of fully functional
gradations that gradually improve image formation.
Both direct and indirect evolutionary processes.
• Most of the “hypothetical” intermediates can still be
found in living organisms, suiting them just fine for
their particular lifestyles.
• No eyes are “perfect”. Human eyes have a blind spot
and an inverted retina, and are relatively poor in terms
of distance sight, spectrum coverage, night vision, and
field of view compared to various other species. But
they suit us just fine for our particular lifestyle.
Class 19
The basis of sexual selection
• Sexual selection arises because of the asymmetry
in reproductive investment in offspring and/or
reproductive potential between males and females.
Eggs are costly. Sperm are cheap.
Also, in species were there is parental care,
females tend to invest a lot more than males.
• Due to the imbalance in reproductive potential,
males and females approach mating very
differently:
• Male fitness is typically maximized by
mating with many females. Males must
therefore compete with one another for access
to females.
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