Download III. Photoreceptors in Protostomes

Photoreception and Vision
I. Overview of photoreception and vision
II. Photoreception in Lower Invertebrates
III. Photoreception in Protostomes
A. Mollusc camera eyes
B. Arthropod compound eyes
IV. Evolution of Eyes
The process involves…..
Transformation
(biochemical energy)
Transduction
(nerve impulse)
Light energy
Integration
Light energy
Rhodopsin
metarhodopsin
+
biochemical energy
Types of photoreceptor cells
(contain rhodopsin)
Ciliary type: photosensitive membrane
derived from or associated with cilia
Rhabdomeric type: photosensitive membrane
derived from microvilli, not cilia
There
has been some
debate
over the
phylogenetic
significance
of receptor
cell types
But some Annelid larvae
have ciliary receptors while
adults have rhabdomeric
II. Photoreceptors in “Lower Invertebrates”
A. Pigmented epithelia: detect light intensity
ex. Corals, most medusae; very small # cells
II. Photoreceptors in “Lower Invertebrates”
B. Eye cup or ocellus: pit of pigmented cells
II. Photoreceptors in “Lower Invertebrates”
B. Eye cup or ocellus: pit of pigmented cells;
discern direction of light
II. Photoreceptors in “Lower Invertebrates”
B. Eye cup or ocellus: pit of pigmented cells
flatworms
Also cnidaria, nemerteans etc.
Phylum Nemertina: eye cups
http://www.discoverlife.org/
II. Photoreceptors in “Lower Invertebrates”
C. Lensed Eyes: Cubomedusae
•Tiny spherical lenses 100 um wide form sharp images free
of spherical aberrations (better peripheral imaging)
• However the focal plane lies behind the retina thus the
image perceived is blurry.
II. Photoreceptors in “Lower Invertebrates”
C. Lensed Eyes: Cubomedusae
Why should evolution have produced such sophisticated
optics that have only poor resolution? Isn’t higher
resolution always better?
In each rhopalium Box jellies have two types of lensed
eyes one looking upward and the other horizontally as
well as two other simple eyes
“Reverse neurobiology” : optics are known but
functions are not
III. Photoreceptors in Protostomes
Many species in the molluscs and arthropods
have only simple eyes or no eyes.
Medial dorsal eye of copepod
III. Photoreceptors in Protostomes
But Mollusca and Arthropoda have independently evolved
eyes that rank among the best:
compound eyes
camera eyes
III. Photoreceptors in Protostomes
A. Mollusca:
2
- most with simple eye cups
- some snails with lensed, image forming eyes
(e.g. queen conch)
- bivalves without ocular organs except for the
scallop Pecten (more on this)
- most cephalopods have image forming
camera eyes and acute vision
III. Photoreceptors in Protostomes
A. Mollusca:
2
- most with simple eye cups
- some snails with lensed, image forming eyes
(e.g. queen conch)
- bivalves without ocular organs except for the
scallop Pecten (more on this)
- most cephalopods have image forming
camera eyes and acute vision
III. Photoreceptors in Protostomes
A. Mollusca:
1. scallops have as many as 60 lensed eyes lining
the edge of the mantle; in some species these eyes are
elaborate and functionally unusual with mirrored
membranes; “ glorified shadow detectors”
III. Photoreceptors in Protostomes
A. Mollusca:
2. nautiloids have a large pair of pin hole eyes
III. Photoreceptors in Protostomes
A.2. nautiloid eyes work by the same
principle as a pinhole camera.
Pin hole
aperture
inverted,
sharp, but
dim image
III. Photoreceptors in Protostomes
A.2. nautiloid eyes work by the same
principle as a pinhole camera.
III. Photoreceptors in Protostomes
A. Mollusca:
Lensed
camera eyes of
cephalopods
form very
sharp images
cornea
‘pupil’
‘sensory
Retina ’
Optic lobe
III. Photoreceptors in Protostomes
Some differences between the cephalopod eye
and the human (chordate) eye:
Direct v. indirect eye arrangement
Rhabdomeric v. ciliary chromophores
Photoreceptors from nervous system
in chordates but from epidermis in
cephalopods
Focusing ability of cornea ( I.e. the
refractive index of water and air)
-- A more dense lens is needed to
focus light originating in a fluid
medium; complications in focusing
cornea
Aquatic lens
eye
Cornea lens
eye
III. Photoreceptors in Protostomes
B. Arthropod Compound eyes
Amphipod
crustacean
with simple
eye and
compound
eye
Made up of hundreds to thousands
independent optical units: ommatidia
facets
Single
ommatidium
Lens and cone
help direct the light
Retinula cells produce
the photosensitive
rhabdomes
Pigment cells can keep
light from moving into
adjacent ommatidia
“pupils”
The
performance
of compound
eyes
•Erect, compound images
•Wide field of vision
• The higher the # of ommatidea the sharper the image
• But no system to adjust focus; image is grainy
• Whole eyes are sensitive to motion
• In some species, lateral inhibition improves sensitivity
Each facet must
Represent 10,000
ommatidea for this
compound eye to
reach the visual
acuity of the
human eye
Among Compound Eyes, the House fly eyes achieve best focus
Among Crustaceans, mantis shrimp
have the most elaborate eyes
Sharp image
Trinocular vision
Use polarized light
Infrared and UV sensitivity
Can be pivoted on
eye stalk, increasing
the visual field to
nearly 360 degrees
IV. How many times did eyes evolve?
1977- Ernst Mayr and colleagues estimate at least 40 times,
although 5 phyla have developed image-forming eyes
(Chordates, Molluscs, Arthropods, Annelids, Cnidarians)
: convergent evolution to vision (image formation)
a.
b.
c.
d.
e.
f.
g.
h.
Eye cups (pits)
Basic compound
Aquatic lens
Corneal lens
Apposition compd
Superposition
Mirrored eyes
Reflecting superposition eyes
IV. How many times did eyes evolve?
1977- Ernst Mayr and colleagues estimate at least 40 times,
although 5 phyla have developed image-forming eyes
(Chordates, Molluscs, Arthropods, Annelids, Cnidarians)
: convergent evolution to vision (image formation)
1993 - Developmental geneticists discover “eye opening”
gene called eyeless in fruit flies
1994 - similar gene found in mice and human eyes
more recently even in squid eyes (pax-6 etc.)
(90% similarity)
1995 - experiments in which eyeless was activated in
other parts of fly body by human and mouse genes
implanted in fly embryos
Work of Walter Gehring and colleagues
When eyeless is turned on
in parts of the body where
it is inactive it could initiate
development of topical eyes
in unusual places... and in
other species!
How many times did eyes evolve?
Gehring calls eyeless a “master control gene” for
eye development. Argues that its occurrence in
many phyla indicates eyes evolved only once
perhaps from a proto-eye: eyes are homologous
Mayr counters that species of worms without eyes
also have eyeless-like genes. Argues that these
are genes that evolved long ago in other functions
( i.e. Nervous system ) then got co-opted into eye
development several times as eyes evolved.
Co-Option
Dickinson:
1. same molecule can assume different functions
2. duplication produces paralogous genes whose
members can encompass a wide number of roles
3. domain shuffling generates molecules with clear
homologies but potentially different functions
Kosmik et al.
Expression of c-opsin in eye
of cubomedusa
What Dan Nilsson
(2004) calls
“genetic promiscuity” in
eye evolution
(Current Opinion in Neurobiology)
melanin is the shielding pigment
Compromise view proposed, but at present
there is no consensus among researchers:
Parallel evolution
Photosensitivity is monophyletic
at some level; pax, eyeless used
in cell, eye development
But image forming eyes
evolved independently, each
time incorporating
developmental pax genes in the
developmental process