In addition to intensity and colour, the retinas of many
invertebrates are capable of light detection based on its
linear polarization (Wehner, 1983). The detection mechanism
permitting this capability is based on the intrinsic dichroism
of chromophores oriented along rhabdomeric microvilli. In
vertebrates, however, except for anchovies (Fineran & Nicol,
1978), such axial dichroism is absent rendering vertebrate
outer segments insensitive to the polarization of axially incident
light. Nonetheless, there is evidence for
polarization sensitivity in a few species of fish (goldfish,
rainbow trout and sunfish). But the findings for goldfish and
rainbow trout appear contradictory to those for the green
sunfish (Parkyn & Hawryshyn, 1993), and a detection mechanism
that could explain polarization sensitivity for lower
vertebrates in general is unknown.
This thesis was undertaken to try to solve some of these unknowns by investigating: 1) the neural polarization signal,
at the level of the optic nerve, in fish species from four
groups with distinct retinal cone mosaics (rainbow trout,
green and pumpkinseed sunfishes, common white sucker, and
northern anchovy), 2) the ultrastructure and light transmission
properties of different cone types (single, twin
and double cones) , and 3) the characteristics of the
underwater polarized light field that could permit the observed laboratory behaviours in nature. I measured compound
action potential (CAP) responses from the optic nerve of live
anaesthetized fish to evaluate the possibility that a fish
could detect the orientation of the electric field of linearly
polarized light (mathematically-designated as the E-vector) .
Results from these studies showed that rainbow trout and the
northern anchovy were polarization-sensitive, but both species
of sunfish and the common white sucker were not. In addition,
CAP measurements conducted with rainbow trout exposed to light
stimuli of varying polarization percentages showed, in
conjunction with underwater polarized light measurements, that
the use of polarized light in this animal was restricted to
crepuscular time periods. To try to understand why some fish
species were polarization-sensitive and others were not, I
carried out microscopy studies of retinal cones. Optical
measurements of transmitted polarized light through the length
of cones showed: 1) small cone birefringence (retardance <
2nm) , and 2) preferential transmission of polarized light that
was parallel to the partition dividing twin and double cones
(single cones were isotropic). In addition, histological
studies showed that the partition in trout double cones was
tilted with respect to the vertical while that of twin cones
in sunfish was straight. We envisioned that the higher index
of refraction of the partition with respect to the surrounding
cell cytoplasm would make it behave as a mirror, reflecting
and polarizing incident light. A large optical model was built to test this idea consisting of two photodiodes evenly spaced
on either side of a cover-slip "partition" upon which
physiologically-relevant illumination was incident.
Measurements using this model and theoretical calculations
with refractive indices approaching those expected for double
cone partitions and cytoplasm (Sidman, 1957) were consistent
with the optical results obtained in situ. Thus the tilt in
the partition of trout double cones relayed different amounts
of light to each outer segment depending on the polarization
of incident light, whereas a straight partition, as in
sunfish, did not. Comparison of signals from orthogonally-arranged
double cones and single cones in the centro-temporal
retina of trout thus became the basis for a model neural network that could reproduce all the polarization sensitivity
results known to date. To support the idea that an ordered
(e.g. orthogonal) arrangement of double cones was a necessity
for polarization detection, I showed that the common white
sucker, a fish with double cones, had these arranged randomly
in the centro-temporal retina (hence its lack of polarization
sensitivity). Finally, the northern anchovy exhibited unique
cones with lipid lamellae parallel to their lengths, forming
a dichroic system for polarization detection somewhat
analogous to that of cephalopods and decapod crustaceans. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/8135 |
| Date | 18 May 2017 |
| Creators | Novales Flamarique, Inĩgo, Novales Flamarique, Inĩgo |
| Contributors | Hawryshyn, Craig W. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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