Light-dependent magnetoreception in zebra finches : - from behaviour to receptor
Author
Summary, in English
It is well known that animals can use a wide variety of information sources to help them orient and navigate as
they move over short or long distances. Birds can rely on information from the sun, the skylight polarization
pattern, the stars, landmarks and the Earth’s magnetic field for orientation and navigation. However, despite the
behavioural evidence supporting the use of the magnetic field as an orientation and navigation cue, the sensory
mechanisms underlying magnetoreception are still missing. In this thesis, I studied one of the proposed models
for magnetoreception: the light-dependent magnetic compass in the zebra finch. The light-dependent magnetic
compass of birds provides directional information about the spatial alignment of the geomagnetic field. It is
proposed to be located in the avian retina, and be mediated by a light-induced, biochemical radical-pair
mechanism involving cryptochromes as putative receptor molecules.
The general goal of my thesis was to study how the light-dependent magnetic compass works in birds by
investigating the proposed mechanism at different levels. I studied the behavioural responses of zebra finches
trained to relocate a food reward inside a 4-arm plus maze, using the magnetic field as the only cue. By testing
birds under different light conditions, I assessed the spectral properties of the magnetic compass. I showed that
the magnetic compass of the zebra finch exhibits the same spectral properties as has been described in
migratory birds, and that the mechanism is most likely mediated by a radical-pair mechanism (Paper I). To
identify which of the three different cryptochromes found in birds is the most likely candidate magnetoreceptor I
studied their gene expression patterns over the circadian day. My assumption was that any cryptochrome
related to circadian tasks should exhibit a daily variation, whereas one involved in magnetoreception is expected
to be expressed constantly over time. I found that Cry1 and Cry2 followed a circadian variation, whereas Cry4
was expressed at equal levels over the day, irrespective of time, which suggested that Cry4 is a better candidate
for magnetoreception than the other cryptochromes (Paper II). With the findings from both the behavioural and
gene expression studies supporting the involvement of cryptochromes in magnetoreception, I next investigated
the location and distribution of Cry1 and Cry4 in the zebra finch retina using immunodetection. I found that Cry1
was expressed in the basal part of the outer segments of the UV cones. Taking into account that Cry1 is likely
not involved in magnetoreception, my findings open up the possibility that Cry1 is involved in a different, yet
undetermined function in the avian UV/V cones (paper III). Cry4, on the other hand, was expressed in a
subpopulation of peripheral horizontal cells in the zebra finch retina. These Cry4-expressing cells form a ring
around the periphery of the retina, suggesting a new model for how the magnetic field may be perceived by
birds. It also provides novel solution to the problem how the magnetoreception system may coexist with the
visual system without interfering with each other (paper IV).
Taken together, the findings presented in the chapters of this thesis show that the light-dependent magnetic
compass in zebra finches works similar to other birds and involves a radical-pair mechanism probably based on
Cry4. The distribution of Cry4 in the zebra finch retina offers a new view on how birds, and maybe other
organisms, can detect the Earth’s magnetic field. Cry1, on the other hand, is likely involved in a novel function in
the avian UV/V cones unrelated to magnetoreception.
they move over short or long distances. Birds can rely on information from the sun, the skylight polarization
pattern, the stars, landmarks and the Earth’s magnetic field for orientation and navigation. However, despite the
behavioural evidence supporting the use of the magnetic field as an orientation and navigation cue, the sensory
mechanisms underlying magnetoreception are still missing. In this thesis, I studied one of the proposed models
for magnetoreception: the light-dependent magnetic compass in the zebra finch. The light-dependent magnetic
compass of birds provides directional information about the spatial alignment of the geomagnetic field. It is
proposed to be located in the avian retina, and be mediated by a light-induced, biochemical radical-pair
mechanism involving cryptochromes as putative receptor molecules.
The general goal of my thesis was to study how the light-dependent magnetic compass works in birds by
investigating the proposed mechanism at different levels. I studied the behavioural responses of zebra finches
trained to relocate a food reward inside a 4-arm plus maze, using the magnetic field as the only cue. By testing
birds under different light conditions, I assessed the spectral properties of the magnetic compass. I showed that
the magnetic compass of the zebra finch exhibits the same spectral properties as has been described in
migratory birds, and that the mechanism is most likely mediated by a radical-pair mechanism (Paper I). To
identify which of the three different cryptochromes found in birds is the most likely candidate magnetoreceptor I
studied their gene expression patterns over the circadian day. My assumption was that any cryptochrome
related to circadian tasks should exhibit a daily variation, whereas one involved in magnetoreception is expected
to be expressed constantly over time. I found that Cry1 and Cry2 followed a circadian variation, whereas Cry4
was expressed at equal levels over the day, irrespective of time, which suggested that Cry4 is a better candidate
for magnetoreception than the other cryptochromes (Paper II). With the findings from both the behavioural and
gene expression studies supporting the involvement of cryptochromes in magnetoreception, I next investigated
the location and distribution of Cry1 and Cry4 in the zebra finch retina using immunodetection. I found that Cry1
was expressed in the basal part of the outer segments of the UV cones. Taking into account that Cry1 is likely
not involved in magnetoreception, my findings open up the possibility that Cry1 is involved in a different, yet
undetermined function in the avian UV/V cones (paper III). Cry4, on the other hand, was expressed in a
subpopulation of peripheral horizontal cells in the zebra finch retina. These Cry4-expressing cells form a ring
around the periphery of the retina, suggesting a new model for how the magnetic field may be perceived by
birds. It also provides novel solution to the problem how the magnetoreception system may coexist with the
visual system without interfering with each other (paper IV).
Taken together, the findings presented in the chapters of this thesis show that the light-dependent magnetic
compass in zebra finches works similar to other birds and involves a radical-pair mechanism probably based on
Cry4. The distribution of Cry4 in the zebra finch retina offers a new view on how birds, and maybe other
organisms, can detect the Earth’s magnetic field. Cry1, on the other hand, is likely involved in a novel function in
the avian UV/V cones unrelated to magnetoreception.
Department/s
Publishing year
2018-04
Language
English
Document type
Dissertation
Publisher
Lund University, Faculty of Science, Department of Biology
Topic
- Natural Sciences
Keywords
- magnetoreception
- radical-pair process
- magnetic compass
- orientation
- cryptochrome
- Taeniopygia guttata
Status
Published
Research group
- Lund Vision Group
Supervisor
ISBN/ISSN/Other
- ISBN: 978-91-7753-682-6
- ISBN: 978-91-7753-681-9
Defence date
25 May 2018
Defence time
10:00
Defence place
Lecture hall “Blå hallen”, Ecology building, Sölvegatan 37, Lund
Opponent
- Leo Peichl (Prof. Dr.)