Asymmetry: Causes, Measurements and Analysis

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Asymmetry Causes, Measurements and Analysis
Kristina Lock, lockkm67_at_mail.buffalostate.edu
Amy M. McMillan, Biology Department, State
University of New York College at Buffalo
Abstract
What is Asymmetry?
Common Loon
Asymmetry is found in every species within the
animal kingdom. Certain asymmetries are
conspicuous, while others are subtle. Asymmetries
can be exhibited internally or externally. Three
kinds of asymmetry have been studied in
vertebrates directional asymmetry (DA),
antisymmetry (AS) and fluctuating asymmetry (FA).
The presence of an asymmetry may be an indicator
as to how the animal handles developmental
stress, or how a population may be evolving.
Asymmetry analyses have evolved into multi-step
processes with strict standards for measurement
error and correction, as well as specialized
tests to account for asymmetry and anomalies.
Upon completion of analysis, conclusions can be
drawn to indicate the presence or absence of an
asymmetry and its implications for the species
studied. Measurements were obtained via samples
provided by the New York State Department of
Environmental Conservation (DEC) from loons that
had washed ashore of Lake Ontario during the fall
of 2006. Overall, I would expect to see
characteristics that serve an important
functional role display less asymmetry than
characteristics that are not critical to the
survival of the bird.
Asymmetry is defined as a deviation from
perfect symmetry. In particular, it is a
departure from symmetry among bilateral
characteristics (Windig 1998). Many previous
studies have been done on organisms to discover
the presence or absence of an asymmetry. In a
natural environment, the presence of an asymmetry
is often seen. The most commonly studied
asymmetries are fluctuating asymmetry,
directional asymmetry and antisymmetry.
Fluctuating Asymmetry
For the purpose of this experiment, the
Common Loon (Gavia immer) will be studied. No
previous asymmetrical studies have been published
on the Common Loon however other avian species
have been studied. These species include the
Taita Thrush (Lens et al. 2000), Finches and
Chickens (Campo et al. 2006). Previous studies on
avian species have been performed to test for the
presence of fluctuating asymmetry. Very little
work has been done with avian species and
directional asymmetry or antisymmetry. The
loon has a wide breeding range throughout
Northern North America and parts of Europe. North
American Loons have a yearly migration pattern
that involves summers in northern Canada, the
northern US and Greenland, and winters on the
east and west coasts of the southern US. Within
recent years, large loon die-offs have occurred
on the Great Lakes. Many of these die-offs have
been confirmed as botulism type E deaths and have
scientists struggling with its implications.
In New York State, even with recent loon
die-offs, the Common Loon is considered a species
of special concern (NYS DEC 2007). Species of
special concern warrant attention and
consideration but current information, collected
by various state agencies, do not justify listing
the loon as either endangered or threatened (NYS
DEC 2007). My study is specifically focusing on
asymmetries of migrating loons found on Lake Erie
and Lake Ontario during fall 2006.
Fluctuating asymmetry (FA) refers to subtle
deviations from perfect symmetry, which is random
in direction, on otherwise bilaterally
symmetrical organisms (Polak et al. 2004). When
bilateral measurements are taken, the left side
is subtracted from the right side (R-L) and
plotted. If there were no asymmetry, this
procedure would result in a vertical line forming
at the zero mark. However, an FA distribution
results in a bell curve where the majority of
measurements are distributed around a mean of
zero with only a few deviations. Fluctuating
asymmetry is considered to arise from an
organisms inability to buffer against
developmental noise (Swaddle 2003). Developmental
noise refers to outside interference and
stressors that can hinder an organisms ability
to develop properly by causing random
fluctuations and mutations at the cellular level
(Swaddle 2003). Ultimately, the presence of FA
reflects on the organisms developmental
instability. Although commonly interchanged with
fluctuating asymmetry and development noise,
developmental instability refers to an organisms
inability to properly develop and buffer against
developmental interruptions as a whole (Swaddle
2003). There are many genetic and
environmental factors that can disrupt
developmental stability and increase
developmental noise and asymmetry. However, the
response of an asymmetry based on stress appears
to be species and trait specific (Leung and
Forbes 1996). Traits of some species may show
signs of FA whereas traits of others do not
appear to be affected, even when severe
stress-induced mortalities are present (Bjorksten
et al. 2001). Due to this, the relationship
between asymmetry, genetics and environmental
stressors are unclear.
Directional Asymmetry
Directional asymmetry (DA) is defined as a fixed
heritable asymmetry (Hallgrimsson and Hall 2005).
All individuals of a species will exhibit the
same chirality, or the same-sided asymmetry, with
few exceptions. When bilateral measurements are
taken, the left side is subtracted from the right
side (R-L) and plotted. A typical DA plot appears
as a skewed bell curve. The skewed nature follows
the direction of the asymmetry, with only a few
points opposite the skew. Directional
asymmetry suggests that symmetry is broken at
some point during development to allow for the
growth of an asymmetrical feature. However,
developmental consequences may abound when a
reversal of the asymmetry occurs, or when the
asymmetry does not fully develop (Hallgrimsson
2005).
Why Do We Care?
1) Presence of an asymmetry could indicate an
adaptation of the bird. - An adaptation
would indicate a direct genetic link to the
asymmetry. - If the bird were to exhibit a
directional asymmetry, this would indicate
an adaptation that explains an aspect of the life
history of the species. 2) Presence of
fluctuating asymmetry may indicate developmental
interference. - Developmental
interference could be a result of genetic
factors, environmental factors, or a
combination of both. - This could indicate
stressors in the breeding lakes and may warrant
further study. 3) An asymmetrys sudden
appearance could have implications for
conservation. - Any factor that may
influence the quality of loon breeding lakes
(e.g., deforestation, new home
construction, chemical dumping) could
create an unstable developmental environment for
an embryo by contributing to excess noise
and/or stress during development. This
interference may prove to cause random
fluctuations at the molecular level
during development which can lead to the presence
of an asymmetry.
Antisymmetry
Antisymmetry (AS), is considered a random
asymmetry. Antisymmetry is considered heritable
in the sense that the asymmetry will be passed
down to the next generation. However, it is not
heritable as to which side the asymmetry will
show up on. In other words, while the parent
will have an asymmetry and will pass that
asymmetry on to its offspring, the chance of the
asymmetry being left-sided (sinistral) or
right-sided (dextral) is equal (Hallgrimsson and
Hall 2005). This too implies that symmetry is
somehow broken during development to allow for
the asymmetry. However, unlike directional
asymmetry, antisymmetry has many scientists
puzzled. The mechanism as to how the sided
asymmetry is determined is currently unknown.
Literature Cited
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during development fails to increase the
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Genetic and phenotypic correlation between
fluctuating asymmetry and two measurements of
fear and stress in chickens. Applied Animal
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Hall B. 2005. Varation A Central Concept in
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2000. Developmental instability and inbreeding in
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levels of habitat disturbance. Journal of
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Developmental Instability Causes and
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