1. UVR and the selective advantage of ebony Drosophila melanogaster
Joy Goodwin and Brianna Smith
Biology 315 Section 03

2. 2
Abstract
As an effective absorber of light, melanin is one of the natural ways in which living
organisms are protected from the genetic damage caused by ultraviolet radiation (UVR). In
this experiment we examine the variation of UVR sensitivity that can be found in yellow
and ebony phenotype Drosophila melanogaster. We hypothesized that the dark pigmented
phenotype, ebony, would confer a lower sensitivity to UVR exposure than would the lighter
yellow phenotype. After measuring the average days of survival post UVR exposure in each
phenotype, we found that the ebony phenotype had a significantly lower sensitivity to UVR
and thus has a selective advantage over the yellow phenotype. However, we also
acknowledged that our experiment and results have limitations.
Introduction
Ultraviolet (UV) light has mutagenic properties that have been known to cause alternations in
DNA (Livneh et al., 1993). As a form of radiation, excessive exposure to UV poses a threat the
genetic material of all living organisms. Due to the “broadband absorption” properties of
melanin, dark pigmentation is one of the most effective protection factors against damage caused
by ultraviolet radiation (UVR) (Brenner & Hearing 2008). Therefore, it is expected that
variation in pigmentation will correlate with variations in the organism’s sensitivity to UV. In
this experiment we evaluated the sensitivity to UVR that two differently pigmented strains of
Drosophila melanogaster have relative to one another. More specifically, we compared the
lifespans of UV exposed yellow (Y) and ebony (E) strains of Drosophila, in order to determine if
darkly pigmented strains have a selective advantage over lightly pigmented strains when exposed
to UVR. We hypothesized that when exposed to UV, the E population would have a longer
lifespan post-exposure (lower sensitivity to UVR), while the Y population (non-pigmented)
would have a shorter lifespan (higher sensitivity to UVR).

3. 3
Methods
Twenty flies from each of the Y and E populations were etherized placed into separate
uncovered petri dishes and subjected to UV for five minutes. After a ten-minute break from the
UV, the same two samples of twenty were re-etherized (to prevent flies from awakening during
the process), and subjected to UV for an additional five minutes. Both populations of twenty
were then transferred to separate vials containing media and labeled as “UV exposed” Y and E,
respectively. Twenty additional flies from each of the original (untreated) populations were then
placed in separate vials labeled as “non-UV exposed” Y and E, respectively, and served as the
controls. All flies used in the experiment were flying adults. Eggs were removed from all vials
when they appeared and media was changed once every seven days.
The number of deaths in each vial was recorded every day until all flies were dead. This was
done in order to monitor how long each fly lived after the UV exposure date, and also to monitor
the lifespan of the controls since we did not know how old the original populations were. If a
death took place during a weekend day (Saturday or Sunday), it was recorded as if they had died
on Sunday. A summary of the number of days survived post exposure date can be seen in table 1.
In order to show that UV caused a significant change in lifespan for each phenotype, t-tests that
compared the lifespans of the non-UV exposed to the UV-exposed flies of the same strain were
run for each phenotype (table 2). A G-test (table 3) was used to determine if the difference in
days of survival between the ebony and the yellow populations was statistically significant. The
average days of survival for the non-UV exposed populations served as the “expected” values,
while the average days of survival for the UV exposed populations served as the “observed”
values.
Results

4. 4
Table 1. Number of days each population survived post-exposure date and the net loss of days due to UV displayed
in table.
Average Days of Survival
Yellow (Y) Ebony (E)
Non-UV Exposed 30.7 25.1
UV Exposed 21.3 20.s6
Total Days Lost 9.4 4.5
Table 2. T-test values and results for same individual strains displayed in table.
Population Degrees of
Freedom
t-value Significance
Level
p-value
Yellow 19 9.50811 .05 <.000001
Ebony 19 3.89541 .05 .000385
Table 3. G-test values and results comparing both strains displayed in table.
Test Degrees of
Freedom
G-test statistic Critical Value Significance
Level
E vs Y 1 -20.1 3.84 .05
Discussion
Based on the knowledge that ebony phenotype Drosophila have higher levels of melanin than
their yellow phenotype counterparts (Wang 2008), we hypothesized that our E population would
have lower sensitivity to UV exposure then the Y population. After subjecting both populations
to UVR and comparing the days of survival of the exposed flies against their respective controls,
we found that the Y population lost an average of 9.4 days of life, while the E population only
lost an average of 4.5 days when exposed. We ran a t-test for each phenotype in order to
establish that the UV had a significant effect on the survival of each strain. The data from both
tests (table 2) indicated that UV did have a significant effect on the lifespan of both the ebony
and the yellow phenotypes. The G-test we conducted (table 3) further supported our hypothesis
by indicating that there was a significant difference between the days of survival of the ebony
and the yellow phenotypes post-UV exposure. Due to the significant difference in the days of
survival between ebony and yellow phenotypes, we accept our hypothesis that the ebony
phenotype confers a lower sensitivity to UVR and thus has a selective advantage over yellow

5. 5
phenotype drosophila. However, we must acknowledge that the experiment design and results
have limitations.
Although the ebony Drosophila had a longer lifespan post UV-exposure, we failed to
acknowledge any other variable besides pigmentation that may have conferred this resistance to
UVR damage. Additionally, we noted that some of the flies may have died due to causes
unrelated to UV exposure. For instance, we believe that some of the flies may have died due to
getting stuck in the media, which would have skewed the average days of life for both the
exposed and non-exposed populations. There was also a lack of replication and a very small
sample size for each population (n=20). In order to confirm the validity of this experiment’s
results, we suggest additional replicates with larger sample sizes, and using flies whose exact age
is known
Literature Cited
Brenner, M., & Hearing, V. J. (2008). The protective role of melanin against UV damage in
human skin. Photochemistry and photobiology, 84(3), 539-549.
Livneh, Z., Cohen-Fix, O., Skaliter, R., & Elizur, T. (1993). Replication of damaged DNA and
the molecular mechanism of ultraviolet light mutagenesis. Critical reviews in biochemistry and
molecular biology, 28(6), 465-513.
Wang, Z., Liu, R., Wang, A., Du, L., & Deng, X. (2008). Phototoxic effect of UVR on wild type,
ebony and yellow mutants of Drosophila melanogaster: Life Span, fertility, courtship and
biochemical aspects. Science in China Series C: Life sciences, 51(10), 885-893.