Mary Gilleland
Science Research
20 November 2014
A Test of Drosophila melanogaster’s Nutritive Intelligence
Purpose:
The purpose of these experiments are to discover if Drosophila melanogaster can detect
the difference between non-nutritive and nutritive sugars and sweeteners by performing
behavioral assays and a series of experiments that will allow me to infer if they are able to
choose which sweet compound they eat based upon its nutritive value.
Background:
We can classify sweeteners in two categories: nutritive and non-nutritive. A non-nutritive
sweetener means that our bodies cannot break down the molecules within it. When they are
nutritive, the molecules can be metabolized and we can receive energy from it, therefore
extracting calories. Artificial sweeteners are made up of molecules that attach onto the glucose
receptors in our bodies and cause us to have the same sweet sensation that real sugar, such as
sucrose, gives us. The difference between sweetener and sugar is that sweeteners mock the sweet
taste of sugars but do not produce as much food energy.
Drosophila melanogaster have gustatory taste receptors, which are their version of taste
buds, on multiple parts of the body including the proboscis, win margins, legs and ovipositor.1
This is because fruit flies rely on direct contact chemosensation to detect nutrient rich foods and
toxins. In order to prevent insect digestion, “plants produce deterrent compounds with low
volatility, referred to as antifeedants, …all of which taste bitter to humans.” It is important that
David A. Yarmolinsky, Charles S. Zuker and Nicholas J.P. Ryba, “Common Sense about Taste:
From Mammals to Insects,” Cell, no. 139, (2009): 239-244. doi: 10.1016/j.cell.2009.10.001.
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flies have gustatory sensilla, or taste receptors, in other places than their proboscis, the
equivalent of a mammalian tongue, because they can detect toxic foods before ingesting them.
The behavioral reactions of Drosophila and humans are the same, leading us to identify taste
receptors the same in both species.2
A study completed in 2008 at the Monell Chemical Senses Center in Philadelphia
concluded that because of human and fruit fly’s common ecologies, omnivorous diets and
overlapping environments throughout evolution, they have similar taste preferences, especially to
sweet tasting compounds. This means that humans and flies both prefer and are responsive to a
“broad repertoire of noncarbohydrate, sweet stimuli.” Through multiple behavioral experiments,
Drosophila are consistently able to detect many of the same sweet and/or bitter compounds that
have been “qualitatively and quantitatively similar to humans’ abilities.” The way to conclude
this is by studying the response of the flies and comparing the taste receptor genes and proteins.
Flies’ response to sweeteners is said to be “more human-like than are those of many mammals.”
This is because all of the sweeteners used in their experiment were able to elicit an appetitive
response, or feeding behaviors, and in their colored agar test, all of the flies ingested a colored
agar containing any of the sugars. While in this experiment they did not test whether or not flies
could detect the nutritive or non-nutritive qualities of the sweeteners, “the need to detect a
variety of nutritive polyols from fruits, vegetable, and perhaps some animal tissues” is necessary
and have “shaped the binding pockets and receptive fields of the sweetener receptors.” Humans
and flies share receptor ancestry; following this, they concluded that the taste receptor genes in
flies have a “strong sequence homology” that “code for the 2 proteins comprising a mammalian
Craig Montell, “A Taste of the Drosophila Gustatory Receptors,” National Institute of Health,
no. 19(4), (2009): 345-353. doi: 10.1016/j.conb.2009.07.001.
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sweet receptor heteromer.” The behavioral responses allowed them to infer that Drosophila’s
sweet preference is most like humans.3
While sugar is a necessary part of human’s diets, people are afraid of it because it’s said
to be worse than bread, salt and even fat. An article from the New York Times explores the
scientific and social difficulties when trying to find a sugar alternative. People do not like
artificial sweeteners because they are not natural, leading scientists to try to discover a natural
sweetener that is not sugar. The leading natural sweetener now is called stevia, which is naturally
sweet plant, whose crushed up leaves work well as a sugar substitute. However, problems still
arise because customers demand the same taste of sugar, without using sugar. While this didn’t
used to be difficult for scientists because they could create something artificial, this is now
difficult because it must be natural in order to be successful. While scientists keep making
progress, for example trying to alter the stevia plant instead of the stevia product, there are new
questions raised with every step forward. For example, how do you draw the line between what
is altered or unchanged, and what isn’t? Even if it is altered “naturally,” does this still make it
“natural?” What are the differences between “all-natural,” “naturally source” and “natureidentical?” How do you know when a product is natural if each consumer and producer has a
different idea of what the word means? If a sweetener is not artificial, can we assume it is
natural? Or, if a substitute is unnatural, does that mean it is artificial? Producers of sugar
substitutes have run into a new problem in order to succeed: their sweetener must be natural.
While what people think they want are fewer calories, they really want the same sweet taste they
Beth Gordesky-Gold, Natasha Rivers, Osama M. Ahmed and Paul A.S. Breslin, “Drosophila
melanogaster Prefers Compounds Perceived Sweet by Humans,” Chem. Senses, no. 33 (2008):
301-309, accessed October 10, 2014. doi: 10.1093/chemse/bjm088.
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get from sugar. Therefore, the industry ultimately only has one competitor: sugar. No matter
what they do or try, if it does not taste as sweet as sugar, they will never win.4
Hypothesis:
With all this information about sugar and artificial sweeteners and the differences
between the varieties of them, many questions arise about the behavioral or interactive reactions,
and their nutritional effects. While there have been few studies done on whether or not
Drosophila can detect a difference between nutritive and non-nutritive sugars and sweeteners,
the evidence provided in studies which perform behavioral assays in response to non-caloric
sweeteners that generate appetitive response from flies, allows one to infer that fruit flies could
tell a difference over time. Because they are able to make decisions about which sweet food to
eat, the higher energy they receive from said choices might allow them to choose which one they
eat next.
Materials:
The sugars that I will be using, consisting of sucrose, fructose, glucose, maltose and
lactose are all natural sugars. Each of them is caloric and can be found in natural foods or
commonly eaten foods.
The artificial sweeteners used in these experiments consist of aspartame and sucralose.
Aspartame is an “artificial, non-saccharide sweetener” used in many trademark names such as
Equal, NutraSweet and Candere. It has a nice sweet taste that is clean but “its time intensity
profile differs from sucrose.” These differences do not affect the taste, just the make up of the
sugar; it is a low calorie sweetener, producing only four calories per gram, used in low calorie
Daniel Engber, “The Quest for a Natural Sugar Substitute,” The New York Times, last modified
January 1, 2014, http://www.nytimes.com/2014/01/05/magazine/the-quest-for-a-natural-sugarsubstitute.html?_r=0.
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foods. Our bodies break aspartame down into “natural residual components.” These components
are safe and can be metabolized, as it would be if derived from another source; it is so low
calorie because it is much more potent than sugar. Sucralose is the closest sweetener to sugar
because it is composed “by a process that substitutes 3 chloride atoms for 3 hydroxyl groups on
the sucrose molecule.” It is 450-650 times sweeter than sucrose, therefore being more potent as
well. It provides zero calories for humans because our bodies to not recognize it as a sugar and
therefore does not metabolize it. It is not toxic and no other health benefits or detriments are
known, and it is safe for human consumption.5
Methods:
In order to infer which sweet food flies prefer and whether or not they choose the caloric
or non-caloric choice, I will perform two different types of behavioral tests. The first test will be
to see where the flies like to be in terms of where the sweet food source is. I will take two
horizontal test tubes, both with agar solutions in them, and record where how many flies are in
each quadrant of the test tubes every minute. In the following test the flies will be put into two
test tubes, one with agar and one with an added sugar or sweetener, and the same procedure will
be followed. This is a behavioral study that will show whether or not the flies are attracted to
something because it is sweet, or if they are motivated by different reasons such as their
knowledge of caloric content, for moving about the test tubes.6
The second test will be a choice test. Two foods, one sweetened and one unsweetened, or
one sweetened with natural sugar and one with a non-caloric artificial sweetener, will be colored
Sanchari Chattopadhyay, Upta Raychaudhuri and Runu Chakraborty, “Artificial Sweeteners – a
review,” Association of Food Scientists & Technologists (India), no. 51(4), (2011): 611-621. doi:
10.1007/s13197-011-0571-1.
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Lar L. Vang, Alexei V. Madvedev and Julius Adler, “Simple Ways to Measure Behavioral
Responses of Drosophila to Stimuli and Use of These Methods to Characterize a Novel Mutant,”
Public Library of Science, no. 7(5), (2012): e.37495. doi: 10.1371/journal.pone.0037495.
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with two different colors of food dye. The flies will be left in a dark chamber for a 45 minutes
after being starved for 18 hours. After 45 minutes, they will be looked at under a microscope and
the colors of their abdomen will be recorded and a percentage will be calculated. This will allow
me to see which food the majority of flies preferred. This test will be repeated with the same
group of flies over the course of my experiments.7
Gordesky-Gold, Rivers, Ahmed and Breslin, “Drosophila melanogaster Prefers Compounds
Perceived Sweet by Humans,” 302.
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Outline:
Paragraph 1: Purpose
Paragraph 2: Nutritive vs. Non nutritive – caloric vs. non-caloric
Paragraph 3: Gustatory taste receptors
Body placement
Purpose
Paragraph 4: Human & fly receptor similarities
Reasons: ancestry, ecology and diet
Same response to sweet/bitter tastes
(Cannot detect nutritive value, but it is necessary)
Paragraph 5: NYT – sugar substitutes
In search for a natural substitute for sugar
Common, important questions about qualifications and requirements for substitutes
Paragraph 6: Hypothesis
Paragraph 7: Method 1
Paragraph 8: Method 2
Paragraph 9: Materials 1 (natural sugars)
Paragraph 10: Materials 2 (artificial sweeteners & important information)
Reordered paragraphs:
1) Paragraph 1: Purpose
2) Paragraph 2: Nutritive vs. Non nutritive – caloric vs. non-caloric
3) Combination of Paragraph 3 and Paragraph 4:
Gustatory taste receptors
Body placement
Purpose
Human & fly receptor similarities
Reasons: ancestry, ecology and diet
Same response to sweet/bitter tastes
(Cannot detect nutritive value, but it is necessary)
5) (OR possible conclusion) Paragraph 5: NYT – sugar substitutes
In search for a natural substitute for sugar
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Common, important questions about qualifications and requirements for substitutes
6) Paragraph 6: Hypothesis
7) Paragraph 9: Materials 1 (natural sugars)
8) Paragraph 10: Materials 2 (artificial sweeteners & important information)
9) Paragraph 7: Method 1
10) Paragraph 8: Method 2
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