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Fruit choice as host preference indicator in Drosophila melanogaster.

Authors: Lee Ware, Krystina Sandefur, John Hargis, Adekele Eko

Introduction

D. melanogaster types (clockwise): brown eyes ...

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Drosophila melanogaster is one of the most widely used model organisms in biology, including research in genetics, physiology and life history evolution (Drosophila melanogaster, 2009). D. melanogaster is a short-winged fruit fly species whose morphology is easily identifiable, once anesthetized, by its yellowish brown body and transverse black rings across its abdomen. D. melanogaster exhibits sexual dimorphism, making it ideal for laboratory experiments in which sex is a factor (Drosophila melanogaster, 2009).

Insects use chemical cues to identify host plants, which suggests that chemosensory perception could be a target of natural selection during host specialization (Whiteman & Pierce, 2008). Because adult female Drosophila must identify and lay eggs on prospective host plants for their offspring that subsequently feed on the microbes living on rotting plant tissue, the discovery of those factors that influence host selection is essential to understanding host specialization (Whiteman & Pierce, 2008). However, basic knowledge regarding D. melanogaster’s chemical ecology and olfactory preferences are currently lacking (Ruebenbauer, Schlyter, Hansson, Lofstedt, & Larsson, 2008). Against this backdrop, we conducted a simple experiment with the polyphag D. melanogaster, a known food / host generalist but close relative of several specialist species (Whiteman & Pierce, 2008), to look for evidence of mild fruit (host) preferences upon which natural selection could work by releasing large numbers of flies into a chambered apparatus with a different fruit sample in each chamber (apples, bananas, oranges and an empty control).

Hypothesis

Female Drosophila melanogaster will show greater selectivity in fruit choice than males because of selective pressures to maximize reproductive success.

Methods

The experimental parameters and apparatus were refined over several test trials, culminating in a definitive experiment using 1,100 Drosophila melanogaster specimens.
The subjects included wild-type and white-eyed D. melanogaster harvested from culture tubes in an environment-controlled incubator (24-hour light cycle, 27° Celsius). The subjects were released from the culture tubes directly into the apparatus fully awake to allow thorough dispersion. The jars containing prepared and measured fruit samples were attached to the apparatus via funnels to insure that the subjects, once in the jar, could not easily exit. The release point was the middle of five chambers, with two fruit jars to each side. The flies were released and allowed a 24-hour selection period inside the incubation chamber to control for light preferences (Rieger et al., 2007). At the end of the 24-hour period, the jars were removed from the apparatus and the contents of each were captured, counted, and sexed. This procedure was performed twice – the pilot group of 250 subjects was followed by a definitive group of 850.

A Chi-Square Goodness of Fit test was used to determine whether Drosophila melanogaster displayed any significant fruit preferences. A Chi-Square Contingency Table was used to determine whether the difference between male and female fruit preference was significant.

Results

The difference between the observed fruit preference and the expected values for random selection were found to be significant across all observed D. melanogaster (X2 = 254.34; df=3; sig.=0.0001) (Figure 6). Five percent of all D. melanogaster chose the empty control, 31% chose the orange sample, 27% chose the apple sample, and 36% chose the banana sample (Figure 6).

A significant difference was confirmed to exist between male and female fruit preferences (X2 = 53.24; df=3; sig=0.0001) (Figure 7). One percent of all observed males chose the empty control, while 10% of females did; 37% of all observed males chose the orange sample, while 24% of females did; 28% of all observed males and 27% of females chose the apple sample, while 34% of all observed males and 39% of females chose the Banana sample (Figure 7).

Discussion

Conclusions: Drosophila melanogaster displays fruit choice preference. Further, male and female D. melanogaster display disparate preferences. Female D. melanogaster are much more likely to select the empty (control) chamber than males. Males are more likely to select the orange chamber than females.

Our hypothesis predicted females to display greater selectivity in fruit / host choice. Our results were seemingly contradictory. While females were more unequally distributed among the three fruit samples, they were also much more likely than males to select the empty control chamber – a behavior inconsistent with our prediction.

Problems: An unexplained disparity between the numbers of male and female collected in the apparatus suggests that our determinations of sex were significantly in error. Future experiments should take special care with sexing techniques and use smaller, more frequent trials to minimize human error.

Future Research: This line if experimentation deserves much more time to properly address genetic inheritance and other possible factors such as light environment and developmental host environment.

Acknowledgements

Drs. Ron Rosen and Marc Rowley both helped with apparatus design and construction.

Figure Captions

Females are usually larger than males, reaching 2.5 millimeters in length (Figure 1). Males are generally darker than females, with a distinctive black patch at the end of their abdomens (Figure 2). Males also have short bristles around the tarsus of their first legs (Figure 3) and a cluster of spiky hairs around their reproductive organs that allow them to attach to females during copulation.

The apparatus consisted of four fruit-containing jars (L-R: Banana, orange, empty control, and apple) taped to funnels, taped to a release chamber, with a release point in the center. The subjects were released en masse into the main chamber and then given 24-hours to “select” a fruit jar (Figure 4). The apparatus was kept in an environment chamber to control temperature and light.

After 24-hours in the environment chamber, the apparatus was removed and disassembled, and the subjects were anesthetized, sexed, and counted. Almost all subjects had “selected” a fruit jar by the 24th hour (Figure 5).

Works Cited

Byrne, P. G., & Rice, W. R. (2006). Evidence for adaptive male mate choice in the fruit fly Drosophila melanogaster. Proceedings of the Royal Society B: Biological Sciences, 273(1589), 917–922. doi: 10.1098/rspb.2005.3372.

Drosophila melanogaster. (2009, May 12). Wikipedia.org. Encyclopedia. Retrieved May 13, 2009, from http://en.wikipedia.org/wiki/Drosophila_melanogaster.

Rieger, D., Fraunholz, C., Popp, J., Bichler, D., Dittmann, R., & Helfrich-Forster, C. (2007). The Fruit Fly Drosophila melanogaster Favors Dim Light and Times Its Activity Peaks to Early Dawn and Late Dusk. J Biol Rhythms, 22(5), 387-399. doi: 10.1177/0748730407306198.

Ruebenbauer, A., Schlyter, F., Hansson, B., Lofstedt, C., & Larsson, M. (2008). Genetic Variability and Robustness of Host Odor Preference in Drosophila melanogaster. Current Biology, 18(18), 1438-1443. doi: 10.1016/j.cub.2008.08.062.

Whiteman, N. K., & Pierce, N. E. (2008). Delicious poison: genetics of Drosophila host plant preference. Trends in Ecology & Evolution, 23(9), 473-478. doi: 10.1016/j.tree.2008.05.010.

Discussion

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