Drosophila as a Model Organism

The common fruit fly, Drosophila melanogaster, is a widely used model organism in biology, though it may be more commonly recognized as a fruit-loving pest. There are multiple reasons that make D. melanogaster an excellent experimental organism: their roughly two-week generation time allows studying multiple generations, they are easily maintained in very small tubes, and their sexual dimorphism enables investigators to easily distinguish between males that have black coloration towards the posterior and females that have banded abdomens.

Thomas Hunt Morgan and his students were among the first to make use of Drosophila, in the early 20^th century. In 1933, Morgan received a Nobel Prize for his discovery of the role of chromosomes in heredity, which was founded on his observations of mutant Drosophila. Morgan continued his studies by generating new mutants with radiation. The presence of multiple mutants with discrete effects on the appearance of the fly enabled him to look at patterns of multifactor inheritance. A key aspect of his early work was the discovery of apparent violations of Mendel’s second law, which states that genes segregate independently of one another. Morgan’s observations indicated that genes will not segregate independently if they are joined together on the same chromosome. Furthermore, it became possible to determine gene locations on a chromosome relative to each other through analyses of how often they recombined thus triggering gene mapping studies to identify the location of a gene and distances between genes. Consequently, associates of Morgan’s lab received two additional Nobel Prizes for other studies of Drosophila genetics. Morgan’s success and the accumulating information about Drosophila inspired others to adopt them as experimental organisms, which added to our knowledge of them and progressively increased their value as a model organism. Other scientists have developed many mutations. Over 27,000 unique Drosophila lines are now maintained as stock cultures at the Drosophila stock centers and are readily available to laboratories that use them as tools for studying diverse aspects of biology, including behavior¹.

Drosophila Ethology

Animal behavior, or ethology, is the study of how animals interact with each other and their environments. Although it is a relatively young biological field, ethology has important implications since an organism’s survival depends on how it behaves with regard to possible mates, food sources, and natural obstacles. Studying these organisms helps scientists to understand the contexts of these behaviors and the underlying mechanisms as to how they have evolved. Thus, a goal of the study of animal behavior is to find a genetic basis for the behaviors exhibited by individuals or groups of individuals. For example, kin selection, first proposed by William Hamilton, suggests that individuals will behave altruistically when making a sacrifice for a family member rather than a stranger². This is because related individuals share more genes than unrelated ones, and the future reproduction by their relatives contributes in a way to their own fitness, or reproductive success. Hence, observing how organisms like Drosophila interact with their environment can allow us to understand how these behaviors are hard-wired. Additionally, these behaviors provide insight into how these organisms have adapted to cope with their environment and survive.

Directional behaviors are commonly studied to understand the basis of how and why an organism moves in its environment. Kinesis and taxis are two forms of directional behaviors. Kinesis involves stimulus-induced movements in random directions, such as the random movement of flies when chased away. On the other hand, taxis involves stimulus-induced movement towards a specific direction, such as moths flying towards a lightbulb. Taxis behaviors are positive if the animal moves towards the stimulus and negative if the animal moves away from the stimulus. Furthermore, taxis behaviors are named based on the specific stimulus that induces them. For example, geotaxis is a response to gravity, where positive geotaxis means that the organism moves with gravity. Phototaxis is movement in response to light, such as positive phototaxis of moths towards a lightbulb. Similarly, chemotaxis is attraction to or avoidance of an airborne chemical cue, such as the negative chemotaxis of many predators, including humans, in response to the smell of skunk spray. Furthermore, there are many other types of taxes that are studied by scientists, including aerotaxis, barotaxis, hydrotaxis, and magnetotaxis, that are movements in response to oxygen, pressure, water, and magnetic fields, respectively.

Study of Drosophila taxis behavior

A common and simple method to study Drosophila taxis behavior is to use a choice chamber, which allows the observation of a directional response of the fly to a particular stimulus, such as gravity, light, or a chemical. Drosophila are inserted in the center of the choice chamber and then allowed to wander freely about the chamber, which is designed so that alternative stimuli can be presented at opposing sides. After a period of wandering, the number of flies is counted on either side of the choice chamber. The number of flies on each side of the chamber is then analyzed with the Chi Square test to gauge whether or not there is a preference for a stimulus.

Comparison of directional behavior of mutant and wild-type Drosophila allows researchers to determine the role of the mutated gene in that specific behavior. Moreover, scientists are also trying to understand how animals decide what to do in the presence of combinations of stimuli, including conflicting stimuli³. Together, these studies can help predict how organisms will react to certain stimuli or changes in environmental factors.

References

1. Cook KR, Parks AL, Jacobus LM, Kaufman TC, Matthews K. New research resources at the Bloomington Drosophila Stock Center. Fly. 2010, Vol. 4, 1: 88-91.
2. WD, Hamilton. Altruism and related phenomena, mainly in social insects. Ann Rev Ecol Systematics. 1972, Vol. 3, 193-202.
3. Gepner R, Skanata MM, Bernat NM, Kaplow M, Gershow M. Computations underlying Drosophila photo-taxis, odor-taxis, and multi-sensory integration. eLife. 2015, Vol. 4, e06229.