Have you ever left a fruit bowl out for a while and return to discover a fruit fly settlement? But why is the fruit fly attracted to ripe fruit in the first place? In other words, what is the stimulus that tells them where to go? This analysis is part of a wider field called ethology, which is the study of animal behavior.
A stimulus may cause two types of animal movement. Kinesis, which is non-directional or random, or taxis, which is directional. An example of kinesis would be the movement of the flies in random directions when chased away. Conversely, an example of taxis would be the directed movement of the flies back towards the ripe fruit. This specific type of taxis is referred to as chemotaxis and since it is towards the stimulant, food, it is further defined as positive chemotaxis. An example of negative chemotaxis would be the fly moving away from an insect repellent. Another common type of taxis is phototaxis, which is movement is response to a light stimulus. Moths are showing positive phototaxis when they move towards a light source. On the contrary, cockroaches that scuttle away when the light is turned on are being negatively phototactic. Geotaxis, also known as gravitaxis, is movement in response to gravity. Moving in the direction of gravity is positive geotaxis, while movement in the opposite direction is negative geotaxis. There are many other types of taxis, including aerotaxis, barotaxis, hydrotaxis, and magnetotaxis – which are movement in response to oxygen, pressure, water and magnetic fields respectively.
So why do scientists study animal behaviors like these? One of the goals may be to find the genetic basis for the behaviors exhibited by individuals or groups within a species. Sometimes, a behavior can even be traced to a specific mutation in a single gene. On a broader scale, understanding how genetics and behavioral traits are linked can also help us to gain a better understanding of behaviors across the animal kingdom, including our own.
In this lab, you will assess the geotactic, phototactic and chemotactic responses of the fruit fly Drosophila melanogaster, using choice chambers.
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 20th 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.
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.
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.
