Video tracking fruit fly adults

Those tiny flies that seem to like your fruit and take over your garbage cans during the summer? Those are fruit flies, or Drosophila melanogaster. They have a lot more in common with us than some might think. Researchers already know this. Having 75% of the genes that cause human disease in common with us, and being very low-maintenance, makes you a popular animal model, so many labs devote their research time to these little insects. 

Many of these studies focus on assessing general locomotor behavior, or more detailed behaviors such as courtship. Video tracking makes this job a lot easier. On this page, you will find some examples of studies that used video tracking to assess foraging behavior, circadian rhythmicity, courtship behavior, and thigmotaxis (wall-hugging) in Drosophila, as well as the reasons why these behaviors are studied. 

Early life adversity

What happens early in life may affect how you do later in life. While this seems logical, we don’t always know how the developmental process works. There are many discussions about interaction between biology and environment (nature vs nurture) and how this effects human development. Due to the similarity in genomes, investigations into Drosophila melanogaster development may help to answer some of these questions. 

Sitters and rovers

Burns and his colleagues used Drosophila as their animal model to investigate the effects of early life adversity. To see the interplay of early life circumstance and genes on later behavior and health, two strains, called sitters and rovers, were used. As their names imply, these strains normally show different behaviors in the presence of food: one primarily sits, while the other roves through the environment. 

Food deprivation activates sitters

The adverse environmental challenge presented to young Drosophila in this study was food deprivation. When rearing circumstances were good, rovers show the normal darting exploration to obtain food, while sitters, as their name implies, remain much more still. When deprived of food early in life, rovers still show the same amount of darting exploration, but sitters up their game and move around much more, fighting against their genetic propensity to sit still and instead rove as needed for food. 

Parameters of exploration from video tracking

Exploration was measured in a petri dish, and animals were video tracked with EthoVision XT. By using an automated tracking system, velocity, distance moved, time moving vs not-moving, and position in the dish were automatically measured in detail. 

Night-time activity

We have something else in common with fruit flies: our day-night rhythmicity. Like us, Drosophila are diurnal – they are active during the daytime, and inactive at night. Fuiji and colleagues from the Duke University Medical Center in North Carolina investigated how social context influences this rhythm. 

Comparing the behaviors of individuals and of pairs (male-male, female-female, and male-female), they found that all animals showed this normal diurnal activity rhythm, except for the heterosexual couple. 

Courtship behavior 

Researchers measured locomotion and the distance between the flies automatically with EthoVision. When a male and female were put together, increased nightly activity including more time spent in proximity, and other behaviors reflecting courtship, were recorded.  The data also showed that this behavioral change was dependent on the male, and a change in his behavior, and that a properly functioning olfactory system was crucial for this.

For more research on courtship behavior, see this page. 

Male sex drive on the clock

In a following study, Fuiji and Amrein took a closer look at the circadian rhythmicity of the male sex drive. Fruit flies have multiple circadian clocks: the main clock in the brain and several others in the testes, eyes, antennae, and other organs. In this experiment, researchers manipulated the circadian clock in different subsets of neurons throughout the fly body. Fly locomotor activity and close-proximity encounters were measured in a petri dish with EthoVision video tracking software; researchers ultimately discovered that the one in the ventral lateral neurons of the brain were essential for the intact, properly displayed, male sex drive. 

Hugging the walls 

Like many other animals, fruit flies also show thigmotaxis or wall-hugging behavior. This is an evolutionarily protective behavior; as open spaces can lead to more predation or negative outcomes. Soibam and his colleagues from the University of Houston, Texas, wanted to take a closer look at how Drosophila interact with their environment, in regards to explorative behavior; to do this, they designed a detailed computer model. 

Not all walls are equal

The researchers tracked fruit flies in several types of arenas: a concentric circular arena, an hourglass-shaped arena, a double-spiral arena, and an arena shaped like their home state of Texas. Interestingly, fruit flies did not like all walls equally: external walls were much more appreciated than internal walls, within the arena. They show the greatest preference for corners; for example, the Texas arena (having irregularly shaped walls) showed that the more enclosed a corner was, the better the flies liked it. 

Tracking vs a computer model

To track the movement of the animals during the trial, EthoVision XT was used. The animal path results from EthoVision XT were compared with a computer model that was based on directional persistence and local wall force. The model well predicted actual fruit fly behavior in most arenas, but as the arenas became more complex, the model could not as accurately predict Drosophila behavior.

Read more about this research in this blog post.

Who smells better?

Despite being such visually-dependent creatures, even we as humans reply upon smell as an important factor in how much we like someone. Fruit flies, more than humans, rely heavily on olfaction; in particular, gustatory receptors on the male antennae are crucial for sex drive and courtship behavior displays. In fact, there are several types of receptors that have different functions in this matter.

For more research on courtship behavior, see this page.

Age matters

For fruit flies, age also matters, as Hu and colleagues (Tongji University, Shanghai) found in their study. While younger and older females were equally liked by male fruit flies separately, when given a choice between the two, they prefer younger flies. 

Measuring courtship

Courtship behaviors of these fruit flies was measured by placing the male in an arena with two older and two younger females. Their behavior was recorded; general movement was tracked with EthoVision XT, and courtship specific behaviors were characterized by time in proximity with each specific female. A courtship index was calculated as the total time spent in showing courtship behavior as a percentage of the total time in the arena. 

Sniffing out the younger female

The preference for younger mates seems mediated by the specific subtype 33a of the gustatory receptors, as loss of function of this receptor does not decrease the male sex drive, but does eliminate the preference for younger females. Apparently, the scent a female gives off depends on her age. It is probably wise not to draw comparisons to human features here… for now. 

References 

Other publications that might interest you: