Here we present a protocol to automatically determine the locomotor performance of Drosophila at changing temperatures using a programmable temperature-controlled arena that produces fast and accurate temperature changes in time and space.
Temperature is a ubiquitous environmental factor that affects how species distribute and behave. Different species of Drosophila fruit flies have specific responses to changing temperatures according to their physiological tolerance and adaptability. Drosophila flies also possess a temperature sensing system that has become fundamental to understanding the neural basis of temperature processing in ectotherms. We present here a temperature-controlled arena that permits fast and precise temperature changes with temporal and spatial control to explore the response of individual flies to changing temperatures. Individual flies are placed in the arena and exposed to pre-programmed temperature challenges, such as uniform gradual increases in temperature to determine reaction norms or spatially distributed temperatures at the same time to determine preferences. Individuals are automatically tracked, allowing the quantification of speed or location preference. This method can be used to rapidly quantify the response over a large range of temperatures to determine temperature performance curves in Drosophila or other insects of similar size. In addition, it can be used for genetic studies to quantify temperature preferences and reactions of mutants or wild-type flies. This method can help uncover the basis of thermal speciation and adaptation, as well as the neural mechanisms behind temperature processing.
Temperature is a constant environmental factor that affects how organisms function and behave1. Differences in latitude and altitude lead to differences in the type of climates organism are exposed to, which results in evolutionary selection for their responses to temperature2,3. Organisms respond to different temperatures through morphological, physiological, and behavioral adaptations that maximize performance under their particular environments4. For instance, in the fruit fly Drosophila melanogaster, populations from different regions have different temperature preferences, body sizes, developmental times, longevity, fecundity, and walking performance at different temperatures2,5,6,7. The diversity observed between flies of different origins is explained in part by genetic variation and plastic gene expression8,9. Similarly, Drosophila species from different areas distribute differently among temperature gradients and show differences in resistance to extreme heat and cold tests10,11,12.
Drosophila has also recently become the model of choice to understand the genetic and neural basis of temperature perception13,14,15,16,17. Broadly, adult flies perceive temperature through cold and hot peripheral temperature sensors in the antennae and through temperature sensors in the brain13,14,15,16,17,18,19,20. The periphery receptors for hot temperatures express Gr28b.d16 or Pyrexia21, while the periphery cold receptors are characterized by Brivido14. In the brain, temperature is processed by neurons expressing TrpA115. Behavioral studies on mutants of these pathways are improving our understanding of how temperature is processed and give insights into mechanisms that vary among populations of Drosophila from different regions.
Here we describe a temperature-controlled arena that produces fast and precise temperature changes. Investigators can pre-program these changes, which allows for standardized and repeatable temperature manipulations without human intervention. Flies are recorded and tracked with specialized software to determine their position and speed at different phases of an experiment. The main measurement presented in this protocol is the walking speed at different temperatures, because it is an ecologically relevant index of physiological performance that can identify individual thermal adaptability5. Together with temperature receptor mutants, this technique can help reveal the mechanisms of thermal adaptation at cellular and biochemical levels.
1. Preparation of Fly Food Medium
2. Preparation of Flies
3. Frame of Lights
4. Temperature-Controlled Arena
5. Temperature Behavioral Experiments
6. Video Tracking and Data Analysis
The temperature-controlled arena (Figure 1A) consists of three copper tiles whose temperature can be individually controlled through a programmable circuit. Each copper tile possesses a temperature sensor that gives feedback to the programmable circuit. The circuit activates a power supply to increase the temperature of each tile. Passive thermoelectric elements act as constant heating elements to maintain the desired temperature, while a heat sink cooled by a fan provides constant cooling. The magnitude of temperature change determines the speed of the process in a non-linear manner. An increase of 2 °C requires only 0.1 s, and an increase of 18 °C requires 4 s. A screen connected to the programmable circuit (Figure 1C) informs the user of the temperature measured by the temperature sensors in each of the tiles. The copper tiles are surrounded by an aluminum ring constantly heated to 50 °C (Figure 1B and 1C) by semiconductors around the periphery. This ring forms the edges of the Fly Arena (Figure 1C), the area in which flies are to be placed. The Fly Arena is covered by a siliconized glass cover (Figure 1A and 1C), which provides a 3 mm high space which ensures that flies can walk but not fly. Next to the Fly Arena are two red LEDs (Figure 1C) that can be programmed to mark different experimental phases. For example, for the results shown in Figure 2A, each LED is associated with a different temperature, while in Figure 2B, each LED indicates 60 s. The FlySteps software can register when each of the indicative LEDs is on, and the researcher can then use this information to automatically determine the experimental phases based on temperature or time.
The temperature-controlled arena can be used to compare the behavioral response of flies from different genetic backgrounds to dynamic temperature changes. For example, flies from different species can be exposed to gradually increasing temperatures (Figure 3) to compare differences in thermal performance. The speed of all species increases as temperature increases until reaching a point of maximum performance, after which it decayed and perished. However, each species has a particular response curve with specific maximum response speeds and thermal tolerances. Previous reports have shown that Drosophila from different species differ among developmental timing, longevity, fecundity, body dimensions, sexual communication, and temperature tolerance3,6,7,8,22. Thus, our description of species-specific locomotion in a temperature gradient adds to this body of work.
The temperature-controlled arena can also be used to explore the response to conditioning experiments based on temperature. The simplest form of this approach is an operant conditioning paradigm in which flies are trained to prefer one side of the arena over the other, by warming up the side that will be avoided23,24,25. We exposed individual flies to 40 °C in the middle and one of the side tiles, while leaving the other side tile at a comfortable 22 °C (Figure 4). Wild-type flies quickly stopped moving along the arena and remained in the comfortable location. In contrast, the classic memory mutant Dunce kept exploring the arena and spent less time than controls in the comfortable location. The differences between performance of the wild-type flies and Dunce mutants became larger when all tiles were set to 22 °C and comparisons were made between the treatment groups. Dunce mutants also showed greater differences between training and test phases in comparison to the wild-type flies (Figure 4). These results suggest an effect of memory on remaining in the comfortable location.
Combinations of temperature and location are also useful to understand the function of different temperature receptors during dynamic temperature changes. We exposed individual D. melanogaster Gr28b.d and TrpA1GAL4 mutants to increasing temperatures (2 °C increase every 60 s) while providing a comfortable location at 22 °C (Figure 5). The comfortable location shifted from left to right, and vice versa, per iteration. Results show that the periphery temperature receptor Gr28b.d mutants behave as the control, as they spend more time in the comfortable location as temperature increases. However, brain temperature receptor TrpA1GAL4 mutants are not affected by increasing temperatures and do not change their locations in the arena. The increases and decrease in the curve of TrpA1GAL4 mutants show the effect in flies that were already sitting in the comfortable location before it became comfortable and remained there during that phase. The consistency of peaks and valleys of the curve of TrpA1GAL4 suggest that these flies remained still for most of the experiment; hence, they were constantly counted when their location was the one considered comfortable. This conclusion was confirmed by visual inspection of the recorded videos. These results support previous physiological reports suggesting that periphery perception of fast and large changes does not depend on Gr28b.d17 and that flies possess a main central mechanism to sense temperature based on TrpA114,21.
Figure 1: Diagram of temperature controlled-arena. (A) A lateral view of the temperature-controlled arena. A programmable circuit connects a power supply and temperature sensors to heating elements under copper tiles to control their temperature. Tiles are constantly cooled down through a heat sink connected to a fan. A heated aluminum ring over which a glass cover rests surrounds the tiles. (B) Thermal imaging showing the tiles set at 24 °C (top) and side tiles at 24 °C with a middle tile at 30 °C (bottom). (C) A top view of the arena. A camera records the copper tiles, aluminum ring, and red LEDs, then automatically determines experimental phases. A screen in the corner of the box, not recorded by the camera, displays the current tile temperature. (D) Ring of light: two warm white LED strips inside a wooden box covered in white paper ensure constant and symmetric illumination of the whole arena. Please click here to view a larger version of this figure.
Figure 2: Flies must acclimate to the arena before starting the temperature protocol. (A) Single male flies were introduced to the arena and allowed to explore at a constant 16 °C for 1 min, after which the temperature started increasing. (B) Single flies exposed to 16 °C, 20 °C, or 24 °C (no group differences; two-way ANOVA F (2,570) = 4.156, p = 0.162) have a higher locomotion at the beginning of the experiment than after 5 min (two-way RM ANOVA F (9,570) = 7.803, p < 0.0001). Data are mean and standard error of the mean (± SEM) of 20 virgin female flies 5 to 7 days old tested over multiple days. Asterisk indicates significant difference among groups (****p < 0.0001; Tukey's multiple comparison test, p = 0.05). Please click here to view a larger version of this figure.
Figure 3: Locomotion of 5 Drosophila species exposed to gradually increasing temperatures. Individual male flies from temperate (blue), tropical (red), and cosmopolitan (brown) Drosophila species were exposed to an increasing temperature gradient (2 °C every 60 s) between 16 and 46 °C. The first 7 min were constantly at 22 °C to allow flies to explore the arena. Species were significantly different (two-way RM ANOVA F(4,70) = 28.46, p < 0.001). (a) D. melanogaster (brown; filled circles) was faster when introduced to the arena. (b) D. yakuba (red; empty squares) was faster as temperature increased. (c) D. suzukii (brown; filled square) was slower than the other cosmopolitan flies at its maximum performance point. (d) D. simulans (brown; empty circles) was in decay at the maximum point of D. melanogaster. Each point represents the mean (± SEM) of 15 male flies 5 to 7 days old tested over several days. Significance indicated by symbols (♦ = difference from all, p < 0.0001; †= difference from all except D. melanogaster, p < 0.0001; • = difference from D. melanogaster, p < 0.01; ¢ = difference from D. melanogaster, p < 0.001; **** = difference between named groups, p < 0.0001; Tukey's multiple comparison test, p = 0.05). Please click here to view a larger version of this figure.
Figure 4: The temperature-controlled arena can be used for operant conditioning. D. melanogaster Canton-S strain (wild-type; black border) and dnc1 (Dunce; red border) mutants were trained to prefer a lateral tile at 22 °C after warming the middle and opposite lateral tiles to 40 °C for 4 min (training, no pattern). Memory of the heated areas is then tested by setting all tiles to 22 °C (test; grid pattern). Flies were conditioned to prefer tiles on the left in half of the experiments, then tiles on the right in the other half. The percentage of total time inside the tile at 22 °C during training and testing was measured to compare performances. Groups were significantly different (one-way ANOVA F(3,76) = 23.23, p < 0.0001), with Dunce performing worse than wild-type overall. Data are mean (± SEM) of 20 virgin female flies 5 to 7 days old tested over several days. Asterisks indicate significance difference among groups (****p > 0.0001; ***p > 0.001; **p > 0.01; Tukey's multiple comparison test, p = 0.05) Please click here to view a larger version of this figure.
Figure 5: Response of temperature mutants to increasing temperature when a comfortable location is provided. Temperature mutants Gr28b.d (green; squares) respond as controls (w1118, black; circles) by increasing the percentage of time in the comfortable area as temperature increases (two-way RM ANOVA F (1,38) = 0.5107, p = 0.479). TrpA1GAL4 mutants (yellow; triangles) are different from controls (w1118, black), as they do not increase the time in the comfortable area as temperature increases (two-way RM ANOVA F (1,38) = 1.670, p = 0.019). Data are mean (± SEM) of 20 male flies 5 to 7 days old tested over several days. TrpA1GAL4 is significantly different from Gr28b.d and the control (p < 0.05; Tukey's multiple comparison test, p = 0.05). Please click here to view a larger version of this figure.
Here we have presented an automated temperature-controlled arena (Figure 1) that produces precise temperature changes in time and space. This method allows exposure of individual Drosophila not only to pre-programmed gradual increases of temperature (Figure 2 and Figure 3), but also to dynamic temperature challenges in which each tile of the fly arena was heated independently to a different temperature (Figure 4 and Figure 5).
The temperature-controlled arena uses an innovative approach to the heating process. Instead of producing temperature changes in the tiles through thermoelectric Peltier heating elements used in traditional methods, the temperature-controlled arena uses current to warm up a copper mass with the copper tiles, and flies are placed at the top. The copper mass is constantly cooled down by a heat sink block connected to a fan. Peltier-like elements are used to maintain the desired temperature of the copper mass once it has been warmed up. Because these elements are not the main temperature generators, they suffer less stress, which extends their life span and permits faster temperature changes. A programmable circuit that receives feedback from temperature sensors under each of the copper tile, which can also activate the low voltage power supply, coordinates the heating mechanism. Researchers can specify when and where temperature changes occur and determine the intensity and direction of such changes. Furthermore, coupling the method with specialized tracking software, such as FlySteps, permits analysis of all aspects relating to Drosophila's movement, such as the overall speed at certain temperatures or time spent in certain locations (Figure 2, Figure 3, Figure 4, Figure 5). Nevertheless, all results must consider characteristics inherent to fly behavior that might affect their locomotion. For example, if flies are not allowed to explore the arena and settle before changing the temperature, speed measurements might be artificially high (Figure 2). Flies can also leave odorants that affect subsequent flies; hence, the glass cover must be cleaned, and tape covering the tiles must be changed between subjects. Given that locomotion declines as flies age26, it is important that flies are standardized for age to avoid variation in results. In our arena, flies have also shown centrophobism, preferring edges over the middle area. Experimenters must control for this by changing the location of comfortable areas to prevent overestimating site preference.
The current characteristics of the arena and requirements of the tracking process could limit some experimental procedures. For example, the close environment of the arena does not include access points through which odours could be introduced, which prevents studies in which this stimulus is important. Similarly, the FlyStepts tracker necessitates videos with uniform backgrounds, which limits the possibility of adding food or other items to the fly's environment. The arena could be adapted to include a connection to a gas valve, and software developments exist that may allow for more objects to be present. Future projects may take advantage of these possibilities to adapt the temperature-controlled arena to specific experimental needs.
Finally, we have shown in the results that different species of Drosophila perform differently as temperature increases (Figure 3) and that temperature mutants do not respond in the same way as controls (Figure 5). This shows that this new method may be used to explore Drosophila's thermal behavior and how it is affected by natural selection and functional characteristics. Finally, it illustrates that our method may help further understanding of thermal adaptation and speciation as well as the interactions of temperature receptors with other stimuli in future studies.
The authors have nothing to disclose.
This work was supported in part by a scholarship from the Behavioural and Cognitive Neuroscience Program of the University of Groningen and a graduate scholarship from the Consejo Nacional de Ciencia y Tecnología (CONACyT) from Mexico, granted to Andrea Soto-Padilla, and a grant from the John Templeton Foundation for the study of time awarded to Hedderik van Rijn and Jean-Christophe Billeter. We are also thankful to Peter Gerrit Bosma for his participation in developing the FlySteps tracker.
Scripts TemperaturePhases,FlySteps, and FlyStepAnalysis can be found as supplementary information and in the following temporary and publicly available link:
https://dataverse.nl/privateurl.xhtml?token=c70159ad-4d92-443d-8946-974140d2cb78
Arduino Due | Arduino | A000062 | Software RUG |
Electronics Board | Ruijsink Dynamic Engineering | FF-Main-02-2014 | |
Power supply Boost | XP-Power 48. V 65 W | ECS65US48 | Set to 53 Volt |
Power supply Tile Heating | XP-Power 15. V 80 W | VFT80US15 | |
Power supply Cooling | XP-Power 15. V 130 W | ECS130U515 | |
Peltier elements | Marlow Industries | RC12-4 | 2 Elements, controlled DC feed |
Heat sink | Fisher Technik | LA 9/150-230V | Decoupled for vibration |
Temperature sensors | Measurement Specialties | MCD_10K3MCD1 | Micro Thermistor Probe |
Copper block/tiles | Ruijsink Dynamic Engineering | FF-CB-01-2014 | |
Auminum ring | Ruijsink Dynamic Engineering | FF-RoF-02-2015 | |
Tesa 4104 white tape 25 x 66 mm | RS Components | 111-2300 | White conductive tape |
Red LEDs | Lucky Ligt | ll-583vc2c-v1-4da | Wavelength between 625 nm, 20 mAmp and 6 V |
Warm white LED strip | Ledstripkoning | HQ-3528-SMD | 60 LEDs per meter |
Switch Power Supply | Generic | T-36-12 | |
Logitech c920 | Logitech Europe S.A | PN960-001055 | |
QuickTime Player | Apple Computer | Recording program | |
Tracking analysis software | R | Packages: pacman | |
Tracking analysis software | MATLAB | ||
Thermal Imaging | FLIR T400sc | ||
Graphs and Statisticts Software | Graph Pad Prism | ||
Sigmacote | Sigma-Aldrich | SL2-100ML | Siliconising agent |
Fly rearing bottles | Flystuff | 32-130 | 6oz Drosophila stock bottle |
Flypad | Flystuff | 59-114 | |
Fly rearing vials | Dominique Dutscher | 789008 | Drosophila tubes narrow 25×95 mm |
Incubator | Sanyo | MIR-154 | |
Magnetic hot plate | Heidolph | 505-20000-00 | MR Hei-Standard |
Agar | Caldic Ingredients B.V. | 010001.26.0 | |
Glucose | Gezond&wel | 1019155 | Dextrose/Druivensuiker |
Sucrose | Van Gilse | Granulated sugar | |
Cornmeal | Flystuff | 62-100 | |
Wheat germ | Gezond&wel | 1017683 | |
Soy flour | Flystuff | 62-115 | |
Molasses | Flystuff | 62-117 | |
Active dry yeast | Red Star | ||
Tegosept | Flystuff | 20-258 | 100% |