Drosophila gravitaxis maze is used to observe positive or negative gravitaxis movement in Drosophila species. It is a choice maze that allows the subjects to move up or down based on their preference. The results obtained using gravitaxis maze are analyzed for studying the molecular and genetic basis of gravitaxis.

Gravitaxis, also known as geotaxis, is the directed movement represented by organisms in response to gravity. Gravity helps living beings to orient themselves with their surroundings. It is integral for an organism to respond to gravitational force either positively or negatively to sustain in the environment. The movement of an organism towards gravity is called positive gravitaxis or geopositive response, whereas, the movement against gravity is termed as negative gravitaxis or geonegative response. Like other organisms, Drosophila melanogaster also displays a strong gravitaxis behavior.

The Drosophila gravitaxis maze consists of an entrance tube fitted into the maze made of polypropylene tubes along with T and Y shaped connectors. The maze has one entrance and nine exits. The maze makes use of an attractant in the exit collection tubes in addition to the light source present near the exit of the maze. As a result, the Drosophila navigate the maze by choosing either to walk upward or downward to reach their preferred choice exit. The number of up or down choices made by the flies in the maze is used to deduce the gravitaxis behavior of Drosophila.

Other apparatuses that make use of the Drosophila model are the Drosophila Y Maze, the Drosophila Shallow Chamber, the Drosophila Maze Array, and the Drosophila Olfactory Operant Conditioning.

The gravitaxis maze consists of the entrance tube, the maze, and the exit collection tubes. The entrance channel of the maze is fitted with an 11 cm long plastic tube covered with opaque black tape. The maze is composed of 4.76 mm diameter polypropylene tubing and transparent plastic Y, and T shaped connectors. The nine exit areas comprise 15 ml collection tubes made of acrylic, fitted through small plastic pipes and rubber stoppers. These collection tubes contain the attractant for flies such as yeast.  The maze is fixed to a 0.79 mm thick supporting base for rigidity. An illuminated source is placed seven inches from the collection tubes at the maze end.

Before the trial begins, number the collection tubes from top to bottom as 1 to 9 for calculating the result. An external tracking and recording systems such as Noldus Ethovision XT can be used with the Gravitaxis maze for observing the flies’ gravitaxis behavior.

Pretraining

Transfer the subjects in empty vials 20 to 30 min before loading into the gravitaxis maze to avoid any transfer of food or debris inside the maze.

The Gravitaxis Assay

Place an attractant such as yeast paste in the collection tubes at the exit of the maze. Transfer the flies in the entrance tube and attach it to the maze entrance point. Flick the entrance tube after 1 hour to motivate the flies to leave the tube and enter the maze. Allow the flies to run through the maze and gather in the collection tubes under the influence of light source near the exit. After 3 hours, remove the collection tubes and plug them with cotton. Count the number of flies present in each collection tube.

Identification of QTLs contribution in Gravitaxis behavior of Drosophila melanogaster

Desroches, Busto, Riedl, Mackay, and Sokolowski (2010) performed Quantitative Trait Locus (QTL) mapping in 88 recombinant inbred (RI) lines of male Drosophila melanogaster. The   Gravitaxis maze was used to identify the significant QTLs responsible for the gravitaxis behavior observed. Subjects were tested every day for 5 consecutive days with strain and maze location randomized for each day. Behavioral variation was observed in all RI lines in adult gravitaxis. Further, the mean exit tube position was approximately 4 across all lines of Drosophila melanogaster. QTL mapping revealed four significant gravitaxis QTLs on the second and third chromosomes.  Of the four QTLs, gravitaxis QTL A had the highest level of significance in the QTL analysis.

The following parameters can be recorded in the Drosophila Gravitaxis maze:

• Number of flies at each exit point
• Average choices made in the upward direction
• Average choices made in the downward direction

Strengths

The gravitaxis maze helps in assessing gravitaxis behavior and furnish the basic data required for evaluating the molecular, cellular and genetic assessment of the gravitaxis behavior in flies. The maze has a simple yet effective design for assessment of gravitaxis behaviors. The maze is easy to use and delivers reproducible results. The subjects do not require habituation to the apparatus prior to being tested.

Limitations

The orientation (vertical or horizontal) of the maze is crucial for obtaining unbiased and accurate results. Vertically oriented gravitaxis maze produce more accurate results as more flies appear at the exit points. On the contrary, horizontally oriented mazes show more random movement in the right and left direction instead of towards or away from gravity (Armstrong, Texada, Munjaal, Baker, & Beckingham, 2006).  Factors such as age, sex, species, and strains can also affect the task results. Presence of unnecessary disturbances and stimuli can also affect task performances.

• Drosophila gravitaxis maze is used to observe the positive and negative gravitaxis movement.
• The gravitaxis maze consists of nine choice tubes facing a light source which acts as an attractant.
• The results obtained by using gravitaxis maze can be applied for further research to evaluate the molecular and genetic basis of gravitaxis along with the gravitaxis defects in the subjects.
• The vertically oriented choice maze produces more accurate results in comparison to the horizontal orientation.

1. Armstrong, J. D., Texada, M. J., Munjaal, R., Baker, D. A., & Beckingham, K. M. (2006). Gravitaxis in Drosophila melanogaster: a forward genetic screen. Genes, Brain and Behavior, 5(3), 222-239. doi: 10.1111/j.1601-183X.2005.00154.x
2. Desroches, C. E., Busto, M., Riedl, C. A., Mackay, T. F., & Sokolowski, M. B. (2010). Quantitative trait locus mapping of gravitaxis behaviour in Drosophila melanogaster. Genetics Research, 92(3), 167-174. doi:10.1017/S0016672310000194