A Rapid Food-Preference Assay in Drosophila

Summary

We present a protocol for a two-choice feeding assay for flies. This feeding assay is fast and easy to run and is suitable not only for small-scale laboratory research, but also for high-throughput behavioral screens in flies.

Abstract

To select food with nutritional value while avoiding the consumption of harmful agents, animals need a sophisticated and robust taste system to evaluate their food environment. The fruit fly, Drosophila melanogaster, is a genetically tractable model organism that is widely used to decipher the molecular, cellular, and neural underpinnings of food preference. To analyze fly food preference, a robust feeding method is needed. Described here is a two-choice feeding assay, which is rigorous, cost-saving, and fast. The assay is Petri-dish-based and involves the addition of two different foods supplemented with blue or red dye to the two halves of the dish. Then, ~70 prestarved, 2-4-day-old flies are placed in the dish and allowed to choose between blue and red foods in the dark for about 90 min. Examination of the abdomen of each fly is followed by the calculation of the preference index. In contrast to multiwell plates, each Petri dish takes only ~20 s to fill and saves time and effort. This feeding assay can be employed to quickly determine whether flies like or dislike a particular food.

Introduction

Despite dramatic differences in the anatomical structure of taste organs between flies and mammals, the flies' behavioral responses to many tastant substances are strikingly similar to those of mammals. For example, flies prefer sugar^{1,2,3,4,5,6,7,8}, amino acids^9,10, and low salt¹¹, which indicate nutrients, but reject....

Protocol

1. Assembling the assay chambers

NOTE: While this protocol describes the use of a 35 mm Petri dish (Figure 1A), the desired effect can be achieved using any watertight, smooth-bottomed vessel that can be bisected and covered.

1. First, bisect a lidded 35 mm Petri dish by fixing a length of plastic (5 mm in width and 3 mm in height) down the midline with waterproof adhesive, forming two watertight compartments. Confirm that the seal is complete to avoid leakage that can lead to mixing of the two food substrates being assayed.
   ​NOTE: After assembly, reuse this apparatus as long as the seal h....

Results

In this assay, a 35 mm dish was divided into two equal feeding compartments, with each half of the dish containing agarose food coupled with either blue or red dye (Figure 1A). To exclude dye bias, the blue and red dye concentrations were carefully refined to yield an approximate "0" PI when only these two dyes were added (Figure 1B). Once the Petri dish was loaded with tested food, ~70 wet-starved, 2-4-day-old adult flies were transferred to the dish, allowing .......

Discussion

This method involves several crucial steps where problems can occur. First, make sure flies ingest a sufficient amount of food to provide stable data. If flies eat poorly, ensure that the flies have been wet-starved for at least 24 h, and that the experimental media contains at least a minimal sucrose concentration (2 mM). To further stimulate food consumption, prolong the wet-starvation period beyond 24 h, depending on the flies' physiological condition. If too many flies fail to survive the prolonged starvation, en.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
35 mm Petri dish	Fisher Scientific	08-772E
Agarose	Thomas Scientific	C756P56
Clear adhesive	Fisher Scientific	NC9884114
Conical centrifuge tubes	Fisher Scientific	05-527-90
Dissection microscope	Amscope	SM-2T-6WB-V331
FCF Brilliant Blue	Wako Chemical	3844-45-9
Fly CO2 anesthesia setup	Genesee Scientfic	59-114/54-104M
Fly incubator with programmable day/night cycle	Powers Scientific Inc.	IS33SD
Fly lines
Glass dish (microwave-safe)
Kimwipes	Fisher Scientific	06-666A
Media storage bottle	Fisher Scientific	50-192-9998
Plastic divider cut to fit the dish from a sheet no thicker than 5 mm
Plastic fly vials	Genesee Scientific	32-116
Sucrose	Millipore Sigma	S9378
Sulforhodamine B	Millipore Sigma	S9012
Tastant compound of interest
Vortex mixer	Benchmark Scientific	BV1000
Water bath	Fisher Scientific	FSGPD05