Abstract
Drosophila larvae exhibit klinotaxis when placed in a gradient of temperature, chemicals, or light. The larva samples environmental stimuli by casting its head from side to side. By comparing the results of two consecutive samples, it decides the direction of movement, appearing as a turn proceeded by one or more head casts. Here by analyzing larval behavior in a light-spot-based phototaxis assay, we showed that, in addition to turns with a single cast (1-cast), turns with multiple head casts (n-cast) helped to improve the success of light avoidance. Upon entering the light spot, the probability of escape from light after the first head cast was only ~30%. As the number of head casts increased, the chance of successful light avoidance increased and the overall chance of escaping from light increased to >70%. The amplitudes of first head casts that failed in light avoidance were significantly smaller in n-cast turns than those in 1-cast events, indicating that n-cast turns might be planned before completion of the first head cast. In n-casts, the amplitude of the second head cast was generally larger than that of the first head cast, suggesting that larvae tried harder in later attempts to improve the efficacy of light avoidance. We propose that both 1-cast turns and n-cast turns contribute to successful larval light avoidance, and both can be initiated at the first head cast.
Highlights
Multiple head cast in Drosophila larval phototaxis experiments, a beam of blue light (460 nm) was projected through a round hole in a foil sheet to form a 2-cm diameter light spot on an agar plate
We investigated the location of single and multiple head-cast turns, which was defined as the position where a stop period started. 1-casts and n-casts appeared to occur with approximately equal probability at a range of distances from the edge of the light field (Fig 2B)
No obvious correlation was seen between the magnitude of head casts and the larval orientation angle with respect to the light spot, for both the first and second head casts (S5 Fig)
Summary
Fly culture w1118 flies were raised at 25 ̊C on standard medium in a 12:12 h light: dark cycle. Third instar larvae of 72–96 hours after egg laying were used. The setup, placed in a dark room, consisted of: an LED (Thorlabs Inc. Newton, NJ, USA) for blue light illumination; a web camera with an 850±30 nm band-pass infrared filter in front of the lens to allow infrared imaging; LED generating infrared light placed aside the test arena; an 1% agar plate (1mm in thickness) as arena for behavior test (S1 Fig). Multiple head cast in Drosophila larval phototaxis experiments, a beam of blue light (460 nm) was projected through a round hole in a foil sheet to form a 2-cm diameter light spot on an agar plate. The light intensity was 0.44 μw/cm in the light spot (S1 Fig).Light intensity changed for ~40 folds within 2-3mm at the rim of the light spot (S2 Fig)
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