Turns with multiple and single head cast mediate <i>Drosophila</i> larval light avoidance

<div><p><i>Drosophila</i> 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.</p></div

Typical 1-cast and n-cast turns in light-spot based phototaxis.

<p>The tailspeed (top row), headomega (second row), headtheta (third row) and bodyomega (bottom row) time series of a larval 1-cast (left column) and a 4-cast (right column) turning events. Rectangular frames show the stop periods. Arrows in tailspeed point to the onset of the stop period. Arrows in headomega point to headomega peaks indicating head casts following stop. Arrows in headtheta point to the largest body angles. Arrows in bodyomega point to the bodyomega peaks which indicate larval turning. In 1-cast, the larva entered the light spot at 2.87 second. In 4-cast, the larva entered the light spot at 2.0 second, the stop period indicated by the rectangular frame is the second stop period after larva entered light spot.</p

n-cast turns are initiated independently of 1-cast turns.

<p>(A) Percentages of successful light avoidance of the first head cast in 1-casts and n-casts in larval fast phototaxis. n = 28 for 1-cast, n = 18 for n-cast. (B) Percentages of larvae that rejected failed first and second head casts. n = 30 for 1^st cast, n = 12 for 2^nd cast. (C) Rates of acceptance of first and second head casts that were successful in light avoidance. n = 16 for 1^st cast, n = 9 for 2^nd cast. (D) The magnitudes of accepted first head casts that failed in light avoidance were significantly larger than those that were rejected. The magnitude of head cast was measured by peak value of headomega. n = 13 for accepted, n = 17 for rejected. (E) The headomega magnitudes of accepted second head casts that failed in light avoidance were similar to those of the rejected casts. The magnitude of head cast was measured by peak value of headomega. n = 7 for accepted, n = 5 for rejected. **, <i>P</i><0.01; n.s., not significant; Fisher’s exact test for A, B, C, <i>t</i>-test for D and E.</p

Second head casts are different from first head casts in n-cast turns.

<p>(A) Success rates of first-cast, second-cast and n> = 3-casts in n-cast turns in light avoidance. n = 19, 19 and 17 for 1^st, 2^nd and > = 3^rd cast.*,<i>P</i><0.05, Fisher’s exact test. (B) Peak headomega values of head casts in n-cast turns. The amplitude of the second head cast is significantly larger than that of the first head cast. n = 19, 19, 7, 5 and 5 for 1^st, 2^nd, 3^rd, 4^thand > = 5^th cast. *,<i>P</i><0.05, n.s. not significant, <i>t</i>-tests.</p

Properties of 1-cast and n-cast turns in larval fast phototaxis.

<p>(A) Frequency of turns with one head cast, two head casts, and three or more head casts. The frequency decreased as the number of head casts increased. n = 46. (B) The distribution of turns with different numbers of head casts in relation to the distance from the edge of the light spot at the time that turning was initiated. There was no obvious correlation between the distance and number of head casts. Positive distance means out of light spot, negative distance means inside light spot. n = 45. (C) The probability of successful light avoidance after 1-cast and n-cast turns was lower than the overall probability of successful light avoidance. n = 28, 18and 35 respectively for 1-cast, n-cast and total. **, <i>P</i><0.01, ***, <i>P</i><0.001, Fisher’s exact test.</p