<i>tph(RNAi)</i> animals lacking eye pigment are slower to orient to light.

<p>Orientation dynamics of control and <i>tph(RNAi</i>) planarians under low (A-C), medium (D-F) and high (G-I) light gradient conditions. Both control and <i>tph(RNAi)</i> worms respond to all three gradients by turning away from the light source, but <i>tph(RNAi)</i> worms react less efficiently, especially in low light gradients. (A, D, G) Center-of-mass tracking results for both control and tph(RNAi) populations (n = 10). Triangles indicate the orientation of the light gradient, with the light source on the right. All points are color-coded for time, as shown by color bar legends. At t = 0 s, the worms are manually oriented toward the light source. (B, E, H) Orientational order parameters as a function of time for both populations (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127074#sec002" target="_blank">Methods</a>). The error bars show SEM at selected time points. The dashed y = 0 line is a guide for the eye showing the threshold between orientation biased toward the light souce (y > 0) and away from the light source (y < 0). Note the decrease in time scale as the gradient strength is increased, indicating a faster negative phototactic response of both populations at increased illumination. (C, F, I) Angular distributions of velocities at different time periods (corresponding to grayed regions in B, E and H) showing the reversal of polarity at different rates between control and <i>tph(RNAi</i>) worms. The size of the wedge indicates the proportion of worms traveling in the given direction in that time period, where 0° is towards the light source, and 180° is away from the light source. A two-way ANOVA test confirms statistical difference at the 1% level for TPH and control animals as well as for the three different gradient settings.