The planarian TRPA1 homolog mediates extraocular behavioral responses to near ultraviolet light

Abstract

Although light is most commonly thought of as a visual cue, many animals possess mechanisms to detect light outside of the eye for various functions, including predator avoidance, circadian rhythms, phototaxis and migration. Here we confirm that planarians (like <i>Caenorhabditis elegans</i>, leeches and <i>Drosophila</i> larvae) are capable of detecting and responding to light using extraocular photoreception. We found that, when either eyeless or decapitated worms were exposed to near-ultraviolet (near-UV) light, intense wild-type photophobic behaviors were still observed. Our data also revealed that behavioral responses to green wavelengths were mediated by ocular mechanisms, whereas near-UV responses were driven by extraocular mechanisms. As part of a candidate screen to uncover the genetic basis of extraocular photoreception in the planarian species <i>Schmidtea mediterranea</i>, we identified a potential role for a homolog of the transient receptor potential channel A1 (<i>TRPA1</i>) in mediating behavioral responses to extraocular light cues. RNA interference (RNAi) to <i>Smed-TrpA</i> resulted in worms that lacked extraocular photophobic responses to near-UV light, a mechanism previously only identified in <i>Drosophila</i> These data show that the planarian <i>TRPA1</i> homolog is required for planarian extraocular-light avoidance and may represent a potential ancestral function of this gene. TRPA1 is an evolutionarily conserved detector of temperature and chemical irritants, including reactive oxygen species that are byproducts of UV-light exposure. Our results suggest that planarians possess extraocular photoreception and display an unconventional TRPA1-mediated photophobic response to near-UV light.