Phase response curve and RNA-sequencing demonstrate spiders' sensitivity to light and pinpoint candidate light-responsive genes
Spiders can maintain a wide range of free-running periods while still being entrained to a 24-hour day. To investigate the underlying mechanism of this entrainment, we constructed the phase-response curve (PRC) for the orb weaver, Metazygia wittfeldae, by subjecting the spiders to one-hour light pulses at various times throughout the circadian day. The resulting type 0 PRC showed high amplitude (> 6 hour) phase advance and delays when the light pulse was applied during circadian time (CT) 16-18, with a break point of advances to delays at CT 17. We then investigated the genetic mechanism of the phase response to light by splitting M. wittfeldae adult females entrained to 12 hours light:12 hours dark (LD 12:12) into two groups. One group received a 1-hour light pulse 5 hours after lights off (CT17), and one group did not. We then sacrificed spiders for RNA isolations 1 and 10 hours after the light pulse. We identified numerous genes that were downregulated by the light pulse 1 hour after the pulse relative to no pulse group. Intriguingly, many of these genes had a flipped pattern of expression 9 hours later - the pulse group had higher expression than the no pulse group. This pattern is consistent with the shifted phase of locomotor activity expected after the light pulse application. We also identified clock gene homologs in M. wittfeldae that had distinct expression patterns from other arthropods.