Photoperiodic plasticity of pigment-dispersing factor immunoreactive fibers projecting toward prothoracicotropic hormone neurons in flesh fly Sarcophaga similis larvae

Abstract

Larvae of the flesh fly, Sarcophaga similis exhibit photoperiodic responses to control pupal diapause. Although the external coincidence model is applicable to S. similis photoperiodic responses, it remains unknown how circadian clock system integrates day-length information. To explore the mechanisms underlying the photoperiodic control of pupal diapause, we examined the neural circuitry involving circadian clock lateral neurons (LNs) and prothoracicotropic hormone (PTTH) neurons, the latter possibly controlling the ecdysteroid production. We also examined the photoperiodic effects on LN-fiber patterns in third-instar S. similis larvae. Immunohistochemistry showed that PERIOD and a neuropeptide pigment-dispersing factor (PDF) were co-localized in four cells per hemisphere, and we named these PDF-LNs of S. similis. Single-cell polymerase chain reaction of backfilled neurons from the ring gland showed that two pairs of pars lateralis neurons with contralateral axons (PL-c neurons) expressed ptth. Double labeling with immunohistochemistry and backfills revealed that PDF-immunoreactive varicose fibers projected in the proximity of fibers from PL-c neurons. However, pdf receptor was not but glutamate-gated chloride channel was expressed in most PL-c neurons. L-glutamate but not PDF acutely inhibited the spontaneous firing activity of PL-c neurons. The number of PDF-immunoreactive varicosities of PDF-LNs in the dorsal protocerebrum was significantly higher under short-day than that under long-day conditions in a time-dependent manner. These results suggest that PDF-LNs, PTTH neurons, and glutamate signaling form a potential neural circuity for the photoperiodic control of pupal diapause and that photoperiod modifies the connectivity strength between PDF-LNs and their post- or pre-neurons in the circuitry.