Mechanisms of insect photoperiodism: analysis in Protophormia terraenovae

Abstract

Most of the animals know seasons via environmental signals and adapt the development and reproduction to cyclic seasonal changes. Among those, length of a day (photoperiod) is the most reliable because of constancy through years, and thus many animals use photoperiod as a seasonal cue. The physiological response of organisms to photoperiod is called photoperiodism. The blow fly, Protophormia terraenovae has relatively large body size of 1cm length, and live in high-latitude regions such as Hokkaido or Aomori prefecture in Japan. Females of the blow fly have adult diapause induced under the short-day and low-temperature conditions in autumn. It is believed that the diapausing flies overwinter, for instances, among stacked fallen leaves beneath snow. Overwintered flies mate and lay eggs from spring to early summer. Now we have accumulated data about photoperiodic photoreceptors (photoreceptors in the compound eye), an endocrine organ (corpus allatum), and brain neurosecretory cells (PI neurons and PL neurons) regulating diapause induction and reproduction in P. terraenovae . Furthermore, it has been demonstrated that circadian clock neurons (small ventral lateral neuron, s-LN v ) driving circadian rhythm are involved in photoperiodism in this species, and that LN v including s-LN v synapses upon PL neurons crucial for diapause induction. Besides, both the expression pattern of a circadian clock gene ( period ) in the brain and subcellular localization of PERIOD in LN v alter in a photoperiod-dependent manner. Light information received in the compound eyes is probably translated into photoperiodic information, such as long days or short days in LN v . It seems that PL neurons receiving the short-day information from LN v change own electrophysiological properties to induce diapause. In this review, I introduce neural bases underlying photoperiodism of insects, focusing on the knowledge revealed in a non-model organism, P. terraenovae .