A comparative view of insect circadian clock systems

Abstract

Recent studies revealed that the neuronal network controlling overt rhythms shows striking similarity in various insect orders. The pigment-dispersing factor seems commonly involved in regulating locomotor activity. However, there are considerable variations in the molecular oscillatory mechanism, and input and output pathways among insects. In Drosophila, autoregulatory negative feedback loops that consist of clock genes, such as period and timeless are believed to create 24-h rhythmicity. Although similar clock genes have been found in some insects, the behavior of their product proteins shows considerable differences from that of Drosophila. In other insects, mammalian-type cryptochrome (cry2) seems to work as a transcriptional repressor in the feedback loop. For photic entrainment, Drosophila type cryptochrome (cry1) plays the major role in Drosophila while the compound eyes are the major photoreceptor in others. Further comparative study will be necessary to understand how this variety of clock mechanisms derived from an ancestral one.

Fig. 1

Schematic diagrams showing Drosophila (a) and cockroach (b) cerebral cells expressing PERIOD (redrawn from [23, 125]). a In Drosophila, there are seven main groups of neurons. Three groups (DN1, DN2, and DN3) are located in the dorsal region, and the remaining four (LNd, l-LNv, s-LNv, and LPN) are located laterally. The s-LNv and l-LNv express a neuropeptide, PDF. l-LNvs have their processes in the optic lobe and send their axonal projection to the contralateral optic lobe through the posterior optic tract (POT). s-LNvs have axonal projection to the dorsomedial region of the protocerebrum. b In the cockroach, Blattera germanica, three groups of neurons are located in the optic lobe (G-I, G-II, and G-III) and three groups in the dorsal protocerebrum (G-IV, G-V, and G-VI). There is another group (G-D) in the deutocerebrum. PDF is coexpressed in G-I, -II, -III, -IV and -D cells. OL Optic lobe, Pr protocerebrum, De deutocerebrum

Fig. 2

A scheme illustrating current knowledge about molecular machinery of the Drosophila circadian clock. The circadian clock consists of three feedback loops that are interlocked to oscillate with a period of about 24 h. See text for details