Molecular characterization of the circadian clock genes in the bean bug, Riptortus pedestris, and their expression patterns under long- and short-day conditions

Although the molecular mechanisms and the diversity of insect circadian clocks have been well investigated in holometabolous insects, hemimetabolous insects have received little attention. In the present study, we isolated the circadian clock genes, period (per), cycle (cyc), vrille (vri), and mammalian-type cryptochrome (cry-m) from the bean bug Riptortus pedestris. This is the first report of vri and cry-m in hemimetabolous insects. All of the genes showed high similarities to respective homologous genes in other insects. The discovery of cry-m in R. pedestris indicates that the clockwork of hemimetabolous insects is similar to that in insects having CRY-m, including the monarch butterfly Danaus plexippus and the honey bee Apis mellifera, and not to insects lacking it, such as Drosophila melanogaster. Real-time PCR showed that mRNAs of these circadian clock genes exhibited extremely weak diel oscillations at day 9 in the head of R. pedestris, and their expression levels under long- and short-day conditions were nearly identical. In addition, expression levels of per mRNA were almost stable from days 0 to 15 under both photoperiodic conditions. The difference between long-day and short-day conditions in the mRNA level seems too small to distinguish photoperiodic conditions clearly. These results suggest that transcriptional regulations of circadian clock genes would not play an important role in the diapause programming in R. pedestris.