Adaptive differences in circadian clock gene expression patterns and photoperiodic diapause induction in Nasonia vitripennis

Day length (photoperiod) and temperature oscillate daily and seasonally and are important cues for season-dependent behaviour. Larval diapause of the parasitoid Nasonia vitripennis is maternally induced following a certain number of days (switch point) of a given critical photoperiod (CPP). Both the switch point and CPP follow a latitudinal cline in European N. vitripennis populations,. We previously showed that allelic frequencies of the clock gene period correlate with this diapause induction cline. Here, we report that circadian expression of four clock genes, period (per), cryptochrome-2 (cry-2), clock (clk) and cycle (cyc), oscillates as a function of photoperiod and latitude of origin in wasps from populations from the extremes of the cline. Expression amplitudes are lower in northern wasps, indicating a weaker, more plastic, clock. Northern wasps also have a later onset of activity and longer free running rhythms in constant conditions. Per RNAi caused speeding up of the circadian clock, changed the expression of other clock genes and delayed diapause in both southern and northern wasps. These results point towards adaptive latitudinal clock-gene expression differences and to a key role of per in the timing of photoperiodic diapause induction of N. vitripennis.