The temporal expression pattern of the circadian clock gene period was compared between heads of the linden bug, Pyrrhocoris apterus , kept under diapause-promoting short days (SD) and diapause-preventing long days (LD) using a real-time PCR quantification. Diapause or reproduction was programmed by photoperiod during the larval stage, but the first difference in per mRNA abundance between SD and LD insects was observed only after adult ecdysis. The expression level of per mRNA was markedly higher, up to more than 10-fold, in the destined-to diapause animals compared with those scheduled for reproduction. Up-regulation of per transcript was restricted to an early diapause peak, with the maximum expression on days 3 to 5 after adult ecdysis. Starvation reduced the peak level of per mRNA to about 50% of the value found in feeding females in the SD conditions, but per mRNA abundance was similarly low in fasting and feeding females in LD. Photoperiodic refractoriness in either wild-type postdiapause adults or in a selected nondiapause variant of P. apterus was associated with reproduction and low, LD-like levels of per mRNA under both SD and LD. Overall, the data suggest that the photoperiodic programming itself has no direct effect on per mRNA abundance, but it does determine the response of per transcript to food signals during subsequent expression of diapause/reproduction physiology.
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