Temperature-induced plasticity of cold tolerance has been reported in many insect species, but cold tolerance can also be affected by changes in day (or night) length. In the present study we elucidate the direct and indirect effects of photoperiod on the cold tolerance of females of two Drosophila montana strains--one which possesses a robust photoperiodic diapause and another which does not. In the diapause-strain the time needed for recovery from chill coma showed a positive correlation with day length, but diapause itself played only a minor role in photoperiodic acclimation. The strain that was not able to enter to diapause as a response to day length also lacked photoperiodic cold acclimation ability indicating that this strain has deficiencies in its photoperiodic time measurement system. In the diapause-strain, the expression level of regucalcin gene was more than two times higher in diapausing than in non-diapausing females maintained in a single photoperiod, but day length per se did not cause significant changes in expression levels of this gene in either of the strains. In the non-diapausing strain this gene showed no expression changes in any comparison. Overall, the study shows that a decrease in day length can induce cold acclimation in D. montana, while changes in regucalcin expression are linked with photoperiodic diapause.
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