I always feel a little sad when I have to start putting on the lights almost as soon as I get home at the end of the day, but shorter day lengths signify a far more dramatic lifestyle change for many other species. When the number of hours of daylight falls too low, many northern insects place their reproductive systems into suspended animation – known as reproductive diapause – to wait out winter. Some species fine-tune the timing of the switch depending on their latitude, with females from higher latitudes triggering reproductive diapause when the days are longer than those in more southern locations. Yet, how insects synchronize the physiological processes that must be coordinated to harmonise their physiology with the seasons, remained a mystery.
With a long-standing interest in how insects predict the onset of winter, Anneli Hoikkala from the University of Jyväskylä, Finland – with colleagues from her university, Oulu University, Finland, and St Andrews, UK – took advantage of the insect's versatility. They artificially evolved populations of Drosophila montana, collected in northern Finland by shortening the amount of daylight experienced by each subsequent generation by 0.5 h to begin unravelling the masterplan that coordinates the flies’ seasonal reproductive off switch. Once they were sure that the diapause trigger had been reprogrammed by the shorter day lengths, the team looked for differences between the new population and flies that had been maintained for generations on a daily schedule of 24 h light, simulating the northern summer.
Presenting the new population with days where the number of hours of light varied from 13–21 h over a 24 h period, Hannele Kauranen and Johanna Kinnunen found that 50% of the females went into diapause when they had 17.5 h of light, in contrast to their northerly ancestors, which initiated reproductive diapause at 19.4 h. Then Pekka Lankinen tinkered with the length of the full day – fixing the amount of daylight at 12 h while varying the length of the night from 6 to 72 h – he found that their internal body clocks – circadian rhythms – were not contributing to triggering the flies’ reproductive shut down.
Having checked factors that could activate reproductive diapause, the team knew that the wild insects tend to bear cold better as the days draw in, so Anna-Lotta Hiillos measured the temperature at which the evolved flies toppled into a coma when it became too cold. She found that females from the evolved population that had gone into diapause were only able to able to tolerate temperatures down to around –1.8°C while the flies that had experienced normal 24 h days across the generations were able to hold out to –2.2°C; the evolved flies were less cold tolerant. Finally Axel Wiberg and Michael Ritchie analysed the flies’ genome to search for differences that could explain the reproductive diapause shift and found changes in genes involved in the insect's ability to sense light, their hormone signalling, growth and egg development. ‘Overall, our study shows that photoperiodic selection for reproduction under short photoperiods affects diapause-associated traits without disrupting the central clock network generating circadian rhythms in fly locomotor activity’, says Kauranen.