DIAPAUSE'S SHOCKING SWITCH

Abstract

Growing up on a nicely decaying lump of flesh is the best start in life a Sarcophaga crassipalpis larva can get. Rotting flesh, gaping wounds, Sarcophaga parents will deposit their larvae on any old piece of meat, making them true flesh flies, by name and nature. But there's one thing this insect doesn't like, and that's winter. At the first sign of shorter days, the adults trigger a developmental program that will see their next brood safely through to spring by arresting the pupae's development in a process called diapause. And once triggered there's no turning back, the diapause program must run its course; only then will the pupae mature into next spring's adults. For more than thirty years, David Denlinger has been fascinated by the mechanisms that maintain the flies' suspended state. Having originally focused on the diapausing insect's ability to survive icy temperatures, Denlinger and Scott Hayward have now turned their attention to the pupae's ability to handle drought. Exposing both developing and diapausing pupae to a desiccating environment, Hayward tracked the insect's stress protein responses as they dried out, and was amazed when he found that some stress proteins could be a major component of the insect's diapause regulating machinery (p. 963).The proteins Hayward and Denlinger decided to track were the Heat Shock Proteins (Hsp), which despite their name protect cells from many more stresses than heat. Hayward triggered diapause in some of the pupae by shortening their day length, while leaving the other pupae to continue developing with 15 hours of light per day. Placing the pupae in low humidity chambers, he waited to see how their heat shock proteins responded to the stress.Working with Joseph Rinehart, Hayward realised that as the developing flies dehydrated, they began producing high levels of two heat shock proteins, Hsp23 and Hsp70. However, the diapausing pupae were already producing high levels of the proteins before they began dehydrating, and the added stress didn't alter their Hsp23 or Hsp70 levels. Interestingly, a completely different group of Hsps responded to stress recovery during rehydration, Hsp90 and Hsc70, and this response was the same in both developing and diapausing flies.Hayward also looked at the insect's responses to dehydration just after they had quit diapause and resumed developing into adults. But he was surprised to see that Hsp23 and Hsp70 had vanished from the dehydrated insects. How could this be? Both proteins are sure-fire indicators of dehydration stress, and these insects were experiencing a serious drought. If Hsp23 and Hsp70 had vanished, even under these dry conditions, maybe Hsp23 and Hsp70 were doing something else. Maybe both proteins were a component of the insect's diapause machinery, and development could only be resumed once the proteins had vanished.This time, Hayward aroused the dehydrated pupae from diapause with a dose of hexane to see how the proteins responded. Again Hsp23 and Hsp70 vanished,and it was only after the flies' developmental program was fully initiated that their Hsp23 and Hsp70 levels rose to protect them from the desiccating conditions. Hayward realised that if the two heat shock proteins were there,the pupae couldn't escape diapause. Development could only be resumed if both Hsp23 and Hsp70 had vanished. The proteins seem to be a key component of the pupae's diapause machinery, keeping the pupae safe from harm until danger passes.