DROPPING LIKE FLIES

Abstract

When the going gets hot, fruit flies turn on heat shock proteins (HSPs) to protect themselves. But when physiologist Donna Folk teamed up with evolutionary biologist George Gilchrist to study heat tolerance in the humble fruit fly, she found that HSP protection during a heatwave comes at a price. Curious about the protective roles of HSPs in insects and the different methods used to assess insects' heat tolerance, Folk joined Gilchrist at the College of William and Mary in Virginia to study fruit fly populations that can tolerate different temperatures. High tolerance flies had been artificially selected to cope with high temperatures. Flies selected for low tolerance, on the other hand, collapsed at much milder temperatures. But what role did the insects' heat shock responses play in their physiological responses to a hot blast (p. 3964)?Folk explains that animals that have experienced a previous heatwave have already churned out HSPs, so they have them onboard for protection when the mercury rises later; the insects are said to have `hardened'. Knowing that high tolerance flies usually trigger HSP production at high temperatures, Folk expected that a mild heatwave would only trigger the low tolerance insects'heat shock systems, leaving the high tolerance flies more vulnerable to heat shock than their low tolerance counterparts. To find out, Folk first hardened the flies by giving them a mild heat shock to build up the insects' HSP reserves. She then gave the flies a serious heat shock, and counted how many flies were alive a day later. To her surprise, the high tolerance flies survived better after hardening; their heat shock response had unexpectedly been triggered by the previous mild heatwave.Knowing that flies lose the ability to move around at very high temperatures, Folk wondered whether hardened high temperature tolerant flies would retain their ability to move at higher temperatures compared with their non-hardened counterparts. Using a 5 foot long column, surrounded by a heated water jacket to control the column's air temperature, Folk allowed flies inside the column to move freely as she slowly raised the temperature,counting the numbers of flies that became incapacitated as the air temperature rose in 0.5°C increments. Pouring hardened and non-hardened flies into the column, Folk was startled to see that the hardened high tolerance flies fell out of the column at a cooler temperature than their non-hardened counterparts. Despite their improved survival, the high tolerance flies'ability to move in hot air suffered more from the hardening process than unhardened insects from the same population.This was a big surprise, Folk admits, `because people assume that hardening improves heat tolerance. This really made us stop and think about what could be going on. ' Like all good things, it seems HSP production comes with a cost; Folk suspects that high tolerance flies struggle with energetic challenges when the mercury rises. `Their metabolic rate can shoot up by 70%,' says Folk, `and HSPs also use up lots of energy to work. ' This may cause the flies to `overheat' more quickly, she concludes.Folk also noticed that high tolerance females survived better than low tolerance females after a heat shock. To explain this, Folk turned to HSC70,an HSP that is not produced in response to heat stress but is found naturally in flies. Folk found that HSC70 levels drop off sharply in low tolerance females during the first minutes of a heat shock. If high tolerance females are somehow slowing their HSC70 degradation rate, Folk says, this might explain their better survival.