Local adaptation in the plasticity of cold tolerance in the eastern spruce budworm

Abstract

Temperature affects many aspects of spruce budworm (SBW) fitness, including survival and development; therefore, there we hypothesized there would be strong selection for local adaptation of cold tolerance and the capacity to increase it We know that regional populations are capable of plastic responses to stressful low temperature conditions, however we have limited knowledge of the biochemical mechanisms underlying these responses. Cold tolerance is closely tied to glycerol production as glycerol is the most prevalent cryoprotectant in SBW. Yet glycerol synthesis must come at the cost of glycogen reserves, which also form the only overwintering energy store. We hypothesized that local adaptation for increased glycerol synthesis has occurred in SBW, and predicted that fluctuating temperatures (which increase glycerol synthesis) would induce increased enzyme activity in the pathway responsible for glycerol synthesis, and that more northerly populations would have higher enzyme activity than more southerly populations. We developed enzyme activity assays for three key enzymes in the SBW glycerol synthesis pathway: glucose-6-phosphate dehydrogenase (G6PDH), phosphoglucoisomerase (PGI) and phosphofructokinase (PFK). We then assayed the activity of these enzymes in SBW from five populations (NWT, Alberta, New Brunswick, Quebec, and IPQL) reared in common garden conditions and then following fluctuating cold exposure to assess enzymatic reaction rate (Vmax) and enzyme substrate affinity (Km) to characterize changes in potential glycerol synthesis. Here we show that Km was not changed by either population origin or experimental conditions, but Vmax is frequently affected by both, illuminating a mechanism of adaptation to extreme temperatures.