Abstract
The activities of three enzymes involved in polyol biosynthesis (aldose reductase, AR; ketose reductase, KR; and polyol dehydrogenase, PDH) were studied in adult females of the linden bug, Pyrrhocoris apterus, collected from the field during 2005/2006. While the activities of three enzymes were low in reproductive females, activities greater by one or two orders were seen in reproductively arrested females. AR and KR showed similar seasonal trends in activity. Activities were low during diapause initation and later increased and stabilized during autumnal diapause development. Further increases of AR and KR activities were seen during low temperature quiescence and finally the activities sharply decreased during vernal resumption of direct development. The activity of PDH was relatively high (but fluctuating) during diapause, then decreased in quiescent insects and almost disapeared in reproductively active females. Insects collected in February were subjected to laboratory de-acclimation (exposure to high temperatures) followed by re-acclimation (exposure to low temperatures) which resulted in loss of activity in all three enzymes and no regain. High activities of AR, KR and PDH in reproductively arrested females thus conform well with their previously observed high capacity to synthesize and accumulate polyol cryoprotectants.
Highlights
Pervasive effects of low temperature on ectotherm physiology have had a deep influence on evolution of insect overwintering strategies in temperate habitats
Glycogen reserves in the fat body serve as the main source for polyol biosynthesis (Hayakawa & Chino, 1981; Storey & Storey, 1981; Koštál et al, 2004a)
Regulation of glycogen phosphorylase (GPase) activity most probably exerts the primary control over the activation of polyol synthesis (Ziegler et al, 1979; Hayakawa, 1985)
Summary
Pervasive effects of low temperature on ectotherm physiology have had a deep influence on evolution of insect overwintering strategies in temperate habitats. Freeze-tolerant species survive freezing of their body fluids provided it is restricted to extracellular compartments (Lee & Denlinger, 1991; Sinclair et al, 2003). Many insects accumulate low molecular weight sugars and polyols during overwintering. They function as either colligative or noncolligative cryoprotectants, improve cold tolerance and enhance survival during unfavourable winter conditions (Zachariassen, 1985; Storey & Storey, 1991). Accumulated cryoprotectants cause colligative depression of melting and supercooling points in freeze-susceptible insects, regulate the minimum cell volume during dehydration caused by extracellular ice formation in freeze-tolerant species and protect functional structures of biological membranes and proteins (Crowe et al, 1987; Carpenter & Crowe, 1988; Storey & Storey, 1988, 1991). Other environmental cues are involved, for instance photoperiod, food and water availability (Furusawa et al, 1982; Storey & Storey, 1986; Rojas et al, 1986; Hodková & Hodek, 2004)
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