Chill tolerance in insects

(Funded by the Sapere Aude program from DFF)

The chill tolerance of insects represents a vital temperature response that markedly influences their ability to persist in cold environments. Susceptibility to chill tolerance is associated with the inability to maintain ion and water homeostasis which causes muscular dysfunction and ultimately chill-injury and death. However, little is known about the underlying physiological mechanisms of chill tolerance in insects. The overall objective of this project is to identify the key physiological processes determining chill tolerance, to investigate how these processes are influenced by acclimation and other environmental factors and to investigate their evolutionary origin and molecular basis of chill tolerance.

 

Insects and cold hardiness 

Truly cold hardy insects can either tolerate freezing of their body fluids or survive winter by maintaining their body fluids in a super-cooled state. Many temperate, sub-tropical and tropical insects (including for example bees, grasshoppers, mosquitoes and flies) do, however, not possess any significant cold tolerance and are therefore referred to as chill susceptible/tolerant. The physiological correlates of chill tolerance are poorly understood, but seems to be related to an inability to maintain trans-membrane ion homeostasis at low temperatures. Dissipation of ion gradients and resting membrane potential cause a disruption of nervous- and particularly muscular-excitability and as a consequence, the chilled insect loses its ability to maintain posture and enters a state termed chill coma.

The evolution of environmental resilience has allowed for a global dispersal of insects ranging from the warm and wet tropics to the cold Arctic climates and to the driest deserts. Thus, the ability of particular insect species to inhabit specific environments is inherently linked to their environmental stress tolerance. This is also the case for chill tolerance where our recent study demonstrates how the cold tolerance of 100 drosophila species has been exceptionally important for defining their realised niches and geographical distributions(Fig. 1).

 
Relation between minimal temperature of the environment and chill resistance measured as the temperature of cold coma in 100 species of Drosophila

Relation between minimal temperature of the environment and chill resistance measured as the temperature of cold coma in 100 species of Drosophila

 

The working hypothesis for the current project is that disruption of ion balance causes chill coma and chill injury, and that such disruption is a result of a mismatch between active and passive transport processes. Figure 2 depicts the working model illustrating how the ability to maintain muscle function and excitability is dependent on active and passive ion transport processes. These processes include ion transport in muscle tissue itself, but also processes related to regulation of water and ion secretion/reabsorption, transpiratory/respiratory water loss and effects of diet and water intake.