Doctor of Philosophy
Dr. Brent J. Sinclair
A mechanistic understanding of how temperature limits insect performance is needed to accurately model insect distribution and abundance. Upon crossing the temperature of their critical thermal minimum (CTmin), insects enter a state of paralysis (chill-coma). Chill-susceptible insects accumulate injuries (termed chilling injury) during prolonged exposure to low temperatures. My objective was to determine the mechanisms by which both chill-coma and chilling injury manifest in chill-susceptible insects. In aquatic animals, critical thermal limits are associated with a temperature-induced failure of oxygen supply relative to demand (oxygen- and capacity- limitation of thermotolerance; OCLT), which leads to reliance on anaerobic metabolism at thermal extremes. However, using open-flow respirometry and biochemical techniques, I found that fall field crickets (Gryllus pennsylvanicus) in chill-coma continued to exchange gases through the tracheal system and did not accumulate anaerobic byproducts, which suggests OCLT does not set the CTmin of insects. To characterize the patterns of ion balance disruption at low temperatures, I estimated water and ion content of the hemolymph and tissues of G. pennsylvanicus in chill-coma using gravimetric methods and atomic absorption spectrometry. Exposure to low temperatures caused a movement of Na+ and water from the hemolymph to the gut in G. pennsylvanicus, which increased hemolymph [K+] and depolarized muscle resting potential. When removed from the cold, crickets rebalanced ions and water, and the restoration of hemolymph [K+] (and muscle equilibrium potential) was coincident with the recovery of neuromuscular function. Although crickets recover the ability to move rapidly after removal from the cold, complete recovery of ion and water homeostasis requires additional time and metabolic investment. There is both inter- and intraspecific variation in cold tolerance in flies of the genus Drosophila. Using ion-selective microelectrodes, I found that cold-tolerant Drosophila species and cold- acclimated D. melanogaster maintain low concentrations of [Na+] and [K+] in their hemolymph. Drosophila cold tolerance was also associated with low Na+/K+-ATPase activity on a whole-organism level. Together, these studies allow me to construct a conceptual model of how the direct effects of temperature on ion homeostasis may drive chill-coma, chill-coma recovery and chilling injury in insects.
MacMillan, Heath A., "Ionic and Osmotic Mechanisms Of Insect Chill-Coma And Chilling Injury" (2013). Electronic Thesis and Dissertation Repository. 1410.