Doctor of Philosophy
Brent J. Sinclair
Water and ion homeostasis has emerged as an important factor limiting chill-susceptible insects at low temperatures; loss of this homeostasis in the cold likely contributes to chronic chilling injury, and reestablishment of homeostasis is required for recovery from chilling. Both plastic and interspecific variation in cold tolerance correlates with enhanced defense of water and ion homeostasis during cold exposure, however the mechanisms are poorly understood. Using Gryllus crickets, I generated and tested hypotheses about the mechanisms underlying this variation in transport function. I first related interspecific variation in cold tolerance to water and ion balance in early chill coma. A rapid influx of Na+ to the hemolymph suggests that Na+ first leaks from the tissues, and could drive migration of Na+ and water to the gut. Gryllus veletis (a more cold-tolerant species) may avoid or slow this Na+ leak by maintaining lower hemolymph Na+ content and lower osmotic pressure between the gut and hemolymph, compared to G. pennsylvanicus. Plasticity in defense of water and ion homeostasis during cold exposure is thought to involve enhanced active transport function and/or decreased permeability of ionoregulatory tissues. Using G. pennsylvanicus I identified specific candidate mechanisms related to these transport function modifications by comparing the hindgut and Malpighian tubule transcriptomes of warm- and cold-acclimated individuals. Cold acclimation modified the expression of hindgut and Malpighian tubule ion transporters, and hindgut structural (cytoskeletal and cell junction) genes. Rectal macromorphology and rectal pad scalariform complex ultrastructure were unchanged (suggesting that modified permeability does not involve these structural elements), however cytoskeletal modifications do protect rectal pad actin stability during cold shock. Cold acclimation decreases excretion rate (i.e. active transport) across the Malpighian tubules, which may be driven by modified activity of Na+-K+ ATPase but not of V-ATPase. Increased expression of hindgut Na+-K+ ATPase did not alter the activity of this enzyme in the rectum. Overall I show that cold acclimation modifies active transport function in the Malpighian tubules and modifies rectal pad structure to enhance cytoskeletal stability during cold exposure.
Des Marteaux, Lauren E., "Mechanisms Underlying Variation in Insect Chill Tolerance: The Role of Ion and Water Transport" (2017). Electronic Thesis and Dissertation Repository. 4536.