Fluctuating thermal regimes prevent chill injury but do not change patterns of oxidative stress in the alfalfa leafcutting bee, Megachile rotundata

Authors: TORSON, ALEX - University Of Western Ontario; Yocum, George; Rinehart, Joe; NASH, SEAN - North Dakota State University; BOWSHER, JULIA - North Dakota State University

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2019
Publication Date: 8/28/2019
Citation: Torson, A.S., Yocum, G.D., Rinehart, J.P., Nash, S.A., Bowsher, J.H. 2019. Fluctuating thermal regimes prevent chill injury but do not change patterns of oxidative stress in the alfalfa leafcutting bee, Megachile rotundata. Journal of Insect Physiology. https://doi.org/10.1016/j.jinsphys.2019.103935.
DOI: https://doi.org/10.1016/j.jinsphys.2019.103935

Interpretive Summary: The alfalfa leafcutting bee is the primary pollinator used in the production of alfalfa seed in North America. A number of other specialty crops such as carrot, onion, and blueberry are also pollinated by this bee. To aid in the availability of the alfalfa leafcutting bee for pollination services, they are subjected to low-temperature storage at various times during their life cycle. Low-temperature storage of insects have historically been conducted using constant temperatures. We have demonstrated that using a fluctuating temperature regime, in which insects are given a daily high temperature pulse, improves both bee survival and pollinator health as compared to the standard practice. The molecular mechanism by which fluctuating temperature regimes improve survival is unclear. The current hypothesis suggests that prolonged exposure of insects to low temperatures induces oxidative damage and this leads to mortality. We tested this idea using the alfalfa leafcutting bee, but were unable to confirm this hypothesis. These results will help narrow the focus of future research aimed to understand how fluctuating temperature regimes improve the survival of alfalfa leafcutting bees. This understanding will lead to better storage procedures for the bee industry and improved health and availability of pollinators for agricultural producers.

Technical Abstract: In insects, prolonged exposure to unseasonably low temperatures can lead to detrimental physiological effects known as chill injury. Changes in membrane phase during low temperature exposure drive the collapse of ion gradients, metabolic imbalance and oxidative stress. In the alfalfa leafcutting bee, Megachile rotundata, transcriptomic evidence provides support for these responses at the level of gene expression, but variable expression profiles between life stages in M. rotundata indicate that different mechanisms could be responsible repairing and protecting against chill injuries across development. Herein, we test the hypothesis that exposure to a fluctuating thermal regime (FTR) promotes an increased and protective oxidative stress response, relative to a chill-injury-inducing static thermal regime (STR) in M. rotundata. We measured the expression of several transcripts with products known to have antioxidant properties as well as total antioxidant capacity and lipid peroxidation during both extended overwintering in prepupae and low temperature stress during pupal development. We observed differential gene expression for the antioxidant glutathione peroxidase and several transcripts with known antioxidant properties including vitellogenin, apolipoprotein D, glutathione S-transferase, and nuclear protein 1. However, the expression of transcripts coding for other enzymatic antioxidants did not change between treatments. Neither life stage varied in their capacity to cope with an induced oxidative stress after FTR exposure and we did not observe evidence of lipid peroxidation in chill injured (STR) prepupae. These results did not support our initial hypothesis and suggest that either 1) oxidative damage occurs in other structural components besides lipids or 2) oxidative damage is not a factor in FTR responses to chill injury in Megachile rotundata or 3) the damage caused by chill injury is highly tissue-specific and is masked by our whole-organism homogenates.