Physiological responses to fluctuating temperatures are characterized by distinct transcriptional profiles in a solitary bee

Authors: TORSON, ALEX S - North Dakota State University; Yocum, George; Rinehart, Joseph - Joe; NASH, SEAN - North Dakota State University; KVIDERA, KALLY - North Dakota State University; BOWSHER, JULIA - North Dakota State University

Submitted to: Journal of Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2017
Publication Date: 9/15/2017
Publication URL: http://handle.nal.usda.gov/10113/5852153
Citation: Torson, A., Yocum, G.D., Rinehart, J.P., Nash, S.A., Kvidera, K.M., Bowsher, J.H. 2017. Physiological responses to fluctuating temperatures are characterized by distinct transcriptional profiles in a solitary bee. Journal of Experimental Biology. 220(18):3372-3380. https://doi.org/10.1242/jeb.156695.

Interpretive Summary: Large numbers of healthy insects are needed to support research into pest control and for pollination services. To meet the need for the various insect species used within agriculture, improved low-temperature storage protocols are being developed. Exposing insects to stressful, low temperatures during development can result in the accumulation of harmful physiological effects called chill injury. Metabolic imbalances, disruptions in ion balance, and oxidative stress contribute to the increased mortality of chill-injured insects. Interestingly, survival can be significantly increased when chill susceptible insects are exposed to a daily warm-temperature pulse during chilling. We hypothesize that warm pulses allow for the repair of damage associated with chill injury. Here, we describe gene regulation during exposure to a fluctuating thermal regime (FTR), relative to constant chilled temperatures, during pupal development in the alfalfa leafcutting bee, Megachile rotundata, using a combination of various gene screening methods. Pupae were exposed to either a constant, chilled temperature of 6°C, or 6°C with a daily pulse of 20°C for seven days. Gene expression patterns isolated after experimental treatment revealed differential expression of genes involved in construction of cell membranes, oxidation-reduction and various metabolic processes. This result provides support for physiological responses to chill injury seen across other insect species. The large number of differentially expressed genes observed on day seven suggests that the initial divergence in expression profiles between the two treatments occurred upstream of the time point sampled. Additionally, the differential expression profiles observed in this study show little overlap with those differentially expressed during temperature stress in the diapause state, suggesting that the mechanisms governing a robust physiological low-temperature stress response depend upon life stage.

Technical Abstract: Exposure to stressful, low temperatures during development can result in the accumulation of deleterious physiological effects called chill injury. Metabolic imbalances, disruptions in ion homeostasis, and oxidative stress contribute to the increased mortality of chill-injured insects. Interestingly, survival can be significantly increased when chill susceptible insects are exposed to a daily warm-temperature pulse during chilling. We hypothesize that warm pulses allow for the repair of damage associated with chill injury. Here, we describe transcriptional regulation during exposure to a fluctuating thermal regime (FTR), relative to constant chilled temperatures, during pupal development in the alfalfa leafcutting bee, Megachile rotundata using a combination of RNA-seq and qPCR. Pupae were exposed to either a constant, chilled temperature of 6°C, or 6°C with a daily pulse of 20°C for seven days. RNA-seq after experimental treatment revealed differential expression of transcripts involved in construction of cell membranes, oxidation-reduction and various metabolic processes. This result provides support for physiological responses to chill injury seen across taxa. The large number of differentially expressed transcripts observed on day seven suggests that the initial divergence in expression profiles between the two treatments occurred upstream of the time point sampled. Additionally, the differential expression profiles observed in this study show little overlap with those differentially expressed during temperature stress in the diapause state, suggesting that the mechanisms governing a robust physiological low-temperature stress response depend upon life stage.