Immediate transcriptional response to a temperature pulse under a fluctuating thermal regime

Authors: Melicher, Dacotah; TORSON, ALEX - Western University; ANDERSON, TANNER - University Of Oregon; Yocum, George; Rinehart, Joe; BOWSHER, JULIA - North Dakota State University

Submitted to: Integrative and Comparative Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2019
Publication Date: 6/7/2019
Citation: Melicher, D.M., Torson, A.S., Anderson, T.J., Yocum, G.D., Rinehart, J.P., Bowsher, J.H. 2019. Immediate transcriptional response to a temperature pulse under a fluctuating thermal regime. Integrative & Comparative Biology. 59(2):320-337. https://doi.org/10.1093/icb/icz096.
DOI: https://doi.org/10.1093/icb/icz096

Interpretive Summary: Because insects are cold-blooded, their internal temperature changes quickly with the environment. This allows them to tolerate a wide variety of temperatures but can also lead to stress when the temperature is constant for a long period of time. For instance, exposure to low temperatures leads to slower cellular functions and saved energy reserves, but also causes accumulated damage due to cold stress. Under warmer temperatures they are able to resume activity and recover from cold damage. This is economically important because insects are often stored by agriculture and pollination service providers for long periods of time until they are needed, and improving storage protocols may help ensure the availability and quality of stored insects. Our group has been working on improving storage protocols for the alfalfa leafcutting bee, an important pollinator of alfalfa and other seed crops. Previously, we have shown that regularly changing the storage temperature of this species improves its health. In this study, we rapidly changed the temperature to identify genes that respond to these conditions. This allowed us to see both how the insects are stressed by cold exposure and how they recover during warm periods. Understanding what genes are expressed, and when they are expressed, will help us to develop storage protocols that improve the health and efficiency of pollinators.

Technical Abstract: The response of ectotherms to temperature stress is complex, non-linear and is influenced by life stage and previous thermal exposure. Mortality is higher under static temperatures than under fluctuating thermal regimes which periodically increase the temperature. Long term exposure to fluctuating thermal regimes has been shown to increase transcription of genes involved in oxidative stress, immune function, and metabolic pathways which may aid in recovery from chill injury and oxidative damage. Pervious research suggests that gene expression changes that protect against sub-lethal damage emerge rapidly under exposure to fluctuating temperatures. However, the existing studies have only examined gene expression after a week or over many months. Here we characterize gene expression during a single temperature cycle under a fluctuating thermal regime. Pupal alfalfa leafcutting bees (Megachile rotundata) were maintained at 6°C with a one-hour pulse to 20°C and returned to 6°C. RNA was collected before, during, and after the temperature pulse and compared to bees maintained at a static 6°C. The greatest number of differentially expressed genes were upregulated during the warm pulse. These included genes associated with developmental patterning, suggesting that one hour at 20C was enough to resume development in the pupae. Once the pupae were returned to 6°C these differences in expression were maintained. Other genes upregulated during the warm pulse include genes associated with membrane homeostasis and X metabolism. This pattern of differentially expressed genes indicates that even brief exposures to warm temperatures have a significant protective effect on insects exposed stressful cold temperatures. The transcriptional response extended beyond the warm period, allowing the protective effects to persist. M. rotundata’s senstitivity to temperature fluctuations indicate that short exposures to temperature changes can maintain lasting effects on development and physiology.