Overwintering conditions impact insulin pathway gene expression in diapausing Megachile rotundata

Authors: CAMBRON, LIZZETTE - North Dakota State University; Yocum, George; Yeater, Kathleen; GREENLEE, KENDRA - North Dakota State University

Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2021
Publication Date: 3/15/2021
Citation: Cambron, L.D., Yocum, G.D., Yeater, K.M., Greenlee, K. 2021. Overwintering conditions impact insulin pathway gene expression in diapausing Megachile rotundata. Comparative Biochemistry and Physiology. 256. Article 110937. https://doi.org/10.1016/j.cbpa.2021.110937.
DOI: https://doi.org/10.1016/j.cbpa.2021.110937

Interpretive Summary: Alfalfa is the leading cultivated perennial forage crop in the world. In the United States, alfalfa is valued at about $8.8 billion placing it just under corn, soybeans and wheat in total value. The alfalfa leafcutting bee (ALCB), Megachile rotundata, is the primary pollinator of alfalfa in North America. Currently, it is not possible to maintain the numbers of ALCBs needed to pollinate alfalfa in the United States, requiring American farmers to import ALCBs from Canada at significant costs. Under current management protocols the ALCB are stored (overwintered) as dormant (diapausing) bees under constant temperature for 9 to 10 months of the year. The bees stored under such conditions do not receive the natural environmental cues that enable field bees to adjust their physiology to optimize survival and long-term health. In order to improve the overwintering storage of the ALCBs we compared key diapause regulatory genes in ALCBs stored under current management protocols against ALCBs stored under more natural field conditions. We demonstrated that gene expression patterns are different between these two groups of bees. This indicates that current management protocols result in ALCBs that are physiologically different from ALCBs maintained under field conditions. Understanding when the key genes controlling diapause change expression levels under field conditions will provide insights as to how we may need to change the storage conditions of managed ALCBs to improve their overwintering survival and long-term health.

Technical Abstract: Diapause is a non-feeding state that many insects undergo to survive the winter months. With fixed resources, overall metabolism and insulin signaling (IIS) are maintained at low levels, but whether those change in response to seasonal temperature fluctuations remains unknown. The focus of this study is to determine 1) how insulin signaling varies throughout diapause and 2) if that variation changes in response to temperature. To test the hypothesis that IIS is responsible for allocating energy in response to temperature fluctuations during overwintering, alfalfa leafcutting bees, Megachile rotundata, were overwintered at either a constant 4°C or in naturally fluctuating temperatures. Expression of genes in the IIS pathway, cell cycle regulators, and transcription factors were measured. Overall our findings showed that a few key targets of the insulin signaling pathway along with growth regulators change during overwintering, suggesting that only cell cycle regulators, and not the IIS pathway as a whole, change across the phases of diapause. To answer our second question, we compared gene expression levels between temperature treatments at each month for a given gene. We observed significantly more differences in expression of IIS pathway targets, indicating temperature does influence insulin signaling. With differences seen between temperature treatment groups, these findings indicate that constant temperatures like those used in agricultural storage protocols, lead to different expression profiles and possibly different phenotypes for alfalfa leafcutting bees.