Timing of diapause initiation and overwintering conditions alter gene expression profiles in Megachile rotundata

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

Submitted to: Frontiers in Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2022
Publication Date: 3/8/2022
Citation: Cambron-Kopco, Yocum, G.D., Yeater, K.M., Greenlee, K. 2022. Timing of diapause initiation and overwintering conditions alter gene expression profiles in Megachile rotundata. Frontiers in Physiology. https://doi.org/10.3389/fphys.2022.844820.
DOI: https://doi.org/10.3389/fphys.2022.844820

Interpretive Summary: The alfalfa leafcutting bee (ALCB), Megachile rotundata, is the primary pollinator used for alfalfa seed production in North America. The valuation of the annual production of alfalfa hay is around $8.8 billion placing it just under corn, soybeans, and wheat in total value. Therefore, having an adequate source of healthy ALCB each spring is essential for maintaining the supply of alfalfa needed for dairy and livestock production. Under current management practices ALCB are overwintered for 9 to 10 months in a state of dormancy known as diapause. This critical period of the ALCB life cycle has received very little attention in the scientific literature. One central question that needs to be answered is, how does the environmental history of the bees impact how they should be overwintered to ensure high quality bees for the next year alfalfa seed production. To start to answer this question researchers in the Insect Genetics and Biochemistry Research Unit, Fargo, ND, in collaboration with researchers at the North Dakota State University, Department of Biological Sciences examined gene expression in overwintering bees that entered diapause early or late in the field season. We demonstrated that key diapause regulating genes were differentially expressed between these two groups of bees. This indicates that the early-and late-season bees were physiologically distinct from each other and therefore may need different overwintering storage conditions to ensure optimal survival and post-storage quality. This study lays the fundamental foundation for optimizing the overwintering storage of not only ALCB but also for other insect species of agricultural importance.

Technical Abstract: Within the United States and Canada, the primary pollinator of alfalfa is the alfalfa leafcutting bee (ALCB), Megachile rotundata. Our previous findings showed that ALCB overwintering conditions impacted gene expression profile in bees that entered diapause early in the season. However, ALCB are a bivoltine species, leaving the question if bees exposed to summer temperatures and entering diapause later in the season also show this trend. To better understand the effects of the timing of diapause initiation, we analyzed mRNA copy number of genes known to be involved in diapause regulation in early- and late-season diapausing ALCB that were overwintered in field conditions or using current agricultural management conditions. We hypothesized that overwintering conditions for late diapausing bees also affects gene expression profiles. Our results showed that expression profiles were altered by both overwintering condition and timing of diapause initiation, with bees that entered diapause earlier in the season showing different expression patterns than those that entered later in the season. This trend was seen in expression of members of the cyclin family and several targets of the insulin signaling pathway, including forkhead box protein O (FOXO), which is known to be important for diapause regulation and stress responses. But, of the genes screened, the proto-oncogene, Myc, was the most impacted by the timing of diapause initiation. Under field (fluctuating temperatures) conditions there were significant differences in Myc expression between the early and late season samples in all months except for November and February. This same general trend in Myc expression was also seen in the laboratory-maintained (constant temperature) bees with significant difference in expression in all months except for November, February, and May. These results support our previous conclusions showing that the molecular regulation of diapause development in ALCB is not a simple canonical cascade of gene expression but a highly plastic response that varies between bees depending upon their environmental history.