Environmental history impacts gene expression during diapause development in the alfalfa leafcutting bee, Megachile rotundata

Authors: Yocum, George; Childers, Anna; Rinehart, Joseph - Joe; Rajamohan, Arun; Pitts Singer, Theresa; GREENLEE, KENDRA - North Dakota State University; BOWSHER, JULIA - North Dakota State University

Submitted to: Journal of Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2018
Publication Date: 7/6/2018
Citation: Yocum, G.D., Childers, A.K., Rinehart, J.P., Rajamohan, A., Pitts Singer, T., Greenlee, K.J., Bowsher, J.H. 2018. Environmental history impacts gene expression during diapause development in the alfalfa leafcutting bee, Megachile rotundata. Journal of Experimental Biology. 221:jeb173443. https://doi.org/10.1242/jeb.173443.
DOI: https://doi.org/10.1242/jeb.173443

Interpretive Summary: The alfalfa leafcutting bee, Megachile rotundata, is a managed solitary bee used as the primary pollinator of alfalfa for seed production in North America. There is a very limited seasonal window (early June to late September) where the resources the bees need for development and reproduction are available. Therefore, under current management protocols the bees need to be stored (overwinter) as dormant (diapausing) bees for the rest of the year. How the bees are stored can have a major impact on the ability of the adult bee to pollinate the next season’s crop. This critical period of the alfalfa leafcutting bee life cycle has received very little attention. A question that has never been raised in the scientific literature before is how the past seasonal conditions may affect the bees during storage. For example, if the growing season is cooler than normal will the storage conditions need to be altered to ensure optimal survival of the bees and pollination services the following season? To begin addressing these shortfalls in our knowledge, a gene expression study was conducted. The Insect Genetics and Biochemistry Research Unit, Fargo, ND, in collaboration with researchers at USDA-ARS Logan, UT, and the North Dakota State University, Department of Biology, collected alfalfa leafcutter bees entering diapause early or late in the growing season. These two groups were further subdivided into laboratory and field maintained groups, resulting in four experimental groups of diapausing bees, based on prewintering and overwintering treatments. Gene expression screening revealed that the four groups of bees were physiologically distinct from each other throughout the period of overwintering storage. These results demonstrate that past seasonal environmental conditions do indeed affect the bees physiologically during overwintering storage. The implication of this investigation is that overwintering storage conditions need to be tailored to bees’ environmental history to ensure the delivery of high quality pollinators to the end users. To provide future impact, the genetic information about alfalfa leafcutter genes has been deposited into ARS i5K insect genome project. This information can be used by other researchers to design future genetic experiments.

Technical Abstract: Our understanding of the mechanisms controlling insect diapause has increased dramatically with the introduction of global gene expression techniques, such as RNAseq. However, little attention has been given to how ecologically relevant field conditions may affect gene expression during diapause development because previous studies have focused on laboratory reared and maintained insects. To determine whether gene expression differs between lab and field conditions, prepupae of the alfalfa leafcutting bee, Megachile rotundata, entering diapause early or late in the growing season were collected. These two groups were further subdivided in early autumn into laboratory and field maintained groups, resulting in four experimental treatments of diapausing prepupae, early and late field, and early and late laboratory. RNAseq and differential expression analysis were performed on bees from the four treatment groups in November, January, March and May. The number of treatment-specific differentially expressed genes (97 to 1249) outnumbered the number of differentially regulated genes common to all four treatments (14 to 229), indicating that exposure to laboratory or field conditions had a major impact on gene expression during diapause development. Principle component analysis and hierarchical cluster analysis yielded similar grouping of treatments, confirming that the treatments result in distinct clusters. Our results support the conclusion that gene expression during the course of diapause development is not a simple ordered sequence, but rather a highly plastic response determined primarily by the environmental history of the individual insect.