Life stage and the environment as effectors of transposable element activity in two bee species

Authors: SIGNOR, SARAH - North Dakota State University; Yocum, George; BOWSHER, JULIA - North Dakota State University

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2022
Publication Date: 1/19/2022
Citation: Signor, S., Yocum, G.D., Bowsher, J. 2022. Life stage and the environment as effectors of transposable element activity in two bee species. Journal of Insect Physiology. 137. Article 104361. https://doi.org/10.1016/j.jinsphys.2022.104361.
DOI: https://doi.org/10.1016/j.jinsphys.2022.104361

Interpretive Summary: Because of the alarming decline of the honey bee, Apis mellifera, colonies worldwide, there is an increasing need for the pollination services provided by non-Apis bee species such as the alfalfa leafcutting bee, Megachile rotundata and the blue orchard bee, Osmia lignaria. To improve the quality of non-Apis bees used within the agricultural systems, improved management protocols are needed. Both the alfalfa leafcutting bee and the blue orchard bee spend up to 9 months of the year in a state of dormancy known as diapause. How the diapausing bees are stored for overwintering can have a major impact on the ability of the adult bee to pollinate the next season’s crop. Therefore, understanding the physiology of diapause is essential for developing management protocols that optimize overwintering storage of these bees. One area of diapause research that has been nearly completely neglected is the activity of transposable elements during diapause. Transposable elements are short sections of DNA that can move around in an organism’s genome and therefore may have a significant impact on an organism’s physiology. To begin addressing this shortfall in our knowledge, we screened various nondiapausing and diapausing developmental stages in the alfalfa leafcutting bee and the blue orchard bee. It was demonstrated that diapause stage and temperature stress had large effects on transposable element expression, and that the change in expression of transposable elements tended to be larger in those expressed during diapause. This implies that the bees’ physiology is changing over the course of overwintering storage and that the bees are also adjusting their physiology to counteract environmental stresses. The implication of this investigation is that overwintering storage conditions need to be tailored to bees’ changing physiology over the course of overwintering storage to ensure the delivery of high-quality pollinators to the end users.

Technical Abstract: Diapause is a complex physiological phenomenon that allows insects to weather stressful environmental conditions. The regulation of diapause is accordingly complex, including signaling pathways that involve both small RNA and mRNA and affect the cell cycle, metabolism, stress resistance, and developmental timing. Transposable elements (TE), mobile genetic elements that replicate within the genome, are also thought to be stress responsive and regulated by the small RNA pathway. Therefore we asked what the relationship was between environmental stress, diapause status, and TE expression in two species of agriculturally important bees, M. rotundata and O. lignaria. We characterized the TE content of the genomes of both species, then evaluated the expression of TEs during temperature stress (M. rotundata), general environmental stress (M. rotundata, lab versus field), and diapause stage (O. lignaria and M. rotundata). We found that the TE content of the two species varied widely, with M. rotundata having a much higher TE content than O. lignaria. We also found that both diapause stage and temperature stress had large effects on TE expression, and while the fold change of TEs tended to be larger in those expressed during diapause, there was no overall bias in the direction of change. This suggests that stress, and genomic resistance to stress, do not break down to a simple up-regulation or down-regulation of TEs, but rather that the TE, the genomic position of its insertions, and the exact heterochromatin formation of the organism at any given environmental state or life stage will affect overall expression of TEs.