The effects of the neonicotinoid imidacloprid on gene expression and DNA methylation in the buff-tailed bumblebee Bombus terrestris

Abstract

Neonicotinoids are effective insecticides used on many important arable and horticultural crops. They are nicotinic acetylcholine receptor agonists which disrupt the function of insect neurons and cause paralysis and death. In addition to direct mortality, there are numerous sublethal effects of low doses of neonicotinoids on bees. We hypothesize that some of these large array of effects could be a consequence of epigenetic changes in bees induced by neonicotinoids. We compared whole methylome (BS-seq) and RNA-seq libraries of the brains of buff-tailed bumblebee Bombus terrestris workers exposed to field-realistic doses of the neonicotinoid imidacloprid to libraries from control workers. We found numerous genes which show differential expression between neonicotinoid-treated bees and control bees, but no differentially methylated cytosines in any context. We found CpG methylation to be focused mainly in exons and associated with highly expressed genes. We discuss the implications of our results for future legislation.

Keywords: BS-seq; RNA-seq; epigenetics; methylome; pesticide; social insects.

Figure 1.

Methylated Cs distribution. Average proportion of methylation reads ±s.d. per CpG (a), CHG (b), and CHH (c) positions over genomic features. Control samples in black and neonicotinoid-treated samples in grey.

Figure 2.

Average percentage of methylated CpG per gene. Differentially expressed genes (DEG) and non-differentially expressed genes (non-DEG) are plotted separately. Dots represent genes.

Figure 3.

The proportion of methylated CpGs is plotted against gene expression rank. One hundred ‘bins’ of progressively increasing level of expression were generated and genes with a similar level of expression have been grouped in the same bin. Solid lines represent control samples and dotted lines neonicotinoid-treated samples. The grey shading represents 95% confidence intervals.