Disruption of quercetin metabolism by fungicide affects energy production in honey bees (Apis mellifera)
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Contributed by May R. Berenbaum, January 6, 2017 (sent for review September 7, 2016; reviewed by Zachary Y. Huang and Blair Siegfried)

Significance
The western honey bee, Apis mellifera, provides essential crop pollination services, but for 10 years, US beekeepers have experienced substantial colony losses. Although insecticides have been implicated in these losses, triazole fungicides affect bees by inhibiting cytochrome P450 monooxygenases that detoxify insecticides. These enzymes also detoxify phytochemicals, including the flavonol quercetin, in their nectar- and pollen-based diet. RNA-Seq analysis of bee larvae consuming quercetin revealed that it down-regulates multiple mitochondrion-related nuclear genes involved in energy production. Bees consuming quercetin together with the triazole myclobutanil produce less thoracic ATP and thus less energy for flight muscles. Therefore, agricultural use of triazole fungicides in combination with insecticides can potentially harm bees by compromising their capacity to extract sufficient energy from their natural diet.
Abstract
Cytochrome P450 monooxygenases (P450) in the honey bee, Apis mellifera, detoxify phytochemicals in honey and pollen. The flavonol quercetin is found ubiquitously and abundantly in pollen and frequently at lower concentrations in honey. Worker jelly consumed during the first 3 d of larval development typically contains flavonols at very low levels, however. RNA-Seq analysis of gene expression in neonates reared for three days on diets with and without quercetin revealed that, in addition to up-regulating multiple detoxifying P450 genes, quercetin is a negative transcriptional regulator of mitochondrion-related nuclear genes and genes encoding subunits of complexes I, III, IV, and V in the oxidative phosphorylation pathway. Thus, a consequence of inefficient metabolism of this phytochemical may be compromised energy production. Several P450s metabolize quercetin in adult workers. Docking in silico of 121 pesticide contaminants of American hives into the active pocket of CYP9Q1, a broadly substrate-specific P450 with high quercetin-metabolizing activity, identified six triazole fungicides, all fungal P450 inhibitors, that dock in the catalytic site. In adults fed combinations of quercetin and the triazole myclobutanil, the expression of five of six mitochondrion-related nuclear genes was down-regulated. Midgut metabolism assays verified that adult bees consuming quercetin with myclobutanil metabolized less quercetin and produced less thoracic ATP, the energy source for flight muscles. Although fungicides lack acute toxicity, they may influence bee health by interfering with quercetin detoxification, thereby compromising mitochondrial regeneration and ATP production. Thus, agricultural use of triazole fungicides may put bees at risk of being unable to extract sufficient energy from their natural food.
Footnotes
- ↵1To whom correspondence should be addressed. Email: maybe{at}illinois.edu.
Author contributions: W.M., M.A.S., and M.R.B. designed research; W.M. performed research; W.M. and M.R.B. analyzed data; and W.M., M.A.S., and M.R.B. wrote the paper.
Reviewers: Z.Y.H., Michigan State University; and B.S., University of Florida–Gainesville.
The authors declare no conflict of interest.
Data deposition: The data reported in this paper have been deposited in the Gene Expression Omnibus (GEO) database, www.ncbi.nlm.nih.gov/geo (accession no. GSE83437).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1614864114/-/DCSupplemental.
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