Abstract
The honey bee, Apis mellifera L., is a major pollinator insect that lacks novel “selenoprotein genes”, rendering it susceptible to elevated levels of Selenium (Se) occurring naturally in the environment. We investigated the effects of two inorganic forms of Se on biological traits, oxidative stress, and gene regulation. Using bioassay arenas in the laboratory, one-day old sister bees were fed ad libitum 4 different concentrations of selenate and selenite, two common inorganic forms of Se. The transcription levels of 4 honey bee antioxidant genes were evaluated, and three putative selenoprotein-like genes (SELENOT, SELENOK, SELENOF) were characterized as well as Sbp2, a Selenium binding protein required for the translation of selenoproteins mRNA. Oxidative stress and Se residues were subsequently quantified in honey bee bodies throughout the experiment.Se induced higher oxidative stress in treated honey bees leading to a significantly elevated protein carbonyl content, particularly at the highest studied concentrations. Early upregulations of Spb2 and MsrA were identified at day 2 of the treatment while all genes except SELENOT were upregulated substantially at day 8 to alleviate the Se-induced oxidative stress levels. We determined that doses between 60 and 600 mg.Se.L−1 were acutely toxic to bees (<48 h) while doses between 0.6 and 6 mg.Se.L−1 led to much lower mortality (7–16)%. Furthermore, when fed ad libitum, Se residue data indicated that bees tolerated accumulation up to 0.12 µg Se bee−1 for at least 8 days with a Se LC50 of ∼6 mg/L, a field realistic concentration found in pollen of certain plants in a high Se soil environment.
Highlights
Selenium (Se) is a nonmetal element that occurs naturally in some alkaline soils, plants and flowers
The EPA maximum contaminant level of Se in drinking water is 0.05 mg/L and Se was identified in honey bees (Apis mellifera L.) at 14.8 μg/g and bumble bees (Bombus spp.) at 251 μg/g of their dry weight (Quinn et al, 2011)
Using laboratory bioassay arenas, we studied the effects of selenate and selenite at various concentrations on honey bee biological traits and oxidative stress, as well as on regulation of major antioxidant genes
Summary
Selenium (Se) is a nonmetal element that occurs naturally in some alkaline soils, plants and flowers. Se is a trace element nutrient for humans and other living organisms and functions as a cofactor for reduction of antioxidant enzymes, but it can be toxic at large doses. Soil with high Se content, such as in Nebraska, Kansas, Dakotas, Wyoming (> 4–5 mg/kg), eventually results in high levels of Se plant uptake leading to toxicity in herbivores (Rosenfeld and Beath, 1964). Elevated levels of Se in flowers, especially in Se-hyper-accumulating plants, can be toxic to many insect herbivores. Since there is no evidence that honey bees and other floral visitors can avoid high-Se flowers (Quinn et al, 2011), it is conceivable that the latter could put those economically important pollinators at great risk. Honey bee foragers sampled from non-contaminated Se urban areas contained 0.73 mg Se kg−1. Se was identified in forager bees in Poland at a concentration of 7.03 mg Se kg−1 dw (Roman, 2010)
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