Chronic Cadmium Exposure Induces Impaired Olfactory Learning and Altered Brain Gene Expression in Honey Bees (Apis mellifera)

SIMPLE SUMMARY: The honey bee (Apis mellifera) is kept all over the world and plays a dominant role in the pollination of crops. Honey bees may be exposed to different levels of cadmium through the collection of contaminated nectar during their foraging activities. In this study, honey bees were chronically exposed to cadmium to investigate the effects of sublethal cadmium doses on the olfactory learning and brain gene expression profiles in honey bees. Honey bees exhibited significantly impaired olfactory learning performances after being chronically exposed to cadmium and had a significantly lower head weight in comparison with control bees. Furthermore, genes involved in oxidative stress response and odor sensing were dysregulated in the brain of cadmium-treated bees. These results suggest that cadmium exposure exerted oxidative stress and decreased gene expression levels of chemoreceptors in honey bees, which probably resulted in the impaired olfactory learning of honey bees. ABSTRACT: The honey bee (Apis mellifera) plays vital ecological roles in the pollination of crops and the maintenance of ecological balance, and adult honey bees may be exposed to exogenous chemicals including heavy metals during their foraging activities. Cadmium (Cd) is regarded as a nonessential toxic metal and is readily accumulated in plants; honey bees can therefore acquire Cd through the collection of contaminated nectar. In the present study, honey bees were chronically exposed to Cd to investigate the effects of sublethal cadmium doses on the olfactory learning and brain gene expression profiles of honey bees. The results showed that Cd-treated bees exhibited significantly impaired olfactory learning performances in comparison with control bees. Moreover, the head weight was significantly lower in Cd-treated bees than in control bees after chronic exposure to Cd. Gene expression profiles between the Cd treatment and the control revealed that 79 genes were significantly differentially expressed. Genes encoding chemoreceptors and olfactory proteins were downregulated, whereas genes involved in response to oxidative stress were upregulated in Cd-treated bees. The results suggest that Cd exposure exerts oxidative stress in the brain of honey bees, and the dysregulated expression of genes encoding chemoreceptors, olfactory proteins, and cytochrome P450 enzymes is probably associated with impaired olfactory learning in honey bees.