One size does not fit all: Caste and sex differences in the response of bumblebees (<i>Bombus impatiens</i>) to chronic oral neonicotinoid exposure

Neonicotinoid insecticides have been implicated in the rapid global decline of bumblebees over recent years, particularly in agricultural and urban areas. While there is much known about neonicotinoid toxicity effects at the colony stage of the bumblebee annual cycle, far less is known about such effects at other stages critical for the maintenance of wild populations. In the present work, individual-based feeding assays were used to show that chronic consumption of the widely used neonicotinoid clothianidin at a field-realistic average rate of 3.6 and 4.0 ng/g·bee/day reduces survival of queen and male bumblebees, respectively, within a 7-day period. In contrast, worker survival was unaffected at a similar consumption rate of 3.9 ng/g·bee/day. To test the hypothesis that males have a lower tolerance for oral clothianidin exposure than workers due to their haploid genetic status, RNAseq analysis was used to compare the transcriptomic responses of workers and males to chronic intake of clothianidin at a sub-lethal dose of 0.37ng/bee/day for 5 days. Surprisingly, clothianidin consumption only altered the expression of 19 putative detoxification genes in a sex-specific manner, with 11/19 genes showing increased expression in workers. Sub-lethal clothianidin exposure also altered the expression of 40 genes associated with other major biological functions, including locomotion, reproduction, and immunity. Collectively, these results suggest that chronic oral toxicity effects of neonicotinoids are greatest during mating and nest establishment phases of the bumblebee life cycle. Chronic oral toxicity testing on males and queens is therefore required in order to fully assess the impact of neonicotinoids on wild bumblebee populations.近年来，新烟碱类杀虫剂(Neonicotinoid insecticides)被证实与全球范围内熊蜂种群的快速衰退密切相关，在农业与城市生境中尤为显著。尽管学界对熊蜂年生活史中蜂群阶段的新烟碱类毒性效应已有较为充分的研究，但对维持野生种群至关重要的其他发育阶段的此类效应却知之甚少。本研究通过基于个体的取食生物测定证实：当以符合田间实际浓度的平均摄入速率（3.6 ng/蜂·天、4.0 ng/蜂·天）长期摄入广泛使用的新烟碱类杀虫剂噻虫胺(Clothianidin)时，熊蜂蜂王与雄蜂的存活率会在7天内分别出现显著下降。与之相对，当工蜂摄入剂量为3.9 ng/蜂·天时，其存活率未受显著影响。为验证“由于雄蜂具有单倍体遗传特征，其对噻虫胺口服暴露的耐受性低于工蜂”这一假说，本研究采用RNA测序(RNAseq)技术，对比了工蜂与雄蜂在连续5天摄入亚致死剂量（0.37 ng/蜂·天）噻虫胺后的转录组响应差异。令人意外的是，噻虫胺仅以性别特异性方式调控了19个推定解毒基因的表达，其中11/19个基因在工蜂体内的表达量上调。亚致死剂量的噻虫胺暴露还改变了40个与其他主要生物学功能相关的基因表达，这些功能涵盖运动、繁殖与免疫等。综合来看，上述结果表明新烟碱类杀虫剂的慢性口服毒性效应在熊蜂生活史的交配与筑巢建立阶段最为显著。因此，为全面评估新烟碱类杀虫剂对野生熊蜂种群的影响，需针对雄蜂与蜂王开展慢性口服毒性测试。