Megachile rotundata RNA-seq from post-diapause quiescent prepupae. Megachile rotundata

The genetic component of insect response to long-term, ecologically relevant temperature stress is poorly understood. Long-term low temperature exposure can lead to physiological effects collectively known as chill injury. Periodically increasing temperatures during long-term chilling has been shown to increase survival in many insects. However, the transcripts responsible for this increase in survival have never been characterized. Here we present the first transcriptome-level analysis of increased longevity under fluctuating temperatures during chilling. Overwintering post-diapause quiescent alfalfa leafcutting bees, Megachile rotundata, were exposed to a constant temperature of 6°C or 6°C with a daily fluctuation to 20°C. RNA was collected at two different time points, before and after mortality rates began to diverge between temperature treatments. Expression analysis identified differentially regulated transcripts between pairwise comparisons of both treatments and time points. Transcripts functioning in ion homeostasis, metabolic pathways, and oxidative stress response were up-regulated in individuals exposed to periodic temperature fluctuations during chilling. These provide genetic support for the hypotheses that fluctuating temperatures protect against chill injury by reducing oxidative stress and returning ion concentrations and metabolic function to more favorable levels. Additionally, exposure to fluctuating temperatures lead to increased expression of transcripts functioning in immune response and neurogenesis, providing evidence for additional mechanisms associated with increased longevity during chilling in M. rotundata.

昆虫对长期生态相关温度胁迫的遗传响应机制目前仍知之甚少。长期低温暴露可引发一系列生理损伤，这类损伤统称为冷伤害（chill injury）。已有研究证实，长期低温胁迫期间周期性升高环境温度，能够提升多数昆虫的存活率，但介导该存活率提升的关键转录本（transcript）尚未得到系统表征。本研究首次针对低温胁迫下温度波动延长昆虫寿命的现象开展转录组（transcriptome）水平分析。实验对象为滞育后越冬静息的苜蓿切叶蜂（Megachile rotundata），将其分别置于恒定6℃环境，或每日波动至20℃的6℃环境中暴露处理。研究分别在两组死亡率开始出现显著组间差异的前后两个时间点收集RNA样本。转录组表达分析鉴定出两种温度处理组间，以及不同采样时间点间的差异表达转录本。结果显示，在低温胁迫期间接受周期性温度波动的个体中，参与离子稳态（ion homeostasis）、代谢通路及氧化应激反应（oxidative stress response）的转录本表达显著上调。上述发现为相关假说提供了遗传层面的支持：温度波动可通过降低氧化应激水平、使离子浓度与代谢功能恢复至更适宜状态，从而抵御冷伤害。此外，周期性温度波动暴露还可上调参与免疫应答（immune response）与神经发生（neurogenesis）的转录本表达，为苜蓿切叶蜂低温胁迫期间寿命延长的额外潜在机制提供了实验证据。