Elucidating mechanisms for insect body size: partial support for the oxygen-dependent induction of moulting hypothesis

Body size is a key life history trait and knowledge of its mechanistic basis is crucial in life history biology. Such knowledge is accumulating in holometabolous insects, whose growth is characterised and body size affected by moulting. According to the oxygen-dependent induction of moulting (ODIM) hypothesis, moult is induced at a critical mass where oxygen demand of growing tissues overrides the supply that principally grows only at moults. Support for the ODIM hypothesis is controversial partly because of a lack of proper data to explicitly test the hypothesis. The ODIM hypothesis predicts that the critical mass is positively correlated with oxygen partial pressure (pO2) and negatively with temperature. To resolve the controversy that surrounds the ODIM hypothesis, we rigorously test these predictions by exposing penultimate-instar Orthosia gothica (Lepidoptera: Noctuidae) larvae to temperature and moderate pO2 manipulations in a factorial experiment. The relative mass increment in the focal instar increased along with increasing pO2, as predicted, but there was only weak suggestive evidence of the temperature effect. Probably due to a high measurement error in the trait, pO2 effect on the critical mass was sex-specific; high pO2 had a positive effect only in females, whereas low pO2 had a negative effect only in males. Critical mass was independent of temperature. Support for the ODIM hypothesis is partial because of only suggestive evidence of a temperature effect on moulting, but the role of oxygen in moult induction seems unambiguous. The ODIM mechanism, thus, seems worth considering in body size analyses.

体型是核心生活史性状，阐明其机制基础对生活史生物学研究至关重要。这方面的认知在全变态昆虫（holometabolous insects）中逐渐积累，这类昆虫的生长以蜕皮为特征，其体型亦受蜕皮过程调控。根据氧依赖蜕皮诱导（oxygen-dependent induction of moulting, ODIM）假说，当生长组织的氧需求超过主要仅在蜕皮阶段才提升的氧供应阈值时，生物体便会触发蜕皮。目前学界对ODIM假说的支持态度尚存争议，部分原因在于缺乏可直接验证该假说的恰当实验数据。ODIM假说预测，临界体重与氧分压（oxygen partial pressure, pO2）呈正相关，而与温度呈负相关。为解决ODIM假说相关的争议，本研究通过析因实验，将倒数第二龄的栎夜蛾（Orthosia gothica，鳞翅目：夜蛾科）幼虫暴露于不同温度和适度调控的氧分压环境中，严格验证上述假说预测。正如假说预测，目标龄期的相对体重增量随pO2升高而增加，但仅能观察到温度效应的微弱暗示性证据。该性状的测量误差较高可能是原因之一：pO2对临界体重的影响存在性别特异性——高氧分压仅对雌性个体产生正向效应，而低氧分压仅对雄性个体产生负向效应。临界体重与温度无关。由于仅能获得温度对蜕皮存在影响的暗示性证据，本研究对ODIM假说的支持仅为部分性，但氧在蜕皮诱导中的作用似乎明确无疑。因此，在体型分析中纳入ODIM机制似有必要。