Data from: Neural dysfunction correlates with heat coma and CTmax in Drosophila but does not set the boundaries for heat stress survival

When heated, insects lose coordinated movement followed by the onset of heat coma (critical thermal maximum, CTmax). These traits are popular measures to quantify interspecific and intraspecific differences in insect heat tolerance, and CTmax correlates well with current species distributions of insects, including Drosophila. Here, we examined the function of the central nervous system (CNS) in five species of Drosophila with different heat tolerances, while they were exposed to either constant high temperature or a gradually increasing temperature (ramp). Tolerant species were able to preserve CNS function at higher temperatures and for longer durations than sensitive species, and similar differences were found for the behavioural indices (loss of coordination and onset of heat coma). Furthermore, the timing and temperature (constant and ramp exposure, respectively) for loss of coordination or complete coma coincided with the occurrence of spreading depolarisation (SD) events in the CNS. These SD events disrupt neurological function and silence the CNS, suggesting that CNS failure is the primary cause of impaired coordination and heat coma. Heat mortality occurs soon after heat coma in insects; to examine whether CNS failure could also be the proximal cause of heat death, we used selective heating of the head (CNS) and abdomen (visceral tissues). When comparing the temperature causing 50% mortality (LT50) of each body part versus that of the whole animal, we found that the head was not particularly heat sensitive compared with the abdomen. Accordingly, it is unlikely that nervous failure is the principal/proximate cause of heat mortality in Drosophila.当昆虫受热时，会先丧失协调运动能力，随后进入热昏迷状态，此时对应的临界温度即被称为临界热最大值（critical thermal maximum, CTmax）。这类性状常被用于量化昆虫耐热性的种间与种内差异，且CTmax与包括果蝇属（Drosophila）在内的当前昆虫物种分布具有良好相关性。本研究针对5种耐热性存在差异的果蝇属昆虫，探究了其在持续高温或渐升温度（梯度升温）暴露下的中枢神经系统（central nervous system, CNS）功能变化。相较于耐热性敏感的物种，耐热性较强的果蝇能够在更高温度下、更长时间内维持中枢神经系统功能；行为指标（协调运动丧失与热昏迷发作）也呈现出类似的差异。此外，协调运动丧失或完全昏迷的发生时间与温度（分别对应持续高温暴露与梯度升温暴露场景），与中枢神经系统内的去极化扩散（spreading depolarisation, SD）事件的发生时刻高度吻合。这类去极化扩散事件会破坏神经功能并使中枢神经系统陷入沉默，这表明中枢神经系统功能衰竭是导致协调运动受损与热昏迷的主要原因。昆虫通常会在热昏迷后不久出现热死亡；为探究中枢神经系统功能衰竭是否同样是热死亡的直接诱因，我们采用了选择性加热头部（中枢神经系统所在部位）与腹部（内脏组织）的实验方案。对比单独加热两个身体部位时的半致死温度（LT50）与整只动物的半致死温度后，我们发现相较于腹部，头部的热敏感性并未显著更高。据此推测，在果蝇属昆虫中，神经功能衰竭不太可能是热死亡的主要/直接诱因。