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Abstract：
Drosophila, like most insects, are susceptible to low temperatures, and will succumb to temperatures above the freezing point of their hemolymph. For these insects, cold exposure causes a loss of extracellular ion and water homeostasis, leading to chill injury and eventually death. Chill-tolerant species are characterized by lower hemolymph [Na(+)] than chill-susceptible species and this lowered hemolymph [Na(+)] is suggested to improve ion and water homeostasis during cold exposure. It has therefore also been hypothesized that hemolymph Na(+) is replaced by other \'cryoprotective\' osmolytes in cold-tolerant species. Here, we compared the hemolymph metabolite profiles of five drosophilid species with marked differences in chill tolerance. All species were examined under \'normal\' thermal conditions (i.e. 20°C) and following cold exposure (4 h at 0°C). Under benign conditions, total hemolymph osmolality was similar among all species despite chill-tolerant species having lower hemolymph [Na(+)]. Using NMR spectroscopy, we found that chill-tolerant species instead have higher levels of sugars and free amino acids in their hemolymph, including classical \'cryoprotectants\' such as trehalose and proline. In addition, we found that chill-tolerant species maintain a relatively stable hemolymph osmolality and metabolite profile when exposed to cold stress while sensitive species suffer from large increases in osmolality and massive changes in their metabolic profiles during a cold stress. We suggest that the larger contribution of classical cryoprotectants in chill-tolerant Drosophila plays a non-colligative role for cold tolerance that contributes to osmotic and ion homeostasis during cold exposure and, in addition, we discuss how these comparative differences may represent an evolutionary pathway toward more extreme cold tolerance of insects.
摘要：
果蝇,像大多数昆虫一样，易受低温影响，会屈服于血淋巴冰点以上的温度。对于这些昆虫来说,冷暴露会导致细胞外离子和水稳态的丧失，导致寒冷的伤害并最终死亡。耐寒物种的特征是血淋巴[Na（）]比易感物种低，并且这种降低的血淋巴[Na（）]被建议改善冷暴露期间的离子和水稳态。因此，也有人假设，在耐寒物种中，血淋巴Na（）被其他“冷冻保护性”渗透剂代替。这里,我们比较了五种果蝇的血淋巴代谢产物谱，它们在耐寒性方面存在明显差异。所有物种均在正常热条件下（即20°C）和冷暴露（0°C下4小时）进行检查。在良性条件下,尽管耐寒物种的血淋巴[Na（）]较低，但所有物种的总血淋巴渗透压浓度相似。使用NMR光谱，我们发现耐寒物种的血淋巴中的糖和游离氨基酸含量更高，包括经典的“冷冻保护剂”，如海藻糖和脯氨酸。此外,我们发现,耐寒物种在暴露于冷应激时保持相对稳定的血淋巴渗透压和代谢谱,而敏感物种在冷应激期间,其渗透压大幅增加和代谢谱发生巨大变化.我们建议，经典冷冻保护剂在耐寒果蝇中的更大贡献对耐寒性起着非依数作用，这有助于在冷暴露期间的渗透和离子稳态，此外,我们讨论了这些比较差异如何代表昆虫更极端耐寒的进化途径。
