Center for Evolutionary Hologenomics, The Globe Institute, University of Copenhagen, Copenhagen, Denmark. Electronic address: https://twitter.com/askmobjerg.
Center for Evolutionary Hologenomics, The Globe Institute, University of Copenhagen, Copenhagen, Denmark.
Comp Biochem Physiol A Mol Integr Physiol. 2022 Sep;271:111245. doi: 10.1016/j.cbpa.2022.111245. Epub 2022 May 29.
Subzero temperatures are among the most significant factors defining the distribution of organisms, yet, certain taxa have evolved to overcome this barrier. The microscopic tardigrades are among the most freeze-tolerant animals, with selected species reported to survive milli-Kelvin temperatures. Here, we estimate survival of fully hydrated eutardigrades of the species Ramazzottius varieornatus following exposures to -20 °C and -80 °C as well as -196 °C with or without initial cooling to -80 °C. The tardigrades easily survive these temperatures, yet with a significant decrease in viability following rapid cooling by direct exposure to -196 °C. Hence, post-freeze recovery of R. varieornatus seems to rely on cooling rate and thus controlled ice formation. Cryophilic organisms are renowned for having cold-active enzymes that secure appropriate reaction rates at low temperatures. Hence, extreme freeze-tolerance in R. varieornatus could potentially involve syntheses of cryoprotectants and de novo transcription. We therefore generated a reference transcriptome for this cryophilic R. varieornatus population and explored for differential gene expression patterns following cooling to -80 °C as compared to active 5 °C controls. Specifically, we tested for fast transcription potentially occurring within 25 min of cooling from room temperature to a supercooling point of ca. -20 °C, at which the tardigrades presumably freeze and enter into the ametabolic state of cryobiosis. Our analyses revealed no evidence for differential gene expression. We, therefore, conclude that extreme freeze-tolerance in R. varieornatus relies on controlled extracellular freezing with any freeze-tolerance related genes being constitutively expressed.
零下温度是决定生物分布的最重要因素之一，但某些类群已经进化到能够克服这一障碍。微小的缓步动物是最能耐受冷冻的动物之一，有报道称某些选定的物种能在毫开尔文温度下存活。在这里，我们估计了完全水合的 Ramazzottius varieornatus 缓步动物在暴露于-20°C 和-80°C 以及-196°C 时的存活情况，其中包括有无初始冷却至-80°C 的情况。缓步动物很容易在这些温度下存活，但在直接暴露于-196°C 快速冷却后，其活力显著下降。因此，R. varieornatus 的冷冻后恢复似乎依赖于冷却速度，从而控制冰晶的形成。嗜冷生物以具有低温活性酶而闻名，这些酶可确保在低温下适当的反应速率。因此，R. varieornatus 的极端抗冻性可能涉及到冷冻保护剂的合成和新转录的产生。因此，我们为这种嗜冷的 R. varieornatus 种群生成了一个参考转录组，并研究了冷却至-80°C 与 5°C 活性对照相比后的差异基因表达模式。具体来说，我们测试了在从室温冷却到约-20°C 的过冷点时，快速转录的可能性，在这个温度下，缓步动物可能会冻结并进入无代谢状态的冷冻生物状态。我们的分析没有发现差异基因表达的证据。因此，我们得出结论，R. varieornatus 的极端抗冻性依赖于受控的细胞外冷冻，任何与抗冻性相关的基因都被持续表达。
