Department of Biology, University of Modena and Reggio Emilia, via Campi 213/D, 41125 Modena, Italy.
J Insect Physiol. 2011 May;57(5):567-76. doi: 10.1016/j.jinsphys.2011.03.003. Epub 2011 Mar 21.
In this review we analyze the dormancy strategies of metazoans inhabiting "hostile to life" habitats, which have a strong impact on their ecology and in particular on the traits of their life history. Tardigrades are here considered a model animal, being aquatic organisms colonizing terrestrial habitats. Tardigrades evolved a large variety of dormant stages that can be ascribed to diapause (encystment, cyclomorphosis, resting eggs) and cryptobiosis (anhydrobiosis, cryobiosis, anoxibiosis). In tardigrades, diapause and cryptobiosis can occur separately or simultaneously, consequently the adoption of one adaptive strategy is not necessarily an alternative to the adoption of the other. Encystment and cyclomorphosis are characterized by seasonal cyclic changes in morphology and physiology of the animals. They share several common features and their evolution is strictly linked to the molting process. A bet-hedging strategy with different patterns of egg hatching time has been observed in a tardigrade species. Four categories of eggs have been identified: subitaneous, delayed-hatching, abortive and diapause resting eggs, which needs a stimulus to hatch (rehydration after a period of desiccation). Cryptobiotic tardigrades are able to withstand desiccation (anhydrobiosis) and freezing (cryobiosis) at any stage of their life-cycle. This ability involves a complex array of factors working at molecular (bioprotectans), physiological and structural levels. Animal survival and the accumulation of molecular damage are related to the time spent in the cryptobiotic state, to the abiotic parameters during the cryptobiotic state, and to the conditions during initial and final phases of the process. Cryptobiosis evolved independently at least two times in tardigrades, in eutardigrades and in echiniscoids. Within each evolutionary line, the absence of cryptobiotic abilities is more related to selective pressures to local habitat adaptation than to phylogenetic relationships. The selective advantages of cryptobiosis (e.g. persistency in "hostile to life" habitats, reduction of competitors, parasites and predators, escaping in time from stressful conditions) could explain the high tardigrade species diversity and number of specimens found in habitats that dry out compared to freshwater habitats.
在这篇综述中，我们分析了栖息在“不利于生命”生境中的后生动物的休眠策略，这些策略对它们的生态学，特别是它们的生活史特征有很大的影响。缓步动物在这里被视为一种模式动物，是在水生栖息地中殖民的生物体。缓步动物进化出了多种休眠阶段，可以归因于休眠（包囊形成、循环形态变化、休眠卵）和隐生（无水生、低温生、缺氧生）。在缓步动物中，休眠和隐生可以单独或同时发生，因此采用一种适应策略不一定是另一种适应策略的替代。包囊形成和循环形态变化的特征是动物形态和生理的季节性循环变化。它们有几个共同的特征，它们的进化与蜕皮过程密切相关。在一种缓步动物中观察到了不同孵化时间的卵孵化时间的贝叶斯策略。已经确定了四类卵：即时卵、延迟孵化卵、流产卵和休眠卵，这些卵需要刺激才能孵化（在干燥一段时间后再水合）。隐生的缓步动物能够在其生命周期的任何阶段耐受干燥（无水生）和冷冻（低温生）。这种能力涉及到分子（生物保护剂）、生理和结构水平上的一系列复杂因素。动物的生存和分子损伤的积累与它们在隐生状态下的时间、隐生状态下的非生物参数以及初始和最终阶段的条件有关。隐生至少在缓步动物的两个进化支中独立进化，即真缓步动物和棘头虫。在每个进化支中，缺乏隐生能力更多地与对当地栖息地适应的选择压力有关，而与系统发育关系无关。隐生的选择优势（例如，在“不利于生命”的生境中持久存在、减少竞争者、寄生虫和捕食者、及时逃避压力条件）可以解释为什么在干燥的栖息地中发现的缓步动物物种多样性和标本数量比淡水栖息地多。
