Data from: The chicken or the egg? Adaptation to desiccation and salinity tolerance in a lineage of water beetles

Transitions from fresh to saline habitats are restricted to a handful of insect lineages, as the colonization of saline waters requires specialized mechanisms to deal with osmotic stress. Previous studies have suggested that tolerance to salinity and desiccation could be mechanistically and evolutionarily linked, but the temporal sequence of these adaptations is not well established for individual lineages. We combined molecular, physiological and ecological data to explore the evolution of desiccation resistance, hyporegulation ability (i.e., the ability to osmoregulate in hyperosmotic media) and habitat transitions in the water beetle genus Enochrus subgenus Lumetus (Hydrophilidae). We tested whether enhanced desiccation resistance evolved before increases in hyporegulation ability or vice versa, or whether the two mechanisms evolved in parallel. The most recent ancestor of Lumetus was inferred to have high desiccation resistance and moderate hyporegulation ability. There were repeated shifts between habitats with differing levels of salinity in the radiation of the group, those to the most saline habitats generally occurring more rapidly than those to less saline ones. Significant and accelerated changes in hyporegulation ability evolved in parallel with smaller and more progressive increases in desiccation resistance across the phylogeny, associated with the colonization of meso- and hypersaline waters during global aridification events. All species with high hyporegulation ability were also desiccation-resistant, but not vice versa. Overall, results are consistent with the hypothesis that desiccation resistance mechanisms evolved first and provided the physiological basis for the development of hyporegulation ability, allowing these insects to colonize and diversify across meso- and hypersaline habitats.从淡水生境向盐水生境的演化过渡仅局限于少数昆虫支系，因为盐水水域的定殖需要特化机制以应对渗透胁迫。既往研究表明，盐度耐受性与脱水耐受性可能在机制与演化层面存在关联，但针对单个支系而言，这两类适应性特征的演化时序尚未得到明确阐释。我们整合了分子、生理与生态数据，以探究水龟虫科（Hydrophilidae）牙甲属（Enochrus）的Lumetus亚属内的脱水耐受性、低渗调节能力（hyporegulation ability，即高渗介质中的渗透调节能力）与生境过渡的演化历程。我们开展了如下检验：增强型脱水耐受性是否先于低渗调节能力提升出现，抑或反之，亦或是这两类机制协同演化。研究推断，Lumetus亚属的最近共同祖先具备较高的脱水耐受性与中等水平的低渗调节能力。在该类群的辐射演化过程中，不同盐度等级的生境间存在多次转换；相较于低盐生境，向高盐生境的转换通常发生得更快。在整个系统发育框架中，低渗调节能力出现了显著且加速的变化，同时脱水耐受性呈现出幅度更小但更为渐进的提升，二者协同演化，这一特征与全球干旱化事件期间类群向中盐度及高盐度水域的定殖过程相关联。所有具备高水准低渗调节能力的物种同时具备脱水耐受性，但反之则不成立。总体而言，本研究结果与如下假说相符：脱水耐受性机制率先演化，并为低渗调节能力的形成提供了生理基础，从而使得这些昆虫能够在中盐度与高盐度生境中完成定殖与物种分化。