Cuticle Hydrocarbons Show Plastic Variation under Desiccation in Saline Aquatic Beetles

SIMPLE SUMMARY: Desiccation resistance and physiological plasticity are key traits for species persistence in the context of aridification under global change. One of the main mechanisms of desiccation resistance in insects is the control of cuticular transpiration through changes in the quantity and composition of epicuticular hydrocarbons (CHCs), which have been well studied in terrestrial insects. Our study provides novel information regarding the capacity to cope with desiccation stress through plastic changes in the composition of cuticle hydrocarbons in aquatic insects from saline intermittent streams. We demonstrate that the plasticity of CHCs is an effective mechanism to regulate water loss rate under desiccation stress in the most saline-tolerant species studied. These traits, so far largely unexplored in aquatic insects, are relevant to understanding different biochemical adaptations to deal with drought stress in inland saline waters in an evolutionary and ecological context. ABSTRACT: In the context of aridification in Mediterranean regions, desiccation resistance and physiological plasticity will be key traits for the persistence of aquatic insects exposed to increasing desiccation stress. Control of cuticular transpiration through changes in the quantity and composition of epicuticular hydrocarbons (CHCs) is one of the main mechanisms of desiccation resistance in insects, but it remains largely unexplored in aquatic ones. We studied acclimation responses to desiccation in adults of two endemic water beetles from distant lineages living in Mediterranean intermittent saline streams: Enochrus jesusarribasi (Hydrophilidae) and Nebrioporus baeticus (Dytiscidae). Cuticular water loss and CHC composition were measured in specimens exposed to a prior non-lethal desiccation stress, allowed to recover and exposed to a subsequent desiccation treatment. E. jesusarribasi showed a beneficial acclimation response to desiccation: pre-desiccated individuals reduced cuticular water loss rate in a subsequent exposure by increasing the relative abundance of cuticular methyl-branched compounds, longer chain alkanes and branched alkanes. In contrast, N. baeticus lacked acclimation capacity for controlling water loss and therefore may have a lower physiological capacity to cope with increasing aridity. These results are relevant to understanding biochemical adaptations to drought stress in inland waters in an evolutionary and ecological context.