Experience and memory of environmental stimuli that indicate future stress can prepare (prime) organismic stress responses even in species lacking a nervous system. The process through which such organisms prepare their phenotype for an improved response to future stress has been termed 'priming'. However, other terms are also used for this phenomenon, especially when considering priming in different types of organisms and when referring to different stressors. Here we propose a conceptual framework for priming of stress responses in bacteria, fungi and plants which allows comparison of priming with other terms, e.g. adaptation, acclimation, induction, acquired resistance and cross protection. We address spatial and temporal aspects of priming and highlight current knowledge about the mechanisms necessary for information storage which range from epigenetic marks to the accumulation of (dormant) signalling molecules. Furthermore, we outline possible patterns of primed stress responses. Finally, we link the ability of organisms to become primed for stress responses (their 'primability') with evolutionary ecology aspects and discuss which properties of an organism and its environment may favour the evolution of priming of stress responses.

SUMMARYThe ectoparasitic mite Varroa destructor and honey bee pathogenic viruses have been implicated in the recent demise of honey bee colonies. Several studies have shown that the combination of V. destructor and deformed wing virus (DWV) poses an especially serious threat to honey bee health. Mites transmitting virulent forms of DWV may cause fatal DWV infections in the developing bee, while pupae parasitised by mites not inducing or activating overt DWV infections may develop normally. Adult bees respond to brood diseases by removing affected brood. This hygienic behaviour is an essential part of the beesʼ immune response repertoire and is also shown towards mite-parasitised brood. However, it is still unclear whether the bees react towards the mite in the brood cell or rather towards the damage done to the brood. We hypothesised that the extent of mite-associated damage rather than the mere presence of parasitising mites triggers hygienic behaviour. Hygienic behaviour assays performed with mites differing in their potential to transmit overt DWV infections revealed that brood parasitised by ʻvirulentʼ mites (i.e. mites with a high potential to induce fatal DWV infections in parasitised pupae) were removed significantly more often than brood parasitised by ʻless virulentʼ mites (i.e. mites with a very low potential to induce overt DWV infections) or non-parasitised brood. Chemical analyses of brood odour profiles suggested that the bees recognise severely affected brood by olfactory cues. Our results suggest that bees show selective, damage-dependent hygienic behaviour, which may be an economic way for colonies to cope with mite infestation. Supplementary material available online at

The role of phenotypical plasticity in ecological speciation and the evolution of sexual isolation remains largely unknown. We investigated whether or not divergent host plant use in an herbivorous insect causes assortative mating by phenotypically altering traits involved in mate recognition. We found that males of the mustard leaf beetle Phaedon cochleariae preferred to mate with females that were reared on the same plant species to females provided with a different plant species, based on divergent cuticular hydrocarbon profiles that serve as contact pheromones. The cuticular hydrocarbon phenotypes of the beetles were host plant specific and changed within 2 weeks after a shift to a novel host plant species. We suggest that plant-induced phenotypic divergence in mate recognition cues may act as an early barrier to gene flow between herbivorous insect populations on alternative host species, preceding genetic divergence and thus, promoting ecological speciation.

We investigated the role that cuticular hydrocarbons (CHC) play in sexual communication by the mustard leaf beetle, Phaedon cochleariae (Coleoptera: Chrysomelidae). In laboratory bioassays, male P. cochleariae attempted to copulate with living or freeze-killed females as often as with males. However, the duration of copulation with females was longer than with males. To elucidate the impact of CHC on this behavior, cuticular compounds of adults of both sexes were extracted with dichloromethane. Male mating attempts with glass beads treated with the dichloromethane extract were nearly as frequent as with living beetles. The dichloromethane extract was fractionated by silica gel chromatography, and the biological activity of the fractions was tested by applying them to glass beads. A non-polar hexane fraction significantly elicited mating behavior, whereas the polar methanol fraction did not, likely because it contained defensive compounds from exocrine glands located in the elytra and pronota. Interestingly, a mixture of both the non-polar and polar fraction tended to elicit more mating attempts than did the non-polar hexane fraction alone. Further fractionation of the significantly active hexane fraction by silver nitrate column chromatography revealed that saturated CHC elicited mating behavior, but the olefins did not. GC-MS analyses of dichloromethane cuticular extracts showed that the male and female CHC profiles were qualitatively identical, but differed in their relative composition. Canonical discriminant analysis showed that CHC profiles of males and females formed separate clusters. Nevertheless, the results of our bioassays demonstrated that male and female CHC did not elicit sex discriminative male behavior, but induced mating by males regardless of the sex of the partner.

The insect integument is covered by cuticular hydrocarbons (CHCs) which provide protection against environmental stresses, but are also used for communication. Here we review current knowledge on environmental and insect-internal factors which shape phenotypic plasticity of solitary living insects, especially herbivorous ones. We address the dynamics of changes which may occur within minutes, but may also last weeks, depending on the species and conditions. Two different modes of changes are suggested, i.e. stepwise and gradual. A switch between two distinct environments (e.g. host plant switch by phytophagous insects) results in stepwise formation of two distinct adaptive phenotypes, while a gradual environmental change (e.g. temperature gradients) induces a gradual change of numerous adaptive CHC phenotypes. We further discuss the ecological and evolutionary consequences of phenotypic plasticity of insect CHC profiles by addressing the question at which conditions is CHC phenotypic plasticity beneficial. The high plasticity of CHC profiles might be a trade-off for insects using CHCs for communication. We discuss how insects cope with the challenge to produce and "understand" a highly plastic, environmentally dependent CHC pattern that conveys reliable and comprehensible information. Finally, we outline how phenotypic plasticity of CHC profiles may promote speciation in insects that rely on CHCs for mate recognition.