Phenotypic Plasticity of Cuticular Hydrocarbon Profiles in Insects.

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.

Phenotypic plasticity of mate recognition systems prevents sexual interference between two sympatric leaf beetle species.

Maladaptive sexual interactions among heterospecific individuals (sexual interference) can prevent the coexistence of animal species. Thus, the avoidance of sexual interference by divergence of mate recognition systems is crucial for a stable coexistence in sympatry. Mate recognition systems are thought to be under tight genetic control. However, we demonstrate that mate recognition systems of two closely related sympatric leaf beetle species show a high level of host-induced phenotypic plasticity. Mate choice in the mustard leaf beetles, Phaedon cochleariae and P. armoraciae, is mediated by cuticular hydrocarbons (CHCs). Divergent host plant use causes a divergence of CHC phenotypes, whereas similar host use leads to their convergence. Consequently, both species exhibit significant behavioral isolation when they feed on alternative host species, but mate randomly when using a common host. Thus, sexual interference between these syntopic leaf beetles is prevented by host-induced phenotypic plasticity rather than by genotypic divergence of mate recognition systems.

The effect of dietary fatty acids on the cuticular hydrocarbon phenotype of an herbivorous insect and consequences for mate recognition.

The cuticular hydrocarbon (CHC) profile of the mustard leaf beetle Phaedon cochleariae is known to mediate mate recognition and is dependent on food plant species; beetles previously were shown to prefer mates that fed on the same plant species and which have a similar CHC pattern. In order to elucidate whether the pattern of ingested fatty acids affects the CHC pattern of P. cochleariae adults, we fed beetles: (a) with two different host plant species differing in fatty acid profile; and (b) artificial diets differing mainly in their composition of mono-, di-, and triunsaturated fatty acids. Analyses of the beetles' CHCs revealed that ingestion of different fatty acid blends results in quantitative effects on the beetle's straight-chain and methyl-branched CHCs. Interestingly, CHC patterns of males and females were affected differently by ingestion of fatty acids. In contrast to the effect on mating caused by feeding on different host plant species, beetles that were fed with different artificial diets, leading to different beetle CHC profiles, did not exhibit mating preference. We suggest that the occurrence of CHC-dependent assortative mating in P. cochleariae does not depend on the dietary fatty acids offered to the beetles in this study, but on other food constituents that affect CHC biosynthesis.

Monitoring of microbially mediated corrosion and scaling processes using redox potential measurements.

The use of redox potential measurements for corrosion and scaling monitoring, including microbially mediated processes, is demonstrated. As a case study, monitoring data from 10years of operation of an aquifer thermal energy storage (ATES) site located in Berlin, Germany, were examined. (Fe(2+))-activities as well as [Fe(3+)]-build up rates were calculated from redox potential, pH, conductivity, temperature and dissolved oxygen measurements. Calculations are based on assuming (Fe(3+))-activity being controlled by Fe(OH)3-solubility, the primary iron(III)-precipitate. This approach was tested using a simple log-linear model including dissolved oxygen besides major Fe(2+)-ligands. Measured redox potential values in groundwater used for thermal storage are met within ±8mV. In other systems comprising natural groundwater and in heating and cooling systems in buildings, quantitatively interpretable values are obtained also. It was possible to calculate particulate [Fe(3+)]-loads in the storage fluids in the order of 2µM and correlate a decrease in filter lifetimes to [Fe(3+)]-build up rates, although observations show clear signs of microbially mediated scaling processes involving iron and sulphur cycling.

Looking for a similar partner: host plants shape mating preferences of herbivorous insects by altering their contact pheromones.

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.

Insect egg deposition induces indirect defense and epicuticular wax changes in Arabidopsis thaliana.

Egg deposition by the Large Cabbage White butterfly Pieris brassicae on Brussels sprouts plants induces indirect defense by changing the leaf surface, which arrests the egg parasitoid Trichogramma brassicae. Previous studies revealed that this indirect defense response is elicited by benzyl cyanide (BC), which is present in the female accessory reproductive gland (ARG) secretion and is released to the leaf during egg deposition. Here, we aimed (1) to elucidate whether P. brassicae eggs induce parasitoid-arresting leaf surface changes in another Brassicacean plant, i.e., Arabidopsis thaliana, and, if so, (2) to chemically characterize the egg-induced leaf surface changes. Egg deposition by P. brassicae on A. thaliana leaves had similar effects to egg deposition on Brussels sprouts with respect to the following: (a) Egg deposition induced leaf surface changes that arrested T. brassicae egg parasitoids. (b) Application of ARG secretion of mated female butterflies or of BC to leaves had the same inductive effects as egg deposition. Based on these results, we conducted GC-MS analysis of leaf surface compounds from egg- or ARG-induced A. thaliana leaves. We found significant quantitative differences in epicuticular waxes compared to control leaves. A discriminant analysis separated surface extracts of egg-laden, ARG-treated, untreated control and Ringer solution-treated control leaves according to their quantitative chemical composition. Quantities of the fatty acid tetratriacontanoic acid (C34) were significantly higher in extracts of leaf surfaces arresting the parasitoids (egg-laden or ARG-treated) than in respective controls. In contrast, the level of tetracosanoic acid (C24) was lower in extracts of egg-laden leaves compared to controls. Our study shows that insect egg deposition on a plant can significantly affect the quantitative leaf epicuticular wax composition. The ecological relevance of this finding is discussed with respect to its impact on the behavior of egg parasitoids.

The role of cuticular hydrocarbons in male mating behavior of the mustard leaf beetle, Phaedon cochleariae (F.).

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.

Starving the competition: a proximate cause of reproductive skew in burying beetles (Nicrophorus vespilloides).

Proximate mechanisms underlying reproductive skew are obscure in many animals that breed communally. Here, we address causes of reproductive skew in brood-parasitic associations of burying beetles (Nicrophorus vespilloides). Male and female burying beetles feed and defend their larvae on buried carcasses. When several females locate the same small carcass, they engage in violent physical altercations. The subordinate then acts as an intraspecific brood parasite, laying eggs, but not providing care. The dominant female largely monopolizes access to the carcass; she alone provides parental care and her share of the brood is much larger than the subordinate's. On larger carcasses, subordinates have greater access to the carcass than on small ones, and reproductive skew is reduced. Differential fecundity, ovicide and larvicide have been suggested as causes of skew on small carcasses. Here, we report the results of the experiments pertaining to the first two of these potential mechanisms. Ovicide did not significantly contribute to reproductive skew on small carcasses, but differential fecundity did. Fecundity differences were due to dominance status, not body size per se. Fecundity differences disappeared when supplemental food was available, suggesting that reduced access to the carcass limits fecundity by causing nutritional deficiencies. Supplemental food prevented such nutritional deficiencies and allowed subordinates to produce as many eggs as dominants. Apparently, aggressive behaviour by dominants functions in the context of reproductive competition, limiting subordinate reproduction by preventing food intake on the carcass.

Perceptual filling-in from the edge of the blind spot.

Looking at the world with one eye, we do not notice a scotoma in the receptor-free area of the visual field where the optic nerve leaves the eye. Rather we perceive the brightness, color, and texture of the adjacent area as if they were actually there. The mechanisms underlying this kind of perceptual filling-in remain controversial. To better understand these processes, we determined the minimum region around the blind spot that needs to be stimulated for filling-in by carefully mapping the blind spot and presenting individually fitted stimulus frames of different width around it. Uniform filling-in was observed with frame widths as narrow as 0.05 degrees visual angle for color and 0.2 degrees for texture. Filling-in was incomplete, when the frame was no longer contiguous with the blind spot border due to an eye movement. These results are consistent with the idea that perceptual filling-in of the blind spot depends on local processes generated at the physiological edge of the cortical representation.

Surround-induced foveal afterimage pulsation: evidence for a long-range neural effect.

We demonstrate that an afterimage resulting from a strong foveal light flash can be made to pulsate by luminance modulation of a surrounding annulus as far as 8 deg away. Afterimage pulsation persists even if all artifacts due to pupil size, stray light and simultaneous contrast are ruled out. This suggests an origin by a long-range neural process acting from the remote surround. The effect is interpreted in terms of an adaptive gain control optimizing the responses of visual cells.