Epicuticular wax of large and small white butterflies, Pieris brassicae and P. rapae crucivora: qualitative and quantitative comparison between diapause and non-diapause pupae

Cuticular Lipid Topology on Insect Body Surfaces Studied by Synchrotron Radiation FTIR ATR Microspectroscopy

The cuticular lipid covering the integument of insects is exposed to the environment and involved in a variety of functions offered by insect body surfaces, ranging from protection against the environment, such as the control of water transpiration, the reduction of abrasive damage, and the prevention of pathogen intrusion, to the communication between insects from intraspecific to interspecific interactions. In comparison with the importance of their physiological functions, there is remarkably little information on the structure and physical property of cuticular lipids on insect body surfaces. The lipid layer on the outer exoskeleton is very thin, estimated on the order of 0.01-1 μm or less, and this has led to a lack of practical methodologies for detailed structural analyses. To fill this devoid, we have exploited the characteristics of Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy, which allows us to conduct a chemical analysis on insect body surfaces and also to investigate depth-dependent structural changes. We have applied a combination of FTIR ATR microspectroscopy with IR radiation provided by a synchrotron facility to obtain in situ two-dimensional (2D) information of the cuticular lipid layer on the surface of the integument. The 2D FTIR spectra measured on the two-spotted cricket and the American cockroach show that the IR bands due to the cuticular lipid, such as CH₂ symmetric and antisymmetric stretch, ν_a(CH₂) and ν_s(CH₂), change in intensity significantly, depending on the location of measurements. As if to keep pace with this, the bands of the amide group for the underlying cuticular layer also change in intensity significantly, although the changes are in the opposite direction; as the lipid bands increase in intensity, the amide band decreases, and vice versa. The ATR spectral analysis, which takes into account the characteristics of the evanescent wave, points out that the lipid layer would vary tens of times in the range of 0.01-1 μm significantly. The ν_a(CH₂) and ν_s(CH₂) bands show frequency shifts, which correlate to some extent with their intensity changes, suggesting that the drastic uneven distribution of the cuticular lipid would be related to the solid-liquid phase separation and also the coarsening of the solid phase domains. The formation of such topological features, significant heterogeneity in the lipid layer thickness, and solid-liquid phase ratios would be accompanied by the partitioning of lipid components according to molecular structures and physicochemical properties. Considering that each lipid component in insect body surface lipids is involved in various physiological roles, the segregation of lipid components during the formation of such heterogeneous structures is thought to have a significant impact on the functionality of the insect body surface.

Diapause affects cuticular hydrocarbon composition and mating behavior of both sexes in Drosophila montana

Environmental cues, mainly photoperiod and temperature, are known to control female adult reproductive diapause in several insect species. Diapause enhances female survival during adverse conditions and postpones progeny production to the favorable season. Male diapause (a reversible inability to inseminate receptive females) has been studied much less than female diapause. However, if the males maximized their chances to fertilize females while minimizing their energy expenditure, they would be expected to be in diapause at the same time as females. We investigated Drosophila montana male mating behavior under short-day conditions that induce diapause in females and found the males to be reproductively inactive. We also found that males reared under long-day conditions (reproducing individuals) court reproducing postdiapause females, but not diapausing ones. The diapausing flies of both sexes had more long-chain and less short-chain hydrocarbons on their cuticle than the reproducing ones, which presumably increase their survival under stressful conditions, but at the same time decrease their attractiveness. Our study shows that the mating behavior of females and males is well coordinated during and after overwintering and it also gives support to the dual role of insect cuticular hydrocarbons in adaptation and mate choice.

Supercooling and freezing as eco-physiological alternatives rather than mutually exclusive strategies: A case study in Pyrrhocoris apterus

Overwintering insects are categorized either as freeze tolerant or freeze avoiding (supercooling) based on their ability or inability, respectively, to tolerate the formation of ice in their body. The freeze tolerant insects set their supercooling point (SCP) higher for winter to stimulate freezing at higher temperatures, while freeze avoiding insects survive winter in a supercooled state by depressing their SCP. Some supercooling insects, however, were found to survive in frozen state when freezing occurred through inoculation by external ice at mild subzero temperatures. Here, we assessed the potential relevance of inoculative freezing and freeze tolerance strategy in an insect that was so far considered as a classical example of a 'supercooler', the linden bug (Pyrrhocoris apterus). Microclimatic conditions of the overwintering microhabitat of P. apterus (leaf litter layer with buffered temperature fluctuations, mild sub-zero extremes, high humidity, and presence of ice) present a potentially high risk of inoculative freezing. We found that P. apterus is highly susceptible to inoculation by external ice. The temperature at which inoculative freezing occurred (above -3°C) was much higher compared to SCP (-16 °C to -20 °C in winter). The insects were inoculated through body openings and across cuticle and were able to survive after freezing. There was, however, a distinct critical ice fraction, corresponding to 38.7-42.8% of total body water, beyond which survival rapidly decreased to zero. We found that P. apterus adaptively reduces the actual ice fraction below critical ice fraction for winter season. Since many insect species overwinter in habitats similar to that of P. apterus, the ability to tolerate freezing after inoculation by external ice crystals could be much more common among 'supercooling' insects than it is currently appreciated.

Early transcriptional events linked to induction of diapause revealed by RNAseq in larvae of drosophilid fly, Chymomyza costata

Background

Diapause is a developmental alternative to direct ontogeny in many invertebrates. Its primary adaptive meaning is to secure survival over unfavourable seasons in a state of developmental arrest usually accompanied by metabolic suppression and enhanced tolerance to environmental stressors. During photoperiodically triggered diapause of insects, the ontogeny is centrally turned off under hormonal control, the molecular details of this transition being poorly understood. Using RNAseq technology, we characterized transcription profiles associated with photoperiodic diapause induction in the larvae of the drosophilid fly Chymomyza costata with the goal of identifying candidate genes and processes linked to upstream regulatory events that eventually lead to a complex phenotypic change.

Results

Short day photoperiod triggering diapause was associated to inhibition of 20-hydroxy ecdysone (20-HE) signalling during the photoperiod-sensitive stage of C. costata larval development. The mRNA levels of several key genes involved in 20-HE biosynthesis, perception, and signalling were significantly downregulated under short days. Hormonal change was translated into downregulation of a series of other transcripts with broad influence on gene expression, protein translation, alternative histone marking by methylation and alternative splicing. These changes probably resulted in blockade of direct development and deep restructuring of metabolic pathways indicated by differential expression of genes involved in cell cycle regulation, metabolism, detoxification, redox balance, protection against oxidative stress, cuticle formation and synthesis of larval storage proteins. This highly complex alteration of gene transcription was expressed already during first extended night, within the first four hours after the change of the photoperiodic signal from long days to short days. We validated our RNAseq differential gene expression results in an independent qRT-PCR experiment involving wild-type (photoperiodic) and NPD-mutant (non-photoperiodic) strains of C. costata.

Conclusions

Our study revealed several strong candidate genes for follow-up functional studies. Candidate genes code for upstream regulators of a complex change of gene expression, which leads to phenotypic switch from direct ontogeny to larval diapause.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1907-4) contains supplementary material, which is available to authorized users.

Sex-specific divergence for adaptations to dehydration stress in Drosophila kikkawai

Several studies on diverse Drosophila species have reported higher desiccation resistance of females, but the physiological basis of such sex-specific differences has received less attention. We tested whether sex-specific differences in cuticular traits (melanic females and non-melanic males) of Drosophila kikkawai correspond with divergence in their water balance mechanisms. Our results are interesting in several respects. First, positive clinal variation in desiccation resistance was correlated with cuticular melanisation in females but with changes in cuticular lipid mass in males, despite a lack of differences between the sexes for the rate of water loss. Second, a comparative analysis of water budget showed that females of the northern population stored more body water as well as hemolymph content and exhibited greater dehydration tolerance than flies from the southern tropics. In contrast, we found no geographical variation in the males for water content and dehydration tolerance. Third, an ~10-fold increase in the rate of water loss after organic solvent treatment of male D. kikkawai suggested a role of cuticular lipids in cuticular transpiration, but had no effect in the females. Fourth, geographical differences in the storage of carbohydrate content (metabolic fuel) were observed in females but not in males. Interestingly, in females, the rate of utilization of carbohydrates did not vary geographically, but males from drier localities showed a 50% reduction compared with wetter localities. Thus, body melanisation, increased body water, hemolymph, carbohydrate content and greater dehydration tolerance confer greater desiccation resistance in females, but a reduced rate of water loss is the only possible mechanism to cope with drought stress in males. Finally, acclimated females showed a significant increase in drought resistance associated with higher trehalose content as well as dehydration tolerance, while males showed no acclimation response. Thus, sex-specific differences in desiccation resistance of D. kikkawai are associated with divergence in some water balance strategies, despite a lack of differences in the rate of water loss between the two sexes.

The molecular physiology of increased egg desiccation resistance during diapause in the invasive mosquito, Aedes albopictus

Photoperiodic diapause is a crucial adaptation to seasonal environmental variation in a wide range of arthropods, but relatively little is known regarding the molecular basis of this important trait. In temperate populations of the mosquito Aedes albopictus, exposure to short-day (SD) lengths causes the female to produce diapause eggs. Tropical populations do not undergo a photoperiodic diapause. We identified a fatty acyl coA elongase transcript that is more abundant under SD versus long-day (LD) photoperiods in mature oocyte tissue of replicate temperate, but not tropical, A. albopictus populations. Fatty acyl CoA elongases are involved in the synthesis of long chain fatty acids (hydrocarbon precursors). Diapause eggs from a temperate population had one-third more surface hydrocarbons and one-half the water loss rates of non-diapause eggs. Eggs from a tropical population reared under SD and LD photoperiods did not differ in surface hydrocarbon abundance or water loss rates. In both a temperate and tropical population, composition of hydrocarbon chain lengths did not differ between eggs from SD versus LD conditions. These results implicate the expression of fatty acyl coA elongase and changes in quantity, but not composition, of egg surface hydrocarbons as important components of increased desiccation resistance during diapause in A. albopictus.

The chemistry of the postpharyngeal gland of female European beewolves

Females of the European beewolf, Philanthus triangulum, possess a large glove-shaped gland in the head, the postpharyngeal gland (PPG). They apply the content of the PPG to their prey, paralyzed honeybees, where it delays fungal infestation. Here, we describe the chemical composition of the gland by using combined GC-MS, GC-FTIR, and derivatization. The PPG of beewolves contains mainly long-chain unsaturated hydrocarbons (C23-C33), lower amounts of saturated hydrocarbons (C14-C33), and minor amounts of methyl-branched hydrocarbons (C17-C31). Additionally, the hexane-soluble gland content is comprised of small amounts of an unsaturated C25 alcohol, an unknown sesquiterpene, an octadecenylmethylester, and several long-chain saturated (C25, C27) and unsaturated (C23-C27) ketones, some of which have not yet been reported as natural products. Surprisingly, we found a dimorphism with regard to the major component of the PPG with some females having (Z)-9-pentacosene, whereas others have (Z)-9-heptacosene as their predominant component. The biological relevance of the compounds for the prevention of fungal growth on the prey and the significance of the chemical dimorphism are discussed.

Cuticular lipids and odors induce sex-specific behaviors in the male cricket Gryllus bimaculatus

Male crickets display sex-specific (e.g., mating and agonistic) behaviors towards conspecific individuals. One of the key signals for these behaviors is the chemical substance on the cricket body surface. In the present study, we analyzed female and male cuticular substances in behavioral assays. Antennal contact stimulation using female forewings elicited a mating behavior in males, while that using male forewings elicited an agonistic behavior in males. Thin-layer-chromatographic and other techniques analysis showed that saturated cuticular lipids were present in both female and male cuticles and that unsaturated lipids were present only in the male cuticle. Filter papers soaked with saturated or unsaturated cuticular lipids were applied to antennae of male crickets. Males showed mating behavior in response to stimulation with saturated lipids from both females and males but showed avoidance behavior in response to stimulation with male unsaturated lipids. Because cuticular lipids did not induce agonistic behavior in males, we collected odors from male crickets and found that these odors induced agonistic behavior in males. Therefore, we concluded that the key signals for mating, avoidance and agonistic behaviors of male crickets are comprised of at least three different components, saturated and unsaturated cuticular lipids and male odors, respectively.

A simple method to discriminate diapause from non-diapause pupae in large and small white butterflies, Pieris brassicae and P. rapae crucivora

Differences in colour and shape have been used to discriminate diapause pupae from non-diapause pupae in butterflies. In the present study, we describe a simple discrimination method based on differences in the specific gravity of diapause and non-diapause pupae of large and small white butterflies, Pieris brassicae and Pieris rapae crucivora. When put into water, diapause pupae sink to the bottom (specific gravity is above 1.0), whereas non-diapause pupae float on the surface (specific gravity is below 1.0). Nuclear magnetic resonance microimaging revealed that this difference in specific gravity is due to a difference in the volume of an internal cavity located between the thorax and the abdomen in the pupae. The cavity appears quite early in development. We also examined the cavity in pupae of the unrelated swallowtail butterfly, Papilio xuthus, and found a similar cavity. However, in this species, the cavity is very small and, as a consequence, non-diapause as well as diapause pupae sink in water.