Within-population variability in a moth sex pheromone blend: genetic basis and behavioural consequences

Morph-linked variation in female pheromone signalling and male response in a polymorphic moth

Understanding the maintenance of genetic variation in reproductive strategies and polymorphisms in the wild requires a comprehensive examination of the complex interactions between genetic basis, behaviour and environmental factors. We tested the association between three colour genotypes and variation in female pheromone signalling and male antennal morphology in the wood tiger moth (Arctia plantaginis). These moths have genetically determined white (WW, Wy) and yellow (yy) hindwings that are linked to mating success and fitness, with heterozygotes (Wy) having an advantage. We hypothesized that attractiveness and reproductive success are correlated, with Wy females being more attractive than the other two genotypes which could contribute to maintaining the polymorphism. Female attractiveness was tested by baiting traps with females of the three colour genotypes both in low- (i.e. field set-up) and in high-population density (i.e. large enclosure set-up). Male's ability to reach females was correlated to their own colour genotype and antennal morphology (length, area and lamellae count). Contrary to our prediction, morph-related reproductive success and attractiveness were not correlated. Heavier Wy females attracted a lower proportion of males compared to WW and yy females. Specifically, an increase in weight corresponded to a decreased Wy but increased yy female attractiveness. yy females were generally more attractive than others likely due to earlier pheromone release. In males, lamellae count and genetic colour morph were linked to the male's ability to locate females. Furthermore, male traits affected their ability to reach females in a context-specific way. Males with denser antennae (i.e. higher lamellae count) and white males reached the females faster than yellows in the enclosure, while yellow males located females faster than whites in the field. Our results indicate that higher yy female attractiveness was likely affected by the combined effect of early pheromone release, female weight and higher population density. Males' searching success was affected by morph-specific behavioural strategies and local population density. Ultimately, the combined effect of genotype-related pheromone signalling strategies of females together with environment-dependent male behaviour affect male response and potentially contribute to maintaining variation in fitness-related traits.

Ecological immunology: do sexual attraction and immunity trade-off through a desaturase?

Given the limited availability of resources in nature, sexual attractiveness may trade off with immunocompetence, as the immunocompetence handicap hypothesis (ICHH) posits. In invertebrates, a direct link between trade-offs through hormonal/molecular effectors in sexual signals and immunity has not been found so far. Here, we assessed how variation in sexual signals affected parasite infection in two sex pheromone selected lines of the moth Chloridea virescens: an attractive line with a low ratio of 16:Ald/Z11-16:Ald and an unattractive line with a high ratio. When infecting these lines with an apicomplexan parasite, we found that the attractive Low line was significantly more susceptible to the parasite infection than the unattractive High line. Since the ratio difference between these two lines is determined by a delta-11-desturase, we hypothesized that this desaturase may have a dual role, i.e., in the quality of the sexual signal as well as an involvement in immune response, comparable to testosterone in vertebrates. However, when we used CRISPR/cas9 to knockout delta-11-desturase in the attractive Low line, we found that the pheromonal phenotype did change to that of the High line, but the infection susceptibility did not. Notably, when checking the genomic location of delta-11-desaturase in the C. virescens, we found that mucin is adjacent to delta-11-desaturase. When comparing the mucin sequences in both lines, we found four nonsynonymous SNPs in the coding sequence, as well as intronic variation between the two lines. These differences suggest that genetic hitchhiking may explain the variation in susceptibility to parasitic infection.

Behavioural consequences of intraspecific variability in a mate recognition signal

Mate recognition is paramount for sexually reproducing animals, and many insects rely on cuticular hydrocarbons (CHCs) for close-range sexual communication. To ensure reliable mate recognition, intraspecific sex pheromone variability should be low. However, CHCs can be influenced by several factors, with the resulting variability potentially impacting sexual communication. While intraspecific CHC variability is a common phenomenon, the consequences thereof for mate recognition remain largely unknown. We investigated the effect of CHC variability on male responses in a parasitoid wasp showing a clear-cut within-population CHC polymorphism (three distinct female chemotypes, one thereof similar to male profiles). Males clearly discriminated between female and male CHCs, but not between female chemotypes in no-choice assays. When given a choice, a preference hierarchy emerged. Interestingly, the most attractive chemotype was the one most similar to male profiles. Mixtures of female CHCs were as attractive as chemotype-pure ones, while a female-male mixture negatively impacted male responses, indicating assessment of the entire, complex CHC profile composition. Our study reveals that the evaluation of CHC profiles can be strict towards 'undesirable' features, but simultaneously tolerant enough to cover a range of variants. This reconciles reliable mate recognition with naturally occurring variability.

Factors affecting the biosynthesis and emission of a Drosophila pheromone

The most studied pheromone in Drosophila melanogaster, cis-vaccenyl acetate (cVA), is synthesized in the male ejaculatory bulb and transferred to the female during copulation. Combined with other chemicals, cVA can modulate fly aggregation, courtship, mating and fighting. We explored the mechanisms underlying both cVA biosynthesis and emission in males of two wild types and a pheromonal mutant line. The effects of ageing, adult social interaction, and maternally transmitted cVA and microbes - both associated with the egg chorion - on cVA biosynthesis and emission were measured. While ageing and genotype changed both biosynthesis and emission in similar ways, early developmental exposure to maternally transmitted cVA and microbes strongly decreased cVA emission but not the biosynthesis of this molecule. This indicates that the release - but not the biosynthesis - of this sex pheromone strongly depends on early developmental context. The mechanism by which the preimaginal effects occur is unknown, but reinforces the significance of development in determining adult physiology and behaviour.

Experimental evolution of a pheromone signal

Sexual signals are important in speciation, but understanding their evolution is complex as these signals are often composed of multiple, genetically interdependent components. To understand how signals evolve, we thus need to consider selection responses in multiple components and account for the genetic correlations among components. One intriguing possibility is that selection changes the genetic covariance structure of a multicomponent signal in a way that facilitates a response to selection. However, this hypothesis remains largely untested empirically. In this study, we investigate the evolutionary response of the multicomponent female sex pheromone blend of the moth Heliothis subflexa to 10 generations of artificial selection. We observed a selection response of about three-quarters of a phenotypic standard deviation in the components under selection. Interestingly, other pheromone components that are biochemically and genetically linked to the components under selection did not change. We also found that after the onset of selection, the genetic covariance structure diverged, resulting in the disassociation of components under selection and components not under selection across the first two genetic principle components. Our findings provide rare empirical support for an intriguing mechanism by which a sexual signal can respond to selection without possible constraints from indirect selection responses.

Toward an understanding of the chemical ecology of alternative reproductive tactics in the bulb mite (Rhizoglyphus robini)

BACKGROUND

Under strong sexual selection, certain species evolve distinct intrasexual, alternative reproductive tactics (ARTs). In many cases, ARTs can be viewed as environmentally-cued threshold traits, such that ARTs coexist if their relative fitness alternates over the environmental cue gradient. Surprisingly, the chemical ecology of ARTs has been underexplored in this context. To our knowledge, no prior study has directly quantified pheromone production for ARTs in a male-polymorphic species. Here, we used the bulb mite-in which males are either armed fighters that kill conspecifics, or unarmed scramblers (which have occasionally been observed to induce mating behavior in other males)-as a model system to gain insight into the role of pheromones in the evolutionary maintenance of ARTs. Given that scramblers forgo investment into weaponry, we tested whether scramblers produce higher quantities of the putative female sex-pheromone α-acaridial than fighters, which would improve the fitness of the scrambler phenotype through female mimicry by allowing avoidance of aggression from competitors. To this end, we sampled mites from a rich and a poor nutritional environment and quantified their production of α-acaridial through gas chromatography analysis.

RESULTS

We found a positive relationship between pheromone production and body size, but males exhibited a steeper slope in pheromone production with increasing size than females. Females exhibited a higher average pheromone production than males. We found no significant difference in slope of pheromone production over body size between fighters and scramblers. However, scramblers reached larger body sizes and higher pheromone production than fighters, providing some evidence for a potential female mimic strategy adopted by large scramblers. Pheromone production was significantly higher in mites from the rich nutritional environment than the poor environment.

CONCLUSION

Further elucidation of pheromone functionality in bulb mites, and additional inter- and intrasexual comparisons of pheromone profiles are needed to determine if the observed intersexual and intrasexual differences in pheromone production are adaptive, if they are a by-product of allometric scaling, or diet-mediated pheromone production under weak selection. We argue chemical ecology offers a novel perspective for research on ARTs and other complex life-history traits.

Emergence of Maruca vitrata as a Major Pest of Food Legumes and Evolution of Management Practices in Asia and Africa

Legume pod borer, Maruca vitrata, has emerged as a major pest on food legumes in Asia and Africa. It is an oligophagous pest, feeding on over 70 species in Fabaceae. We examine the species complex in Asia, Africa, Oceania, and the Americas, with an emphasis on molecular taxonomy. Studies on pheromone production and perception suggest the existence of pheromone polymorphism, especially in Asia and Africa. No Maruca-resistant varieties are available in the major food legumes including cowpea, pigeonpea, mungbean, and yard-long bean. Legume growers use chemical pesticides indiscriminately, leading to the development of pesticide resistance. However recent developments in habitat management, classical biocontrol with more efficient parasitoids, biopesticides, and judicious use of insecticides pave the way for sustainable management of M. vitrata, which can reduce the pesticide misuse. Active engagement of the private sector and policy makers can increase the adoption of integrated pest management approaches in food legumes.

Clustering of loci controlling species differences in male chemical bouquets of sympatric Heliconius butterflies

The degree to which loci promoting reproductive isolation cluster in the genome-that is, the genetic architecture of reproductive isolation-can influence the tempo and mode of speciation. Tight linkage between these loci can facilitate speciation in the face of gene flow. Pheromones play a role in reproductive isolation in many Lepidoptera species, and the role of endogenously produced compounds as secondary metabolites decreases the likelihood of pleiotropy associated with many barrier loci. Heliconius butterflies use male sex pheromones to both court females (aphrodisiac wing pheromones) and ward off male courtship (male-transferred antiaphrodisiac genital pheromones), and it is likely that these compounds play a role in reproductive isolation between Heliconius species. Using a set of backcross hybrids between H. melpomene and H. cydno, we investigated the genetic architecture of putative male pheromone compound production. We found a set of 40 significant quantitative trait loci (QTL) representing 33 potential pheromone compounds. QTL clustered significantly on two chromosomes, chromosome 8 for genital compounds and chromosome 20 for wing compounds, and chromosome 20 was enriched for potential pheromone biosynthesis genes. There was minimal overlap between pheromone QTL and known QTL for mate choice and color pattern. Nonetheless, we did detect linkage between a QTL for wing androconial area and optix, a color pattern locus known to play a role in reproductive isolation in these species. This tight clustering of putative pheromone loci might contribute to coincident reproductive isolating barriers, facilitating speciation despite ongoing gene flow.

A life-history perspective on sexual selection in a polygamous species

Background

Ever since Darwin, evolutionary biologists have studied sexual selection driving differences in appearance and behaviour between males and females. An unchallenged paradigm in such studies is that one sex (usually the male) signals its quality as a mate to the other sex (usually the female), who is choosy in accepting a partner. Here, we hypothesize that in polygamous species these roles change dynamically with the mating status of males and females, depending on direct reproductive costs and benefits of multiple matings, and on sperm competition. We test this hypothesis by assessing fitness costs and benefits of multiple matings in both males and females in a polygamous moth species, as in moths not males but females are the signalers and males are the responders.

Results

We found that multiple matings confer fitness costs and benefits for both sexes. Specifically, the number of matings did not affect the longevity of males or females, but only 67% of the males and 14% of the females mated successfully in all five nights. In addition, the female’s reproductive output increased with multiple matings, although when paired with a new virgin male every night, more than 3 matings decreased her reproductive output, so that the Bateman gradient for females fit a quadratic model better than a linear model. The male’s reproductive success was positively affected by the number of matings and a linear regression line best fit the data. Simulations of the effect of sperm competition showed that increasing last-male paternity increases the steepness of the male Bateman gradient and thus the male’s relative fitness gain from additional mating. Irrespective of last-male paternity value, the female Bateman gradient is steeper than the male one for up to three matings.

Conclusion

Our results suggest that choosiness in moths may well change throughout the mating season, with males being more choosy early in the season and females being more choosy after having mated at least three times. This life-history perspective on the costs and benefits of multiple matings for both sexes sheds new light on sexual selection forces acting on sexual signals and responses.

First come, first served: precopulatory mate-guarding behavior and male-male contests by a hymenopteran saproxylic parasitoid

Precopulatory mate-guarding behavior is a common strategy that maximizes male reproductive success when female receptivity to copulation is low. This behavior has been demonstrated in vertebrates, aquatic crustaceans, terrestrial isopods, and some species of insects, but there is very little available information about hymenopteran insects. A few studies have clarified the factor that determines the outcome of a contest between a guarding male and an invader male. We investigated the male-male contest and mating behavior of a saproxylic parasitoid wasp, Ibalia japonica (Hymenoptera: Cynipoidea: Ibaliidae) using field observations in Japan. These observations indicated that I. japonica males show precopulatory mate-guarding behavior and that four types of male-male contests occur on the Magnolia liliiflora (Magnoliales: Magnoliaceae) tree that virgin females emerge from. We show that the arrival order of I. japonica males that found the future emergence point of a female was key factor that allowed males to secure virgin females.

Within-population variability in a moth sex pheromone blend, part 2: selection towards fixation

To understand how variation in sexual communication systems evolves, the genetic architecture underlying sexual signals and responses needs to be identified. Especially in animals where mating signals are important for mate recognition, and signals and responses are governed by independently assorting genes, it is difficult to envision how signals and preferences can (co)evolve. Moths are a prime example of such animals. In the noctuid moth Heliothis virescens, we found within-population variation in the female pheromone. In previous selection experiments followed by quantitative trait locus (QTL) analysis and expression analysis of candidate desaturase genes, we developed a model involving a trans-acting repressor of the delta-11-desaturase. In our current study with new selection lines, we fixed the most extreme phenotype and found a single underlying mutation: a premature stop codon in the first coding exon of delta-11-desaturase, which we could trace back to its origin in the laboratory. Interestingly, we found no pleiotropic effects of this knock-out mutation on the male physiological or behavioural response, or on growth or fertility. This finding is in contrast to Drosophila melanogaster, where a single desaturase gene affects both female pheromone production and male behavioural response, but similar to other Lepidoptera where these traits are under independent genetic control. To our knowledge, this is the first time that a single point mutation has been identified that underlies the phenotypic variation in the pheromone signal of a moth.

Covariation and phenotypic integration in chemical communication displays: biosynthetic constraints and eco-evolutionary implications

Chemical communication is ubiquitous. The identification of conserved structural elements in visual and acoustic communication is well established, but comparable information on chemical communication displays (CCDs) is lacking. We assessed the phenotypic integration of CCDs in a meta-analysis to characterize patterns of covariation in CCDs and identified functional or biosynthetically constrained modules. Poorly integrated plant CCDs (i.e. low covariation between scent compounds) support the notion that plants often utilize one or few key compounds to repel antagonists or to attract pollinators and enemies of herbivores. Animal CCDs (mostly insect pheromones) were usually more integrated than those of plants (i.e. stronger covariation), suggesting that animals communicate via fixed proportions among compounds. Both plant and animal CCDs were composed of modules, which are groups of strongly covarying compounds. Biosynthetic similarity of compounds revealed biosynthetic constraints in the covariation patterns of plant CCDs. We provide a novel perspective on chemical communication and a basis for future investigations on structural properties of CCDs. This will facilitate identifying modules and biosynthetic constraints that may affect the outcome of selection and thus provide a predictive framework for evolutionary trajectories of CCDs in plants and animals.

Evolutionary ecology of chemosensation and its role in sensory drive

All behaviors of an organism are rooted in sensory processing of signals from its environment, and natural selection shapes sensory adaptations to ensure successful detection of cues that maximize fitness. Sensory drive, or divergent selection for efficient signal transmission among heterogeneous environments, has been a useful hypothesis for describing sensory adaptations, but its current scope has primarily focused on visual and acoustic sensory modalities. Chemosensation, the most widespread sensory modality in animals that includes the senses of smell and taste, is characterized by rapid evolution and has been linked to sensory adaptations to new environments in numerous lineages. Yet, olfaction and gustation have been largely underappreciated in light of the sensory drive hypothesis. Here, we examine why chemosensory systems have been overlooked and discuss the potential of chemosensation to shed new insight on the sensory drive hypothesis and vice versa. We provide suggestions for developing a framework to better incorporate studies of chemosensory adaptation that have the potential to shape a more complete, coherent, and holistic interpretation of the sensory drive.

Alcohol Contributes to Attraction of Heliothis (= Chloridea) virescens Males to Females

Female-emitted volatile sex pheromones in most moths are composed of biosynthetically related blends of fatty acid derivatives, such as aldehydes, acetate esters and alcohols. In many moths, as in the noctuid Heliothis (Chloridea) virescens, the pheromone gland contains alcohols (e.g., (Z)-11-hexadecen-1-ol, hereafter Z11-16:OH) that may serve dual functions as pheromone components as well as precursors of other pheromone components. The relative importance of Z11-16:OH to male attraction in H. virescens has been controversial. It occurs in the pheromone gland in relatively large amounts, but several studies could neither detect Z11-16:OH in gland emissions nor attribute any conspecific behavioral function to it in flight- tunnel assays. Trapping assays in the field, however, have more consistently documented that the addition of Z11-16:OH increased trap catch. Using a short section of thick film megabore column, in combination with derivatization and GC-CI-SIM-MS, we determined that Z11-16:OH is emitted from the sex pheromone gland during calling. Field trapping studies demonstrated that trap catch increased when Z11-16:OH was added to a 2-component minimal blend and to a 6-component blend. Behavioral observations in the field confirmed that more males responded to a pheromone blend that contained a low blend ratio of Z11-16:OH, but ≥5% Z11-16:OH depressed both male behavior and trap catch. We conclude that Z11-16:OH should be considered a component of the sex pheromone of H. virescens females.

Age Influence on Sexual Behavior of the Lesser Cornstalk Borer, Elasmopalpus lignosellus (Zeller) (Lepidoptera: Pyralidae)

The objective of this work was to evaluate the reproductive behavior and response of Elasmopalpus lignosellus (Zeller) males to calling females. Frequency of mating was recorded in couples during the first 7 days of the adult stage. Calling behavior of females was observed during the first 4 days of the adult stage and responses of males, in the same age intervals, to calling females were recorded in wind tunnel bioassays. The maximum number of matings occurred when the couple was between 24 and 48 h old. The scotophase period significantly influenced mating behavior, which peaked between 6 and 8 h of darkness and the mean mating duration was 93.9 ± 4.2 min. Calling females, when evaluated in a wind tunnel, attracted significantly more males than in bioassays with clean air (control). The number of individuals in calling behavior was significantly lower for females that were between 0 to 24 h old compared to the other females evaluated, but this did not influence male response. A lower proportion of males between 48 to 72 h old responded to calling females and these responses were delayed in comparison with males of other ages (0 to 24, 24 to 48, and 72 to 96 h old). These results indicate that the age of E. lignosellus males influences the response to conspecific calling females.

Larval Diet Affects Male Pheromone Blend in a Laboratory Strain of the Medfly, Ceratitis capitata (Diptera: Tephritidae)

The Mediterranean fruit fly (medfly) Ceratitis capitata is a polyphagous pest of fruits and crops with a worldwide distribution. Its ability to use different larval hosts may have multiple effects, including impacts on adult reproductive biology. The male sex pheromone, which plays a key role in attracting both other males to lekking arenas and females for mating, is a mixture of chemical compounds including esters, acids, alkanes and terpenes known to differ between laboratory strains and wild-type populations. The relationship between larval diet and adult pheromone composition remains unexplored. Here, we investigated the effect of larval diet, including laboratory media and fresh fruits, on the composition of the male pheromone mixture. Using Headspace Solid Phase Microextraction we collected the pheromone emitted by males reared as larvae on different substrates and found both qualitative and quantitative differences. A number of alkanes appeared to be typical of the pheromone of males reared on wheat bran-based larval medium, and these may be cuticular hydrocarbons involved in chemical communication. We also detected differences in pheromone composition related to adult male age, suggesting that variations in hormonal levels and/or adult diet could also play a role in determining the chemical profile emitted. Our findings highlight the plasticity of dietary responses of C. capitata, which may be important in determining the interactions of this pest with the environment and with conspecifics. These results also have applied relevance to increase the mating competitiveness of mass-reared C. capitata used in Sterile Insect Technique programs.

Key genes of the sex pheromone biosynthesis pathway in female moths are required for pheromone quality and possibly mediate olfactory plasticity in conspecific male moths in Spodoptera litura

Ninety sex pheromone biosynthesis genes in Spodoptera litura were identified in transcriptome data and were investigated and classified into the following five groups: fatty acid synthase, beta oxidase, fatty acyl-coenzyme A (CoA) reductase, desaturase and acetyl-CoA acetyltransferase. Fourteen female-specific genes were identified through semi-quantitative PCR, and 15 additional genes had expression levels that were 3- to 10-fold higher in females than in males. The majority of the genes had higher expression levels in the sex pheromone glands. Injection of double-stranded RNA (dsRNA) against nine selected genes showed that down-regulation of Desaturase 3 (Des3), Des5 or fatty acyl coenzyme A reductase 17 (FAR17) significantly changed the ratio of the four sex pheromone components (Z,E) -9,11-tetradecadienyl acetate (Z9E11-14:Ac), (Z,E)-9,12-Tetradecadienyl acetate(Z9E12-14:Ac), (Z)-9-tetradecenyl acetate (Z9-14:Ac), (E)-11-Tetradecenyl acetate(E11-14:Ac). These key genes were differentially expressed in female moths collected from different geographical regions. Furthermore, field bioassays demonstrated geographical variation in the olfactory profile of male moths in response to the different sex pheromone mixtures, which therefore indicates that a significant variation in the sex pheromone components exists in the natural population. Our results suggest that a change in the expression of these key genes, Des3, Des5 and FAR17, in the sex pheromone biosynthesis pathway could change the ratio of the sex pheromone components. We surmise that the differential expression levels of the key genes of the sex pheromone biosynthesis pathway may lead to differential ratios of the sex pheromones in the field. Our field trapping experiment suggested that the change of the ratio of the sex pheromone components may have been adapted by the olfactory system and possibly mediate olfactory plasticity in conspecific male moths.

Differential gene expression in the evolution of sex pheromone communication in New Zealand's endemic leafroller moths of the genera Ctenopseustis and Planotortrix

Background

Sex pheromone communication in moths has attracted the attention of evolutionary biologists due to the vast array of pheromone compounds used, addressing questions of how this diversity arose and how male reception has evolved in step with the female signal. Here we examine the role of changing gene expression in the evolution of mate recognition systems in leafroller moths, particularly focusing on genes involved in the biosynthetic pathways of sex pheromones in female pheromone glands and the peripheral reception repertoire in the antennae of males. From tissue-specific transcriptomes we mined and compared a database of genes expressed in the pheromone glands and antennae of males and females of four closely related species of leafroller moths endemic to New Zealand, Ctenopseutis herana and C. obliquana, and Planotortrix excessana and P. octo. The peculiarity of this group, compared to other Lepidoptera, is the use of (Z)-5-tetradecenyl acetate, (Z)-7-tetradecenyl acetate, and (Z)-8-tetradecenyl acetate as sex pheromone components.

Results

We identify orthologues of candidate genes from the pheromone biosynthesis pathway, degradation and transport, as well as genes of the periphery olfactory repertoire, including large families of binding proteins, receptors and odorant degrading enzymes. The production of distinct pheromone blends in the sibling species is associated with the differential expression of two desaturase genes, deast5 and desat7, in the pheromone glands. In male antennae, three odorant receptors, OR74, OR76a and OR30 are over-expressed, but their expression could not be clearly associated with the detection of species-specific pheromones components. In addition these species contain duplications of all three pheromone binding proteins (PBPs) that are also differentially expressed among species.

Conclusions

While in females differences in the expression of desaturases may be sufficient to explain pheromone blend differences among these New Zealand leafroller species, in males differential expression of several genes, including pheromone binding proteins, may underpin differences in the response by males to changing pheromone components among the species.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4451-1) contains supplementary material, which is available to authorized users.

Proximity of signallers can maintain sexual signal variation under stabilizing selection

How sexual communication systems can evolve under stabilizing selection is still a paradox in evolutionary biology. In moths, females emit a species-specific sex pheromone, consisting of a blend of biochemically related components, to which males are attracted. Although males appear to exert strong stabilizing selection on female pheromone, these blends seem to have evolved rapidly, as evidenced by ~120,000 moth species. Here we propose and test a “proximity model” wherein two females that vary in their relative attractiveness to males, can both benefit from calling in close proximity to each other. In a field study, we show that (1) artificially selected unattractive females can achieve mating rates comparable to attractive females if they signal in close proximity to attractive females, and (2) attractive females benefit from higher mating rates when signalling in close proximity to unattractive females. We propose that frequency-dependent behavioural and spatial interactions can sustain signal variation within populations even when these signals are under stabilizing selection.

Experimental evidence for chemical mate guarding in a moth

In polyandrous species, males seek to maximize their reproductive output by monopolizing their mate. Often the male transfers substances to the female that suppress her sexual receptivity or antagonize the behavior of competing males; both are usually transferred in seminal fluids and represent forms of chemical mate guarding. In moths, more long-range female sex pheromones have been identified than in any other animal group, and males often display with close-range sex pheromones, yet odor-based post-copulatory mate guarding has not been described in moths so far. We tested the hypothesis that the male sex pheromone in the noctuid moth Heliothis virescens perfumes the female and functions as an anti-aphrodisiac. Indeed, virgin females perfumed with male pheromone extract, or with its main component, mated significantly less than control virgin females, and this effect persisted for two successive nights. This chemical mate guarding strategy was disadvantageous for H. virescens females, because the reproductive output of twice-mated females was significantly higher than that of once-mated females. Since the female and male sex pheromones are biosynthetically related in this and other moth species, chemical mate guarding may also impose selection pressure on the long-range female sex pheromone channel and consequently affect the evolution of sexual communication.

Individual Pheromone Signature in Males: Prerequisite for Pheromone-Mediated Mate Assessment in the Central American Locust, Schistocerca Piceifrons

Living in high-density groups of animals has advantages and disadvantages for mating. The advantage of facilitated mate finding is compromised by difficulties in protecting a suitable partner from competitors. Thus, males regularly are faced with increased competition for sperm, and females with harassment by males at high population densities. To cope with these problems, mating tactics and mate choice mechanisms have to be adjusted. An adaptation to gregarious condition observed in locusts includes the use of male-emitted pheromones. Males of the Central American locust, Schistocerca piceifrons, release sex-specific volatiles, which were identified as phenethyl alcohol (synonym: phenyl-ethyl-alcohol, 2-phenyl-1-ethanol, 2-phenylethanol, PEA), (Z)-3-nonen-1-ol (3-Nol), and (Z)-2-octen-1-ol (2-Ool). The emission of the two major compounds, PEA and 3-Nol, was restricted to crowded conditions. Furthermore, the release of both volatiles was coupled to males reaching sexual maturity, indicating a function in reproductive behavior. However, neither the single substances nor their mixtures were attractive or repellent to the locusts. Instead, females prefer the sperm of high pheromone-emitting males to fertilize their ova. In this way, the male-specific volatiles act as mate assessment pheromones utilized in a context of cryptic female choice. This function is well supported by the highly variable but individual-specific emission rates of the three compounds. Schistocerca piceifrons males release a virtually unique personal pheromone signature, a prerequisite for mate assessment pheromones.

Epimerisation of chiral hydroxylactones by short-chain dehydrogenases/reductases accounts for sex pheromone evolution in Nasonia

Males of all species of the parasitic wasp genus Nasonia use (4R,5S)-5-hydroxy-4-decanolide (RS) as component of their sex pheromone while only N. vitripennis (Nv), employs additionally (4R,5R)-5-hydroxy-4-decanolide (RR). Three genes coding for the NAD⁺-dependent short-chain dehydrogenases/reductases (SDRs) NV10127, NV10128, and NV10129 are linked to the ability of Nv to produce RR. Here we show by assaying recombinant enzymes that SDRs from both Nv and N. giraulti (Ng), the latter a species with only RS in the pheromone, epimerise RS into RR and vice versa with (4R)-5-oxo-4-decanolide as an intermediate. Nv-derived SDR orthologues generally had higher epimerisation rates, which were also influenced by NAD⁺ availability. Semiquantitative protein analyses of the pheromone glands by tandem mass spectrometry revealed that NV10127 as well as NV10128 and/or NV10129 were more abundant in Nv compared to Ng. We conclude that the interplay of differential expression patterns and SDR epimerisation rates on the ancestral pheromone component RS accounts for the evolution of a novel pheromone phenotype in Nv.

Disposable Polydimethylsiloxane (PDMS)-Coated Fused Silica Optical Fibers for Sampling Pheromones of Moths

In the past decades, the sex pheromone composition in female moths has been analyzed by different methods, ranging from volatile collections to gland extractions, which all have some disadvantage: volatile collections can generally only be conducted on (small) groups of females to detect the minor pheromone compounds, whereas gland extractions are destructive. Direct-contact SPME overcomes some of these disadvantages, but is expensive, the SPME fiber coating can be damaged due to repeated usage, and samples need to be analyzed relatively quickly after sampling. In this study, we assessed the suitability of cheap and disposable fused silica optical fibers coated with 100 μm polydimethylsiloxane (PDMS) by sampling the pheromone of two noctuid moths, Heliothis virescens and Heliothis subflexa. By rubbing the disposable PDMS fibers over the pheromone glands of females that had called for at least 15 minutes and subsequently extracting the PDMS fibers in hexane, we collected all known pheromone compounds, and we found a strong positive correlation for most pheromone compounds between the disposable PDMS fiber rubs and the corresponding gland extracts of the same females. When comparing this method to volatile collections and the corresponding gland extracts, we generally found comparable percentages between the three techniques, with some differences that likely stem from the chemical properties of the individual pheromone compounds. Hexane extraction of cheap, disposable, PDMS coated fused silica optical fibers allows for sampling large quantities of individual females in a short time, eliminates the need for immediate sample analysis, and enables to use the same sample for multiple chemical analyses.

Two fatty acyl reductases involved in moth pheromone biosynthesis

Fatty acyl reductases (FARs) constitute an evolutionarily conserved gene family found in all kingdoms of life. Members of the FAR gene family play diverse roles, including seed oil synthesis, insect pheromone biosynthesis, and mammalian wax biosynthesis. In insects, FAR genes dedicated to sex pheromone biosynthesis (pheromone-gland-specific fatty acyl reductase, pgFAR) form a unique clade that exhibits substantial modifications in gene structure and possesses unique specificity and selectivity for fatty acyl substrates. Highly selective and semi-selective ‘single pgFARs’ produce single and multicomponent pheromone signals in bombycid, pyralid, yponomeutid and noctuid moths. An intriguing question is how a ‘single reductase’ can direct the synthesis of several fatty alcohols of various chain lengths and isomeric forms. Here, we report two active pgFARs in the pheromone gland of Spodoptera, namely a semi-selective, C14:acyl-specific pgFAR and a highly selective, C16:acyl-specific pgFAR, and demonstrate that these pgFARs play a pivotal role in the formation of species-specific signals, a finding that is strongly supported by functional gene expression data. The study envisages a new area of research for disclosing evolutionary changes associated with C₁₄- and C₁₆-specific FARs in moth pheromone biosynthesis.

Beyond species recognition: somatic state affects long-distance sex pheromone communication

Long-range sex pheromones have been subjected to substantial research with a particular focus on their biosynthesis, peripheral perception, central processing and the resulting orientation behaviour of perceivers. Fundamental to the research on sex attractants was the assumption that they primarily coordinate species recognition. However, especially when they are produced by the less limiting sex (usually males), the evolution of heightened condition dependence might be expected and long-range sex pheromones might, therefore, also inform about a signaller's quality. Here we provide, to our knowledge, the first comprehensive study of the role of a male's long-range pheromone in mate choice that combines chemical analyses, video observations and field experiments with a multifactorial manipulation of males' condition. We show that the emission of the long-distance sex pheromone of the burying beetle, Nicrophorus vespilloides is highly condition-dependent and reliably reflects nutritional state, age, body size and parasite load--key components of an individual's somatic state. Both, the quantity and ratio of the pheromone components were affected but the time invested in pheromone emission was largely unaffected by a male's condition. Moreover, the variation in pheromone emission caused by the variation in condition had a strong effect on the attractiveness of males in the field, with males in better nutritional condition, of older age, larger body size and bearing less parasites being more attractive. That a single pheromone is influenced by so many aspects of the somatic state and causes such variation in a male's attractiveness under field conditions was hitherto unknown and highlights the need to integrate indicator models of sexual selection into pheromone research.

Sex-specific consequences of an induced immune response on reproduction in a moth

BACKGROUND

Immune response induction benefits insects in combatting infection by pathogens. However, organisms have a limited amount of resources available and face the dilemma of partitioning resources between immunity and other life-history traits. Since males and females differ in their life histories, sex-specific resource investment strategies to achieve an optimal immune response following an infection can be expected. We investigated immune response induction of females and males of Heliothis virescens in response to the entomopathogenic bacterium Serratia entomophila, and its effects on mating success and the female sexual signal.

RESULTS

We found that females had higher expression levels of immune-related genes after bacterial challenge than males. However, males maintained a higher baseline expression of immune-related genes than females. The increased investment in immunity of female moths was negatively correlated with mating success and the female sexual signal. Male mating success was unaffected by bacterial challenge.

CONCLUSIONS

Our results show that the sexes differed in their investment strategies: females invested in immune defense after a bacterial challenge, indicating facultative immune deployment, whereas males had higher baseline immunity than females, indicating immune maintenance. Interestingly, these differences in investment were reflected in the mate choice assays. As female moths are the sexual signallers, females need to invest resources in their attractiveness. However, female moths appeared to invest in immunity at the cost of reproductive effort.

The Genetic Basis of Pheromone Evolution in Moths

Moth sexual pheromones are widely studied as a fine-tuned system of intraspecific sexual communication that reinforces interspecific reproductive isolation. However, their evolution poses a dilemma: How can the female pheromone and male preference simultaneously change to create a new pattern of species-specific attraction? Solving this puzzle requires us to identify the genes underlying intraspecific variation in signals and responses and to understand the evolutionary mechanisms responsible for their interspecific divergence. Candidate gene approaches and functional analyses have yielded insights into large families of biosynthetic enzymes and pheromone receptors, although the factors controlling their expression remain largely unexplored. Intra- and interspecific crosses have provided tantalizing evidence of regulatory genes, although, to date, mapping resolution has been insufficient to identify them. Recent advances in high-throughput genome and transcriptome sequencing, together with established techniques, have great potential to help scientists identify the specific genetic changes underlying divergence and resolve the mystery of how moth sexual communication systems evolve.

Evolution of moth sex pheromone composition by a single amino acid substitution in a fatty acid desaturase

For sexual communication, moths primarily use blends of fatty acid derivatives containing one or more double bonds in various positions and configurations, called sex pheromones (SPs). To study the molecular basis of novel SP component (SPC) acquisition, we used the tobacco hornworm (Manduca sexta), which uses a blend of mono-, di-, and uncommon triunsaturated fatty acid (3UFA) derivatives as SP. We identified pheromone-biosynthetic fatty acid desaturases (FADs) MsexD3, MsexD5, and MsexD6 abundantly expressed in the M. sexta female pheromone gland. Their functional characterization and in vivo application of FAD substrates indicated that MsexD3 and MsexD5 biosynthesize 3UFAs via E/Z14 desaturation from diunsaturated fatty acids produced by previously characterized Z11-desaturase/conjugase MsexD2. Site-directed mutagenesis of sequentially highly similar MsexD3 and MsexD2 demonstrated that swapping of a single amino acid in the fatty acyl substrate binding tunnel introduces E/Z14-desaturase specificity to mutated MsexD2. Reconstruction of FAD gene phylogeny indicates that MsexD3 was recruited for biosynthesis of 3UFA SPCs in M. sexta lineage via gene duplication and neofunctionalization, whereas MsexD5 representing an alternative 3UFA-producing FAD has been acquired via activation of a presumably inactive ancestral MsexD5. Our results demonstrate that a change as small as a single amino acid substitution in a FAD enzyme might result in the acquisition of new SP compounds.

Selection and validation of reference genes for qRT-PCR expression analysis of candidate genes involved in olfactory communication in the butterfly Bicyclus anynana

Real-time quantitative reverse transcription PCR (qRT-PCR) is a technique widely used to quantify the transcriptional expression level of candidate genes. qRT-PCR requires the selection of one or several suitable reference genes, whose expression profiles remain stable across conditions, to normalize the qRT-PCR expression profiles of candidate genes. Although several butterfly species (Lepidoptera) have become important models in molecular evolutionary ecology, so far no study aimed at identifying reference genes for accurate data normalization for any butterfly is available. The African bush brown butterfly Bicyclus anynana has drawn considerable attention owing to its suitability as a model for evolutionary ecology, and we here provide a maiden extensive study to identify suitable reference gene in this species. We monitored the expression profile of twelve reference genes: eEF-1α, FK506, UBQL40, RpS8, RpS18, HSP, GAPDH, VATPase, ACT3, TBP, eIF2 and G6PD. We tested the stability of their expression profiles in three different tissues (wings, brains, antennae), two developmental stages (pupal and adult) and two sexes (male and female), all of which were subjected to two food treatments (food stress and control feeding ad libitum). The expression stability and ranking of twelve reference genes was assessed using two algorithm-based methods, NormFinder and geNorm. Both methods identified RpS8 as the best suitable reference gene for expression data normalization. We also showed that the use of two reference genes is sufficient to effectively normalize the qRT-PCR data under varying tissues and experimental conditions that we used in B. anynana. Finally, we tested the effect of choosing reference genes with different stability on the normalization of the transcript abundance of a candidate gene involved in olfactory communication in B. anynana, the Fatty Acyl Reductase 2, and we confirmed that using an unstable reference gene can drastically alter the expression profile of the target candidate genes.

Intraspecific Variation in Female Sex Pheromone of the Codling Moth Cydia pomonella

The codling moth, Cydia pomonella L. (Lepidoptera, Tortricidae), is a major pest of apple, pear and walnut orchards worldwide. This pest is often controlled using the biologically friendly control method known as pheromone-based mating disruption. Mating disruption likely exerts selection on the sexual communication system of codling moth, as male and female moths will persist in their attempt to meet and mate. Surprisingly little is known on the intraspecific variation of sexual communication in this species. We started an investigation to determine the level of individual variation in the female sex pheromone composition of this moth and whether variation among different populations might be correlated with use of mating disruption against those populations. By extracting pheromone glands of individual females from a laboratory population in Canada and from populations from apple orchards in Spain and Italy, we found significant between- and within-population variation. Comparing females that had been exposed to mating disruption, or not, revealed a significant difference in sex pheromone composition for two of the minor components. Overall, the intraspecific variation observed shows the potential for a shift in female sexual signal when selection pressure is high, as is the case with continuous use of mating disruption.