Sexual dimorphism and courtship behavior in Drosophila prolongata

Experimental evolution reveals trade-offs between sexual selection and heat tolerance in Drosophila prolongata

Sexual selection promotes traits that enhance mating or fertilization success, but these traits can be very costly under harsh environmental conditions. The extent to which differential investment in costly traits under varying intensities of sexual selection is related to their susceptibility to environmental stress remains unclear. This study explored how experimental evolution under different operational sex ratios (OSRs) shapes traits and reproductive success of male Drosophila prolongata, and how developmental and/or adult heat stress affect the expression of these traits. We found males from even and slightly male-biased OSRs to be larger and display greater reduction in body size under developmental heat stress, suggesting pre-mating sexual selection on body size and condition-dependent thermal sensitivity. These populations also exhibited consistently high mating and fertilization success across temperatures, potentially indicating selection for robust phenotypes with "good genes" that perform well regardless of temperature. Conversely, males from strongly male-biased OSR populations experienced more pronounced decline in sperm competitiveness following exposure to developmental or adult heat stress. These results highlight how environmental stressors differentially impact populations, shaped by varying strengths of pre- and post-mating sexual selection. These observed patterns suggest potential interactions between past selection and the ability to adapt to changing environments.

Regulatory changes in the fatty acid elongase eloF underlie the evolution of sex-specific pheromone profiles in Drosophila prolongata

BACKGROUND

Pheromones play a key role in regulating sexual behavior throughout the animal kingdom. In Drosophila and other insects, many cuticular hydrocarbons (CHCs) are sexually dimorphic, and some are known to perform pheromonal functions. However, the genetic control of sex-specific CHC production is poorly understood outside of the model species D. melanogaster. A recent evolutionary change is found in D. prolongata, which, compared to its closest relatives, shows greatly increased sexual dimorphism in both CHCs and the chemosensory system responsible for their perception. A key transition involves a male-specific increase in the proportion of long-chain CHCs.

RESULTS

Perfuming D. prolongata females with the male-biased long-chain CHCs reduces copulation success, suggesting that these compounds function as sex pheromones. The evolutionary change in CHC profiles correlates with a male-specific increase in the expression of multiple genes involved in CHC biosynthesis, including fatty acid elongases, reductases and other key enzymes. In particular, elongase F, which is responsible for producing female-specific pheromones in D. melanogaster, is strongly upregulated in D. prolongata males compared both to females and to males of the sibling species. Mutations in eloF reduce the amount of long-chain CHCs, resulting in a partial feminization of pheromone profiles in D. prolongata males. Transgenic experiments show that sex-biased expression of eloF is caused in part by a putative transposable element honghaier insertion in its regulatory region.

CONCLUSIONS

These results show that cis-regulatory changes in the eloF gene, along with other changes in the CHC synthesis pathway, contribute to the evolution of sexual communication.

Socio-ecological context modulates the significance of territorial contest competition in Drosophila prolongata

The intensity and direction of sexual selection are intricately linked to the social and ecological context. Both operational sex ratios (OSRs) and population densities can affect the ability of males to monopolize resources and mates, and thus the form and intensity of sexual selection on them. Here, we studied how the mating system of the promiscuous and strongly sexually dimorphic fruit fly Drosophila prolongata responds to changes in the OSR and population density. We recorded groups of flies over five days and quantified territory occupancy, mating success (MS) and competitive fertilization success. Although sexual selection was stronger under male-biased than even OSRs but unrelated to density, realized selection on morphological traits was higher under even OSRs and increased with density. Larger and more territorial males achieved both higher MS and competitive fertilization success, but only under even OSRs. Our combined results also support a shift in the mating system from territorial contest competition to scramble competition under male-biased OSRs and potentially at low density, where there was no clear contribution of the measured traits to reproductive success. Our study emphasizes the limitations of traditional selection metrics and the role of the socio-ecological context in predicting adaptation to a changing environment.

Regulatory Changes in the Fatty Acid Elongase eloF Underlie the Evolution of Sex-specific Pheromone Profiles in Drosophila prolongata

Pheromones play a key role in regulating sexual behavior throughout the animal kingdom. In Drosophila and other insects, many cuticular hydrocarbons (CHCs) are sexually dimorphic, and some are known to perform pheromonal functions. However, the genetic control of sex-specific CHC production is not understood outside of the model species D. melanogaster. A recent evolutionary change is found in D. prolongata, which, compared to its closest relatives, shows greatly increased sexual dimorphism in both CHCs and the chemosensory system responsible for their perception. A key transition involves a male-specific increase in the proportion of long-chain CHCs. Perfuming D. prolongata females with the male-biased CHCs reduces copulation success, suggesting that these compounds function as sex pheromones. The evolutionary change in CHC profiles correlates with a male-specific increase in the expression of multiple genes involved in CHC biosynthesis, including fatty acid elongases and reductases and other key enzymes. In particular, elongase F, which is responsible for producing female-specific pheromones in D. melanogaster, is strongly upregulated in D. prolongata males compared both to females and to males of the sibling species. Induced mutations in eloF reduce the amount of long-chain CHCs, resulting in a partial feminization of pheromone profiles in D. prolongata males while having minimal effect in females. Transgenic experiments show that sex-biased expression of eloF is caused in part by a putative transposable element insertion in its regulatory region. These results reveal one of the genetic mechanisms responsible for a recent evolutionary change in sexual communication.

Highly contiguous genome assembly of Drosophila prolongata-a model for evolution of sexual dimorphism and male-specific innovations

Drosophila prolongata is a member of the melanogaster species group and rhopaloa subgroup native to the subtropical highlands of Southeast Asia. This species exhibits an array of recently evolved male-specific morphological, physiological, and behavioral traits that distinguish it from its closest relatives, making it an attractive model for studying the evolution of sexual dimorphism and testing theories of sexual selection. The lack of genomic resources has impeded the dissection of the molecular basis of sex-specific development and behavior in this species. To address this, we assembled the genome of D. prolongata using long-read sequencing and Hi-C scaffolding, resulting in a highly complete and contiguous (scaffold N50 2.2 Mb) genome assembly of 220 Mb. The repetitive content of the genome is 24.6%, the plurality of which are long terminal repeats retrotransposons (33.2%). Annotations based on RNA-seq data and homology to related species revealed a total of 19,330 genes, of which 16,170 are protein-coding. The assembly includes 98.5% of Diptera BUSCO genes, including 93.8% present as a single copy. Despite some likely regional duplications, the completeness of this genome suggests that it can be readily used for gene expression, genome-wide association studies (GWAS), and other genomic analyses.

Highly Contiguous Genome Assembly of Drosophila prolongata - a Model for Evolution of Sexual Dimorphism and Male-specific Innovations

Drosophila prolongata is a member of the melanogaster species group and rhopaloa subgroup native to the subtropical highlands of southeast Asia. This species exhibits an array of recently evolved male-specific morphological, physiological, and behavioral traits that distinguish it from its closest relatives, making it an attractive model for studying the evolution of sexual dimorphism and testing theories of sexual selection. The lack of genomic resources has impeded the dissection of the molecular basis of sex-specific development and behavior in this species. To address this, we assembled the genome of D. prolongata using long-read sequencing and Hi-C scaffolding, resulting in a highly complete and contiguous (scaffold N50 2.2Mb) genome assembly of 220Mb. The repetitive content of the genome is 24.6%, the plurality of which are LTR retrotransposons (33.2%). Annotations based on RNA-seq data and homology to related species revealed a total of 19,330 genes, of which 16,170 are protein-coding. The assembly includes 98.5% of Diptera BUSCO genes, including 93.8% present as a single copy. Despite some likely regional duplications, the completeness of this genome suggests that it can be readily used for gene expression, GWAS, and other genomic analyses.

Reproductive advantage of the winners of male-male competition in Drosophila prolongata

In the resource-defence mating system, where males compete for limited resources to acquire females, male traits associated with fighting ability are selected, leading to the evolution of sexual dimorphism. However, the evolution of sexual dimorphism is also driven by other mechanisms, such as female selection. Therefore, to elucidate the evolutionary mechanisms of male traits, it is necessary to clarify their contribution to fitness through male-male competition. In this regard, it is surprising that numerous studies on sexually dimorphic species have assumed the resource-defence mating system without directly examining the relationship between resource-defending behaviour and mating success. In a sexually dimorphic fruit fly, Drosophila prolongata, the presence of the resource-defence mating system has been suggested, but technical difficulties had prevented spatial quantification of the resource-defending behaviour. In this study, by using an automated behaviour analysis tool previously developed, we located the occurrence of male-male competition and courtship to investigate their relationship in D. prolongata, considering the position of food resources. We found that the male-male competition led to the exclusive occupation of resources, increasing the courtship opportunities of the resource holders. These results illustrate the importance of resource-defending for reproductive success in D. prolongata.

Fight outcome influences male mating success in Drosophila prolongata

The outcomes of preceding fights can influence the probability of winning a subsequent fight, known as the winner/loser effect. However, we know relatively little about how the experience of a preceding fight influences subsequent mating success. Here, we investigated the influence of preceding fight outcomes on subsequent mating behavior in a fruit fly Drosophila prolongata. Subordinate males mated less in two-choice mating assays, showing that the fight outcome predicts male mating success in this species. This tendency remained in a no-choice mating assay where direct interaction between the dominant and subordinate males was eliminated, suggesting that the mating disadvantage of the subordinate males was dependent on the experience of the previous fight rather than the direct interference by the dominant male. When a no-choice mating assay was performed before the fight, the prospective subordinate males mated at the same rate as the dominant males, confirming that the intrinsic male qualities in fighting and mating performances were independent of each other in our experiments. These results indicated that the experience-dependent changes in the subordinate males led to the reduced mating success.

Strongly sexually dimorphic forelegs are not more condition-dependent than less dimorphic traits in Drosophila prolongata

Directional sexual selection drives the evolution of traits that are most closely linked to reproductive success, giving rise to trait exaggeration and sexual dimorphism. Exaggerated structures are often costly and, therefore, thought to be expressed in a condition-dependent manner. Sexual selection theory thus predicts a direct link between directional sexual selection, sexual dimorphism, and sex-specific condition dependence. However, only a handful of studies investigate the relationship between sexual dimorphism and condition dependence. Using 21 genetic lines of Drosophila prolongata, we here compared the degree of sexual dimorphism and sex-specific condition dependence, measured as allometric slopes, in sexually selected and non-sexual traits. Our data revealed male-biased sexual dimorphism in all traits examined, most prominently in the sexually selected forelegs. However, there was no relationship between the degree of sex-specific condition dependence and sexual dimorphism across traits and genetic lines. Our results contradict theoretical predictions and highlight the importance of understanding the role of exaggerated traits in the context of both sexual and natural selection.

An Indirect Cost of Male-Male Aggression Arising from Female Response

Animal behavior is often polymorphic between individuals within a population. A cost/benefit balance of a particular behavioral pattern may be influenced by social interaction with other individuals with different behavioral patterns. Males of a fruitfly, Drosophila prolongata, show genetically defined polymorphism in aggressiveness and boldness against rival males. Males of the H strain are highly aggressive, and their fights tend to escalate into boxing, the highest level of aggressive interaction. H males are also bold against sneaker males and do not hesitate to perform leg vibration (LV), a courtship behavior that is vulnerable to interception of the female by surrounding rival males. In contrast, males of the L strain rarely engage in boxing and do not perform LV in the presence of rival males. We examined their mating success in small experimental populations. The mating success of L males was higher in a pure L population than in a mixed population with H males, whereas that of H males was higher in a mixed population than in a pure H population. Notably, this 'cost of aggression' in a pure H population seemed not directly derived from the male-to-male interaction but was imposed by the female's response of escaping from fighting males, compromising the benefit of the resource monopolization as territory.

Sex-specific evolution of a Drosophila sensory system via interacting cis- and trans-regulatory changes

The evolution of gene expression via cis-regulatory changes is well established as a major driver of phenotypic evolution. However, relatively little is known about the influence of enhancer architecture and intergenic interactions on regulatory evolution. We address this question by examining chemosensory system evolution in Drosophila. Drosophila prolongata males show a massively increased number of chemosensory bristles compared to females and males of sibling species. This increase is driven by sex-specific transformation of ancestrally mechanosensory organs. Consistent with this phenotype, the Pox neuro transcription factor (Poxn), which specifies chemosensory bristle identity, shows expanded expression in D. prolongata males. Poxn expression is controlled by nonadditive interactions among widely dispersed enhancers. Although some D. prolongata Poxn enhancers show increased activity, the additive component of this increase is slight, suggesting that most changes in Poxn expression are due to epistatic interactions between Poxn enhancers and trans-regulatory factors. Indeed, the expansion of D. prolongata Poxn enhancer activity is only observed in cells that express doublesex (dsx), the gene that controls sexual differentiation in Drosophila and also shows increased expression in D. prolongata males due to cis-regulatory changes. Although expanded dsx expression may contribute to increased activity of D. prolongata Poxn enhancers, this interaction is not sufficient to explain the full expansion of Poxn expression, suggesting that cis-trans interactions between Poxn, dsx, and additional unknown genes are necessary to produce the derived D. prolongata phenotype. Overall, our results demonstrate the importance of epistatic gene interactions for evolution, particularly when pivotal genes have complex regulatory architecture.

Condition- and context-dependent alternative reproductive tactic in Drosophila prolongata

Species with intense male–male competition for access to females often show alternative reproductive tactics (ARTs) where males of lower competitive ability adopt a sneaky behavior to gain access to mates. These ARTs are usually associated with intrasexual dimorphisms, in that males with distinct morphologies show different tactics. In some cases, however, males adopt different tactics without being dimorphic. Male Drosophila prolongata exhibit continuous variation in body size and shape, with enlarged forelegs that they use in male-male contests and in courtship, including stimulation of the female’s abdomen. During this “leg vibration,” however, nearby males can intercept the courted female and mate without their own courtship. Here, we studied the causes and consequences of these different mating tactics in competitive mating trials between males varying in their size and shape. We found that male mating tactics were condition-dependent. Whereas large, high-condition males were more likely to show leg vibration in their courtship, smaller, lower-condition males were more likely to intercept. However, the number of offspring produced was independent of male condition and reproductive tactic. We discuss possible scenarios for the evolution and maintenance of the ARTs and some future directions for the study of ARTs in this species and in general.

Wing-waving behaviors are used for conspecific display in the Japanese scorpionfly, Panorpa japonica

Species of scorpionfly (Mecoptera) in the family Panorpidae perform wing-waving behaviors, whereby they rotate their front and rear wings at the same time. Previous studies have suggested that a male, which carries food for use as nuptial gifts for females, performs the wing-waving behavior when the male gives the gift to a female or competes with other males. However, when and how the wing-waving behavior occurs during a series of nuptial giftings and male–male competitions have not been investigated. Therefore, we here observed the role of wing-waving behavior during the processes of giving nuptial gifts and male–male competition in the Japanese scorpionfly Panorpa japonica in the laboratory and field. Unlike previous studies, only males performed wing-waving behavior toward females, while females did not exhibit the behavior in the wild. Also, males always performed wing-waving behavior before male–male competition. After a male–male competition, winner males continued wing-waving behavior, but loser males never performed the behavior against the winner male. A comparison of wing-waving behaviors before competitions between winner and loser males showed that the frequencies of wing-waving behaviors were higher in winner than in loser males. The present results suggest that the wing-waving behavior functions in the inter-sexual and intra-sexual selection in P. japonica. Digital video images related to the article are available at http://www.momo-p.com/showdetail-e.php?movieid=momo210513pj01a and http://www.momo-p.com/showdetail-e.php?movieid=momo210513pj02a and http://www.momo-p.com/showdetail-e.php?movieid=momo210513pj03a.

Intra- Versus Inter-Sexual Selection on Sexually Dimorphic Traits in Drosophila prolongata

Sexual dimorphism, such as sexual size dimorphism (SSD) and sexually dimorphic exaggerated traits, often evolves via sexual selection. In many species, evolution of sexual dimorphism is thought to be driven by either of the two forms of sexual selection: intra- and inter-sexual selection. In some species, however, intra- and inter-sexual selection act simultaneously on the same sexually dimorphic trait. Therefore, it is important to consider the effects of both forms of sexual selection to fully understand the evolution of sexual dimorphism. Drosophila prolongata is a fruit fly that shows male-biased SSD and has enlarged forelegs only in males. In this study, the relationship between body size parameters and aggression/mating behavior was examined. Our results showed that aggressive behavior was influenced by body weight and foreleg size, whereas mating success was not influenced by any size parameters, suggesting that intra-sexual selection is the primary mechanism that maintains the sexual dimorphism in the current D. prolongata population.

Evolution of wing pigmentation in Drosophila: Diversity, physiological regulation, and cis-regulatory evolution

Fruit flies (Drosophila and its close relatives, or “drosophilids”) are a group that includes an important model organism, Drosophila melanogaster, and also very diverse species distributed worldwide. Many of these species have black or brown pigmentation patterns on their wings, and have been used as material for evo‐devo research. Pigmentation patterns are thought to have evolved rapidly compared with body plans or body shapes; hence they are advantageous model systems for studying evolutionary gains of traits and parallel evolution. Various groups of drosophilids, including genus Idiomyia (Hawaiian Drosophila), have a variety of pigmentations, ranging from simple black pigmentations around crossveins to a single antero‐distal spot and a more complex mottled pattern. Pigmentation patterns are sometimes obviously used for sexual displays; however, in some cases they may have other functions. The process of wing formation in Drosophila, the general mechanism of pigmentation formation, and the transport of substances necessary for pigmentation, including melanin precursors, through wing veins are summarized here. Lastly, the evolution of the expression of genes regulating pigmentation patterns, the role of cis‐regulatory regions, and the conditions required for the evolutionary emergence of pigmentation patterns are discussed. Future prospects for research on the evolution of wing pigmentation pattern formation in drosophilids are presented, particularly from the point of view of how they compare with other studies of the evolution of new traits.

Evolution of sexually dimorphic pheromone profiles coincides with increased number of male-specific chemosensory organs in Drosophila prolongata

Binary communication systems that involve sex-specific signaling and sex-specific signal perception play a key role in sexual selection and in the evolution of sexually dimorphic traits. The driving forces and genetic changes underlying such traits can be investigated in systems where sex-specific signaling and perception have emerged recently and show evidence of potential coevolution. A promising model is found in Drosophila prolongata, which exhibits a species-specific increase in the number of male chemosensory bristles. We show that this transition coincides with recent evolutionary changes in cuticular hydrocarbon (CHC) profiles. Long-chain CHCs that are sexually monomorphic in the closest relatives of D. prolongata (D. rhopaloa, D. carrolli, D. kurseongensis, and D. fuyamai) are strongly male-biased in this species. We also identify an intraspecific female-limited polymorphism, where some females have male-like CHC profiles. Both the origin of sexually dimorphic CHC profiles and the female-limited polymorphism in D. prolongata involve changes in the relative amounts of three mono-alkene homologs, 9-tricosene, 9-pentacosene, and 9-heptacosene, all of which share a common biosynthetic origin and point to a potentially simple genetic change underlying these traits. Our results suggest that pheromone synthesis may have coevolved with chemosensory perception and open the way for reconstructing the origin of sexual dimorphism in this communication system.

Sex-specific evolution of relative leg size in Drosophila prolongata results from changes in the intersegmental coordination of tissue growth

Evolution of relative organ size is the most prolific source of morphological diversity, yet the underlying molecular mechanisms that modify growth control are largely unknown. Models where organ proportions have undergone recent evolutionary changes hold the greatest promise for understanding this process. Uniquely among Drosophila species, Drosophila prolongata displays a dramatic, male-specific increase in the size of its forelegs relative to other legs. By comparing leg development between males and females of D. prolongata and its closest relative Drosophila carrolli, we show that the exaggerated male forelegs are produced by a sex- and segment-specific increase in mitosis during the final larval instar. Intersegmental compensatory control, where smaller leg primordia grow at a faster rate, is observed in both species and sexes. However, the equilibrium growth rates that determine the final relative proportion between the first and second legs have shifted in male D. prolongata compared both to conspecific females and to D. carrolli. We suggest that the observed developmental changes that produce new adult proportions reflect an interplay between conserved growth coordination mechanisms and evolving organ-specific growth targets.

Artificial selection on walking distance suggests a mobility-sperm competitiveness trade-off

Securing matings is a key determinant of fitness, and in many species, males are the sex that engages in mate searching. Searching for mates is often associated with increased mobility. This elevated investment in movement is predicted to trade-off with sperm competitiveness, but few studies have directly tested whether this trade-off occurs. Here, we assessed whether artificial selection on mobility affected sperm competitiveness and mating behavior, and if increased mobility was due to increased leg length in red flour beetles (Tribolium castaneum). We found that, in general, males selected for decreased mobility copulated for longer, stimulated females more during mating, and tended to be better sperm competitors. Surprisingly, they also had longer legs. However, how well males performed in sperm competition depended on females. Males with reduced mobility always copulated for longer than males with high mobility, but this only translated into greater fertilization success in females from control populations and not the selection populations (i.e. treatment females). These results are consistent with a mate-searching/mating-duration trade-off and broadly support a trade-off between mobility and sperm competitiveness.

Chemosensory sensilla of the Drosophila wing express a candidate ionotropic pheromone receptor

The Drosophila wing was proposed to be a taste organ more than 35 years ago, but there has been remarkably little study of its role in chemoreception. We carry out a differential RNA-seq analysis of a row of sensilla on the anterior wing margin and find expression of many genes associated with pheromone and chemical perception. To ask whether these sensilla might receive pheromonal input, we devised a dye-transfer paradigm and found that large, hydrophobic molecules comparable to pheromones can be transferred from one fly to the wing margin of another. One gene, Ionotropic receptor (IR)52a, is coexpressed in neurons of these sensilla with fruitless, a marker of sexual circuitry; IR52a is also expressed in legs. Mutation of IR52a and optogenetic silencing of IR52a⁺ neurons decrease levels of male sexual behavior. Optogenetic activation of IR52a⁺ neurons induces males to show courtship toward other males and, remarkably, toward females of another species. Surprisingly, IR52a is also required in females for normal sexual behavior. Optogenetic activation of IR52a⁺ neurons in mated females induces copulation, which normally occurs at very low levels. Unlike other chemoreceptors that act in males to inhibit male–male interactions and promote male–female interactions, IR52a acts in both males and females, and can promote male–male as well as male–female interactions. Moreover, IR52a⁺ neurons can override the circuitry that normally suppresses sexual behavior toward unproductive targets. Circuit mapping and Ca²⁺ imaging using the trans-Tango system reveals second-order projections of IR52a⁺ neurons in the subesophageal zone (SEZ), some of which are sexually dimorphic. Optogenetic activation of IR52a⁺ neurons in the wing activates second-order projections in the SEZ. Taken together, this study provides a molecular description of the chemosensory sensilla of a greatly understudied taste organ and defines a gene that regulates the sexual circuitry of the fly.

piggyBac- and phiC31 integrase-mediated transgenesis in Drosophila prolongata

The development of transgenesis systems in non-model organisms provides a powerful tool for molecular analysis and contributes to the understanding of phenomena that are not observed in model organisms. Drosophila prolongata is a fruit fly that has unique morphology and behavior not found in other Drosophila species including D. melanogaster. In this study, we developed a phiC31 integrase-mediated transgenesis system for D. prolongata. First, using piggyBac-mediated transgenesis, 37 homozygous attP strains were established. These strains were further transformed with the nosP-Cas9 vector, which was originally designed for phiC31-mediated transgenesis in D. melanogaster. The transformation rate varied from 0% to 3.4%. Nine strains with a high transformation rate of above 2.0% were established, which will serve as host strains in future transformation experiments in D. prolongata. Our results demonstrate that genetic tools developed for D. melanogaster are applicable to D. prolongata with minimal modifications.

Triggers of the Postural Display of Courtship in Drosophila persimilis Flies

D. persimilis courtship shows some flexibility and courting males sometimes perform an elaborate postural display in addition to the standard courtship behaviours shared by most Drosophila species. This postural display includes the acrobatic contortion and tremulation of their abdomen, accompanied by the generation of substrate-borne vibrations, and they proffer a nutritional droplet to the female. Here, we use courtship and choice assays to ask what triggers this display and what advantages males may gain from it during courtship. In pair assays, we found no differences in the courtship duration and copulation success between displaying and non-displaying males. In trio assays, however, the female always mated with the male who performed the display. To investigate what promotes the male display, we varied the level of receptivity of the female and studied the impact of a second male. We found that rejection by the female does not induce the male to display, contrary to what was previously suggested. We present evidence that the male display is in fact promoted by the presence of an attentive and sexually receptive female and the absence of male competition, with the greatest exhibition rate obtained if the courted female is starved. These findings provide valuable information about the social ecology of flies, and how internal and external cues influence sexual behaviours and mate choice.

Effect of social condition on behavioral development during early adult phase in Drosophila prolongata

Behavioral changes during early development provide useful insights into the internal mechanisms that generate complex behavior expressed by mature individuals. At the same time, social conditions during early adult phase can influence behavior in later stages of development even in holometabolous insects. In this study, age-dependent changes in courtship behavior and the effect of social conditions were examined in a fruit fly, Drosophila prolongata. Younger males showed lower mating activity and simpler courtship behavior. Mating activity reached a maximum level by 5 days after eclosion, whereas expression of complex courtship behavior was not yet fully developed at that time, suggesting that they are controlled by different mechanisms. When two males were maintained in the same vial, not only mating activity but also courtship complexity was reduced, demonstrating for the first time that preceding social experience, not current social conditions, influenced the complexity of male courtship. The effect of social experience was completely erased by 1 day of isolation, however, showing that social experience did not suppress or promote behavioral development itself. Rather, these results suggest that the observed effect of social experience was a plastic response of males that reduced investment in courtship effort by anticipating increased male–male competition.

Comparative analysis of the brain transcriptome in a hyper-aggressive fruit fly, Drosophila prolongata

Aggressive behavior is observed in many animals, but its intensity differs between species. In a model animal of genetics, Drosophila melanogaster, genetic basis of aggressive behavior has been studied intensively, including transcriptome analyses to identify genes whose expression level was associated with intra-species variation in aggressiveness. However, whether these genes are also involved in the evolution of aggressiveness among different species has not been examined. In this study, we performed de novo transcriptome analysis in the brain of Drosophila prolongata to identify genes associated with the evolution of aggressiveness. Males of D. prolongata were hyper-aggressive compared with closely related species. Comparison of the brain transcriptomes identified 21 differentially expressed genes in males of D. prolongata. They did not overlap with the list of aggression-related genes identified in D. melanogaster, suggesting that genes involved in the evolution of aggressiveness were independent of those associated with the intra-species variation in aggressiveness in Drosophila. Although females of D. prolongata were not aggressive as the males, expression levels of the 21 genes identified in this study were more similar between sexes than between species.

Quantitative analysis of visually induced courtship elements in Drosophila subobscura

We developed a new paradigm for quantitative analysis of courtship behavior in flies, Fly Motion-detector with an Actuator-Coupled Stimulator (FlyMacs), in which the stimulation of a fly with a moving visual target and recording of induced behaviors are automated under computer control. We employ FlyMacs for the identification of motion features that trigger specific courtship elements in Drosophila subobscura, whose mating is suggested to be strongly vision dependent. A female abdomen attached to the actuator, when moved in an appropriate pattern, evokes in the test male tapping-like foreleg motions, midleg swing and proboscis extension, which are considered to be elementary actions in male courtship behavior. Tapping is primarily induced when the target is moving, whereas midleg swing and proboscis extension are most frequently observed after the target stops moving. In contrast to midleg swing, which tends to occur immediately after target cessation (∼3000 ms), the incidence of proboscis extension gradually increases with time after target cessation, reaching a plateau at 3000 ms. The results suggest that tapping, midleg swing and proboscis extension are each induced by different movement features of the visual target. These findings do not support the view that a single key stimulus induces the entire courtship ritual. Rather, courtship behaviors in D. subobscura are correlated with movement and position of the target, which suggests that D. subobscura uses sensory information to pattern its courtship.

Social context-dependent modification of courtship behaviour in Drosophila prolongata

Induction of alternative mating tactics by surrounding conditions, such as the presence of conspecific males, is observed in many animal species. Satellite behaviour is a remarkable example in which parasitic males exploit the reproductive investment by other males. Despite the abundance of parasitic mating tactics, however, few examples are known in which males alter courtship behaviour as a counter tactic against parasitic rivals. The fruit fly Drosophila prolongata shows prominent sexual dimorphism in the forelegs. When courting females, males of D. prolongata perform 'leg vibration', in which a male vibrates the female's body with his enlarged forelegs. In this study, we found that leg vibration increased female receptivity, but it also raised a risk of interception of the female by rival males. Consequently, in the presence of rivals, males of D. prolongata shifted their courtship behaviour from leg vibration to 'rubbing', which was less vulnerable to interference by rival males. These results demonstrated that the males of D. prolongata adjust their courtship behaviour to circumvent the social context-dependent risk of leg vibration.