Differential investment in pre- vs. post-copulatory sexual selection reinforces a cross-continental reversal of sexual size dimorphism inSepsis punctum(Diptera: Sepsidae)

Experimental evolution under predation reduces body size in dung flies but courtship displays persist in males (Diptera: Sepsidae)

Exaggerated sexual traits, such as ornaments and courtship displays, are crucial for mate acquisition in many species and are often subject to directional runaway selection. However, in the face of high predation risk, natural selection can result in a reduction of conspicuous precopulatory displays to avoid detection by potential predators. Sexual selection may then favour increased investment in inconspicuous postcopulatory traits. Here, we investigated the transgenerational effects of predation on precopulatory male courtship and postcopulatory sexual traits (testes size, sperm length) in a dung fly, Sepsis punctum (Sepsidae). Behavioural assays prior to selection document a marked decrease in male courtship displays in the presence of a predator, the Asian Ant Mantis (Odontomantis planiceps). However, after ten generations of experimental evolution, flies exhibited a marked increase in courtship, both in the absence and presence of a predator. Additionally, under sustained predation pressure, male and female body size decreased but male postcopulatory traits were not significantly affected. These results suggest that precopulatory courtship can be under strong sexual selection even in the face of predation pressure. Larger flies were more susceptible to predation, and there could be canalisation of postcopulatory traits that are crucial for fertilisation.

Weak sex-specific evolution of locomotor activity of Sepsis punctum (Diptera: Sepsidae) thermal experimental evolution lines

Elevated temperatures are expected to rise beyond what the physiology of many organisms can tolerate. Behavioural responses facilitating microhabitat shifts may mitigate some of this increased thermal selection on physiology, but behaviours are themselves mediated by physiology, and any behavioural response may trade-off against other fitness-related activities. We investigated whether experimental evolution in different thermal regimes (Cold: 15 °C; Hot: 31 °C; Intergenerational fluctuation 15/31 °C; Control: 23 °C) resulted in genetic differentiation of standard locomotor activity in the dung fly Sepsis punctum. We assessed individual locomotor performance, an integral part of most behavioral repertoires, across eight warm temperatures from 24 °C to 45 °C using an automated device. We found no evidence for generalist-specialist trade-offs (i.e. changes in the breadth of the performance curve) for this trait. Instead, at the warmest assay temperatures hot-selected flies showed somewhat higher maximal performance than all other, especially cold-selected flies, overall more so in males than females. Yet, the flies' temperature optimum was not higher than that of the cold-selected flies, as expected under the 'hotter-is-better' hypothesis. Maximal locomotor performance merely weakly increased with body size. These results suggest that thermal performance curves are unlikely to evolve as an entity according to theory, and that locomotor activity is a trait of limited use in revealing thermal adaptation.

Discovering novel reproductive genes in a non-model fly using de novo GridION transcriptomics

Gene discovery has important implications for investigating phenotypic trait evolution, adaptation, and speciation. Male reproductive tissues, such as accessory glands (AGs), are hotspots for recruitment of novel genes that diverge rapidly even among closely related species/populations. These genes synthesize seminal fluid proteins that often affect post-copulatory sexual selection-they can mediate male-male sperm competition, ejaculate-female interactions that modify female remating and even influence reproductive incompatibilities among diverging species/populations. Although de novo transcriptomics has facilitated gene discovery in non-model organisms, reproductive gene discovery is still challenging without a reference database as they are often novel and bear no homology to known proteins. Here, we use reference-free GridION long-read transcriptomics, from Oxford Nanopore Technologies (ONT), to discover novel AG genes and characterize their expression in the widespread dung fly, Sepsis punctum. Despite stark population differences in male reproductive traits (e.g.: Body size, testes size, and sperm length) as well as female re-mating, the male AG genes and their secretions of S. punctum are still unknown. We implement a de novo ONT transcriptome pipeline incorporating quality-filtering and rigorous error-correction procedures, and we evaluate gene sequence and gene expression results against high-quality Illumina short-read data. We discover highly-expressed reproductive genes in AG transcriptomes of S. punctum consisting of 40 high-quality and high-confidence ONT genes that cross-verify against Illumina genes, among which 26 are novel and specific to S. punctum. Novel genes account for an average of 81% of total gene expression and may be functionally relevant in seminal fluid protein production. For instance, 80% of genes encoding secretory proteins account for 74% total gene expression. In addition, median sequence similarities of ONT nucleotide and protein sequences match within-Illumina sequence similarities. Read-count based expression quantification in ONT is congruent with Illumina's Transcript per Million (TPM), both in overall pattern and within functional categories. Rapid genomic innovation followed by recruitment of de novo genes for high expression in S. punctum AG tissue, a pattern observed in other insects, could be a likely mechanism of evolution of these genes. The study also demonstrates the feasibility of adapting ONT transcriptomics for gene discovery in non-model systems.

Male mating success evolves in response to increased levels of male-male competition

Male-biased operational sex ratios can increase male-male competition and can potentially select for both increased pre- and postcopulatory male success. In the present study, using populations of Drosophila melanogaster evolved under male-biased (M) or female-biased (F) sex ratios, we asked whether (a) male mating success can evolve, (b) males are better at mating females that they have coevolved with, (c) males mating success is affected by female mating status, and (d) male mating success is correlated with their courtship effort. We directly competed M and F males for mating with (a) virgin ancestral (common) females, (b) virgin females from the M and F populations, and (c) singly mated females from the M and F populations. We also assessed the courtship frequency of the males when paired with mated M or F females. Our results show that M males, evolving under an increased level of male-male competition, have higher mating success than F males irrespective of the female evolutionary history. However, the difference in mating success is more pronounced if the females had mated before. M males also have a higher courtship frequency than F males, but we did not find any correlation between mating success and courtship frequency.

Does female resistance to mating select for live-fast-die-young strategies in males? A comparative analysis in the genus Drosophila

Female promiscuity is a pervasive selective force on male reproductive traits, and the strength of sexual selection is predicted to influence the trade-off between lifespan and reproduction. In species where sexual selection is intense, males are predicted to invest in sexual strategies that shorten their lifespan, potentially resulting in female-biased sexual dimorphism in longevity. However, comparative analyses have provided contrasting results, potentially due to the use of broad mating system categories or sexual size dimorphism as a proxy for sexual selection. Here, we used female remating rate (i.e. female promiscuity) as a more direct measure of sexual selection strength and conducted a phylogenetic comparative analysis of the relationship between female remating rate and sexual dimorphism in lifespan in 29 species of Drosophila. We did not find strong evidence that female remating rate was correlated with sexual dimorphism in lifespan. However, we found that male and female lifespans are positively correlated among species and that phylogeny and residual variance (i.e. variation in non-phylogenetic factors) are important in determining female remating rate, male and female lifespans separately, and the correlation between male and female lifespan. We suggest that variation in the nature of sexual competition and variation between studies could account for some of the unexplained variation among species in the relation between female remating rate and sexual dimorphism in lifespan.

Effects of body size divergence on male mating tactics in the ground beetle Carabus japonicus

Animal body size is involved in reproduction in various ways. Carabus japonicus exhibits considerable variation in adult body size across geographical locations depending on the larval environment. To investigate the effects of body size divergence on male mating traits, spermatophore deposition and weight, copulation duration, and post-copulatory mounting were observed using male-female pairs from C. japonicus populations with different body sizes. Then, variables with high predictive power on the mating traits were identified from individual characteristics. When the male was slightly smaller than his mate, spermatophore deposition likely succeeded, suggesting that mechanical size-assortative insemination determined male body size. Although male reproductive organ size was positively correlated with male body size, spermatophore weight was not significantly affected by male body size, whereas copulation duration decreased with increasing male body size. Enlarged males, with a high capacity for spermatophore production, could increase paternity by decreasing copulation duration and increasing mating frequency. Such shifts in mating tactics would alter selection pressures of intra- and intersexual interactions (e.g., sperm competition and sexual conflict). Genital dimensions also affected mating traits other than copulatory duration. Thus, ecological heterogeneity has the potential to lead to divergences in sexual traits, such as genital morphology, through body size divergence.

Evolution of multivariate wing allometry in schizophoran flies (Diptera: Schizophora)

The proximate and ultimate mechanisms underlying scaling relationships as well as their evolutionary consequences remain an enigmatic issue in evolutionary biology. Here, I investigate the evolution of wing allometries in the Schizophora, a group of higher Diptera that radiated about 65 million years ago, by studying static allometries in five species using multivariate approaches. Despite the vast ecological diversity observed in contemporary members of the Schizophora and independent evolutionary histories throughout most of the Cenozoic, size-related changes represent a major contributor to overall variation in wing shape, both within and among species. Static allometries differ between species and sexes, yet multivariate allometries are correlated across species, suggesting a shared developmental programme underlying size-dependent phenotypic plasticity. Static allometries within species also correlate with evolutionary divergence across 33 different families (belonging to 11 of 13 superfamilies) of the Schizophora. This again points towards a general developmental, genetic or evolutionary mechanism that canalizes or maintains the covariation between shape and size in spite of rapid ecological and morphological diversification during the Cenozoic. I discuss the putative roles of developmental constraints and natural selection in the evolution of wing allometry in the Schizophora.

Intraspecific mating system evolution and its effect on complex male secondary sexual traits: Does male-male competition increase selection on size or shape?

Sexual selection is generally held responsible for the exceptional diversity in secondary sexual traits in animals. Mating system evolution is therefore expected to profoundly affect the covariation between secondary sexual traits and mating success. Whereas there is such evidence at the interspecific level, data within species remain scarce. We here investigate sexual selection acting on the exaggerated male fore femur and the male wing in the common and widespread dung flies Sepsis punctum and S. neocynipsea (Diptera: Sepsidae). Both species exhibit intraspecific differences in mating systems and variation in sexual size dimorphism (SSD) across continents that correlates with the extent of male-male competition. We predicted that populations subject to increased male-male competition will experience stronger directional selection on the sexually dimorphic male foreleg. Our results suggest that fore femur size, width and shape were indeed positively associated with mating success in populations with male-biased SSD in both species, which was not evident in conspecific populations with female-biased SSD. However, this was also the case for wing size and shape, a trait often assumed to be primarily under natural selection. After correcting for selection on overall body size by accounting for allometric scaling, we found little evidence for independent selection on any of these size or shape traits in legs or wings, irrespective of the mating system. Sexual dimorphism and (foreleg) trait exaggeration is therefore unlikely to be driven by direct precopulatory sexual selection, but more so by selection on overall size or possibly selection on allometric scaling.

Disruptive sexual selection on male body size in the polyphenic black scavenger fly Sepsis thoracica

Sexual selection has 2 main components, female preference and male–male competition, which can lead males to adopt alternative reproductive tactics to optimize their reproductive success. Two traits that significantly influence reproductive success are body size and coloration, as they can facilitate access to females through male contests or as female attractors. We investigated whether, and if so which mechanism of sexual selection contributes to the maintenance, and possibly even the establishment, of 2 almost discrete male morphs in the polyphenic black scavenger fly Sepsis thoracica (Diptera: Sepsidae): small and black, or large and amber. We performed 2 complementary laboratory experiments to evaluate the mating success of the different male morphs and the behaviors (of both males and females) presumably mediating their mating success. We found evidence for intraspecific disruptive sexual selection on male body size that is mediated by male–male interactions, and significant positive directional selection on body size that interacted with (directional) selection on coloration, likely contributing to the origin and/or maintenance of the threshold relationship between the 2 traits in this species. The simultaneous occurrence of disruptive selection and polyphenism in S. thoracica supports the role of sexual selection in the intraspecific diversification of coupled traits (here body size and coloration), which could be a speciation starting point.

Evolutionary Trade-Off between Secondary Sexual Traits and Ejaculates

Recent theoretical models predict that the evolutionary diversification of the weapons and ornaments of pre-mating sexual selection should be influenced by trade-offs with male expenditure on ejaculates. However, the patterns of association between secondary sexual traits and ejaculate expenditure are frequently inconsistent in their support of this prediction. We show why consideration of additional life-history, ecological, and mating-system variables is crucial for the interpretation of associations between secondary sexual traits and ejaculate production. Incorporation of these 'missing variables' provides evidence that interactions between pre- and post-mating sexual selection can underlie broad patterns of diversification in male weapons and ornaments. We call for more experimental and genetic approaches to uncover trade-offs, as well as for studies that consider the costs of mate-searching.

Reproductive senescence: new perspectives in the wild

According to recent empirical studies, reproductive senescence, the decline in reproductive success with increasing age, seems to be nearly ubiquitous in the wild. However, a clear understanding of the evolutionary causes and consequences of reproductive senescence is still lacking and requires new and integrative approaches. After identifying the sequential and complex nature of female reproductive senescence, we show that the relative contributions of physiological decline and alterations in the efficiency of parental care to reproductive senescence remain unknown and need to be assessed in the light of current evolutionary theories of ageing. We demonstrate that, although reproductive senescence is generally studied only from the female viewpoint, age-specific female reproductive success strongly depends on male-female interactions. Thus, a reduction in male fertilization efficiency with increasing age has detrimental consequences for female fitness. Lastly, we call for investigations of the role of environmental conditions on reproductive senescence, which could provide salient insights into the underlying sex-specific mechanisms of reproductive success. We suggest that embracing such directions should allow building new bridges between reproductive senescence and the study of sperm competition, parental care, mate choice and environmental conditions.

Plastic and evolutionary responses to heat stress in a temperate dung fly: negative correlation between basal and induced heat tolerance?

Extreme weather events such as heat waves are becoming more frequent and intense. Populations can cope with elevated heat stress by evolving higher basal heat tolerance (evolutionary response) and/or stronger induced heat tolerance (plastic response). However, there is ongoing debate about whether basal and induced heat tolerance are negatively correlated and whether adaptive potential in heat tolerance is sufficient under ongoing climate warming. To evaluate the evolutionary potential of basal and induced heat tolerance, we performed experimental evolution on a temperate source population of the dung fly Sepsis punctum. Offspring of flies adapted to three thermal selection regimes (Hot, Cold and Reference) were subjected to acute heat stress after having been exposed to either a hot-acclimation or non-acclimation pretreatment. As different traits may respond differently to temperature stress, several physiological and life history traits were assessed. Condition dependence of the response was evaluated by exposing juveniles to different levels of developmental (food restriction/rearing density) stress. Heat knockdown times were highest, whereas acclimation effects were lowest in the Hot selection regime, indicating a negative association between basal and induced heat tolerance. However, survival, adult longevity, fecundity and fertility did not show such a pattern. Acclimation had positive effects in heat-shocked flies, but in the absence of heat stress hot-acclimated flies had reduced life spans relative to non-acclimated ones, thereby revealing a potential cost of acclimation. Moreover, body size positively affected heat tolerance and unstressed individuals were less prone to heat stress than stressed flies, offering support for energetic costs associated with heat tolerance. Overall, our results indicate that heat tolerance of temperate insects can evolve under rising temperatures, but this response could be limited by a negative relationship between basal and induced thermotolerance, and may involve some but not other fitness-related traits.

Experimental evolution for generalists and specialists reveals multivariate genetic constraints on thermal reaction norms

Theory predicts the emergence of generalists in variable environments and antagonistic pleiotropy to favour specialists in constant environments, but empirical data seldom support such generalist-specialist trade-offs. We selected for generalists and specialists in the dung fly Sepsis punctum (Diptera: Sepsidae) under conditions that we predicted would reveal antagonistic pleiotropy and multivariate trade-offs underlying thermal reaction norms for juvenile development. We performed replicated laboratory evolution using four treatments: adaptation at a hot (31 °C) or a cold (15 °C) temperature, or under regimes fluctuating between these temperatures, either within or between generations. After 20 generations, we assessed parental effects and genetic responses of thermal reaction norms for three correlated life-history traits: size at maturity, juvenile growth rate and juvenile survival. We find evidence for antagonistic pleiotropy for performance at hot and cold temperatures, and a temperature-mediated trade-off between juvenile survival and size at maturity, suggesting that trade-offs associated with environmental tolerance can arise via intensified evolutionary compromises between genetically correlated traits. However, despite this antagonistic pleiotropy, we found no support for the evolution of increased thermal tolerance breadth at the expense of reduced maximal performance, suggesting low genetic variance in the generalist-specialist dimension.

Analysing small insect glands with UV-LDI MS: high-resolution spatial analysis reveals the chemical composition and use of the osmeterium secretion in Themira superba (Sepsidae: Diptera)

For many insect species, pheromones are important communication tools, but chemical analysis and experimental study can be technically challenging because they require the detection and handling of complex chemicals in small quantities. One drawback of traditional mass spectrometry methods such as gas chromatography mass spectrometry is that whole-body extractions from one to several hundred individuals are required, with the consequence that intra- and interindividual differences cannot be detected. Here, we used the recently introduced UV-LDI MS (ultraviolet laser desorption/ionization mass spectrometry) to profile the 'osmeterium' of the sepsid fly Themira superba that is located on the edge of the hind tibia of males. Based on analyses of individual legs, we established that the gland produced a secretion that consisted of oxygenated hydrocarbons and putative isoprenoids. The secretion was first detected 24 h after eclosion, and its transfer to the wings of females during mating was demonstrated using UV-LDI MS. We then tested whether the secretion had an anti-aphrodisiac function, but experimental transfer of the secretion to virgin females did not affect mating success or copulation duration. Throughout the study, UV-LDI MS proved invaluable, because it allowed tracking the natural and experimental transfer of small quantities of pheromones to specific body parts of small flies.

Ivermectin sensitivity is an ancient trait affecting all ecdysozoa but shows phylogenetic clustering among sepsid flies

Avermectins are potent and popular veterinary pharmaceuticals used globally to fight parasites of livestock and humans. By disturbing ion channel transport through the membrane, avermectins are effective against endo- and ectoparasitic round and horsehair worms (Nematoida), insects, or ticks (Arthropoda), but not against Plathelminthes, including flatworms (Trematoda) and tapeworms (Cestoda), or segmented worms (Annelida). Unfortunately, excreted avermectins have strong nontarget effects on beneficial arthropods such as the insect community decomposing livestock dung, ultimately impeding this important ecosystem function to the extent that regulators mandate standardized eco-toxicological tests of dung organisms worldwide. We show that the ancient phylogenetic pattern and qualitative mechanism of avermectin sensitivity is conserved and compatible with most recent phylogenomic hypotheses grouping the Nematoida with the Arthropoda as Ecdysozoa (molting animals). At the species level, we demonstrate phylogenetic clustering in ivermectin sensitivities of 23 species of sepsid dung flies (Diptera: Sepsidae). This clustered 500-fold quantitative variation in sensitivity may indicate recent lineage-specific responses to selection, but more likely reflects pre-existing genetic variation with pleiotropic effects on eco-toxicological responses to pollutants. Regardless, our results question the common practice in eco-toxicology of choosing single test species to infer detrimental effects on entire species communities, which should ideally assess a representative taxonomic sample.