Mating success in fruit flies: courtship interference versus female choice

Beyond simple vs. complex: exploring the nuanced and unexpected effects of spatial environmental complexity on mating patterns and female fecundity

The features of the physical environment set the stage upon which sexual selection operates, and consequently can have a significant impact on variation in realized individual fitness, and influence a population's evolutionary trajectory. This phenomenon has been explored empirically in several studies using fruit flies (Drosophila melanogaster) which have found that changing the spatial complexity of the mating environment influenced male-female interaction dynamics, (re)mating rates, and realized female fecundities. However, these studies did not explore mating patterns, which can dramatically alter the genetic composition of the next generation, and frequently only compared a single, small "simple" environment to a single larger "complex" environment. While these studies have shown that broadly changing the characteristics of the environment can have big effects on reproductive dynamics, the plasticity of this outcome to more subtle changes has not been extensively explored. Our study set out to compare patterns of mating and courtship between large- and small-bodied males and females, and female fecundities in both a simple environment and 2 distinctly different spatially complex environments. We found that realized offspring production patterns differed dramatically between all 3 environments, indicating that the effects of increasing spatial complexity on mating outcomes are sensitive to the specific type of environmental complexity. Furthermore, we observed female fecundities were higher for flies in both complex environments compared those in the simple environment, supporting its role as a mediator of sexual conflict. Together, these results show that the union of gametes within a population can be greatly influenced by the specific spatial features of the environment and that while some outcomes of increased environmental complexity are likely generalizable, other phenomena such as mating patterns and courtship rates may vary from one complex environment to another.

Sex ratio affects sexual selection against mutant alleles in a locus-specific way

Higher male:female operational sex ratio (OSR) is often assumed to lead to stronger sexual selection on males. Yet, this premise has been directly tested by very few studies, with mixed outcomes. We investigated how OSR affects the strength of sexual selection against two deleterious alleles, a natural ebony mutant and a transgenic GFP insertion, in Drosophila melanogaster. To this end, we estimated the relative paternity share of homozygous mutant males competing against wild-type males under different OSRs (1:2, 1:1, 2:1). We also manipulated the mating pool density (18, 36, or 54 individuals) and assessed paternity over three consecutive days, during which the nature of sexual interaction changed. The strength of sexual selection against the ebony mutant increased with OSR, became weaker after the first day, and was little affected by density. In contrast, sexual selection against the GFP transgene was markedly affected by density: at the highest density, it increased with OSR, but at lower densities, it was strongest at 1:1 OSR, remaining strong throughout the experiment. Thus, while OSR can strongly affect the strength of sexual selection against "bad genes," it does not necessarily increase monotonically with male:female OSR. Furthermore, the pattern of relationship between OSR and the strength of sexual selection can be locus-specific, likely reflecting the specific phenotypic effects of the mutation.

Mate Selection on Anastrepha curvicauda: Effect of Weight, Age, and Virginity

Sexual selection determines the evolution of the species by favoring some attributes that confer a reproductive advantage to those individuals with those attributes. Tephritidae flies do not always select the same traits when looking for a mating partner. Some aspects of the mating system of Anastrepha curvicauda are known; nevertheless, there is no information on the effect of age, size, and virginity when selecting a mating partner. We set up a series of experiments where a selector (male or female) may select between (a) an old or young partner, (b) a small or large partner, and (c) a virgin or mated partner. Males of A. curvicauda significantly preferred large, young, and virgin females, while females showed no preference for high- or low-quality males. The females' non-preference for a particular male is discussed in the light of their mating system.

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.

Effects of mitochondrial haplotype on pre-copulatory mating success in male fruit flies (Drosophila melanogaster)

While mitochondria have long been understood to be critical to cellular function, questions remain as to how genetic variation within mitochondria may underlie variation in general metrics of organismal function. To date, studies investigating links between mitochondrial genotype and phenotype have largely focused on differences in expression of genes and physiological and life-history traits across haplotypes. Mating display behaviours may also be sensitive to mitochondrial functionality and so may also be affected by sequence variation in mitochondrial DNA, with consequences for sexual selection and fitness. Here, we tested whether the pre-copulatory mating success of male fruit flies (Drosophila melanogaster) varies across six different mitochondrial haplotypes expressed alongside a common nuclear genetic background. We found a significant effect of mitochondrial haplotype on our measure of competitive mating success, driven largely by the relatively poor performance of males with one particular haplotype. This haplotype, termed 'Brownsville', has previously been shown to have complex and sex-specific effects, most notably including depressed fertility in males but not females. Our study extends this disproportionate effect on male reproductive success to pre-copulatory aspects of reproduction. Our results demonstrate that mutations in mitochondrial DNA can plausibly affect pre-copulatory mating success, with implications for future study into the subcellular underpinnings of such behaviours and the information they may communicate.

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.

Evolution of sociability by artificial selection

There has been extensive research on the ecology and evolution of social life in animals that live in groups. Less attention, however, has been devoted to apparently solitary species, even though recent research indicates that they also possess complex social behaviors. To address this knowledge gap, we artificially selected on sociability, defined as the tendency to engage in nonaggressive activities with others, in fruit flies. Our goal was to quantify the factors that determine the level of sociability and the traits correlated with this feature. After 25 generations of selection, the high-sociability lineages showed sociability scores about 50% higher than did the low-sociability lineages. Experiments using the evolved lineages indicated that there were no differences in mating success between flies from the low and high lineages. Both males and females from the low lineages, however, were more aggressive than males and females from the high lineages. Finally, the evolved lineages maintained their sociability scores after 10 generations of relaxed selection, suggesting no costs to maintaining low and high sociability, at least under our settings. Sociability is a complex trait, which we currently assess through genomic work on the evolved lineages.

A shift to shorter cuticular hydrocarbons accompanies sexual isolation among Drosophila americana group populations

Because sensory signals often evolve rapidly, they could be instrumental in the emergence of reproductive isolation between species. However, pinpointing their specific contribution to isolating barriers, and the mechanisms underlying their divergence, remains challenging. Here, we demonstrate sexual isolation due to divergence in chemical signals between two populations of Drosophila americana (SC and NE) and one population of D. novamexicana, and dissect its underlying phenotypic and genetic mechanisms. Mating trials revealed strong sexual isolation between Drosophila novamexicana males and SC Drosophila americana females, as well as more moderate bi-directional isolation between D. americana populations. Mating behavior data indicate SC D. americana males have the highest courtship efficiency and, unlike males of the other populations, are accepted by females of all species. Quantification of cuticular hydrocarbon (CHC) profiles-chemosensory signals that are used for species recognition and mate finding in Drosophila-shows that the SC D. americana population differs from the other populations primarily on the basis of compound carbon chain-length. Moreover, manipulation of male CHC composition via heterospecific perfuming-specifically perfuming D. novamexicana males with SC D. americana males-abolishes their sexual isolation from these D. americana females. Of a set of candidates, a single gene-elongase CG17821-had patterns of gene expression consistent with a role in CHC differences between species. Sequence comparisons indicate D. novamexicana and our Nebraska (NE) D. americana population share a derived CG17821 truncation mutation that could also contribute to their shared "short" CHC phenotype. Together, these data suggest an evolutionary model for the origin and spread of this allele and its consequences for CHC divergence and sexual isolation in this group.

Spatial heterogeneity in resources alters selective dynamics in Drosophila melanogaster

Environmental features can alter the behaviors and phenotypes of organisms, influencing the dynamics of natural and sexual selection. Experimental environmental manipulation, particularly when conducted in experiments where the dynamics of the purging of deleterious alleles are compared, has demonstrated both direct and indirect effects on the strength and direction of selection. However, many of these studies are conducted with fairly simplistic environments, where it is not always clear how or why particular forms of spatial heterogeneity influence behavior or selection. Using Drosophila melanogaster, we tested three different spatial environments designed to determine if spatial constraint of critical resources influences the efficiency of natural and sexual selection. We conducted two allele purging experiments to (1) assess effects of these spatial treatments on selective dynamics of six recessive mutations, and (2) determine how these dynamics changed when sexual selection was relaxed and spatial area reduced for two of the mutants. Allele purging dynamics depended on spatial environment, however the patterns of purging rates between the environments differed across distinct deleterious mutations. We also tested two of the mutant alleles, and demonstrate sexual selection increased the purging rate.

Condition-dependent interaction between mating success and competitive fertilization success in Drosophila melanogaster

Dietary restriction during development can affect adult body size and condition. In many species, larger (high-condition) males gain higher mating success through male-male competition and female choice, and female condition can affect the extent of both female mate choice and male investment in courtship or ejaculates. However, few studies have examined the joint effects and interplay of male and female condition during both the pre- and the postcopulatory phases of sexual selection. We therefore manipulated the larval diet of male and female Drosophila melanogaster to study how body size variation in both sexes biases competitive outcomes at different reproductive stages, from mating to paternity. We did not find a difference in mate preference or mating latency between females of different conditions, nor any interaction between male and female conditions. However, large males were more successful in gaining matings, but only when in direct competition, whereas mating latencies were shorter for low-condition males in noncompetitive settings. Small males also transferred more sperm to nonvirgin females, displaced a larger proportion of resident sperm, and achieved higher paternity shares per mating than large males. In agreement with existing theory, we suggest that small males might partially compensate for their low mating success by strategically investing in larger sperm numbers and potentially other, unmeasured ejaculate traits, when they do have a mating opportunity.

Natural history of social and sexual behavior in fruit flies

The past 2 decades have seen fruit flies being widely adopted for research on social behavior and aggression. This fruitful research, however, has not been well tied to fruit flies' natural history. To address this knowledge gap, I conducted a field study. My goal was to inform future research conducted in artificial surroundings, and to inspire new investigations that can rely more heavily on fruit flies' actual natural behavior. My two main novel findings were first, that flies in the field showed significant sociability, as they formed social groups rather than dispersed randomly among fruits of similar quality. Second, males showed fair levels of aggression towards each other as indicated by a lunging rate of 17 per hour, and lower rates of wing threat and boxing. Courtship was the most prominent activity on fruits, with females rejecting almost all males' advances. This resulted in an estimated mating rate of 0.6 per female per day. Flies showed a striking peak of activity early in the mornings, even at cold temperatures, followed by inactivity for much of the day and night. Flies, however, handled well high temperatures approaching 40 °C by hiding away from fruit and concentrating activity in the cooler, early mornings. My field work highlights a few promising lines of future research informed by fruit flies' natural history. Most importantly, we do not understand the intriguing dynamics that generate significant sociability despite frequent aggressive interactions on fruits. Males' responses to female rejection signals varied widely, perhaps because the signals differed in information content perceived by flies but not humans. Finally, flies tolerated cold early mornings perhaps owing to fitness benefits associated with increased mating and feeding opportunities at this time. Flies were adept at handling very high temperatures under the natural daily temperature fluctuations and availability of shelters, and this can inform more realistic research on the effects of global warming on animals in their natural settings.

Quantifying selection on standard metabolic rate and body mass in Drosophila melanogaster

Standard metabolic rate (SMR), defined as the minimal energy expenditure required for self-maintenance, is a key physiological trait. Few studies have estimated its relationship with fitness, most notably in insects. This is presumably due to the difficulty of measuring SMR in a large number of very small individuals. Using high-throughput flow-through respirometry and a Drosophila melanogaster laboratory population adapted to a life cycle that facilitates fitness measures, we quantified SMR, body mass, and fitness in 515 female and 522 male adults. We used a novel multivariate approach to estimate linear and nonlinear selection differentials and gradients from the variance-covariance matrix of fitness, SMR, and body mass, allowing traits specific covariates to be accommodated within a single model. In males, linear selection differentials for mass and SMR were positive and individually significant. Selection gradients were also positive but, despite substantial sample sizes, were nonsignificant due to increased uncertainty given strong SMR-mass collinearity. In females, only nonlinear selection was detected and it appeared to act primarily on body size, although the individual gradients were again nonsignificant. Selection did not differ significantly between sexes although differences in the fitness surfaces suggest sex-specific selection as an important topic for further study.

Artificial selection on sexual aggression: Correlated traits and possible trade-offs

Forced copulation is an extreme form of sexual aggression that can affect the evolution of sex-specific anatomy, morphology, and behavior. To characterize mechanistic and evolutionary aspects of forced copulation, we artificially selected male fruit flies based on their ability to succeed in the naturally prevalent behavior of forced matings with newly eclosed (teneral) females. The low and high forced copulation lineages showed rapid divergence, with the high lineages ultimately showing twice the rates of forced copulation as the low lineages. While males from the high lineages spent more time aggressively pursuing and mounting teneral females, their behavior toward non-teneral and heterospecific females was similar to that of males from the low lineages. Males from the low and high lineages also showed similar levels of male-male aggression. This suggests little or no genetic correlations between sexual aggression and non-aggressive pursuit of females, and between male aggression toward females and males. Surprisingly however, males from the high lineages had twice as high mating success than males from the low lineages when allowed to compete for consensual mating with mature females. In further experiments, we found no evidence for trade-offs associated with high forced mating rates: males from the high lineages did not have lower longevity than males from the low lineages when housed with females, and four generations of relaxed selection did not lead to convergence in forced mating rates. Our data indicate complex interactions among forced copulation success and consensual mating behavior, which we hope to clarify in future genomic work.

Sexual selection favours good or bad genes for pathogen resistance depending on males' pathogen exposure

Resistance to pathogens is often invoked as an indirect benefit of female choice, but experimental evidence for links between father's sexual success and offspring resistance is scarce and equivocal. Two proposed mechanisms might generate such links. Under the first, heritable resistance to diverse pathogens depends on general immunocompetence; owing to shared condition dependence, male sexual traits indicate immunocompetence independently of the male's pathogen exposure. By contrast, other hypotheses (e.g. Hamilton-Zuk) assume that sexual traits only reveal heritable resistance if the males have been exposed to the pathogen. The distinction between the two mechanisms has been neglected by experimental studies. We show that Drosophila melanogaster males that are successful in mating contests (one female with two males) sire sons that are substantially more resistant to the intestinal pathogen Pseudomonas entomophila-but only if the males have themselves been exposed to the pathogen before the mating contest. By contrast, sons of males sexually successful in the absence of pathogen exposure are less resistant than sons of unsuccessful males. We detected no differences in daughters' resistance. Thus, while sexual selection may have considerable consequences for offspring resistance, these consequences may be sex-specific. Furthermore, contrary to the 'general immunocompetence' hypothesis, these consequences can be positive or negative depending on the epidemiological context under which sexual selection operates.

Winners have higher pre-copulatory mating success but losers have better post-copulatory outcomes

In many animals, the outcomes of competitive interactions can have lasting effects that influence an individual's reproductive success and have important consequences for the strength and direction of evolution via sexual selection. In the fruit fly, Drosophila melanogaster, males that have won previous contests are more likely to win in subsequent conflicts and losers are more likely to lose (winner-loser effects), but the direct fitness consequences and genetic underpinnings of this plasticity are poorly understood. Here, we tested how male genotype and the outcomes of previous male-male conflicts influence male pre- and post-copulatory success. We quantified pre-copulatory success in a choice and no-choice context, and post-copulatory success by quantifying ejaculate offensive and defensive ability. We found that winners have higher reproductive success compared to losers in both pre-copulatory scenarios. However, losers consistently mated for a longer duration, boosted female fecundity and had an increased paternity share when they were the first males to mate, suggesting increased investment into post-copulatory mechanisms. Finally, by using clonal hybrids from the Drosophila Genetic Reference Panel, we documented that genetic variation explained a sizeable proportion of the observed differences between lines, and of the interaction between line and winner and loser effects. Our results place the behavioural data on winner-loser effects in an evolutionary context by documenting the potential fitness gain to males from altering their reproductive strategy based on fighting experience. Our data may also explain the presence and maintenance of trade-offs between different male reproductive strategies.

Designing mate choice experiments

The important role that mate choice plays in the lives of animals is matched by the large and active research field dedicated to studying it. Researchers work on a wide range of species and behaviours, and so the experimental approaches used to measure animal mate choice are highly variable. Importantly, these differences are often not purely cosmetic; they can strongly influence the measurement of choice, for example by varying the behaviour of animals during tests, the aspects of choice actually measured, and statistical power. Consideration of these effects are important when comparing results among studies using different types of test, or when using laboratory results to predict animal behaviour in  natural populations. However, these effects have been underappreciated by the mate choice literature to date. I focus on five key experimental considerations that may influence choice: (i) should mating be allowed to occur, or should a proxy behavioural measure of preference be used instead? (ii) Should subjects be given a choice of options? (iii) Should each subject be tested more than once, either with the same or different stimuli? (iv) When given a choice, how many options should the subject choose between? (v) What form should the experimental stimuli take? I discuss the practical advantages and disadvantages of common experimental approaches, and how they may influence the measurement of mate choice in systematic ways. Different approaches often influence the ability of animals to perceive and compare stimuli presented during tests, or the perceived costs and benefits of being choosy. Given that variation in the design of mate choice experiments is likely unavoidable, I emphasise that there is no single 'correct' approach to measuring choice across species, although ecological relevance is crucial if the aim is to understand how choice acts in natural populations. I also highlight the need for quantitative estimates of the sizes of potentially important effects, without which we cannot make informed design decisions.

Genetic trade-offs between male reproductive traits in Drosophila melanogaster

In Drosophila melanogaster, males engage in both extensive pre- and post-copulatory competition for the opportunity to mate with females and subsequently sire offspring. The selection pressure for increased male reproductive success has resulted in the evolution of a wide diversity of sexual traits. However, despite strong selection, individuals often exhibit considerable phenotypic variation in the expression of these traits, and it is unclear if any of this variation is owing to underlying genetic trade-offs. Here, using hemiclonal flies, we examine how male reproductive success covaries with their ability to induce long-term stimulation of oogenesis and oviposition in their mates, and how this relationship may change over time. We found that males from hemiclone lines with phenotypes that were more successful in a short-term reproductive 'scramble' environment were less effective at stimulating long-term fecundity in females. Furthermore, we observed that males from hemiclone lines which showed the most improvement over a longer reproductive interaction period also tended to stimulate higher long-term fecundity in females. Together, these results indicate the presence of genetic trade-offs between different male reproductive traits and offer insights into the maintenance of their variation.