The condition-dependence of male genital size and shape.

The male genitals of internal fertilisers evolve rapidly and divergently, and sexual selection is generally responsible for this. Many sexually selected traits are condition-dependent-with their expression dependent upon the resources available to be allocated to them-as revealed by genetic or environmental manipulations of condition. However, it is not clear whether male genitals are also condition-dependent. Here we manipulate condition in two ways (via inbreeding and diet) to test the condition-dependence of the genital arch of Drosophila simulans. We found that genital size but not genital shape suffered from inbreeding depression, whereas genital size and shape were affected by dietary manipulation of condition. The differential effects of these treatments likely reflect underlying genetic architecture that has been shaped by past selection: inbreeding depression is only expected when traits have a history of directional selection, while diet impacts traits regardless of historical selection. Nonetheless, our results suggest genitals can be condition-dependent like other sexually selected traits.

Ultimate drivers of forced extra-pair copulations in birds lacking a penis: jackdaws as a case-study.

Forced copulation is common, presumably because it can increase male reproductive success. Forced extra-pair copulation (FEPC) occurs in birds, even though most species lack a penis and are widely thought to require female cooperation for fertilization. How FEPC persists, despite a presumed lack of siring success and likely non-negligible costs to the male, is unknown. Using the jackdaw (Corvus monedula) as a case study, we use SNPs to quantify the extra-pair paternity rate through FEPC and evaluate explanations for the persistence of FEPC in species without a penis. We then collate evidence for FEPC across penis-lacking birds. Combining genetic and behavioural analyses, our study suggests that the most likely explanations for the maintenance of FEPC in jackdaws are that it provides a selective advantage to males or it is a relic. Our literature review shows that across birds lacking a penis, FEPC is taxonomically widespread, and yet, little is known about its evolution. A broader implementation of the approach used here, combining both genetic and behavioural data, may shed light on why this widespread sexual behaviour persists. Additional work is necessary to understand whether a penis is needed for paternity through forced copulation and to quantify the costs of FEPC.

Sperm competition in yellow dung flies: No consistent effect of sperm size.

The male competition for fertilization that results from female multiple mating promotes the evolution of increased sperm numbers and can impact sperm morphology, with theory predicting that longer sperm can at times be advantageous during sperm competition. If so, males with longer sperm should sire more offspring than competitors with shorter sperm. Few studies have directly tested this prediction, and findings are inconsistent. Here we assessed whether longer sperm provide a competitive advantage in the yellow dung fly (Scathophaga stercoraria; Diptera: Scathophagidae). Initially, we let brothers with different temperature-mediated mean sperm lengths compete - thus minimizing confounding effects of genetic background - and found no clear advantage of longer sperm. We then used flies from lines subjected to bidirectional selection on phenoloxidase activity that had shown correlated evolutionary responses in sperm and female spermathecal duct lengths. This experiment also yielded no main effect of sperm size on siring success. Instead, there was a trend for a shorter-sperm advantage, but only when competing in females with longer spermathecal ducts. Our data corroborated many previously reported findings (last-male precedence, effects of copula duration and body size), suggesting our failure to find sperm size effects is not inherently due to our experimental protocols. We conclude that longer sperm are not competitively superior in yellow dung flies under most circumstances, and that, consistent with previous work, in this species competitive fertilization success is primarily determined by the relative numbers of sperm competing.

Digest: Brain or brawn: Trade-offs between brain size and flight mode in birds.

Brain size is extremely variable across species, and its evolution depends upon the calorific trade-offs between it and other organs and activities. Shiomi investigated potential brain size trade-offs with different flight modes in birds. Flight can be energetically expensive, and costs are especially high with powered flight. This comparative study indicated that migratory birds employing less energetic modes of flight had relatively larger brains than migratory birds using powered flight, suggesting that brain size is impacted by the energetic costs of flight.

Mapping sex differences in the effects of protein and carbohydrates on lifespan and reproduction in Drosophila melanogaster: is measuring nutrient intake essential?

Understanding how diet affects reproduction and survival is a central aim in evolutionary biology. Although this relationship is likely to differ between the sexes, we lack data relating diet to male reproductive traits. One exception to this general pattern is Drosophila melanogaster, where male dietary intake was quantified using the CApillary FEeder (CAFE) method. However, CAFE feeding reduces D. melanogaster survival and reproduction, so may distort diet-fitness outcomes. Here, we use the Geometric Framework of Nutrition to create nutrient landscapes that map sex-specific relationships between protein, carbohydrate, lifespan and reproduction in D. melanogaster. Rather than creating landscapes with consumption data, we map traits onto the nutrient composition of forty agar-based diets, generating broad coverage of nutrient space. We find that male and female lifespan was maximised on low protein, high carbohydrate blends (~ 1P:15.9C). This nutrient ratio also maximised male reproductive rates, but females required more protein to maximise daily fecundity (1P:1.22C). These results are consistent with CAFE assay outcomes. However, the approach employed here improved female fitness relative to CAFE assays, while effects of agar versus CAFE feeding on male fitness traits depended on the nutrient composition of experimental diets. We suggest that informative nutrient landscapes can be made without measuring individual nutrient intake and that in many cases, this may be preferable to using the CAFE approach. The most appropriate method will depend on the question and species being studied, but the approach adopted here has the advantage of creating nutritional landscapes when dietary intake is hard to quantify.

Kin-mediated plasticity in alternative reproductive tactics.

Conditional strategies occur when the relative fitness pay-off from expressing a given phenotype is contingent upon environmental circumstances. This conditional strategy model underlies cases of alternative reproductive tactics, in which individuals of one sex employ different means to obtain reproduction. How kin structure affects the expression of alternative reproductive tactics remains unexplored. We address this using the mite Rhizoglyphus echinopus, in which large males develop into aggressive 'fighters' and small males develop into non-aggressive 'scramblers.' Because only fighters kill their rivals, they should incur a greater indirect fitness cost when competing with their relatives, and thus fighter expression could be reduced in the presence of relatives. We raised mites in full-sibling or mixed-sibship groups and found that fighters were more common at higher body weights in full-sibling groups, not less common as we predicted (small individuals were almost exclusively scramblers in both treatments). This result could be explained if relatedness and cue variability are interpreted signals of population density, since fighters are more common at low densities in this species. Alternatively, our results may indicate that males compete more intensely with relatives in this species. We provide the first evidence of kin-mediated plasticity in the expression of alternative reproductive tactics.

Natural selection increases female fitness by reversing the exaggeration of a male sexually selected trait.

Theory shows how sexual selection can exaggerate male traits beyond naturally selected optima and also how natural selection can ultimately halt trait elaboration. Empirical evidence supports this theory, but to our knowledge, there have been no experimental evolution studies directly testing this logic, and little examination of possible associated effects on female fitness. Here we use experimental evolution of replicate populations of broad-horned flour beetles to test for effects of sex-specific predation on an exaggerated sexually selected male trait (the mandibles), while also testing for effects on female lifetime reproductive success. We find that populations subjected to male-specific predation evolve smaller sexually selected mandibles and this indirectly increases female fitness, seemingly through intersexual genetic correlations we document. Predation solely on females has no effects. Our findings support fundamental theory, but also reveal unforseen outcomes-the indirect effect on females-when natural selection targets sex-limited sexually selected characters.

Peaks in bat activity at turbines and the implications for mitigating the impact of wind energy developments on bats.

Wind turbines are a relatively new threat to bats, causing mortalities worldwide. Reducing these fatalities is essential to ensure that the global increase in wind-energy facilities can occur with minimal impact on bat populations. Although individual bats have been observed approaching wind turbines, and fatalities frequently reported, it is unclear whether bats are actively attracted to, indifferent to, or repelled by, the turbines at large wind-energy installations. In this study, we assessed bat activity at paired turbine and control locations at 23 British wind farms. The research focussed on Pipistrellus species, which were by far the most abundant bats recorded at these sites. P. pipistrellus activity was 37% higher at turbines than at control locations, whereas P. pygmaeus activity was consistent with no attraction or repulsion by turbines. Given that more than 50% of bat fatalities in Europe are P. pipistrellus, these findings help explain why Environmental Impact Assessments conducted before the installation of turbines are poor predictors of actual fatality rates. They also suggest that operational mitigation (minimising blade rotation in periods of high collision risk) is likely to be the most effective way to reduce collisions because the presence of turbines alters bat activity.

Sexual selection on the genital lobes of male Drosophila simulans.

Sexual selection is thought to be responsible for the rapid divergent evolution of male genitalia with several studies detecting multivariate sexual selection on genital form. However, in most cases, selection is only estimated during a single episode of selection, which provides an incomplete view of net selection on genital traits. Here, we estimate the strength and form of multivariate selection on the genitalia arch of Drosophila simulans when mating occurs in the absence of a competitor and during sperm competition, in both sperm defence and offense roles (i.e., when mating first and last). We found that the strength of sexual selection on the genital arch was strongest during noncompetitive mating and weakest during sperm offense. However, the direction of selection was similar across selection episodes with no evidence for antagonistic selection. Overall, selection was not particularly strong despite genitals clearly evolving rapidly in this species.

Evolutionary insight from a humble fly: sperm competition and the yellow dungfly.

Studies of the yellow dungfly in the 1960s provided one of the first quantitative demonstrations of the costs and benefits associated with male and female reproductive behaviour. These studies advanced appreciation of sexual selection as a significant evolutionary mechanism and contributed to the 1970s paradigm shift toward individual selectionist thinking. Three behaviours in particular led to the realization that sexual selection can continue during and after mating: (i) female receptivity to remating, (ii) sperm displacement and (iii) post-copulatory mate guarding. These behaviours either generate, or are adaptations to sperm competition, cryptic female choice and sexual conflict. Here we review this body of work, and its contribution to the development of post-copulatory sexual selection theory. This article is part of the theme issue 'Fifty years of sperm competition'.

Offspring sex ratios are stable across the life course in Drosophila simulans.

Within populations, adult sex ratios influence population growth and extinction risk, mating behaviours and parental care. Sex ratio adjustment can also have pronounced effects on individual fitness. Accordingly, it is important that we understand how often, and why, offspring sex ratios deviate from parity. In Drosophila melanogaster, females appear to improve their fitness by producing fewer sons when paired with older males. However, facultative sex ratio adjustment in D. melanogaster is controversial, and our understanding of how sex ratio skew affects fitness is hampered by pronounced sexual conflict in this species. Additionally, it is unclear whether maternal age or quality interacts with paternal age to influence offspring sex ratios. Here, we test whether offspring sex ratios vary as a function of maternal quality, and maternal and paternal age in Drosophila simulans, a sister species of D. melanogaster that lacks overt sexual conflict. We find that offspring sex ratios are slightly male-biased overall, but constant across the female life course, and independent of female quality, or paternal age. To really understand if, how and when females skew offspring sex ratios, we need studies linking offspring sex ratios to paternal and maternal phenotypes that are predicted to shift optimal investment in sons and daughters.

De Novo Genome Assembly of the Meadow Brown Butterfly, Maniola jurtina.

Meadow brown butterflies (Maniola jurtina) on the Isles of Scilly represent an ideal model in which to dissect the links between genotype, phenotype and long-term patterns of selection in the wild - a largely unfulfilled but fundamental aim of modern biology. To meet this aim, a clear description of genotype is required. Here we present the draft genome sequence of M. jurtina to serve as a founding genetic resource for this species. Seven libraries were constructed using pooled DNA from five wild caught spotted females and sequenced using Illumina, PacBio RSII and MinION technology. A novel hybrid assembly approach was employed to generate a final assembly with an N50 of 214 kb (longest scaffold 2.9 Mb). The sequence assembly described here predicts a gene count of 36,294 and includes variants and gene duplicates from five genotypes. Core BUSCO (Benchmarking Universal Single-Copy Orthologs) gene sets of Arthropoda and Insecta recovered 90.5% and 88.7% complete and single-copy genes respectively. Comparisons with 17 other Lepidopteran species placed 86.5% of the assembled genes in orthogroups. Our results provide the first high-quality draft genome and annotation of the butterfly M. jurtina.

Complex interactions between sperm viability and female fertility.

Sperm viability is a major male fitness component, with higher sperm viability associated with enhanced sperm competitiveness. While many studies have focussed on sperm viability from the male fitness standpoint, its impact on female fitness is less clear. Here we used a panel of 32 isogenic Drosophila simulans lines to test for genetic variation in sperm viability (percentage of viable cells). We then tested whether sperm viability affected female fitness by mating females to males from low or high sperm viability genotypes. We found significant variation in sperm viability among genotypes, and consistent with this, sperm viability was highly repeatable within genotypes. Additionally, females mated to high sperm viability males laid more eggs in the first seven hours after mating, and produced more offspring in total. However, the early increase in oviposition did not result in more offspring in the 8 hours following mating, suggesting that mating with high sperm-viability genotypes leads to egg wastage for females shortly after copulation. Although mating with high sperm-viability males resulted in higher female fitness in the long term, high quality ejaculates would result in a short-term female fitness penalty, or at least lower realised fitness, potentially generating sexual conflict over optimal sperm viability.

Sexual conflict.

Evolutionary conflict arises from differences in the fitness interests of replicating entities and has its roots in relatedness asymmetries. Every replicator is related to itself by 100%, but in most cases is less related to other replicators, which generates selfishness and conflicts of interest. Since this basic condition is the norm at many levels of biological organization, conflict is rife in biological systems. Sexual conflict, on which we focus here, is the evolutionary conflict that occurs between males and females because of their divergent fitness interests. Sexual conflict occurs despite sexual reproduction requiring some level of cooperation between males and females because the fitness interests of the sexes are nevertheless never perfectly aligned. In other words, males and females may agree on where they are going, but not necessarily on how to get there. Sexual conflict is a vast topic with relevance to many areas of biology and so here we restrict our focus to matters we think are of broadest interest.

Loser-effect duration evolves independently of fighting ability.

Winning or losing contests can impact subsequent competitive behaviour and the duration of these effects can be prolonged. While it is clear effects depend on social and developmental environments, the extent to which they are heritable, and hence evolvable, is less clear and remains untested. Furthermore, theory predicts that winner and loser effects should evolve independently of actual fighting ability, but again tests of this prediction are limited. Here we used artificial selection on replicated beetle populations to show that the duration of loser effects can evolve, with a realized heritability of about 17%. We also find that naive fighting ability does not co-evolve with reductions in the duration of the loser effect. We discuss the implications of these findings and how they corroborate theoretical predictions.

Wolbachia infection can bias estimates of intralocus sexual conflict.

Males and females share most of their genome and develop many of the same traits. However, each sex frequently has different optimal values for these shared traits, creating intralocus sexual conflict. This conflict has been observed in wild and laboratory populations of insects and affects important evolutionary processes such as sexual selection, the maintenance of genetic variation, and possibly even speciation. Given the broad impacts of intralocus conflict, accurately detecting and measuring it is important. A common way to detect intralocus sexual conflict is to calculate the intersexual genetic correlation for fitness, with negative values suggesting conflict. Here, we highlight a potential confounder of this measure-cytoplasmic incompatibility caused by the intracellular parasite Wolbachia. Infection with Wolbachia can generate negative intersexual genetic correlations for fitness in insects, suggestive of intralocus sexual conflict. This is because cytoplasmic incompatibility reduces the fitness of uninfected females mated to infected males, while uninfected males will not suffer reductions in fitness if they mate with infected females and may even be fitter than infected males. This can lead to strong negative intersexual genetic correlations for fitness, mimicking intralocus conflict. We illustrate this issue using simulations and then present Drosophila simulans data that show how reproductive incompatibilities caused by Wolbachia infection can generate signals of intralocus sexual conflict. Given that Wolbachia infection in insect populations is pervasive, but populations usually contain both infected and uninfected individuals providing scope for cytoplasmic incompatibility, this is an important consideration for sexual conflict research but one which, to date, has been largely underappreciated.

Penis evolution across species: divergence and diversity.

The penis is an incredibly diverse and rapidly evolving structure, such that even in closely related species that otherwise differ very little in their morphology, penis form can be highly differentiated. Penises are also much more complex than their fundamental function - sperm transfer - would seem to require. The rapid divergent evolution of male structures is typically the signature of traits under sexual selection and the current evidence suggests the penis is no different in this regard. Despite the general agreement that sexual selection is the main driver of penis evolution, many questions about penis evolution remain unresolved. Furthermore, the penis might be an ideal characteristic on which to focus in the drive to link phenotype with genotype.

Intralocus sexual conflict can resolve the male-female health-survival paradox.

At any given age, men are more likely to die than women, but women have poorer health at older ages. This is referred to as the "male-female, health-survival paradox", which is not fully understood. Here, we provide a general solution to the paradox that relies on intralocus sexual conflict, where alleles segregating in the population have late-acting positive effects on male fitness, but negative effects on female health. Using an evolutionary modelling framework, we show that male-benefit, female-detriment alleles can spread if they are expressed after female reproduction stops. We provide support for our conflict based solution using experimental Drosophila data. Our results show that selecting for increased late-life male reproductive effort can increase male fitness but have a detrimental effect on female fitness. Furthermore, we show that late-life male fertility is negatively genetically correlated with female health. Our study suggests that intralocus sexual conflict could resolve the health-survival paradox.