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637 results for “Sexual selection”
Data from: MHC-similarity and sexual selection: different doesn't always mean attractive
Females that mate multiply have the possibility to exert postcopulatory choice and select more compatible sperm to fertilize eggs. Prior work suggests that dissimilarity in major histocompatibility complex (MHC) plays an important role in determining genetic compatibility between partners. Favouring a partner with dissimilar MHC alleles would result in offspring with high MHC diversity and therefore with enhanced survival thanks to increased resistance to pathogens and parasites. The high variability of MHC genes may further allow discrimination against the sperm from related males, reducing offspring homozygosity and inbreeding risk. Despite the large body of work conducted at precopulatory level, the role of MHC similarity between partners at postcopulatory level has been rarely investigated. We used an internal fertilizing fish with high level of multiple matings (Poecilia reticulata) to study whether MHC similarity plays a role in determining the outcome of fertilization when sperm from two males compete for the same set of eggs. We also controlled for genomewide similarity by determining similarity at 10 microsatellite loci. Contrary to prediction, we found that the more MHC-similar male sired more offspring while similarity at the microsatellite loci did not predict the outcome of sperm competition. Our results suggest that MHC discrimination may be involved in avoidance of hybridization or outbreeding rather than inbreeding avoidance. This, coupled with similar findings in salmon, suggests that the preference for MHC-dissimilar mates is far from being unanimous and that pre- and postcopulatory episodes of sexual selection can indeed act in opposite directions.
Data from: Reduced genetic variance among high fitness individuals: inferring stabilizing selection on male sexual displays in D. serrata
Directional selection is prevalent in nature yet phenotypes tend to remain relatively constant, suggesting a limit to trait evolution. However, the genetic basis of this limit is unresolved. Given widespread pleiotropy, opposing selection on a trait may arise from the effects of the underlying alleles on other traits under selection, generating net stabilizing selection on trait genetic variance. These pleiotropic costs of trait exaggeration may arise through any number of other traits, making them hard to detect in phenotypic analyses. Stabilizing selection can be inferred, however, if genetic variance is greater among low compared to high fitness individuals. We extend a recently suggested approach to provide a direct test of a difference in genetic variance for a suite of cuticular hydrocarbons (CHCs) in Drosophila serrata. Despite strong directional sexual selection on these traits, genetic variance differed between high and low fitness individuals and was greater among the low fitness males for seven of eight CHCs, significantly more than expected by chance. Univariate tests of a difference in genetic variance were non-significant but likely have low power. Our results suggest that further CHC exaggeration in D. serrata in response to sexual selection is limited by pleiotropic costs mediated through other traits.
Data from: The accumulation of reproductive isolation in early stages of divergence supports a role for sexual selection
Models of speciation by sexual selection propose that male-female coevolution leads to the rapid evolution of behavioral reproductive isolation. Here, we compare the strength of behavioral isolation to ecological isolation, gametic incompatibility, and hybrid inviability in a group of dichromatic stream fishes. In addition, we examine whether any of these individual barriers, or a combined measure of total isolation, is predicted by body shape differences, male color differences, environmental differences, or genetic distance. Behavioral isolation reaches the highest values of any barrier and is significantly greater than ecological isolation. No individual reproductive barrier is associated with any of the predictor variables. However, marginally significant relationships between male color and body shape differences with ecological and behavioral isolation are discussed. Differences in male color and body shape predict total reproductive isolation between species; hierarchical partitioning of these two variables' effects suggests a stronger role for male color differences. Together, these results suggest an important role for divergent sexual selection in darter speciation but raise new questions about the mechanisms of sexual selection at play and the role of male nuptial ornaments.
Data from: Sexual selection on male development time in the parasitoid wasp Nasonia vitripennis
Mating systems are shaped by a species' ecology, which sets the stage for sexual selection. Males of the gregarious parasitoid wasp Nasonia vitripennis compete to mate virgin females at the natal site, before females disperse. Males could increase their fitness by being larger and monopolising female emergence sites, or by emerging earlier pre-empting access to females. We consider sexual selection on male body size and development tine in Nasonia, and a potential trade-off between the two traits. We explored sex-specific patterns of larval and pupal development, finding that smaller wasps developed slower than their host-mates. Using competition experiments between brothers we found that earlier eclosing males mated more females independently of absolute and relative body size. Our data explain the lack of relationship between fitness and body size in male Nasonia, and reinforce the importance of protandry in mating systems where access to mates is time-limited.
Data from: Correlated paternity measures mate monopolization and scales with the magnitude of sexual selection
Indirect measures of sexual selection have been criticized because they can overestimate the magnitude of selection. In particular, they do not account for the degree to which mating opportunities can be monopolized by individuals of the sex that compete for mates. We introduce a measure of mate monopolization (m) based on the magnitude of correlated paternity and evaluate its ability to track changes in the magnitude of sexual selection. Simulation models were used to compare how well m tracked changes in the selection differential (s) for a trait regulating mating success. We further evaluated the association between m and s using ten replicate mating arrays of Sagittaria latifolia in which plants with contrasting alleles at microsatellite loci competed for siring opportunities. The computer models and mating arrays both demonstrated a positive linear association between m and s, supporting the utility of m as an index of sexual selection. Commonly used measures of sexual selection are not easily applied to organisms, such as the flowering plants, for which mating events are difficult to observe. The measure of mate monopolization introduced here could prove to be a useful addition to studies of sexual selection in these organisms.
Data from: Experimental sexual selection and sex comb evolution in Drosophila
Sexual selection can drive rapid evolutionary change in reproductive behaviour, morphology and physiology. This often leads to the evolution of sexual dimorphism, and continued exaggerated expression of dimorphic sexual characteristics, although a variety of other alternative selection scenarios exist. Here, we examined the evolutionary significance of a rapidly evolving, sexually dimorphic trait, sex comb tooth number, in two Drosophila species. The presence of the sex comb in both D. melanogaster and D. pseudoobscura is known to be positively related to mating success, although little is yet known about the sexually selected benefits of sex comb structure. In this study, we used experimental evolution to test the idea that enhancing or eliminating sexual selection would lead to variation in sex comb tooth number. However, the results showed no effect of either enforced monogamy or elevated promiscuity on this trait. We discuss several hypotheses to explain the lack of divergence, focussing on sexually antagonistic coevolution, stabilizing selection via species recognition and nonlinear selection. We discuss how these are important, but relatively ignored, alternatives in understanding the evolution of rapidly evolving sexually dimorphic traits.
Data from: Runaway sexual selection leads to good genes
Mate choice and sexual displays are widespread in nature, but their evolutionary benefits remain controversial. Theory predicts these traits can be favored by runaway sexual selection, in which preference and display reinforce one another due to genetic correlation; or by good genes benefits, in which mate choice is advantageous because extreme displays indicate a well-adapted genotype. However, these hypotheses are not mutually exclusive, and the adaptive benefits underlying mate choice can themselves evolve. In particular, examining how and why sexual displays become indicators of good genes is challenging in natural systems. Here, we use experimental evolution in "digital organisms" to demonstrate the origins of condition-dependent indicator displays following their spread due to a runaway process. Surprisingly, handicap-like costs are not necessary for displays to become indicators of male viability. Instead, a pleiotropic genetic architecture underlies both displays and viability. Runaway sexual selection and good genes benefits should thus be viewed as interacting mechanisms that reinforce one another.
Data from: Male cognitive performance declines in the absence of sexual selection
Sexual selection is responsible for the evolution of male ornaments and armaments, but its role in the evolution of cognition—the ability to process, retain and use information—is largely unexplored. Because successful courtship is likely to involve processing information in complex, competitive sexual environments, we hypothesized that sexual selection contributes to the evolution and maintenance of cognitive abilities in males. To test this, we removed mate choice and mate competition from experimental populations of Drosophila melanogaster by enforcing monogamy for over 100 generations. Males evolved under monogamy became less proficient than polygamous control males at relatively complex cognitive tasks. When faced with one receptive and several unreceptive females, polygamous males quickly focused on receptive females, whereas monogamous males continued to direct substantial courtship effort towards unreceptive females. As a result, monogamous males were less successful in this complex setting, despite being as quick to mate as their polygamous counterparts with only one receptive female. This diminished ability to use past information was not limited to the courtship context: monogamous males (but not females) also showed reduced aversive olfactory learning ability. Our results provide direct experimental evidence that the intensity of sexual selection is an important factor in the evolution of male cognitive ability.
Data from: Sexual selection and assortative mating: an experimental test
Mate choice and mate competition can both influence the evolution of sexual isolation between populations. Assortative mating may arise if traits and preferences diverge in step, and, alternatively, mate competition may counteract mating preferences and decrease assortative mating. Here, we examine potential assortative mating between populations of Drosophila pseudoobscura that have experimentally evolved under either increased ('polyandry') or decreased ('monogamy') sexual selection intensity for 100 generations. These populations have evolved differences in numerous traits, including a male signal and female preference traits. We use a two males: one female design, allowing both mate choice and competition to influence mating outcomes, to test for assortative mating between our populations. Mating latency shows subtle effects of male and female interactions, with females from the monogamous populations appearing reluctant to mate with males from the polyandrous populations. However, males from the polyandrous populations have a significantly higher probability of mating regardless of the female's population. Our results suggest that if populations differ in the intensity of sexual selection, effects on mate competition may overcome mate choice.
Data from: Testing for a genetic response to sexual selection in a wild Drosophila population
In accordance with the consensus that sexual selection is responsible for the rapid evolution of display traits on macroevolutionary scales, microevolutionary studies suggest sexual selection is a widespread and often strong form of directional selection in nature. However, empirical evidence for the contemporary evolution of sexually selected traits remains weak. In this study we used a novel application of quantitative genetic breeding designs to test for a genetic response to sexual selection on eight chemical display traits in a field population of the fly, Drosophila serrata. Using our quantitative genetic approach, we were able to detect a genetically-based difference in means between groups of males descended from either successful or random wild males for one of these display traits, the diene (Z,Z)-5,9-C27:2. Our experimental results, in combination with previous lab studies on this system, suggest that both natural selection and sexual selection may be influencing the evolutionary trajectories of these traits in nature, limiting the capacity for a contemporary evolutionary response.
Data from: Testis asymmetry in birds: the influences of sexual and natural selection
Gonad size and shape asymmetries are particularly common in birds. Although some obvious size and shape differences between the left and right testes in birds were first documented more than a century ago, little is known about what influences the variation across species in either the degree or the direction of these asymmetries. Here we show that a left bias in size is the most likely ancestral state in most orders and families, and that there is a weak but significant negative relation between the degree of size and shape asymmetries. In extant species, testis size and shape symmetries increase with the degree of sperm competition (relative testes mass), but those relations are significant only in those species with left bias in each of these traits. When space is particularly constrained on the left side of the body cavity due to a large gizzard (e.g. in granivores), the left testis is more elongated and the degree of size symmetry is larger. Despite significant patterns, relative testes mass and gizzard complexity together explain < 10% of the variation in testis asymmetry in species with left biases. Thus our analyses suggest that some other factor is largely responsible for the evolution of gonad asymmetry in birds: 1) that a left bias in testis size might be a correlated response to selection for a left bias in the development of a single (left) ovary in females of most bird species, and/or 2) that physiological efficiency due to the dramatic and rapid increase in testis size of most species during the breeding season might favour enlargement of one testis. Our conclusions highlight the need to rethink and improve our understanding of the physiological processes underlying the investment in gonads by male birds.
Data from: Simultaneous age-dependent and age-independent sexual selection in the lekking black grouse (Lyrurus tetrix)
Individuals' reproductive success is often strongly associated with their age, with typical patterns of early-life reproductive improvement and late-life senescence. These age-related patterns are due to the inherent trade-offs between life-history traits competing for a limited amount of resources available to the organisms. In males, such trade-offs are exacerbated by the resource requirements associated with the expression of costly sexual traits, leading to dynamic changes in trait expression throughout their life span. Due to the age dependency of male phenotypes, the relationship between the expression of male traits and mating success can also vary with male age. Hence, using longitudinal data in a lekking species with strong sexual selection – the black grouse Lyrurus tetrix – we quantified the effects of age, life span and age of first lek attendance (AFL) on male annual mating success (AMS) to separate the effects of within-individual improvement and senescence on AMS from selective (dis)appearance of certain phenotypes. Then, we used male AMS to quantify univariate and multivariate sexual selection gradients on male morphological and behavioural traits with and without accounting for age and age-related effects of other traits. Male AMS increased with age, and there was no significant reproductive senescence. Most males never copulated, and of the ones that did, the majority had only one successful year. Life span was unrelated to AMS, but early AFL tended to lead to higher AMS at ages 1–3. AMS was related to most morphological and behavioural traits when male age was ignored. Accounting for age and age-specific trait effects (i.e. the interaction between a trait and age) reduced the magnitude of the selection gradients and revealed that behavioural traits are under consistent sexual selection, while sexual selection on morphological traits is stronger in old males. Therefore, sexual selection in black grouse operates primarily on male behaviour and morphological traits may act as additional cues to supplement female choice. These results demonstrate the multifaceted influence of age on both fitness and sexual traits and highlight the importance of accounting for such effects when quantifying sexual selection.
Data from: Selection for costly sexual traits results in a vacant mating niche and male dimorphism
The expected strong directional selection for traits that increase a male's mating ability conflicts with the frequent observation that within species, males may show extreme variation in sexual traits. These male reproductive polymorphisms are usually attributed to direct male-male competition. It is currently unclear, however, how directional selection for sexually selected traits may convert into disruptive selection, and if female preference for elaborate traits may be an alternative mechanism driving the evolution of male polymorphism. Here we explore this mechanism using the polyandric dwarf spider Oedothorax gibbosus as a model. We first show that males characterized by conspicuous cephalic structures serving as a nuptial feeding device ('gibbosus males') significantly outperform other males in siring offspring of previously fertilized females. However, significant costs in terms of development time of gibbosus males open a mating niche for an alternative male type lacking expensive secondary sexual traits. These 'tuberosus males' obtain virtually all fertilizations early in the breeding season. Individual-based simulations demonstrate a hitherto unknown general principle, by which males selected for high investment to attract females suffer constrained mating opportunities. This creates a vacant mating niche of unmated females for non-investing males and, consequently, disruptive selection on male secondary sexual traits.
Data from: Fluorescent sperm in a transparent worm: validation of a GFP marker to study sexual selection
Background: Sexual selection has initially been thought to occur exclusively at the precopulatory stage in terms of contests among males and female mate choice, but research over the last four decades revealed that it often continues after copulation through sperm competition and cryptic female choice. However, studying these postcopulatory processes remains challenging because they occur internally and therefore are often difficult to observe. In the transparent free-living flatworm Macrostomum lignano, a recently established transgenic line that expresses green fluorescent protein (GFP) in all cell types, including sperm, offers a unique opportunity to non-invasively visualise and quantify the sperm of a GFP-expressing donor inside the reproductive tract of wild-type recipients in vivo. We here test several aspects of the reproductive performance of the transgenic individuals and the accuracy of the techniques involved in assessing the GFP-expressing worms and their sperm. We then show the usefulness of these methods in a study on sperm displacement. Results: GFP-expressing worms do not differ from wild-type worms in terms of morphology, mating rate and reproductive success. In addition, we show that the GFP signal is reliably and unequivocally expressed by all GFP-expressing individuals observed under epifluorescence illumination. However, the intensity of the GFP signal emitted by sperm of GFP expressing donors can vary (which we show to be at least in part due to sperm ageing) and the GFP marker is inherited according to Mendel's laws in most, but not all, of the individuals. Nevertheless, we argue these two issues can be addressed with an appropriate experimental design. Finally, we demonstrate the value of the GFP-techniques by comparing the number of GFP-expressing sperm in a wild-type recipient before and after mating with a competing sperm donor, providing clear experimental evidence for sperm displacement in M. lignano. This result suggests that sperm donors can displace previously stored sperm and replace it with their own. Conclusion: The availability of the GFP-techniques in a transparent organism provide unique opportunities to visualise and quantify internal processes in the female reproductive tract after mating, which opens new avenues in the study of sexual selection.
Data from: Multiple sexual selection pressures drive the rapid evolution of complex morphology in a male secondary genital structure
The genitalia of internally fertilizing taxa represent a striking example of rapid morphological evolution. Although sexual selection can shape variation in genital morphology, it has been difficult to test whether multiple sexual selection pressures combine to drive the rapid evolution of individual genital structures. Here, we test the hypothesis that both pre- and postcopulatory sexual selection can act in concert to shape complex structural variation in secondary genital morphology. We genetically modified the size and shape of the posterior lobes of Drosophila melanogaster males and tested the consequences of morphological variation on several reproductive measures. We found that the posterior lobes are necessary for genital coupling and that they are also the targets of multiple postcopulatory processes that shape quantitative variation in morphology, even though these structures make no direct contact with the external female genitalia or internal reproductive organs during mating. We also found that males with smaller and less structurally complex posterior lobes suffer substantial fitness costs in competitive fertilization experiments. Our results show that sexual selection mechanisms can combine to shape the morphology of a single genital structure and that the posterior lobes of D. melanogaster are the targets of multiple postcopulatory selection pressures.
Data from: Negative frequency-dependent selection of sexually antagonistic alleles in Myodes glareolus
Sexually antagonistic genetic variation, where optimal values of traits are sex-dependent, is known to slow the loss of genetic variance associated with directional selection on fitness-related traits. However, sexual antagonism alone is not sufficient to maintain variation indefinitely. Selection of rare forms within the sexes can help to conserve genotypic diversity. We combined theoretical models and a field experiment with Myodes glareolus to show that negative frequency-dependent selection on male dominance maintains variation in sexually antagonistic alleles. In our experiment, high-dominance male bank voles were found to have low-fecundity sisters, and vice versa. These results show that investigations of sexually antagonistic traits should take into account the effects of social interactions on the interplay between ecology and evolution, and that investigations of genetic variation should not be conducted solely under laboratory conditions.
Data from: To help or punish in the face of unfairness: men and women prefer mutually-beneficial strategies over punishment in a sexual selection context
There is evidence that the evolution of cooperation is, in part, maintained through sexual selection. Supporting this, within a female choice model, men display mutually-beneficial behaviour to attract women, and women prefer men who do so. This evidence is based on a 2-choice -architecture (cooperate or not). Here we extend this to include punishment options within a 4-choice-architecture ('punishing a transgressor', 'compensating a victim', 'both punishing and compensating' or 'doing-nothing'). Both compensation (a self-serving mutually-beneficial behaviour) and self-serving punishment, are associated with positive mate qualities. We test which is preferred by males and which is chosen by female undergraduates in a sexual selection context. We further explore effects of trait empathy and political ideology on these preferences. In a series of three studies using a third-party-punishment-compensation (3PPC) game we show (Study One), that romantically-primed undergraduate males, express a preference to either 'compensate' or 'both compensate and punish', and undergraduate women find males who 'compensate' or 'compensate and punish' the most attractive (Studies Two and Three). Compensating men are perceived as compassionate, fair and strong by undergraduate women (Study Three). High trait empathy (Studies One and Three) and a left-wing political ideology (Study Three) are associated with a preference for compensation. Thus, self-serving mutually-beneficial behaviour can be preferred over self-serving punishment as a signal of mate quality in undergraduates. Implications for the evolution of cooperation are discussed with respect to sexual selection.
Data from: Genetic versus census estimators of the opportunity for sexual selection in the wild
The existence of a direct link between intensity of sexual selection and mating system type is widely accepted. However, the quantification of sexual selection has proven problematical. Several measures of sexual selection have been proposed, including the operational sex ratio (OSR), breeding sex ratio (BSR) and opportunity for sexual selection (Imates). For a wild population of pronghorn (Antilocapra americana), we calculated OSR and BSR. We estimated Imates from census data on the spatial and temporal distribution of receptive females in rut and from a multi-generational genetic pedigree. OSR and BSR indicated weak sexual selection on males, but census and pedigree Imates suggested stronger sexual selection on males than on females. OSR and BSR correlated with census but not pedigree estimates of Imates, and census Imates did not correlate with pedigree estimates. This suggests that the behavioral mating system, as deduced from the spatial and temporal distribution of females, does not predict the genetic mating system of pronghorn. The differences we observed between estimators were primarily due to female mate sampling and choice, and the sex ratio. For most species, behavioral data is not perfectly accurate, and therefore will be an insufficient alternative to using multi-generational pedigrees to quantify sexual selection.
Individual and synergistic effects of male external genital traits in sexual selection
Male genital traits exhibit extraordinary inter-specific phenotypic variation. This remarkable and general evolutionary trend is widely considered to be the result of sexual selection. However, we still do not have a good understanding of whether or how individual genital traits function in different competitive arenas (episodes of sexual selection), or how different genital traits may interact to influence competitive outcomes. Here, we use an experimental approach based on high-precision laser phenotypic engineering to address these outstanding questions, focusing on three distinct sets of micron-scale external (non-intromittent) genital spines in male Drosophila kikkawai Burla (Diptera: Drosophilidae). Elimination of the large pair of spines on the male secondary claspers sharply reduced male ability to copulate, yet elimination of the other sets of spines on the primary and secondary claspers had no significant effects on copulation probability. Intriguingly, both the large spines on the secondary claspers and the cluster of spines on the primary claspers were found to independently promote male competitive fertilization success. Moreover, when large and small secondary clasper spines were simultaneously shortened in individual males, these males suffered greater reductions in fertilization success relative to males whose traits were altered individually, providing evidence for synergistic effects of external genital traits on fertilization success. Overall, the results are significant in demonstrating that a given genital trait (the large spines on the secondary claspers) can function in different episodes of sexual selection, and distinct genital traits may interact in sexual selection. The results offer an important contribution to evolutionary biology by demonstrating an understudied selective mechanism, operating via subtle trait interactions in a post-insemination context, by which genital traits may be co-evolving.
Data from: Disentangling the contribution of sexual selection and ecology to the evolution of size dimorphism in pinnipeds
The positive relationship between sexual size dimorphism (SSD) and harem size across pinnipeds is often cited as a textbook example of sexual selection. It assumes that female aggregation selected for large male size via male-male competition. Yet, it is also conceivable that SSD evolved prior to polygyny due to ecological forces. We analysed eleven life history traits in 35 pinniped species to determine their co-evolutionary dynamics and infer their most likely evolutionary trajectories contrasting these two hypotheses. We find support for SSD having evolved prior to changes in the mating system, either as a consequence of niche partitioning during aquatic foraging or in combination with sexual selection on males to enforce copulations on females. Only subsequently did polygyny evolve, leading to further coevolution as the strength of sexual selection intensified. Evolutionary sequence analyses suggest a polar origin of pinnipeds and indicate that SSD and polygyny are intrinsically linked to a suite of ecological and life history traits. Overall, this study calls for the inclusion of ecological variables when studying sexual selection and argues for caution when assuming causality between coevolving traits. It provides novel insights into the role of sexual selection for the co-evolutionary dynamics of SSD and mating system.
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