Component Of Sexual Selection Research Articles

Behavioral estimates of mating success corroborate genetic evidence for pre-copulatory selection

Abstract In promiscuous species, fitness estimates obtained from genetic parentage may often reflect both pre- and post-copulatory components of sexual selection. Directly observing copulations can help isolate the role of pre-copulatory selection, but such behavioral data are difficult to obtain in the wild and may also overlook post-copulatory factors that alter the relationship between mating success and reproductive success. To overcome these limitations, we combined genetic parentage analysis with behavioral estimates of size-specific mating in a wild population of brown anole lizards (Anolis sagrei). Males of this species are twice as large as females and multiple mating among females is common, suggesting the scope for both pre- and post-copulatory processes to shape sexual selection on male body size. Our genetic estimates of reproductive success revealed strong positive directional selection for male size, which was also strongly associated with the number of mates inferred from parentage. In contrast, a male’s size was not associated with the fecundity of his mates or his competitive fertilization success. By simultaneously tracking copulations in the wild via the transfer of colored powder to females by males from different size quartiles, we independently confirmed that large males were more likely to mate than small males. We conclude that body size is primarily under pre-copulatory sexual selection in brown anoles, and that post-copulatory processes do not substantially alter the strength of this selection. Our study also illustrates the utility of combining both behavioral and genetic methods to estimate mating success to disentangle pre- and post-copulatory processes in promiscuous species.

Effects of male age, body size and mating status on female mate preference and offspring sex ratio in Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae)

Mate choice is one of the main components of sexual selection, with females usually considered to be the selective sex. Male status is an important factor that affects female choice and to a large extent female mating preference. Pachycrepoideus vindemmiae (Rondani) (Hymenoptera: Pteromalidae) is an important solitary ectoparasitic idiobiont parasitoid that attacks several fly species and thus can be used as a biological control agent. We explored the influence of male status on mate selection, fecundity and offspring sex ratio. The results show that (1) P. vindemmiae females prefer to mate with young and large males and tended to choose males with only one mating experience; (2) the age, body size and mating status of males did not significantly affect female fecundity; and (3) the proportion of male offspring was larger when females mated with older, small and more often mated males. These results show that P. vindemmiae females prefer to mate with young and large males that had mated few times, and that mating with these males results in a higher proportion of female offspring. In addition, this study also indicates the way of rearing P. vindemmiae that produce more female offspring for use as biological control agents.

Invasive fish retain plasticity of naturally selected, but diverge in sexually selected traits

Invasive alien species (IAS) have become a major threat to ecosystems worldwide. From an evolutionary ecological perspective, they allow teasing apart the relative contributions of plasticity and evolutionary divergence in driving rapid phenotypic diversification. When IAS spread across extensive geographic ranges, climatic variation may represent a source of strong natural selection through overwinter mortality and summer heat stress. This could favour local adaptation, i.e., evolutionary divergence of certain traits. IAS, however, are likely to show plasticity in survival-related traits, and environmental fluctuation in their new distribution range could favour the maintenance of this pre-existing phenotypic plasticity. By contrast, sexually selected traits are more likely to undergo evolutionary divergence when components of sexual selection differ geographically. Here, using data from a common-garden rearing experiment of Western mosquitofish (Gambusia affinis Baird and Girard, 1853) from five populations across the species' invasive range in China, we show that invasive mosquitofish have retained plasticity in key physiological (thermal tolerances), morphological and life-history traits even 100 years after their introduction to China, but exhibit heritable population differences in several sexually selected traits, including the shape of the male copulatory organ. Adaptive plasticity of traits linked to immediate survival in different thermal environments—while likely responsible for the species' extraordinary invasion success—could slow down genetic evolution. Several sexually selected traits could diverge geographically and show rapid evolutionary change, e.g., because climate alters selective landscapes arising from mate competition as an indirect consequence of variation in overwinter mortality.

Different preference functions act in unison: mate choice and risk-taking behaviour in the Atlantic molly (Poecilia mexicana)

Variation in risk-taking versus risk-averse behaviour can become a target of natural and/or sexual selection. However, the link between animal personality and mate choice—as a major component of sexual selection—remains understudied. We asked (1) whether females and males of the livebearing fish Poecilia mexicana prefer risk-taking mating partners (directional mating preference), (2) or if their preferences are dependent on the choosing individual’s own personality type (assortative mating). We characterized each individual for its risk-taking behaviour, assessed as time to emerge from shelter and enter an unknown area. In dichotomous association preference tests, we offered two potential mating partners that differed in risk-taking behaviour but were matched for other phenotypic traits (body size, shape, and colouration). Females, but not males, exhibited a strong directional preference for risk-taking over risk-averse mating partners. At the same time, the strength of females’ preferences correlated positively with their own risk-taking scores. Our study is the first to demonstrate that an overall preference for risk-taking mating partners does not preclude effects of choosing individuals’ own personality type on (subtle) individual variation in mating preferences. More generally, two different preferences functions appear to interact to determine the outcome of individual mate choice decisions.

Offspring sired by subordinate red deer males under controlled conditions: did some females prefer not to mate with the alpha male?

Both male-male competition and female choice are important forces in sexual selection that may act in concert. In red deer (Cervus elaphus), rutting activities related to male-male competition are highly conspicuous and have received most research attention. However, there is increasing evidence that females can gain by selecting mates. Due to the additive genetic benefits of a sire’s dominance rank, females may prefer them as mates, so that selection for male traits associated with dominance can be reinforced by female choice. On the other hand, recent evidence suggests that females might prefer male features not related to dominance and thus affect the distribution of mating outcomes. This predicts mating with less dominant males, but no study has so far investigated to which extent some females may do so when the dominant male is available. Here we use controlled captivity conditions to study whether females mate with subordinate males when dominant males are present. By means of parentage analyses conducted after genotyping the offspring, we found that dominant males did not sire all the offspring, the mean percentage of offspring sired by subordinate males being 13.03%. This result has consequences for the evolutionary reinforcement of components of sexual selection in red deer and might be used as a starting point for future studies on the red deer mating system and sexual selection.

Male mate choice, female competition, and female ornaments as components of sexual selection.

Male mate choice, female competition, and female ornaments as components of sexual selection.

Rapid dynamic colour change is an intrasexual signal in a lek breeding frog (Litoria wilcoxii)

Bright colouration appearing in one sex only can be driven by components of sexual selection including female choice, male competition or mate recognition. Male Litoria wilcoxii undergo rapid colour change from brown to yellow during amplexus, however, the function, if any, is unknown. We tested possible behavioural functions by observing breeding aggregations and behavioural responses (colour change, movement, call and amplexus duration) to varying stimuli (including model male and female frogs). We also examined whether colour change was a by-product of hormone release by comparing spermatic urine of frogs injected with epinephrine (colour change hormone) and hCG (triggers spermiation). Finally, the predation cost of being bright yellow was examined by placing frog models (yellow and brown) in the field and measuring predator attack rate. The behavioural responses of males to model females, brown/brown models (female with amplexing brown male), and brown/yellow models (female with amplexing yellow male), were similar to reactions towards real females, with the important exception that males did not attempt amplexus with brown/yellow models. Epinephrine injections triggered colour change but not sperm release in male frogs, while hCG induced sperm release but not colour change. Attack rates were low in predation trials with no difference in attack rates between yellow and brown models observed. Our study presents a novel function for rapid dynamic colour change as an intrasexual signal during amplexus that could avert sperm competition and displacement by other males. Colour displays during breeding are believed to have evolved through mechanisms of sexual selection (female choice, male–male competition or sexual recognition). Stony creek frogs (Litoria wilcoxii) have been observed to rapidly change colour from brown (similar to female colouration) to bright yellow during amplexus, which is unusual as the colour change occurs after mate selection. Behavioural experiments were used to test hypotheses on the evolutionary function of colour change in this species. In L.wilcoxii, colour functions as an intrasexual signal during amplexus, we hypothesise that this could avert sperm competition and/or displacement by other males during amplexus. The function presented here is novel among amphibians, however as data on dynamic colour change in amphibians is lacking, this trait may be more common.

Male personality and female spawning consistency in a goby with exclusive male care

Consistent inter-individual behavioral differences in animals, or animal personalities, are increasingly recognized as an important component of sexual selection. Despite growing evidence for personality-mediated effects on reproductive success in species with biparental care, the link between personality and intrasexual competition or mate choice in species with uniparental care remains largely unexplored. Here, the non-caring sex in particular should seek partners with personality traits that promise high quality care. Therefore, we investigated effects of male personality in the common goby, a well-established model species for sexual selection under exclusive male care. Using competitive trials, we investigated how personality affects male ability to monopolize either of two central reproductive resources, nesting sites (male-male competition) and ready-to-spawn females (female choice). Personalities were scored in a feeding context and thus independent of reproduction. We found that slow-feeding individuals better succeeded in monopolizing nests, but that female spawning decisions were independent of the foraging personality of two male competitors. Instead, females showed a remarkably consistent between-female spawning preference for slightly heavier males and males with more elaborate nests. Our findings indicate that fitness benefits of the male personality type only materialize via male-male competition in environments where nesting sites are a limiting resource, whereas female choice favors large males with elaborate nests independent of male personality. Animal personality can strongly affect reproductive success in species with parental care. We studied this link in the common goby, a fish where males build nests and solely take care of the brood until hatching. Hence, females should choose a male whose personality traits promise high quality care. We scored male personalities and conducted nest-competition and mate-choice trials. We found that male personality had an effect on nest monopolization, but not on female mating preferences. Instead, females agreed remarkably well in their mating decisions and consistently favored large males with elaborate nests. Our results show that male personality is important for nest allocation, which is crucial in environments where nesting sites are rare.

Polymorphisms in a desaturase 2 ortholog associate with cuticular hydrocarbon and male mating success variation in a natural population of Drosophila serrata.

Elucidating the nature of genetic variation underlying both sexually selected traits and the fitness components of sexual selection is essential to understanding the broader consequences of sexual selection as an evolutionary process. To date, there have been relatively few attempts to connect the genetic variance in sexually selected traits with segregating DNA sequence polymorphisms. We set out to address this in a well-characterized sexual selection system--the cuticular hydrocarbons (CHCs) of Drosophila serrata--using an indirect association study design that allowed simultaneous estimation of the genetic variance in CHCs, sexual fitness and single nucleotide polymorphism (SNP) effects in an outbred population. We cloned and sequenced an ortholog of the D. melanogaster desaturase 2 gene, previously shown to affect CHC biosynthesis in D. melanogaster, and associated 36 SNPs with minor allele frequencies > 0.02 with variance in CHCs and sexual fitness. Three SNPs had significant multivariate associations with CHC phenotype (q-value < 0.05). At these loci, minor alleles had multivariate effects on CHCs that were weakly associated with the multivariate direction of sexual selection operating on these traits. Two of these SNPs had pleiotropic associations with male mating success, suggesting these variants may underlie responses to sexual selection due to this locus. There were 15 significant male mating success associations (q-value < 0.1), and interestingly, we detected a nonrandom pattern in the relationship between allele frequency and direction of effect on male mating success. The minor-frequency allele usually reduced male mating success, suggesting a positive association between male mating success and total fitness at this locus.

Female responses to experimental removal of sexual selection components in Drosophila melanogaster.

BackgroundDespite the common assumption that multiple mating should in general be favored in males, but not in females, to date there is no consensus on the general impact of multiple mating on female fitness. Notably, very little is known about the genetic and physiological features underlying the female response to sexual selection pressures. By combining an experimental evolution approach with genomic techniques, we investigated the effects of single and multiple matings on female fecundity and gene expression. We experimentally manipulated the opportunity for mating in replicate populations of Drosophila melanogaster by removing components of sexual selection, with the aim of testing differences in short term post-mating effects of females evolved under different mating strategies.ResultsWe show that monogamous females suffer decreased fecundity, a decrease that was partially recovered by experimentally reversing the selection pressure back to the ancestral state. The post-mating gene expression profiles of monogamous females differ significantly from promiscuous females, involving 9% of the genes tested (approximately 6% of total genes in D. melanogaster). These transcripts are active in several tissues, mainly ovaries, neural tissues and midgut, and are involved in metabolic processes, reproduction and signaling pathways.ConclusionsOur results demonstrate how the female post-mating response can evolve under different mating systems, and provide novel insights into the genes targeted by sexual selection in females, by identifying a list of candidate genes responsible for the decrease in female fecundity in the absence of promiscuity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0239-3) contains supplementary material, which is available to authorized users.

Sexual selection according to Darwin: A response to Padian and Horner's interpretation

The interpretation of exaggerated structures in the vertebrate fossil record has been hampered by disagreement over the definition of sexual selection and how it relates to sexual dimorphism. Previous assertions that Darwin placed a requirement of sexual dimorphism on sexual selection are mistaken. Instead, Darwin describes variation within one sex and the exertion of a struggle (expressed as intrasexual competition, intersexual mate choice, or both) as the necessary components of sexual selection. The use of structures by one sex to attract mates or repel rivals for mates occurs independently of any existing sexual differences. Differential mating success is also a requirement of Darwinian sexual selection. Mutual sexual selection is a legitimate concept that was described by Darwin. Sexual selection remains a viable explanatory hypothesis for the presence of exaggerated structures of extinct organisms and need not be dismissed summarily, but should not be employed without support as a default hypothesis.

Alternative reproductive tactics and sex‐biased gene expression: the study of the bulb mite transcriptome

AbstractThe sexes experience different selective pressures, which can lead to highly divergent phenotypes that are achieved via sex‐biased gene expression. The effect of sexual dimorphism on the degree of sex‐bias in gene expression can be studied in species characterized by sexually selected alternative male phenotypes. We analyzed gene expression in the bulb mite Rhizoglyphus robini (Acari, Acaridae), in which more sexually dimorphic, aggressive fighter males, possessing thickened legs of the third pair which are used to kill rivals, coexist with unarmored scrambler males. We sequenced transcriptomes of adult females and both types of males and de‐novo assembled 114,456 transcriptome‐based gene models (TGMs). Significantly more TGMs had male‐biased expression than female‐biased expression. Among TGMs that were over expressed in one, but not both, male morphs (compared to expression in females), we found about four times more fighter‐biased genes than scrambler‐biased genes. This demonstrates that the degree of expression bias reflects the degree of sexually selected dimorphism. However, the number of sex‐biased genes was much higher than the number of genes differentially expressed between male morphs, and most male‐biased genes were shared between morphs, suggesting that selection pressures act similarly on males irrespective of their morph. Furthermore, we found that male‐biased genes evolved at a faster rate than female‐biased genes, as evidenced by a higher rate of both gene‐turnover and amino acid substitution, indicating that sexual selection, acting more strongly on males, accelerates the rate of molecular evolution. Interestingly, gene turnover was relatively higher, but amino acid substitution rate relatively lower among fighter‐biased genes, suggesting that different components of sexual selection may have different effects on the evolution of sex‐biased genes.

Precopulatory sexual selection in the seed bug Lygaeus equestris: a comparison of choice and no-choice paradigms

Mate choice has long been appreciated as a key component of sexual selection. However, how we measure mate choice, both in the field and in the laboratory, remains problematic. Mating preferences may be tested using either no-choice or choice tests, but explicit comparisons between these two experimental paradigms remain limited. It has been suggested that preferences may be stronger in choice tests as they allow simultaneous comparison, and some studies have indeed found stronger mating preferences in choice tests compared to no-choice tests. Here we explicitly tested the effect of experimental choice paradigm on the measurement of sexual selection on male and female morphology in the promiscuous seed bug Lygaeus equestris (Heteroptera, Lygaeidae). We performed mating trials in which we varied the amount of choice presented to each sex, giving four choice treatments: no-choice, male choice, female choice and mutual choice. Overall we found evidence for significant positive directional selection on female body length and stabilizing selection on an overall measure of male body size. However, we found no significant effect of choice paradigm on the patterns of sexual selection for males or females. We suggest this may be because of the method of mate assessment in L. equestris, which appears to be primarily via contact cues, which may limit simultaneous comparison between options. • We investigated sexual selection on male and female morphology in a lygaeid bug. • We varied the level of choice available to males and females during mating trials. • We found significant sexual selection on male and female morphology. • However we found no effect of choice paradigm on the strength of selection. • The mechanism of mate choice in this species may limit comparison between options.

Do Male Activity Level And Territory Quality Affect Female Association Time in The Brown Anole, Anolis sagrei?

AbstractFemale mate preference is an important component of sexual selection because some male traits, such as physiology and proficiency in acquiring resources, are indicators of male quality. In lizards, the importance of female choice remains unclear as results reported from previous experiments are contradictory. Here, we investigated the influence of male activity level, its associated physiology, and territory quality on female preference, estimated as association time, for male brown anoles, Anolis sagrei. Male A. sagrei were first rated for endurance then used in mismatched‐pair female association trials. Preference was scored as the time a female spent near a male relative to the amount of time she was in his half of the trial arena, and both male and female behaviors were recorded. Levels of blood glucose were measured in males before and after the endurance tests, and before and after the association trials. Levels of stored glycogen were measured in leg muscle and liver samples collected from males after an association trial. A path analysis indicates that final blood glucose levels affect male activity and that female preference is influenced by male activity. In a second experiment, females were given a choice between a male in a territory supplemented with plants and a size‐matched male in a bare territory. Male activity levels were recorded when both males interacted with the female. In those cases, females spent more time with the more active of the two males, regardless of territory quality. Combined, these results suggest that female A. sagrei exhibit preference for male activity regardless of territory quality.

Female responses to experimental removal of sexual selection components in Drosophila melanogaster

Female responses to experimental removal of sexual selection components in Drosophila melanogaster

Multiple paternity and sperm competition in the sibling speciesDrosophila buzzatiiandDrosophila koepferae

Sperm competition (SC) is a major component of sexual selection that enhances intra- and intersexual conflicts and may trigger rapid adaptive evolution of sexual characters. The actual role of SC on rapid evolution, however, is poorly understood. Besides, the relative contribution of distinctive features of the mating system to among species variation in the strength of SC remains unclear. Here, we assessed the strength of SC and mating system factors that may account for it in the closely related species Drosophila buzzatii and Drosophila koepferae. Our analyses reveal higher incidence of multiple paternity and SC risk in D.buzzatii wild-inseminated females. The estimated number of fathers per brood was 3.57 in D.buzzatii and 1.95 in D.koepferae. In turn, the expected proportion of females inseminated by more than one male was 0.89 in D.buzzatii and 0.58 in D.koepferae. Laboratory experiments show that this pattern may be accounted for by the faster rate of stored sperm usage observed in D.koepferae and by the greater female remating rate exhibited by D.buzzatii. We also found that the male reproductive cost of SC is also higher in D.buzzatii. After a female mated with a second male, first-mating male fertility was reduced by 71.4% in D.buzzatii and only 33.3% in D.koepferae. Therefore, we may conclude that postmating sexual selection via SC is a stronger evolutionary force in D.buzzatii than in its sibling.

Opposite differential allocation by males and females of the same species

Differential allocation (DA)-the adjustment of an individual's parental investment in relation to its mate's attractiveness-is increasingly recognized as an important component of sexual selection. However, although DA is expected by both sexes of parents in species with biparental care, DA by males has rarely been investigated. We have previously demonstrated a decrease in the feeding rates of female blue tits Cyanistes caeruleus when their mate's UV coloration was experimentally reduced (i.e. positive DA). In this study, we used the same experimental protocol in the same population to investigate DA by male blue tits in relation to their female's UV coloration. Males mated to UV-reduced females had higher feeding rates than those mated to control females (i.e. negative DA). Thus, male and female blue tits display opposite DA for the same component of parental effort (chick provisioning), the first time that this has been reported for any species.

Rapid adaptation to mammalian sociality via sexually selected traits.

BackgroundLaboratory studies show that the components of sexual selection (e.g., mate choice and intrasexual competition) can profoundly affect the development and fitness of offspring. Less is known, however, about the total effects of sexual selection on offspring in normal social conditions. Complex social networks, such as dominance hierarchies, regulate the opportunity for mating success, and are often missing from laboratory studies. Social selection is an extended view of sexual selection that incorporates competition during sexual and nonsexual interactions, and predicts complex evolutionary dynamics. Whether social selection improves or constrains offspring fitness is controversial.ResultsTo identify fitness consequences of social selection, wild-derived mice that had bred under laboratory conditions for eight generations were re-introduced to naturalistic competition in enclosures for three consecutive generations (promiscuous line). In parallel, a control lineage bred in cages under random mate assignment (monogamous line). A direct competition experiment using second-generation animals revealed that promiscuous line males had greater reproductive success than monogamous line males (particularly during extra-territorial matings), in spite of higher mortality and equivalent success in social dominance and sperm competition. There were no major female fitness effects (though promiscuous line females had fewer litters than monogamous line females). This result suggested that selection primarily acted upon a sexually attractive male phenotype in the promiscuous line, a hypothesis we confirmed in female odor and mating preference trials.ConclusionsWe present novel evidence for the strength of sexual selection under normal social conditions, and show rapid male adaptation driven largely by sexual trait expression, with tradeoffs in survivorship and female fecundity. Re-introducing wild-derived mice to competition quickly uncovers sexually selected phenotypes otherwise lost in normal colony breeding.

Localizing Brain Regions Associated with Female Mate Preference Behavior in a Swordtail

Female mate choice behavior is a critical component of sexual selection, yet identifying the neural basis of this behavior is largely unresolved. Previous studies have implicated sensory processing and hypothalamic brain regions during female mate choice and there is a conserved network of brain regions (Social Behavior Network, SBN) that underlies sexual behaviors. However, we are only beginning to understand the role this network has in pre-copulatory female mate choice. Using in situ hybridization, we identify brain regions associated with mate preference in female Xiphophorus nigrensis, a swordtail species with a female choice mating system. We measure gene expression in 10 brain regions (linked to sexual behavior, reward, sensory integration or other processes) and find significant correlations between female preference behavior and gene expression in two telencephalic areas associated with reward, learning and multi-sensory processing (medial and lateral zones of the dorsal telencephalon) as well as an SBN region traditionally associated with sexual response (preoptic area). Network analysis shows that these brain regions may also be important in mate preference and that correlated patterns of neuroserpin expression between regions co-vary with differential compositions of the mate choice environment. Our results expand the emerging network for female preference from one that focused on sensory processing and midbrain sexual response centers to a more complex coordination involving forebrain areas that integrate primary sensory processing and reward.

Ontogenetic colour change in females as a function of antiharassment strategy

Mate choice by females is an important component of sexual selection in many species. Theoretically, female sexual traits may be influenced by selection acting on the females via male mate choice, while the evolutionary consequences of male mate preferences are largely unknown, especially in the context of sexual conflict. We tested whether male mate choice affects the evolution of female colour in the damselfly Ischnura senegalensis in which females exhibit dimorphism consisting of a gynomorph that experiences ontogenetic colour change and an andromorph that does not. We first quantitatively confirmed that only gynomorphs change their body colour in relation to sexual maturation. In field experiments, males were unwilling to mate with sexually immature gynomorphs, although they attempted to mate with immature andromorphs. This is because males changed their mating preference for female colour depending on previous copulation experiences with sexually mature females. As a result, immature andromorphs received more male harassment than sexually immature gynomorphs, and then showed decreased food intake. Immature-specific colour functioned to avoid costly male harassment during female prereproductive stages, suggesting that ontogenetic colour changes in females have evolved as an antiharassment strategy of females via male mate choice. ► Female damselfly experienced ontogenetic colour change. ► Females with immature-specific colour efficiently avoided male harassment. ► Avoiding harassment increased food intake in females. ► Male choice may lead to the evolution of antiharassment strategy in females.