Sex-specific Selective Pressures Research Articles

The effect of intergroup competition outcome on ingroup cooperation: insights from the male warrior hypothesis

IntroductionThe Male Warrior Hypothesis (MWH) proposes that sex-specific selective pressures have promoted male cooperation with the ingroup members to outcompete rival groups. However, intergroup conflicts do not occur in isolation and the outcomes of previous competitions may influence group cooperativeness. Since this phenomenon is not well understood, we aimed to shed light on the effect of previous competition outcome on later cooperative behavior under intergroup conflicts. Based on the MWH, we hypothesized that repeated contests between groups could enhance ingroup cooperation, regardless of the outcome of the previous contest because status is at risk, but when competition is not present, participants would move to the symmetric equilibria.MethodsTo test this hypothesis, we recruited 246 individuals organized in groups of 6 and measured cooperation using a threshold public good game over two rounds, manipulating the outcome in the first round to create groups of winners and losers.ResultsOur results show that intergroup conflict scenarios promoted cooperation in both victory and defeat conditions, whereas, in the control scenario only losers increased their cooperation.DiscussionWe argue that winners under the presence of an external threat may enhance in-group cooperation in order to assure their status; whereas, losers may be attempting to regain it.

Sexual dimorphism in subterranean amphipod crustaceans covaries with subterranean habitat type.

Sexual dimorphism can evolve in response to sex-specific selection pressures that vary across habitats. We studied sexual differences in subterranean amphipods Niphargus living in shallow subterranean habitats (close to the surface), cave streams (intermediate), and cave lakes (deepest and most isolated). These three habitats differ because at greater depths there is lower food availability, reduced predation, and weaker seasonality. Additionally, species near the surface have a near-even adult sex ratio (ASR), whereas species from cave lakes have a female-biased ASR. We hypothesized (a) a decrease in sexual dimorphism from shallow subterranean habitats to cave lake species because of weaker sexual selection derived from changes in the ASR and (b) an increase in female body size in cave lakes because of stronger fecundity selection on account of oligotrophy, reduced predation, and weaker seasonality. We measured body size and two sexually dimorphic abdominal appendages for all 31 species and several behaviours related to male competition (activity, risk-taking, exploration) for 12 species. Species with an equal ASR that live close to the surface exhibited sexual dimorphism in all three morphological traits, but not in behaviour. The body size of females increased from the surface to cave lakes, but no such trend was observed in males. In cave lake species, males and females differed neither morphologically nor behaviourally. Our results are consistent with the possibility that sexual and fecundity selection covary across the three habitats, which indirectly and directly, respectively, shape the degree of sexual dimorphism in Niphargus species.

Ecology and the evolution of sex chromosomes.

Sex chromosomes are common features of animal genomes, often carrying a sex determination gene responsible for initiating the development of sexually dimorphic traits. The specific chromosome that serves as the sex chromosome differs across taxa as a result of fusions between sex chromosomes and autosomes, along with sex chromosome turnover-autosomes becoming sex chromosomes and sex chromosomes 'reverting' back to autosomes. In addition, the types of genes on sex chromosomes frequently differ from the autosomes, and genes on sex chromosomes often evolve faster than autosomal genes. Sex-specific selection pressures, such as sexual antagonism and sexual selection, are hypothesized to be responsible for sex chromosome turnovers, the unique gene content of sex chromosomes and the accelerated evolutionary rates of genes on sex chromosomes. Sex-specific selection has pronounced effects on sex chromosomes because their sex-biased inheritance can tilt the balance of selection in favour of one sex. Despite the general consensus that sex-specific selection affects sex chromosome evolution, most population genetic models are agnostic as to the specific sources of these sex-specific selection pressures, and many of the details about the effects of sex-specific selection remain unresolved. Here, I review the evidence that ecological factors, including variable selection across heterogeneous environments and conflicts between sexual and natural selection, can be important determinants of sex-specific selection pressures that shape sex chromosome evolution. I also explain how studying the ecology of sex chromosome evolution can help us understand important and unresolved aspects of both sex chromosome evolution and sex-specific selection.

Sex-specific plasticity in a trophic polymorphic aquatic predator: a modeling approach.

Phenotypic plasticity is common among animal taxa. While there are clearly limits and likely costs to plasticity, these costs are unknown for most organisms. Further, as plasticity is partially genetically determined, the potential magnitude of exhibited plasticity may vary among individuals. In addition to phenotypic plasticity, various animal taxa also display sexual size dimorphism, a feature ultimately thought to arise due to differential size-dependent fitness costs and benefits between sexes. We hypothesized that differential selection acting on males and females can indirectly select for unequal genetically defined plasticity potential between the sexes. We evaluate this possibility for Eurasian perch (Perca fluviatilis), a species that displays modest sexual size dimorphism and habitat-related morphological plasticity. Using 500-year simulations of an ecogenetic agent-based model, we demonstrate that genetically determined morphological plasticity potential may evolve differently for males and females, leading to greater realized morphological variation between habitats for one sex over the other. Genetically determined potential for plasticity evolved differently between sexes across (a) various sex-specific life-history differences and (b) a variety of assumed costs of plasticity acting on both growth and survival. Morphological analyses of Eurasian perch collected in situ were consistent with model predictions: realized morphological variation between habitats was greater for females than males. We suggest that due to sex-specific selective pressures, differences in male and female genetically defined potential for plasticity may be a common feature across organisms.

Sex-specific plasticity across generations I: Maternal and paternal effects on sons and daughters.

Intergenerational plasticity or parental effects-when parental environments alter the phenotype of future generations-can influence how organisms cope with environmental change. An intriguing, underexplored possibility is that sex-of both the parent and the offspring-plays an important role in driving the evolution of intergenerational plasticity in both adaptive and non-adaptive ways. Here, we evaluate the potential for sex-specific parental effects in a freshwater population of three-spined sticklebacks Gasterosteus aculeatus by independently and jointly manipulating maternal and paternal experiences and separately evaluating their phenotypic effects in sons versus daughters. We tested the adaptive hypothesis that daughters are more responsive to cues from their mother, whereas sons are more responsive to cues from their father. We exposed mothers, fathers or both parents to visual cues of predation risk and measured offspring antipredator traits and brain gene expression. Predator-exposed fathers produced sons that were more risk-prone, whereas predator-exposed mothers produced more anxious sons and daughters. Furthermore, maternal and paternal effects on offspring survival were non-additive: offspring with a predator-exposed father, but not two predator-exposed parents, had lower survival against live predators. There were also strong sex-specific effects on brain gene expression: exposing mothers versus fathers to predation risk activated different transcriptional profiles in their offspring, and sons and daughters strongly differed in the ways in which their brain gene expression profiles were influenced by parental experience. We found little evidence to support the hypothesis that offspring prioritize their same-sex parent's experience. Parental effects varied with both the sex of the parent and the offspring in complicated and non-additive ways. Failing to account for these sex-specific patterns (e.g. by pooling sons and daughters) would have underestimated the magnitude of parental effects. Altogether, these results draw attention to the potential for sex to influence patterns of intergenerational plasticity and raise new questions about the interface between intergenerational plasticity and sex-specific selective pressures, sexual conflict and sexual selection.

Sex-Specific Selection Drives the Evolution of Alternative Splicing in Birds.

Males and females of the same species share the majority of their genomes, yet they are frequently exposed to conflicting selection pressures. Gene regulation is widely assumed to resolve these conflicting sex-specific selection pressures, and although there has been considerable focus on elucidating the role of gene expression level in sex-specific adaptation, other regulatory mechanisms have been overlooked. Alternative splicing enables different transcripts to be generated from the same gene, meaning that exons which have sex-specific beneficial effects can in theory be retained in the gene product, whereas exons with detrimental effects can be skipped. However, at present, little is known about how sex-specific selection acts on broad patterns of alternative splicing. Here, we investigate alternative splicing across males and females of multiple bird species. We identify hundreds of genes that have sex-specific patterns of splicing and establish that sex differences in splicing are correlated with phenotypic sex differences. Additionally, we find that alternatively spliced genes have evolved rapidly as a result of sex-specific selection and suggest that sex differences in splicing offer another route to sex-specific adaptation when gene expression level changes are limited by functional constraints. Overall, our results shed light on how a diverse transcriptional framework can give rise to the evolution of phenotypic sexual dimorphism.

The Male Warrior Hypothesis: Testosterone-related Cooperation and Aggression in the Context of Intergroup Conflict

The Male Warrior Hypothesis (MWH) establishes that men’s psychology has been shaped by inter-group competition to acquire and protect reproductive resources. In this context, sex-specific selective pressures would have favored cooperation with the members of one’s group in combination with hostility towards outsiders. We investigate the role of developmental testosterone, as measured indirectly through static markers of prenatal testosterone (2D:4D digit ratio) and pubertal testosterone (body musculature and facial masculinity), on both cooperation and aggressive behavior in the context of intergroup conflict among men. Supporting the MWH, our results show that the intergroup conflict scenario promotes cooperation within group members and aggression toward outgroup members. Regarding the hormonal underpinnings of this phenomenon, we find that body musculature is positively associated with aggression and cooperation, but only for cooperation when context (inter-group competition) is taken into account. Finally, we did not find evidence that the formidability of the group affected individual rates of aggression or cooperation, controlling for individual characteristics.

Pheomelanin-based coloration is related to individual quality and oxidative stress in blue petrels

In several species, the rusty color of hair or feathers is due to pheomelanin pigments, whose adaptive function is unknown. Pheomelanin may be costly because it is phototoxic and its production consumes a key intracellular antioxidant. Pheomelanin-based traits are, however, positively associated with individual quality in several bird species, where they have thus been suggested to have evolved through sexual selection. Here we investigated the signaling potential of the pheomelanin-based coloration of the crown feathers in the blue petrel. Although this pelagic seabird is nocturnal at the breeding colony and breeds within deep burrows, it might use visual communication when settled on the water during daytime. We tested the correlation between crown color and several fitness-related traits, and we found that higher-quality females displayed less-orange crown than poorer-quality females. This result is inconsistent with an adaptive function of pheomelanin-based coloration in inter-, or intra-, sexual selection in females. We suggest that it might, however, be in line with a signaling function of eumelanin-based coloration, if inter-individual variations in orange coloration are mainly due to eumelanin-to-pheomelanin ratio, rather than to pheomelanin quantity. In contrast to females, we did not find strong evidence for associations between melanin-based coloration and individual quality in males, suggesting sex-specific selective pressures on melanin-based traits in this species.

Does sexual dimorphism exist in flowering phenology traits in anemophilous dioecious species? A test with Rumex acetosa.

Sexual dimorphism in flowering phenology traits may have evolved under sexual selection and vector-mediated selection. The conspicuous sexual dimorphism and sex-specific selection pressures in flowering phenology traits have been investigated mainly in entomophilous dioecious plants, whereas little is known about this in anemophilous plants. We examined sexual dimorphism in flowering onset, flowering peak, flowering duration, maximum proportion of open flowers per inflorescence branch, maximum proportion of newly opening flowers on a given date per branch, and longevity of individual flowers in natural Rumex acetosa populations. Correlations between flowering phenology traits and the degree of flowering overlap with the opposite sex were examined. We also tested whether the overlap of female flowering with male flowering enhanced seed set in female plants. Little sexual dimorphism was observed in flowering onset, peak, duration, and maximum proportion of newly opening flowers. Females had greater floral longevity and greater maximum proportion of open flowers than males. Flowering overlap with the opposite sex significantly increased with the maximum proportion of newly opening flowers and decreased with temporal deviation in the flowering peak in both sexes. Females with greater flowering overlap with males set more seeds in two of the three study populations. In wind-pollinated R. acetosa, little sexual dimorphism in phenological traits may have evolved to achieve synchronous flowering with the opposite sex. Our results suggest that, in angiosperms, not only common selection but also anemophily-specific selection may shape little sexual dimorphism in R. acetosa, unlike in entomophilous plants.

Sexual Dimorphism in the Age of Genomics: How, When, Where.

In mammals, sex chromosomes start to program autosomal gene expression and epigenetic patterns very soon after fertilization. Yet whether the resulting sex differences are perpetuated throughout development and how they connect to the sex-specific expression patterns in adult tissues is not known. There is a dearth of information on the timing and continuity of sex biases during development. It is also unclear whether sex-specific selection operates during embryogenesis. On the other hand, there is mounting evidence that all adult tissues exhibit sex-specific expression patterns, some of which are independent of hormonal influence and due to intrinsic regulatory effects of the sex chromosome constitution. There are many diseases with origins during embryogenesis that also exhibit sex biases. Epigenetics has provided us with viable mechanisms to explain how the genome stores the memory of developmental events. We propose that some of these marks can be traced back to the sex chromosomes, which interact with the autosomes and establish sex-specific epigenetic features soon after fertilization. Sex-biased epigenetic marks that linger after reprograming may reveal themselves at the transcriptional level at later developmental stages and possibly, throughout the lifespan. Detailed molecular information on the ontogeny of sex biases would also elucidate the sex-specific selective pressures operating on embryos and how compensatory mechanisms evolved to resolve sexual conflict.

Plumage melanism is linked to male quality, female parental investment and assortative mating in an alpine songbird

Melanin is a common pigment that can indicate individual quality to potential mates and social dominance to competitors. For many avian species, plumage melanism varies within both males and females. If predictive of individual quality, sexual selection may drive contrasting expressions of melanism for each sex, reflecting their different reproductive roles. For an alpine population of horned larks, Eremophila alpestris, we investigated whether plumage ornament size and darkness predicted morphometric traits, parental care and fitness correlates, and we evaluated support for assortative pairing. Males with darker crowns and larger masks were associated with greater wing length and mass, respectively. Males with darker ornaments were paired with females that provisioned nestlings at a higher rate, began nesting earlier in the season, had larger clutch sizes and ultimately had greater nest success. For females, plumage ornaments did not predict morphometric traits (i.e. wing length, mass), provisioning rate of the male partner, clutch initiation date or reproductive success. Instead, females with darker crowns provisioned nestlings more frequently and had larger clutches. Overall darker males tended to pair with darker females, but plumage ornament size did not correlate within mated pairs. We demonstrate that: (1) the darkness of plumage ornaments is a better predictor of fitness than ornament size and (2) darker males are associated with greater individual quality and reproductive investment of the female mate, while female darkness primarily predicts reproductive investment. These results indicate the potential for sex-specific selection pressures and mate choice based on the predictive value of plumage melanism.

Fluctuating asymmetry as a bio-marker to account for in conservation and management of cave-dwelling species

The applicability of fluctuating asymmetry (FA) in conservation biology as a bio-marker and an early warning system have been examined in various groups of animals. However, cave-dwelling invertebrates have gotten little attention and yet caves have no biological management. Thus, developing a bio-marker to identify early responses of cave-dwelling species to stress provides a valuable tool to account for in future conservation and management programs. We examined whether FA can be used as a bio-marker of relaxed selection for cave-dwelling invertebrates. Specifically, we compared FA levels of pedipalps and two segments of chelicerae (chelicel1 and chelicel2) between cave and surface habitats and among populations in a troglophilic harvestman specie, Paranemastoma sillii sillii (Herman, 1871). Under the assumption that cave environment exerts relaxed selection we hypothesised that cave-dwelling harvestmen express higher degree of FA than surface-dwelling harvestmen. Because female and male harvestmen use the pediplaps differently, we assumed sex-specific selective pressures and further examined the effect of sex and habitat (cave vs surface) and sex interaction on FA levels. We hypothesised no differences in the levels of FA among harvestman cave populations, because of the idea that relaxed selection is responsible for higher levels of FA in cave-dwelling harvestmen rather than environmental stress. We found significant habitat sex interactions in the FA of the pedipalp. Interactions of population and sex had a significant effect on pedipalp and chelicel2 FA. These results partially support the idea that FA responds to differences in the strength of selection. Although we cannot fully disentangle the mechanisms of natural selection affecting FA and the study of trait asymmetry should be combined with studies of other sources of stress (i.e. genetic stress, predation, prey availability) we conclude that FA could be a bioindicator to be used as a tool in conservation and management of cave.

Evolution of sex-specific pace-of-life syndromes: causes and consequences

Males and females commonly differ in their life history optima and, consequently, in the optimal expression of life history, behavioral and physiological traits involved in pace-of-life syndromes (POLS). Sex differences in mean trait expression typically result if males and females exhibit different fitness optima along the same pace-of-life continuum, but the syndrome structure may also differ for the sexes. Due to sex-specific selective pressures imposed by reproductive roles and breeding strategies, the sexes may come to differ in the strength of correlation among traits, or different traits may covary in males and females. Ignorance of these selective forces operating between and within the sexes may lead to flawed conclusions about POLS manifestation in the species, and stand in the way of understanding the evolution, maintenance, and variability of POLS. We outline ways in which natural and sexual selection influence sex-specific trait evolution, and describe potential ultimate mechanisms underlying sex-specific POLS. We make predictions on how reproductive roles and the underlying sexual conflict lead to sex-specific trait covariances. These predictions lead us to conclude that sexual dimorphism in POLS is expected to be highly prevalent, allow us to assess possible consequences for POLS evolution, and provide guidelines for future studies.

Behavioral repeatability of flour beetles before and after metamorphosis and throughout aging

Behavioral repeatability is an important trait relevant to personality research and to behavioral ecology in general. We examined here the behavioral repeatability of two activity-related traits: movement and edge preference (proportion of time spent next to the test arena edge). We used the red flour beetle as our test species in order to determine whether repeatability changes throughout metamorphosis and aging and whether there are inter-sexual differences. Young adults moved more than larvae, but movement activity generally declined with age. Behavioral repeatability was high between two successive measurements for larvae and young adults, but did not persist through metamorphosis. These findings support most of the previous studies on insects and probably reflect the occurrence of phenotypic reorganization during metamorphosis. Neither of our predictions of an increase in behavioral repeatability throughout aging and of a higher repeatability of young adults than larvae held true, as repeatability levels estimates remained similar across ontogeny and aging. Similar to other studies on repeatability, estimates declined with the time interval between two pairs of measurements, suggesting an episodic physiological basis for the documented behaviors. Finally, we detected several inter-sexual differences. Females were more active than males and expressed a shallower decline in movement with aging. Female movement activity was also more repeatable, but female edge preference was less repeatable than that of males. We reason that such differences in repeatability may be driven by sex-specific selection pressures on behavioral tendencies. We investigate several questions regarding behavioral repeatability in insects. The two main questions asked are (1) whether inter-individual behavioral differences remain consistent across larvae and adult stage, and (2) how repeatability changes throughout aging. The first question has been only rarely tested in insects that undergo a full metamorphosis. It has been recognized as a timely question and as an opportunity for researchers studying animal personality. Previous results regarding amphibians and insects that undergo partial metamorphosis were inconclusive. The second question is important, because studies rarely follow individuals throughout their lives. This is essential to understand the consequences of aging. We also report on differences between males and females, probably indicating on differences in motivation for activity and in patterns of aging. Finally, the question, how behavior changes with ontogeny, has been already recognized by Tinbergen as a basic one in animal behavior.

Transcriptome Differences between Alternative Sex Determining Genotypes in the House Fly, Musca domestica.

Sex determination evolves rapidly, often because of turnover of the genes at the top of the pathway. The house fly, Musca domestica, has a multifactorial sex determination system, allowing us to identify the selective forces responsible for the evolutionary turnover of sex determination in action. There is a male determining factor, M, on the Y chromosome (YM), which is probably the ancestral state. An M factor on the third chromosome (IIIM) has reached high frequencies in multiple populations across the world, but the evolutionary forces responsible for the invasion of IIIM are not resolved. To test whether the IIIM chromosome invaded because of sex-specific selection pressures, we used mRNA sequencing to determine whether isogenic males that differ only in the presence of the YM or IIIM chromosome have different gene expression profiles. We find that more genes are differentially expressed between YM and IIIM males in testis than head, and that genes with male-biased expression are most likely to be differentially expressed between YM and IIIM males. We additionally find that IIIM males have a “masculinized” gene expression profile, suggesting that the IIIM chromosome has accumulated an excess of male-beneficial alleles because of its male-limited transmission. These results are consistent with the hypothesis that sex-specific selection acts on alleles linked to the male-determining locus driving evolutionary turnover in the sex determination pathway.

Numerous transitions of sex chromosomes in Diptera.

Many species groups, including mammals and many insects, determine sex using heteromorphic sex chromosomes. Diptera flies, which include the model Drosophila melanogaster, generally have XY sex chromosomes and a conserved karyotype consisting of six chromosomal arms (five large rods and a small dot), but superficially similar karyotypes may conceal the true extent of sex chromosome variation. Here, we use whole-genome analysis in 37 fly species belonging to 22 different families of Diptera and uncover tremendous hidden diversity in sex chromosome karyotypes among flies. We identify over a dozen different sex chromosome configurations, and the small dot chromosome is repeatedly used as the sex chromosome, which presumably reflects the ancestral karyotype of higher Diptera. However, we identify species with undifferentiated sex chromosomes, others in which a different chromosome replaced the dot as a sex chromosome or in which up to three chromosomal elements became incorporated into the sex chromosomes, and others yet with female heterogamety (ZW sex chromosomes). Transcriptome analysis shows that dosage compensation has evolved multiple times in flies, consistently through up-regulation of the single X in males. However, X chromosomes generally show a deficiency of genes with male-biased expression, possibly reflecting sex-specific selective pressures. These species thus provide a rich resource to study sex chromosome biology in a comparative manner and show that similar selective forces have shaped the unique evolution of sex chromosomes in diverse fly taxa.

Disentangling the Size and Shape Components of Sexual Dimorphism

Many organisms are sexually dimorphic, reflecting sex-specific selection pressures. But although sexual dimorphism may consist of different variables from size to shape and physiology, most research emphasizes a single aspect of sexual dimorphism, notably size, without specifying its components and their relationship. Among terrestrial animals, spiders exhibit most extreme sex-specific differences in size and abdominal shape, and therefore represent ideal models to address this question. Here, we dissect sexual dimorphism in spiders at two phylogenetic hierarchical levels. At the species level, we employ comparative phylogenetic tests to explore the association between sexual shape dimorphism (SShD) and sexual size dimorphism (SSD) in the orbweb clade Argiopinae. At the genus level, we then explore such patterns on a phylogeny of orb weavers (Araneoidea). Female argiopines had more diverse abdominal morphotypes than the males and the abdominal shape evolution was only poorly correlated between the sexes. Phylogenetic and comparative data suggested that evolution of SShD in argiopines was related to geographic history, but that sexually shape monomorphic cases arose through selection for male size, perhaps acting against fecundity selection. While in argiopines there was no clear association between SShD and SSD, we detected a significant correlation in all orb weavers at the genus level. The shape and the size components of sexual dimorphism may thus respond independently to selection pressures, but at certain phylogenetic levels SSD may be a prerequisite for SShD. Research on other animal groups is needed to establish whether the here detected patterns on spiders are general.

Evolution of opsin expression in birds driven by sexual selection and habitat.

Theories of sexual and natural selection predict coevolution of visual perception with conspecific colour and/or the light environment animals occupy. One way to test these theories is to focus on the visual system, which can be achieved by studying the opsin-based visual pigments that mediate vision. Birds vary greatly in colour, but opsin gene coding sequences and associated visual pigment spectral sensitivities are known to be rather invariant across birds. Here, I studied expression of the four cone opsin genes (Lws, Rh2, Sws2 and Sws1) in 16 species of New World warblers (Parulidae). I found levels of opsin expression vary both across species and between the sexes. Across species, female, but not male Sws2 expression is associated with an index of sexual selection, plumage dichromatism. This fits predictions of classic sexual selection models, in which the sensory system changes in females, presumably impacting female preference, and co-evolves with male plumage. Expression of the opsins at the extremes of the light spectrum, Lws and Uvs, correlates with the inferred light environment occupied by the different species. Unlike opsin spectral tuning, regulation of opsin gene expression allows for fast adaptive evolution of the visual system in response to natural and sexual selection, and in particular, sex-specific selection pressures.

RETRACTED ARTICLE: Pre-dispersal seed predation in gynodioecious Geranium sylvaticum is not affected by plant gender or flowering phenology

Sex-specific interactions with antagonists may explain female maintenance in gynodioecious populations if seeds produced by hermaphroditic plants are preferred over seeds produced by female plants. Among antagonistic interactions, pre-dispersal seed predators have received relatively little attention even though they may exert sex-specific selective pressures on the evolution of floral and flowering traits. In this work, I investigate temporal variation in seed predation in gynodioecious Geranium sylvaticum, where in addition to female and hermaphrodite individuals, plants with an intermediate sexual expression are also present in most populations. Specifically, I examined whether seed predation is linked to flowering phenology, plant gender, and sexual dimorphism in floral and seed traits over the flowering season using an experimental field population. Within the population, I selected female, intermediate, and hermaphrodite plants with different timing of flowering onset (early, mid, or late), and collected seeds across the fruiting period. Seeds were weighed and examined for seed predator damage. The results show that the three genders experienced similar levels of seed predation attack regardless of their flowering phenology, and that overall seed predation was not related to changes in seed production or seed mass. These results suggest that sexual dimorphism in seed predation cannot be responsible for female maintenance in this species.

Transcriptome assemblies for studying sex-biased gene expression in the guppy, Poecilia reticulata.

BackgroundSexually dimorphic phenotypes are generally associated with differential gene expression between the sexes. The study of molecular evolution and genomic location of these differentially expressed, or sex-biased, genes is important for understanding inter-sexual divergence under sex-specific selection pressures. Teleost fish provide a unique opportunity to examine this divergence in the presence of variable sex-determination mechanisms of recent origin. The guppy, Poecilia reticulata, displays sexual dimorphism in size, ornaments, and behavior, traits shaped by natural and sexual selection in the wild.ResultsTo gain insight into molecular mechanisms underlying the guppy’s sexual dimorphism, we assembled a reference transcriptome combining genome-independent as well as genome-guided assemblies and analyzed sex-biased gene expression between different tissues of adult male and female guppies. We found tissue-associated sex-biased expression of genes related to pigmentation, signal transduction, and spermatogenesis in males; and growth, cell-division, extra-cellular matrix organization, nutrient transport, and folliculogenesis in females. While most sex-biased genes were randomly distributed across linkage groups, we observed accumulation of ovary-biased genes on the sex linkage group, LG12. Both testis-biased and ovary-biased genes showed a significantly higher rate of non-synonymous to synonymous substitutions (dN/dS) compared to unbiased genes. However, in somatic tissues only female-biased genes, including those co-expressed in multiple tissues, showed elevated ratios of non-synonymous substitutions.ConclusionsOur work identifies a set of annotated gene products that are candidate factors affecting sexual dimorphism in guppies. The differential genomic distribution of gonad-biased genes provides evidence for sex-specific selection pressures acting on the nascent sex chromosomes of the guppy. The elevated rates of evolution of testis-biased and female-biased genes indicate differing evolution under distinct selection pressures on the reproductive versus non-reproductive tissues.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-400) contains supplementary material, which is available to authorized users.