Postcopulatory fertilization bias as a form of cryptic sexual selection

On the resolution of sexual conflict over shared traits

Anisogamy, different-sized male and female gametes, sits at the heart of sexual selection and conflict between the sexes. Sperm producers (males) and egg producers (females) of the same species generally share most, if not all, of the same genome, but selection frequently favours different trait values in each sex for traits common to both. The extent to which this conflict might be resolved, and the potential mechanisms by which this can occur, have been widely debated. Here, we summarize recent findings and emphasize that once the sexes evolve, sexual selection is ongoing, and therefore new conflict is always possible. In addition, sexual conflict is largely a multivariate problem, involving trait combinations underpinned by networks of interconnected genes. Although these complexities can hinder conflict resolution, they also provide multiple possible routes to decouple male and female phenotypes and permit sex-specific evolution. Finally, we highlight difficulty in the study of sexual conflict over shared traits and promising directions for future research.

Can extreme climatic events induce shifts in adaptive potential? A conceptual framework and empirical test with Anolis lizards

Multivariate adaptation to climatic shifts may be limited by trait integration that causes genetic variation to be low in the direction of selection. However, strong episodes of selection induced by extreme climatic pressures may facilitate future population-wide responses if selection reduces trait integration and increases adaptive potential (i.e., evolvability). We explain this counter-intuitive framework for extreme climatic events in which directional selection leads to increased evolvability and exemplify its use in a case study. We tested this hypothesis in two populations of the lizard Anolis scriptus that experienced hurricane-induced selection on limb traits. We surveyed populations immediately before and after the hurricane as well as the offspring of post-hurricane survivors, allowing us to estimate both selection and response to selection on key functional traits: forelimb length, hindlimb length, and toepad area. The direct selection was parallel in both islands and strong in several limb traits. Even though overall limb integration did not change after the hurricane, both populations showed a non-significant tendency toward increased evolvability after the hurricane despite the direction of selection not being aligned with the axis of most variance (i.e., body size). The population with comparably lower between-limb integration showed a less constrained response to selection. Hurricane-induced selection, not aligned with the pattern of high trait correlations, likely conflicts with selection occurring during normal ecological conditions that favours functional coordination between limb traits, and would likely need to be very strong and more persistent to elicit a greater change in trait integration and evolvability. Future tests of this hypothesis should use G-matrices in a variety of wild organisms experiencing selection due to extreme climatic events.

Chromosome-scale genome assembly of the brown anole (Anolis sagrei), an emerging model species

Rapid technological improvements are democratizing access to high quality, chromosome-scale genome assemblies. No longer the domain of only the most highly studied model organisms, now non-traditional and emerging model species can be genome-enabled using a combination of sequencing technologies and assembly software. Consequently, old ideas built on sparse sampling across the tree of life have recently been amended in the face of genomic data drawn from a growing number of high-quality reference genomes. Arguably the most valuable are those long-studied species for which much is already known about their biology; what many term emerging model species. Here, we report a highly complete chromosome-scale genome assembly for the brown anole, Anolis sagrei - a lizard species widely studied across a variety of disciplines and for which a high-quality reference genome was long overdue. This assembly exceeds the vast majority of existing reptile and snake genomes in contiguity (N50 = 253.6 Mb) and annotation completeness. Through the analysis of this genome and population resequence data, we examine the history of repetitive element accumulation, identify the X chromosome, and propose a hypothesis for the evolutionary history of fusions between autosomes and the X that led to the sex chromosomes of A. sagrei.

Do female amphibians and reptiles have greater reproductive output if they have more mates?

Abstract

In general, males mate with multiple females to increase individual reproductive success. Whether or not, and under what circumstances, females benefit from multiple mating has been less clear. Our review of 154 studies covering 184 populations of amphibians and reptiles showed that polyandry was widespread and variable among and within taxonomic groups. We investigated whether amphibian and reptile females had greater reproductive output as the number of sires for offspring increased. Meta-analysis revealed significant heterogeneity in the dataset of all taxa. Expected heterozygosity was a significant moderator (covariate) of positive relationships between female reproductive output and the number of sires, but a sensitivity test showed the result was tenuous. Significant heterogeneity remained despite controlling for expected heterozygosity and other variables but was resolved for most taxonomic groups with subgroup meta-analyses. Subgroup meta-analyses showed that only female salamanders (Caudata) had significantly greater reproductive output with an increased number of sires. For many species of Caudata, males cannot coerce females into accepting spermatophores. We therefore suggest that if females control the number of matings, they can use polyandry to increase their fitness. Caudata offers ideal models with which to test this hypothesis and to explore factors enabling and maintaining the evolution of female choice. Outstanding problems may be addressed by expanding taxonomic coverage and data collection and improving data reporting.

Significance Statement

Many factors and combinations of factors drive polyandry. Whether or not females benefit from mating with more than one male remains equivocal. Focusing on amphibians and reptiles, our analyses demonstrate that female salamanders produced more offspring when mated with multiple males, whereas this was not the case for reptiles. Unlike many other species in our dataset, the polyandrous female salamanders fully control sperm intake and have chosen to mate multiple times. We further highlight problems and key directions for future research in the field.

Selection on Sperm Count, but Not on Sperm Morphology or Velocity, in a Wild Population of Anolis Lizards

Sperm competition is a widespread phenomenon that shapes male reproductive success. Ejaculates present many potential targets for postcopulatory selection (e.g., sperm morphology, count, and velocity), which are often highly correlated and potentially subject to complex multivariate selection. Although multivariate selection on ejaculate traits has been observed in laboratory experiments, it is unclear whether selection is similarly complex in wild populations, where individuals mate frequently over longer periods of time. We measured univariate and multivariate selection on sperm morphology, sperm count, and sperm velocity in a wild population of brown anole lizards (Anolis sagrei). We conducted a mark-recapture study with genetic parentage assignment to estimate individual reproductive success. We found significant negative directional selection and negative quadratic selection on sperm count, but we did not detect directional or quadratic selection on any other sperm traits, nor did we detect correlational selection on any trait combinations. Our results may reflect pressure on males to produce many small ejaculates and mate frequently over a six-month reproductive season. This study is the first to measure multivariate selection on sperm traits in a wild population and provides an interesting contrast to experimental studies of external fertilizers, which have found complex multivariate selection on sperm phenotypes.

Sperm morphology and count vary with fine-scale changes in local density in a wild lizard population

Given that sperm production can be costly, theory predicts that males should optimally adjust the quantity and/or quality of their sperm in response to their social environment to maximize their paternity success. Although experiments demonstrate that males can alter their ejaculates in response to manipulations of the social environment and studies show that ejaculate traits covary with social environment across populations, it is unknown whether individual variation in sperm traits corresponds to natural variation found within wild populations. Using an island population of brown anole lizards (Anolis sagrei), we tested the prediction that sperm traits (sperm count, sperm morphology, sperm velocity) respond to natural variation in the risk of sperm competition, as inferred from the local density and operational sex ratio (OSR) of conspecifics. We found that males living in high-density areas of the island produced relatively larger sperm midpieces, smaller sperm heads, and lower sperm counts. Sperm traits were unrelated to OSR after accounting for the covariance between OSR and density. Our findings broaden the implications of sperm competition theory to intrapopulation social environment variation by showing that sperm count and sperm morphology vary with fine-scale differences in density within a single wild population.

Estimating encounter rates as the first step of sexual selection in the lizard Anolis sagrei

How individuals move through their environment dictates which other individuals they encounter, determining their social and reproductive interactions and the extent to which they experience sexual selection. Specifically, females rarely have the option of mating with all males in a population-they can only choose among the males they encounter. Further, quantifying phenotypic differences between the males that females encounter and those that sire females' offspring lends insight into how social and reproductive interactions shape male phenotypes. We used an explicitly spatio-temporal Markov chain model to estimate the number of potential mates of Anolis sagrei lizards from their movement behaviour, and used genetic paternity assignments to quantify sexual selection on males. Females frequently encountered and mated with multiple males, offering ample opportunity for female mate choice. Sexual selection favoured males that were bigger and moved over larger areas, though the effect of body size cannot be disentangled from last-male precedence. Our approach corroborates some patterns of sexual selection previously hypothesized in anoles based on describing them as territorial, whereas other results, including female multiple mating itself, are at odds with territorial polygyny, offering insight into discrepancies in other taxa between behavioural and genetic descriptions of mating systems.

Life-history consequences of bidirectional selection for male morph in a male-dimorphic bulb mite

Intralocus sexual conflict (IASC) arises when males and females have different trait optima. Some males pursue different alternative reproductive tactics (ARTs) with different trait optima, resulting in different strengths of IASC. Consequently, for instance daughter fitness is differentially affected by her sire's morph. We tested if-and which-other life-history traits correlatively change in bidirectional, artificial selection experiments for ARTs. We used the male-dimorphic bulb mite Rhizoglyphus robini, the males of which are high-fitness 'fighters' or low-fitness 'scramblers'. Twice in each of the five generations of selection, we assessed clutch composition (number of mites of the various life stages present) and size (total number of offspring). Furthermore, we tracked offspring from egg to adulthood in the first and final generation to detect differences between selection lines in the size and duration of stages, and in maturation time. We found that selection for male morph increased the frequency of that morph. Furthermore, compared to fighter lines, scrambler lines produced more females, which laid larger eggs (in the final generations), and maintained a higher egg-laying rate for longer. Otherwise, our results showed no consistent differences between the selection lines in clutch size and composition, life stage size or duration, or maturation time. Though we found few correlated life-history trait changes in response to selection on male morph, the differences in egg laying rate and egg size suggest that IASC between fighters is costlier to females than IASC with scramblers. We hypothesize that these differences in reproductive traits allow fighter-offspring to perform better in small, declining populations but scrambler-offspring to perform better in large, growing populations.

Paternity Analysis of Wood Turtles (Glyptemys insculpta) Reveals Complex Mating Patterns

Mating system characteristics are of great importance as they may influence male and female reproductive success and reproductive isolation. The wood turtle (Glyptemys insculpta) is a terrestrial freshwater species listed as endangered by the International Union for Conservation of Nature. Considering its conservation status and the paucity of information currently available on parentage relationship for the species, we performed a microsatellite analysis to study the mating system of wood turtles in the Shawinigan River (Québec). We sampled 38 clutches over 2 years (14 in 2006 and 24 in 2007), for a total of 248 offspring genotyped with 7 microsatellite loci. The reconstructed genotypes of the fathers revealed that reproductive success in the sampled clutches varied greatly between males and are positively correlated with the number of mates and clutches sired. Frequency of multiple paternity was estimated at 37% through a consensus of 3 different estimation methods. Positive correlation was observed between the genetic diversity of clutches and the number of fathers. Repeat paternity, however, was observed in 88% of the clutches by the same female in successive years, which suggests either a frequent use of sperm storage, or remating with the same partner in successive years.

Building a new research framework for social evolution: intralocus caste antagonism

The breeding and non‐breeding ‘castes’ of eusocial insects provide a striking example of role‐specific selection, where each caste maximises fitness through different morphological, behavioural and physiological trait values. Typically, queens are long‐lived egg‐layers, while workers are short‐lived, largely sterile foragers. Remarkably, the two castes are nevertheless produced by the same genome. The existence of inter‐caste genetic correlations is a neglected consequence of this shared genome, potentially hindering the evolution of caste dimorphism: alleles that increase the productivity of queens may decrease the productivity of workers and vice versa, such that each caste is prevented from reaching optimal trait values. A likely consequence of this ‘intralocus caste antagonism’ should be the maintenance of genetic variation for fitness and maladaptation within castes (termed ‘caste load’), analogous to the result of intralocus sexual antagonism. The aim of this review is to create a research framework for understanding caste antagonism, drawing in part upon conceptual similarities with sexual antagonism. By reviewing both the social insect and sexual antagonism literature, we highlight the current empirical evidence for caste antagonism, discuss social systems of interest, how antagonism might be resolved, and challenges for future research. We also introduce the idea that sexual and caste antagonism could interact, creating a three‐way antagonism over gene expression. This includes unpacking the implications of haplodiploidy for the outcome of this complex interaction.

Local and systemic immune responses to different types of phytohemagglutinin in the green anole: Lessons for field ecoimmunologists

The phytohemagglutinin (PHA) skin test is commonly used by ecologists to assess cell-mediated immune function of wild animals. It can be performed quickly and easily in the field, involving injection of PHA and measurement of the resultant swelling. There are multiple formulations of PHA used in ecological studies, with potentially differing outcomes that could produce inconsistent results. We tested two common types of PHA in the green anole (Anolis carolinensis) to identify local and systemic immune responses underlying the resultant swelling at 6, 18, 24, and 48 hr post injection. There were differences in both local (injection site) and systemic (blood) leukocyte responses to PHA-L versus PHA-P. PHA-P injection produced a greater overall increase in local heterophil count at the injection site compared with PHA-L, and this response was greatest at 6 and 24 hr post injection. Systemically, heterophil percentage was higher in the blood of PHA-P- versus PHA-L-injected anoles at 24 hr post injection; the time point at which heterophil percentage peaked in PHA-P-injected anoles. These results indicate that although both PHA types are effective tests of immune function in green anoles, the PHA-P swelling response invokes a much stronger heterophilic response. PHA-L is a more specific test of lymphocyte function, particularly at 24 hr post injection, making it preferable for ecoimmunology studies.

Field estimates of parentage reveal sexually antagonistic selection on body size in a population of Anolis lizards

Sexual dimorphism evolves when selection favors different phenotypic optima between the sexes. Such sexually antagonistic selection creates intralocus sexual conflict when traits are genetically correlated between the sexes and have sex‐specific optima. Brown anoles are highly sexually dimorphic: Males are on average 30% longer than females and 150% heavier in our study population. Viability selection on body size is known to be sexually antagonistic, and directional selection favors large male size whereas stabilizing selection constrains females to remain small. We build on previous studies of viability selection by measuring sexually antagonistic selection using reproductive components of fitness over three generations in a natural population of brown anoles. We estimated the number of offspring produced by an individual that survived to sexual maturity (termed RS^V), a measure of individual fitness that includes aspects of both individual reproductive success and offspring survival. We found directional selection on male body size, consistent with previous studies of viability selection. However, selection on female body size varied among years, and included periods of positive directional selection, quadratic stabilizing selection, and no selection. Selection acts differently in the sexes based on both survival and reproduction and sexual conflict appears to be a persistent force in this species.

Direct benefits of choosing a high-fitness mate can offset the indirect costs associated with intralocus sexual conflict

Intralocus sexual conflict generates a cost to mate choice: high-fitness partners transmit genetic variation that confers lower fitness to offspring of the opposite sex. Our earlier work in the fruit fly, Drosophila melanogaster, revealed that these indirect genetic costs were sufficient to reverse potential "good genes" benefits of sexual selection. However, mate choice can also confer direct fitness benefits by inducing larger numbers of progeny. Here, we consider whether direct benefits through enhanced fertility could offset the costs associated with intralocus sexual conflict in D. melanogaster. Using hemiclonal analysis, we found that females mated to high-fitness males produced 11% more offspring compared to those mated to low-fitness males, and high-fitness females produced 34% more offspring than low-fitness females. These direct benefits more than offset the reduction in offspring fitness caused by intralocus sexual conflict, creating a net fitness benefit for each sex to pairing with a high-fitness partner. Our findings highlight the need to consider both direct and indirect effects when investigating the fitness impacts of mate choice. Direct fitness benefits may shelter sexually antagonistic alleles from selection, suggesting a novel mechanism for the maintenance of fitness variation.

Postmating Female Control: 20 Years of Cryptic Female Choice

Cryptic female choice (CFC) represents postmating intersexual selection arising from female-driven mechanisms at or after mating that bias sperm use and impact male paternity share. Although biologists began to study CFC relatively late, largely spurred by Eberhard's book published 20 years ago, the field has grown rapidly since then. Here, we review empirical progress to show that numerous female processes offer potential for CFC, from mating through to fertilization, although seldom has CFC been clearly demonstrated. We then evaluate functional implications, and argue that, under some conditions, CFC might have repercussions for female fitness, sexual conflict, and intersexual coevolution, with ramifications for related evolutionary phenomena, such as speciation. We conclude by identifying directions for future research in this rapidly growing field.

Hormonally Mediated Increases in Sex-Biased Gene Expression Accompany the Breakdown of Between-Sex Genetic Correlations in a Sexually Dimorphic Lizard

The evolution of sexual dimorphism is predicted to occur through reductions in between-sex genetic correlations (r_mf) for shared traits, but the physiological and genetic mechanisms that facilitate these reductions remain largely speculative. Here, we use a paternal half-sibling breeding design in captive brown anole lizards (Anolis sagrei) to show that the development of sexual size dimorphism is mirrored by the ontogenetic breakdown of r_mf for body size and growth rate. Using transcriptome data from the liver (which integrates growth and metabolism), we show that sex-biased gene expression also increases dramatically between ontogenetic stages bracketing this breakdown of r_mf. Ontogenetic increases in sex-biased expression are particularly evident for genes involved in growth, metabolism, and cell proliferation, suggesting that they contribute to both the development of sexual dimorphism and the breakdown of r_mf. Mechanistically, we show that treatment of females with testosterone stimulates the expression of male-biased genes while inhibiting the expression of female-biased genes, thereby inducing male-like phenotypes at both organismal and transcriptomic levels. Collectively, our results suggest that sex-specific modifiers such as testosterone can orchestrate sex-biased gene expression to facilitate the phenotypic development of sexual dimorphism while simultaneously reducing genetic correlations that would otherwise constrain the independent evolution of the sexes.

Sex-specific allelic transmission bias suggests sexual conflict at MC1R

Sexual conflict arises when selection in one sex causes the displacement of the other sex from its phenotypic optimum, leading to an inevitable tension within the genome - called intralocus sexual conflict. Although the autosomal melanocortin-1-receptor gene (MC1R) can generate colour variation in sexually dichromatic species, most previous studies have not considered the possibility that MC1R may be subject to sexual conflict. In the barn owl (Tyto alba), the allele MC1RWHITE is associated with whitish plumage coloration, typical of males, and the allele MC1RRUFOUS is associated with dark rufous coloration, typical of females, although each sex can express any phenotype. Because each colour variant is adapted to specific environmental conditions, the allele MC1RWHITE may be more strongly selected in males and the allele MC1RRUFOUS in females. We therefore investigated whether MC1R genotypes are in excess or deficit in male and female fledglings compared with the expected Hardy-Weinberg proportions. Our results show an overall deficit of 7.5% in the proportion of heterozygotes in males and of 12.9% in females. In males, interannual variation in assortative pairing with respect to MC1R explained the year-specific deviations from Hardy-Weinberg proportions, whereas in females, the deficit was better explained by the interannual variation in the probability of inheriting the MC1RWHITE or MC1RRUFOUS allele. Additionally, we observed that sons inherit the MC1RRUFOUS allele from their fathers on average slightly less often than expected under the first Mendelian law. Transmission ratio distortion may be adaptive in this sexually dichromatic species if males and females are, respectively, selected to display white and rufous plumages.

Phenotypic shifts in urban areas in the tropical lizard Anolis cristatellus

Urbanization is an increasingly important dimension of global change, and urban areas likely impose significant natural selection on the species that reside within them. Although many species of plants and animals can survive in urban areas, so far relatively little research has investigated whether such populations have adapted (in an evolutionary sense) to their newfound milieu. Even less of this work has taken place in tropical regions, many of which have experienced dramatic growth and intensification of urbanization in recent decades. In the present study, we focus on the neotropical lizard, Anolis cristatellus. We tested whether lizard ecology and morphology differ between urban and natural areas in three of the most populous municipalities on the island of Puerto Rico. We found that environmental conditions including temperature, humidity, and substrate availability differ dramatically between neighboring urban and natural areas. We also found that lizards in urban areas use artificial substrates a large proportion of the time, and that these substrates tend to be broader than substrates in natural forest. Finally, our morphological data showed that lizards in urban areas have longer limbs relative to their body size, as well as more subdigital scales called lamellae, when compared to lizards from nearby forested habitats. This shift in phenotype is exactly in the direction predicted based on habitat differences between our urban and natural study sites, combined with our results on how substrates are being used by lizards in these areas. Findings from a common-garden rearing experiment using individuals from one of our three pairs of populations provide evidence that trait differences between urban and natural sites may be genetically based. Taken together, our data suggest that anoles in urban areas are under significant differential natural selection and may be evolutionarily adapting to their human-modified environments.

Intralocus sexual conflict, adaptive sex allocation, and the heritability of fitness

Intralocus sexual conflict arises when selection favours alternative fitness optima in males and females. Unresolved conflict can create negative between-sex genetic correlations for fitness, such that high-fitness parents produce high-fitness progeny of their same sex, but low-fitness progeny of the opposite sex. This cost of sexual conflict could be mitigated if high-fitness parents bias sex allocation to produce more offspring of their same sex. Previous studies of the brown anole lizard (Anolis sagrei) show that viability selection on body size is sexually antagonistic, favouring large males and smaller females. However, sexual conflict over body size may be partially mitigated by adaptive sex allocation: large males sire more sons than daughters, whereas small males sire more daughters than sons. We explored the evolutionary implications of these phenomena by assessing the additive genetic (co)variance of fitness within and between sexes in a wild population. We measured two components of fitness: viability of adults over the breeding season, and the number of their progeny that survived to sexual maturity, which includes components of parental reproductive success and offspring viability (RS(V) ). Viability of parents was not correlated with adult viability of their sons or daughters. RS(V) was positively correlated between sires and their offspring, but not between dams and their offspring. Neither component of fitness was significantly heritable, and neither exhibited negative between-sex genetic correlations that would indicate unresolved sexual conflict. Rather, our results are more consistent with predictions regarding adaptive sex allocation in that, as the number of sons produced by a sire increased, the adult viability of his male progeny increased.

How did viviparity originate and evolve? Of conflict, co-option, and cryptic choice

I propose that the underlying adaptation enabling the reproductive strategy of birthing live young (viviparity) is retraction of the site of fertilization within the female reproductive tract, and that this evolved as a means of postcopulatory sexual selection. There are three conspicuous aspects associated with viviparity: (i) internal development is a complex trait often accompanied by a suite of secondary adaptations, yet it is unclear how the intermediate state of this trait - egg retention - could have evolved; (ii) viviparity often results in a reduction in fecundity; (iii) viviparity has evolved independently many times across a diverse array of animal groups. Focusing on the Diptera (true flies), I provide explanations for these observations. I further propose that fecundity is not traded-off to enable potential benefits of viviparity, but rather that loss of fecundity is directly selected and egg retention is an indirect consequence - a model that provides a unifying common basis for the ubiquity of viviparity.

Human-induced morphological shifts in an island lizard

Understanding the evolutionary consequences of anthropogenic change is an emerging topic in evolutionary biology. While highly sensitive species may go extinct in response to anthropogenic habitat alteration, those with broader environmental tolerances may persist and adapt to the changes. Here, we use morphological data from the brown anole (Anolis sagrei), a lizard species that lives in both natural and human-disturbed habitats, to examine the impact of anthropogenic habitat alteration. We find populations inhabiting disturbed habitats were significantly larger in snout-vent length, hindspan, and mass and provide evidence that the observed divergence in hindspan is driven by human-induced changes in habitat structure. Populations were found to be genetically distinct among islands but are not genetically differentiated between habitat types on islands. Thus, the observed pattern of intra-island morphological differences cannot be explained by separate founding populations. Rather, our results are consistent with morphological differences between habitats having arisen in situ on each island. Results underscore the significant impact anthropogenic change may have on evolutionary trajectories of populations that persist in human-altered habitats.

Experimentally decoupling reproductive investment from energy storage to test the functional basis of a life-history trade-off

The ubiquitous life-history trade-off between reproduction and survival has long been hypothesized to reflect underlying energy-allocation trade-offs between reproductive investment and processes related to self-maintenance. Although recent work has questioned whether energy-allocation models provide sufficient explanations for the survival cost of reproduction, direct tests of this hypothesis are rare, especially in wild populations. This hypothesis was tested in a wild population of brown anole lizards (Anolis sagrei) using a two-step experiment. First, stepwise variation in reproductive investment was created using unilateral and bilateral ovariectomy (OVX) along with intact (SHAM) control. Next, this manipulation was decoupled from its downstream effects on energy storage by surgically ablating the abdominal fat stores from half of the females in each reproductive treatment. As predicted, unilateral OVX (intermediate reproductive investment) induced levels of growth, body condition, fat storage and breeding-season survival that were intermediate between the high levels of bilateral OVX (no reproductive investment) and the low levels of SHAM (full reproductive investment). Ablation of abdominal fat bodies had a strong and persistent effect on energy stores, but it did not influence post-breeding survival in any of the three reproductive treatments. This suggests that the energetic savings of reduced reproductive investment do not directly enhance post-breeding survival, with the caveat that only one aspect of energy storage was manipulated and OVX itself had no overall effect on post-breeding survival. This study supports the emerging view that simple energy-allocation models may often be insufficient as explanations for the life-history trade-off between reproduction and survival.

Inter- and intrasexual genetic correlations of exaggerated traits and locomotor activity

Exaggerated traits in males can be costly and therefore can negatively affect fitness. Although these costs are thought to be male specific, traits that have a negative effect due to exaggeration are often shared between the sexes as life-history traits. When there are genetic intersexual correlations for these shared characters, the evolution of the exaggerated traits can impose these costs on nonadorned females through the intersexual correlation. Thus, the exaggerated traits can constrain optimum development of female characters, even if the females lack these exaggerations completely. However, investigation of this pattern has been largely ignored, and thus, it is necessary to investigate genetic architectures of these traits within and across the sexes. Male flour beetles, Gnatocerus cornutus, have enlarged mandibles that are used in male-male competition, but females lack this character completely. Using a traditional full-sib/half-sib breeding design, we detected a negative intrasexual genetic correlation between male weapon size and locomotor activity, but not an intersexual genetic correlation for locomotor activity. After subjecting this weapon to 17 generations of bidirectional selection, we found a correlated response to locomotor activity in the male, whereas there was no correlated response in the female. Our results suggest that the costs of exaggerated traits to locomotion are not imposed on females and would be male specific. This is partly explained by genetic decoupling of locomotor activities across the sexes.

Two sexes, one genome: the evolutionary dynamics of intralocus sexual conflict

As the evolutionary interests of males and females are frequently divergent, a trait value that is optimal for the fitness of one sex is often not optimal for the other. A shared genome also means that the same genes may underlie the same trait in both sexes. This can give rise to a form of sexual antagonism, known as intralocus sexual conflict (IASC). Here, a tug-of-war over allelic expression can occur, preventing the sexes from reaching optimal trait values, thereby causing sex-specific reductions in fitness. For some traits, it appears that IASC can be resolved via sex-specific regulation of genes that subsequently permits sexual dimorphism; however, it seems that whole-genome resolution may be impossible, due to the genetic architecture of certain traits, and possibly due to the changing dynamics of selection. In this review, we explore the evolutionary mechanisms of, and barriers to, IASC resolution. We also address the broader consequences of this evolutionary feud, the possible interactions between intra- and interlocus sexual conflict (IRSC: a form of sexual antagonism involving different loci in each sex), and draw attention to issues that arise from using proxies as measurements of conflict. In particular, it is clear that the sex-specific fitness consequences of sexual dimorphism require characterization before making assumptions concerning how this relates to IASC. Although empirical data have shown consistent evidence of the fitness effects of IASC, it is essential that we identify the alleles mediating these effects in order to show IASC in its true sense, which is a “conflict over shared genes.”

A general population genetic framework for antagonistic selection that accounts for demography and recurrent mutation

Antagonistic selection--where alleles at a locus have opposing effects on male and female fitness ("sexual antagonism") or between components of fitness ("antagonistic pleiotropy")--might play an important role in maintaining population genetic variation and in driving phylogenetic and genomic patterns of sexual dimorphism and life-history evolution. While prior theory has thoroughly characterized the conditions necessary for antagonistic balancing selection to operate, we currently know little about the evolutionary interactions between antagonistic selection, recurrent mutation, and genetic drift, which should collectively shape empirical patterns of genetic variation. To fill this void, we developed and analyzed a series of population genetic models that simultaneously incorporate these processes. Our models identify two general properties of antagonistically selected loci. First, antagonistic selection inflates heterozygosity and fitness variance across a broad parameter range--a result that applies to alleles maintained by balancing selection and by recurrent mutation. Second, effective population size and genetic drift profoundly affect the statistical frequency distributions of antagonistically selected alleles. The "efficacy" of antagonistic selection (i.e., its tendency to dominate over genetic drift) is extremely weak relative to classical models, such as directional selection and overdominance. Alleles meeting traditional criteria for strong selection (N(e)s >> 1, where N(e) is the effective population size, and s is a selection coefficient for a given sex or fitness component) may nevertheless evolve as if neutral. The effects of mutation and demography may generate population differences in overall levels of antagonistic fitness variation, as well as molecular population genetic signatures of balancing selection.

Intralocus sexual conflict for fitness: sexually antagonistic alleles for testosterone

Intralocus sexual conflict occurs when a trait encoded by the same genetic locus in the two sexes has different optima in males and females. Such conflict is widespread across taxa, however, the shared phenotypic traits that mediate the conflict are largely unknown. We examined whether the sex hormone, testosterone (T), that controls sexual differentiation, contributes to sexually antagonistic fitness variation in the bank vole, Myodes glareolus. We compared (opposite-sex) sibling reproductive fitness in the bank vole after creating divergent selection lines for T. This study shows that selection for T was differentially associated with son versus daughter reproductive success, causing a negative correlation in fitness between full siblings. Our results demonstrate the presence of intralocus sexual conflict for fitness in this small mammal and that sexually antagonistic selection is acting on T. We also found a negative correlation in fitness between parents and their opposite-sex progeny (e.g. father-daughter), highlighting a dilemma for females, as the indirect genetic benefits of selecting reproductively successful males (high T) are lost with daughters. We discuss mechanisms that may mitigate this disparity between progeny quality.

Sex linkage, sex-specific selection, and the role of recombination in the evolution of sexually dimorphic gene expression

Sex-biased genes--genes that are differentially expressed within males and females--are nonrandomly distributed across animal genomes, with sex chromosomes and autosomes often carrying markedly different concentrations of male- and female-biased genes. These linkage patterns are often gene- and lineage-dependent, differing between functional genetic categories and between species. Although sex-specific selection is often hypothesized to shape the evolution of sex-linked and autosomal gene content, population genetics theory has yet to account for many of the gene- and lineage-specific idiosyncrasies emerging from the empirical literature. With the goal of improving the connection between evolutionary theory and a rapidly growing body of genome-wide empirical studies, we extend previous population genetics theory of sex-specific selection by developing and analyzing a biologically informed model that incorporates sex linkage, pleiotropy, recombination, and epistasis, factors that are likely to vary between genes and between species. Our results demonstrate that sex-specific selection and sex-specific recombination rates can generate, and are compatible with, the gene- and species-specific linkage patterns reported in the genomics literature. The theory suggests that sexual selection may strongly influence the architectures of animal genomes, as well as the chromosomal distribution of fixed substitutions underlying sexually dimorphic traits.

The evolution of sex ratio adjustment in the presence of sexually antagonistic selection

Sex ratio adjustment (SRA) of broods has received widespread interest as a means for optimizing parental investment in offspring. Classical explanations for the evolution of SRA focus on improving offspring fitness in light of resource availability or mate attractiveness. Here, we use genetic models to demonstrate that SRA can evolve to alleviate sexual antagonism by improving the chance that the alleles of a sexually antagonistic trait are transmitted to the sex they benefit. In cases where the trait is autosomally inherited, this result is obtained regardless of whether SRA is based on the mother's or the father's genotype and irrespective of the recombination rate between the trait and SRA loci. SRA also evolves in this manner when the trait is sex-linked, provided that SRA decisions are based on the homogametic genotype (XX mothers or ZZ fathers). By contrast, when based on traits in the heterogametic sex, SRA promotes fixation of the allele that is detrimental to that sex, preventing the evolution of substantial levels of SRA. Our models indicate that the evolution of SRA in nature should be strongly influenced by the genetic architecture of the traits on which it is based and the form of selection affecting them.

Paternal condition drives progeny sex-ratio bias in a lizard that lacks parental care

Sex-allocation theory predicts that females in good condition should preferentially produce offspring of the sex that benefits the most from an increase in maternal investment. However, it is generally assumed that the condition of the sire has little effect on progeny sex ratio, particularly in species that lack parental care. We used a controlled breeding experiment and molecular paternity analyses to examine the effects of both maternal and paternal condition on progeny sex ratio and progeny fitness in the brown anole (Anolis sagrei), a polygynous lizard that lacks parental care. Contrary to the predictions of sex-allocation theory, we found no relationship between maternal condition and progeny sex ratio. By contrast, progeny sex ratio shifted dramatically from female-biased to male-biased as paternal condition increased. This pattern was driven entirely by an increase in the production of sons as paternal condition improved. Despite strong natural selection favoring large size and high condition in both sons and daughters, we found no evidence that progeny survival was related to paternal condition. Our results emphasize the importance of considering the paternal phenotype in studies of sex allocation and highlight the need for further research into the pathways that link paternal condition to progeny fitness.

Molecular evidence for high frequency of multiple paternity in a freshwater shrimp species Caridina ensifera

BACKGROUND

Molecular genetic analyses of parentage provide insights into mating systems. Although there are 22,000 members in Malacostraca, not much has been known about mating systems in Malacostraca. The freshwater shrimp Caridina ensifera blue, is a new species belonging to Malacostraca which was discovered recently in Sulawesi, Indonesia. Due to its small body size and low fecundity, this species is an ideal species to study the occurrence and frequency of multiple paternity and to understand of how the low fecundity species persist and evolve.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we developed four polymorphic microsatellites from C. ensifera and applied them to investigate the occurrence and frequency of multiple paternity in 20 C. ensifera broods caught from Lake Matano, Sulawesi. By genotyping the mother and all offspring from each brood we discovered multiple paternity in all 20 broods. In most of the 20 broods, fathers contributed skewed numbers of offspring and there was an apparent inverse correlation between reproductive success of sires and their relatedness to mothers.

CONCLUSIONS/SIGNIFICANCE

Our results in combination with recent reports on multiple paternity in crayfish, crab and lobster species suggests that multiple paternity is common in Malacostraca. Skewed contribution of fathers to the numbers of offspring and inverse correlation between reproductive success of sires and their relatedness to mothers suggest that sperm competition occurred and/or pre- and postcopulatory female choice happen, which may be important for avoiding the occurrence of inbreeding and optimize genetic variation in offspring and for persistence and evolution of low fecundity species.

Reinforcement of gametic isolation in Drosophila

Reinforcement, a process by which natural selection increases reproductive isolation between populations, has been suggested to be an important force in the formation of new species. However, all existing cases of reinforcement involve an increase in mate discrimination between species. Here, I report the first case of reinforcement of postmating prezygotic isolation (i.e., barriers that act after mating but before fertilization) in animals. On the slopes of the African island of São Tomé, Drosophila yakuba and its endemic sister species D. santomea hybridize within a well-demarcated hybrid zone. I find that D. yakuba females from within this zone, but not from outside it, show an increase in gametic isolation from males of D. santomea, an apparent result of natural selection acting to reduce maladaptive hybridization between species. To determine whether such a barrier could evolve under laboratory conditions, I exposed D. yakuba lines derived from allopatric populations to experimental sympatry with D. santomea, and found that both behavioral and gametic isolation become stronger after only four generations. Reinforcement thus appears to be the best explanation for the heightened gametic isolation seen in sympatry. This appears to be the first example in animals in which natural selection has promoted the evolution of stronger interspecific genetic barriers that act after mating but before fertilization. This suggests that many other genetic barriers between species have been increased by natural selection but have been overlooked because they are difficult to study.