The evolution of male genitalia: patterns of genetic variation and covariation in the genital sclerites of the dung beetle Onthophagus taurus

Divergent Genital Morphologies and Female-Male Covariation in Watersnakes

Genital evolution can be driven by diverse selective pressures. Across taxa we see evidence of covariation between males and females, as well as divergent genital morphologies between closely related species. Quantitative analyses of morphological changes in coevolving male and female genitalia have not yet been shown in vertebrates. This study uses 2D and 3D geometric morphometrics to quantitatively compare the complex shapes of vaginal pouches and hemipenes across three species of watersnakes (the sister taxa Nerodia fasciata, N. sipedon, and a close relative N. rhombifer) to address the relationship between genital morphology and divergence time in a system where sexual conflict may have driven sexually antagonistic coevolution of genital traits. Our pairwise comparisons of shape differences across species show that the sister species have male and female genitalia that are significantly different from each other, but more similar to each other than to N. rhombifer. We also determine that the main axes of shape variation are the same for males and females, with changes that relate to deeper bilobation of the vaginal pouch and hemipenes. In males, the protrusion of the region of spines at the base of the hemipene trades off with the degree of bilobation, suggesting amelioration of sexual conflict, perhaps driven by changes in the relative size of the entrance of the vaginal pouch that could have made spines less effective.

Gender Bias in the Study of Genital Evolution: Females Continue to Receive Less Attention than Males

The evolution of female genitalia has historically received less attention than male reproductive organs. Several papers have underscored the disparities in research efforts, but have calls for change resonated with the scientific community and rectified the skew? A literature review was conducted of journal articles published between 2013 through 2021 that explore genital evolution to determine if gender bias (sex of research subject) and imbalance (sex of researcher) have changed. Of the 334 articles that specifically explored genital evolution, first authors of both sexes published on female genitalia less than half as often as male genitalia, although the majority of authors published on genitalia of both sexes. First authors of both sexes mentioned females after males substantially more often than females before males. Female first authors published the most about genital evolution in all taxa except for insects and arachnids. Female first authors published in high impact journals marginally less often than male first authors. Articles about genital evolution across taxa generally had high impact factors, but how impact factors and number of citations varied by the sex of the subject was not clear. Although the number of studies exploring genital co-evolution between the sexes has increased across taxa and years, female genitalia continue to be researched less often than male genitalia when only one sex is investigated. Both female and male scientists are publishing in the field of genital evolution, although research on female subjects continue to lag behind males, demonstrating continued bias within the discipline.

Direct and indirect effects of male genital elaboration in female seed beetles

Our understanding of coevolution between male genitalia and female traits remains incomplete. This is perhaps especially true for genital traits that cause internal injuries in females, such as the spiny genitalia of seed beetles where males with relatively long spines enjoy a high relative fertilization success. We report on a new set of experiments, based on extant selection lines, aimed at assessing the effects of long male spines on females in Callosobruchus maculatus. We first draw on an earlier study using microscale laser surgery, and demonstrate that genital spines have a direct negative (sexually antagonistic) effect on female fecundity. We then ask whether artificial selection for long versus short spines resulted in direct or indirect effects on female lifetime offspring production. Reference females mating with males from long-spine lines had higher offspring production, presumably due to an elevated allocation in males to those ejaculate components that are beneficial to females. Remarkably, selection for long male genital spines also resulted in an evolutionary increase in female offspring production as a correlated response. Our findings thus suggest that female traits that affect their response to male spines are both under direct selection to minimize harm but are also under indirect selection (a good genes effect), consistent with the evolution of mating and fertilization biases being affected by several simultaneous processes.

Sexual selection on the genital lobes of male Drosophila simulans

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

Multivariate stabilizing sexual selection and the evolution of male and female genital morphology in the red flour beetle

Male genitals are highly divergent in animals with internal fertilization. Most studies attempting to explain this diversity have focused on testing the major hypotheses of genital evolution (the lock‐and‐key, pleiotropy, and sexual selection hypotheses), and quantifying the form of selection targeting male genitals has played an important role in this endeavor. However, we currently know far less about selection targeting female genitals or how male and female genitals interact during mating. Here, we use formal selection analysis to show that genital size and shape is subject to strong multivariate stabilizing sexual selection in both sexes of the red flour beetle, Tribolium castaneum. Moreover, we show significant sexual selection on the covariance between the sexes for specific aspects of genital shape suggesting that male and female genitalia also interact to determine the successful transfer of a spermatophore during mating. Our work therefore highlights the important role that both male and female genital morphologies play in determining mating success and that these effects can occur independently, as well as through their interaction. Moreover, it cautions against the overly simplistic view that the sexual selection targeting genital morphology will always be directional in form and restricted primarily to males.

Coevolution between male and female genitalia in Belostoma angustum Lauck, 1964 (Insecta, Heteroptera, Belostomatidae): disentangling size and shape

Sexual and natural selection mechanisms might drive variation in the genitalia of male animals. All aforementioned mechanisms are known to predict the coevolution of male and female genital morphology. Belostoma angustum is known to have subtle variation in the male and female genitalia of its members. In this species, phallosoma with dorsal arms and ventral diverticulum are assumed to be intromittent male genital traits that interact with the female genital chamber. We thus evaluated the existence of variation after disentangling the size from the shape of male genitalia in B. angustum. Body and genitalia dimensions and photographs of phallosoma with dorsal arms, ventral diverticulum and lateral views of the right paramere (the non-intromittent part) were obtained. Semi-landmarks and landmarks were used to capture phenotypic variation, by eliminating all non-shape variation with a Procrustes superimposition. Male and female specimens collected from the same location or immediate vicinity were grouped, and 12 groups originating from 12 locations were used to conduct two block-Partial Least Squares analyses (PLS). Group structures were also taken into account by adopting a multilevel approach. The male and female genital traits had similarly shallow static allometry slopes, as well as the dispersion values around the mean (i.e. coefficient of variation) and the standard error of the estimate. The correlation between the pooled within-locality covariance matrix of the symmetric component of phallosoma with dorsal arms and the female genital chamber was significant (r-PLS=0.37), as well as that with male body dimensions (r-PLS=0.36), even after controlling for allometry. Specimens with lower PLS shape scores had narrower phallosoma with dorsal arms, with poorly curved outer margins of the dorsal arms, whereas specimens with higher PLS shape scores had slightly shorter dorsal arms, with strongly curved outer margins. Lower shape scores were associated with narrower and especially shorter and narrower female genital chambers. Similar shallow allometric curves among sexes and the correlation between intromittent male parts and the female genital chamber, as well as male dimensions, suggest the coevolution of these contact structures in size and in shape.

Microsurgical manipulation reveals pre-copulatory function of key genital sclerites

The copulatory organs of male insects are generally complex, species-specific arrangements of hardened sclerotized plates bound together by flexible, less sclerotized cuticle. Their extensive morphological diversification is a recurrent pattern in the evolutionary radiation of animals, yet a clear consensus as to what selection pressures drive this divergence is still to emerge. In part, this stems from the fact that the function of individual sclerites that integrate to form the aedeagus are poorly understood. In insects, the male copulatory organ is often bounded by two lateral parameres tipped with setae. In a number of species, these have been observed to brush against the terminal abdominal sclerites of the female, suggesting a role in pre- and/or post-copulatory female choice. However, in the absence of experimental manipulation, their function remains elusive. Here, we used microsurgery to reduce paramere length and show that males with one or both paramere tips removed were less likely to achieve genital coupling than sham-operated male control groups. Where treatment males did achieve copulation, surgical removal of the paramere tip(s) had no detectable effect on copulation duration or on the outcome of sperm competition. Surgical manipulation of the end-plate, a genital sclerite that covers the ostium of the median lobe (the non-intromittent section of the aedeagus), resulted in near-complete failure of males to achieve copulation. Our experimental manipulations show that the parameres and end-plate function during pre-copulatory sexual interactions and thus most likely evolved in response to sexual selection occurring prior to insemination.

The evolution of relative trait size and shape: insights from the genitalia of dung beetles

Insects show relatively little genital variation within species compared to extraordinary and often rapid diversification among species. It has been suggested that selection for reproductive isolation through differences in genital shape might explain this phenomenon. This hypothesis predicts that populations diverge faster in genital shape than in genital size. We tested this prediction in males from 10 dung beetle species with known phylogenetic relationships from the genus Onthophagus (Coleoptera: Scarabaeidae), including four species for which we were able to sample multiple populations. Specifically, we compared intra- and interspecific differentiation in shape and relative sizes of genitalia and calculated their respective evolutionary rates. We compared these rates to two similarly sized non-genital traits, the head and the fore-tibia. We found significant intraspecific variation in genital shape in all four species for which multiple populations were sampled, but for three of them we also identified significant relative size variation. We also found that genital shape evolved at higher rates than relative genital size. Genital shape evolved faster than head shape, but not fore-tibia shape. However, shapes of all measured structures evolved faster than their relative size. We discuss the functional constraints that may bias the developmental evolution of relative size and shape of genitalia and other morphological traits.

Constrained evolution of the sex comb in Drosophila simulans

Male fitness is dependent on sexual traits that influence mate acquisition (precopulatory sexual selection) and paternity (post-copulatory sexual selection), and although many studies have documented the form of selection in one or the other of these arenas, fewer have done it for both. Nonetheless, it appears that the dominant form of sexual selection is directional, although theoretically, populations should converge on peaks in the fitness surface, where selection is stabilizing. Many factors, however, can prevent populations from reaching adaptive peaks. Genetic constraints can be important if they prevent the development of highest fitness phenotypes, as can the direction of selection if it reverses across episodes of selection. In this study, we examine the evidence that these processes influence the evolution of the multivariate sex comb morphology of male Drosophila simulans. To do this, we conduct a quantitative genetic study together with a multivariate selection analysis to infer how the genetic architecture and selection interact. We find abundant genetic variance and covariance in elements of the sex comb. However, there was little evidence for directional selection in either arena. Significant nonlinear selection was detected prior to copulation when males were mated to nonvirgin females, and post-copulation during sperm offence (again with males mated to nonvirgins). Thus, contrary to our predictions, the evolution of the D. simulans sex comb is limited neither by genetic constraints nor by antagonistic selection between pre- and post-copulatory arenas, but nonlinear selection on the multivariate phenotype may prevent sex combs from evolving to reach some fitness maximizing optima.

Directional selection effects on patterns of phenotypic (co)variation in wild populations

Phenotypic (co)variation is a prerequisite for evolutionary change, and understanding how (co)variation evolves is of crucial importance to the biological sciences. Theoretical models predict that under directional selection, phenotypic (co)variation should evolve in step with the underlying adaptive landscape, increasing the degree of correlation among co-selected traits as well as the amount of genetic variance in the direction of selection. Whether either of these outcomes occurs in natural populations is an open question and thus an important gap in evolutionary theory. Here, we documented changes in the phenotypic (co)variation structure in two separate natural populations in each of two chipmunk species (Tamias alpinus and T. speciosus) undergoing directional selection. In populations where selection was strongest (those of T. alpinus), we observed changes, at least for one population, in phenotypic (co)variation that matched theoretical expectations, namely an increase of both phenotypic integration and (co)variance in the direction of selection and a re-alignment of the major axis of variation with the selection gradient.

The total opportunity for sexual selection and the integration of pre- and post-mating episodes of sexual selection in a complex world

It is well known that sexual selection can target reproductive traits during successive pre- and post-mating episodes of selection. A key focus of recent studies has been to understand and quantify how these episodes of sexual selection interact to determine overall variance in reproductive success. In this article, we review empirical developments in this field but also highlight the considerable variability in patterns of pre- and post-mating sexual selection, attributable to variation in patterns of resource acquisition and allocation, ecological and social factors, genotype-by-environment interaction and possible methodological factors that might obscure such patterns. Our aim is to highlight how (co)variances in pre- and post-mating sexually selected traits can be sensitive to changes in a range of ecological and environmental variables. We argue that failure to capture this variation when quantifying the opportunity for sexual selection may lead to erroneous conclusions about the strength, direction or form of sexual selection operating on pre- and post-mating traits. Overall, we advocate for approaches that combine measures of pre- and post-mating selection across contrasting environmental or ecological gradients to better understand the dynamics of sexual selection in polyandrous species. We also discuss some directions for future research in this area.

The Genetic Basis of Baculum Size and Shape Variation in Mice

The rapid divergence of male genitalia is a preeminent evolutionary pattern. This rapid divergence is especially striking in the baculum, a bone that occurs in the penis of many mammalian species. Closely related species often display diverse baculum morphology where no other morphological differences can be discerned. While this fundamental pattern of evolution has been appreciated at the level of gross morphology, nearly nothing is known about the genetic basis of size and shape divergence. Quantifying the genetic basis of baculum size and shape variation has been difficult because these structures generally lack obvious landmarks, so comparing them in three dimensions is not straightforward. Here, we develop a novel morphometric approach to quantify size and shape variation from three-dimensional micro-CT scans taken from 369 bacula, representing 75 distinct strains of the BXD family of mice. We identify two quantitative trait loci (QTL) that explain ∼50% of the variance in baculum size, and a third QTL that explains more than 20% of the variance in shape. Together, our study demonstrates that baculum morphology may diverge relatively easily, with mutations at a few loci of large effect that independently modulate size and shape. Based on a combination of bioinformatic investigations and new data on RNA expression, we prioritized these QTL to 16 candidate genes, which have hypothesized roles in bone morphogenesis and may enable future genetic manipulation of baculum morphology.

Condition dependence of male and female genital structures in the seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae)

Theory predicts that costly secondary sexual traits will evolve heightened condition dependence, and many studies have reported strong condition dependence of signal and weapon traits in a variety of species. However, although genital structures often play key roles in intersexual interactions and appear to be subject to sexual or sexually antagonistic selection, few studies have examined the condition dependence of genital structures, especially in both sexes simultaneously. We investigated the responses of male and female genital structures to manipulation of larval diet quality (new versus once-used mung beans) in the bruchid seed beetle Callosobruchus maculatus. We quantified effects on mean relative size and static allometry of the male aedeagus, aedeagal spines, flap and paramere and the female reproductive tract and bursal spines. None of the male traits showed a significant effect of diet quality. In females, we found that longer bursal spines (relative to body size) were expressed on low-quality diet. Although the function of bursal spines is poorly understood, we suggest that greater bursal spine length in low-condition females may represent a sexually antagonistic adaptation. Overall, we found no evidence that genital traits in C. maculatus are expressed to a greater extent when nutrients are more abundant. This suggests that, even though some genital traits appear to function as secondary sexual traits, genital traits do not exhibit heightened condition dependence in this species. We discuss possible reasons for this finding.

Copulatory wounding and traumatic insemination

Copulatory wounding (CW) is widespread in the animal kingdom, but likely underreported because of its cryptic nature. We use four case studies (Drosophila flies, Siphopteron slugs, Cimex bugs, and Callosobruchus beetles) to show that CW entails physiological and life-history costs, but can evolve into a routine mating strategy that, in some species, involves insemination through the wound. Although interspecific variation in CW is documented, few data exist on intraspecific and none on individual differences. Although defensive mechanisms evolve in the wound recipient, our review also indicates that mating costs in species with CW are slightly higher than in other species. Whether such costs are dose- or frequency-dependent, and whether defense occurs as resistance or tolerance, decisively affects the evolutionary outcome. In addition to sexual conflict, CW may also become a model system for reproductive isolation. In this context, we put forward a number of predictions, including (1) occasional CW is more costly than routine CW, (2) CW is more costly in between- than within-population matings, and (3) in the presence of CW, selection may favor the transmission of sexually transmitted diseases if they induce resource allocation. Finally, we outline, and briefly discuss, several medical implications of CW in humans.

Sexual selection targets cetacean pelvic bones

Male genitalia evolve rapidly, probably as a result of sexual selection. Whether this pattern extends to the internal infrastructure that influences genital movements remains unknown. Cetaceans (whales and dolphins) offer a unique opportunity to test this hypothesis: since evolving from land-dwelling ancestors, they lost external hind limbs and evolved a highly reduced pelvis that seems to serve no other function except to anchor muscles that maneuver the penis. Here, we create a novel morphometric pipeline to analyze the size and shape evolution of pelvic bones from 130 individuals (29 species) in the context of inferred mating system. We present two main findings: (1) males from species with relatively intense sexual selection (inferred by relative testes size) tend to evolve larger penises and pelvic bones compared to their body length, and (2) pelvic bone shape has diverged more in species pairs that have diverged in inferred mating system. Neither pattern was observed in the anterior-most pair of vertebral ribs, which served as a negative control. This study provides evidence that sexual selection can affect internal anatomy that controls male genitalia. These important functions may explain why cetacean pelvic bones have not been lost through evolutionary time.

Sexual and natural selection both influence male genital evolution

Rapid and divergent evolution of male genital morphology is a conspicuous and general pattern across internally fertilizing animals. Rapid genital evolution is thought to be the result of sexual selection, and the role of natural selection in genital evolution remains controversial. However, natural and sexual selection are believed to act antagonistically on male genital form. We conducted an experimental evolution study to investigate the combined effects of natural and sexual selection on the genital-arch lobes of male Drosophila simulans. Replicate populations were forced to evolve under lifetime monogamy (relaxed sexual selection) or lifetime polyandry (elevated sexual selection) and two temperature regimes, 25°C (relaxed natural selection) or 27°C (elevated natural selection) in a fully factorial design. We found that natural and sexual selection plus their interaction caused genital evolution. Natural selection caused some aspects of genital form to evolve away from their sexually selected shape, whereas natural and sexual selection operated in the same direction for other shape components. Additionally, sexual and natural selection tended to favour larger genitals. Thus we find that the underlying selection driving genital evolution is complex, does not only involve sexual selection, and that natural selection and sexual selection do not always act antagonistically.

Shape--but not size--codivergence between male and female copulatory structures in Onthophagus beetles

Genitalia are among the fastest evolving morphological traits in arthropods. Among the many hypotheses aimed at explaining this observation, some explicitly or implicitly predict concomitant male and female changes of genital traits that interact during copulation (i.e., lock and key, sexual conflict, cryptic female choice and pleiotropy). Testing these hypotheses requires insights into whether male and female copulatory structures that physically interact during mating also affect each other's evolution and patterns of diversification. Here we compare and contrast size and shape evolution of male and female structures that are known to interact tightly during copulation using two model systems: (a) the sister species O. taurus (1 native, 3 recently established populations) and O. illyricus, and (b) the species-complex O. fracticornis-similis-opacicollis. Partial Least Squares analyses indicated very little to no correlation between size and shape of copulatory structures, both in males and females. Accordingly, comparing shape and size diversification patterns of genitalia within each sex showed that the two components diversify readily - though largely independently of each other - within and between species. Similarly, comparing patterns of divergence across sexes showed that relative sizes of male and female copulatory organs diversify largely independent of each other. However, performing this analysis for genital shape revealed a signature of parallel divergence. Our results therefore suggest that male and female copulatory structures that are linked mechanically during copulation may diverge in concert with respect to their shapes. Furthermore, our results suggest that genital divergence in general, and co-divergence of male and female genital shape in particular, can evolve over an extraordinarily short time frame. Results are discussed in the framework of the hypotheses that assume or predict concomitant evolutionary changes in male and female copulatory organs.

Multiple quantitative trait loci influence intra-specific variation in genital morphology between phylogenetically distinct lines of Drosophila montana

The evolution of animal genitalia has gained renewed interest because of their potential roles during sexual selection and early stages of species formation. Although central to understanding the evolutionary process, knowledge of the genetic basis of natural variation in genital morphology is limited to a very few species. Using an outbred cross between phylogenetically distinct lines of Drosophila montana, we characterized quantitative trait loci (QTLs) affecting the size and shape of the distiphallus, a prominent part of the male intromittent organ. Our microsatellite-based linkage analysis shows that intra-specific variation in the distiphallus involves several QTLs of largely additive effect and that a highly significant QTL co-localizes with the same inversion where we have earlier localized a large QTL for a sexually selected courtship song trait. The latter indicates that inversions can play an important role in shaping the evolution of rapidly evolving traits with a potential influence on speciation.

Unlocking the "Black box": internal female genitalia in Sepsidae (Diptera) evolve fast and are species-specific

BACKGROUND

The species-specificity of male genitalia has been well documented in many insect groups and sexual selection has been proposed as the evolutionary force driving the often rapid, morphological divergence. The internal female genitalia, in sharp contrast, remain poorly studied. Here, we present the first comparative study of the internal reproductive system of Sepsidae. We test the species-specificity of the female genitalia by comparing recently diverged sister taxa. We also compare the rate of change in female morphological characters with the rate of fast-evolving, molecular and behavioral characters.

RESULTS

We describe the ectodermal parts of the female reproductive tract for 41 species representing 21 of the 37 described genera and define 19 morphological characters with discontinuous variation found in eight structures that are part of the reproductive tract. Using a well-resolved molecular phylogeny based on 10 genes, we reconstruct the evolution of these characters across the family [120 steps; Consistency Index (CI): 0.41]. Two structures, in particular, evolve faster than the rest. The first is the ventral receptacle, which is a secondary sperm storage organ. It accounts for more than half of all the evolutionary changes observed (7 characters; 61 steps; CI: 0.46). It is morphologically diverse across genera, can be bi-lobed or multi-chambered (up to 80 chambers), and is strongly sclerotized in one clade. The second structure is the dorsal sclerite, which is present in all sepsids except Orygma luctuosum and Ortalischema albitarse. It is associated with the opening of the spermathecal ducts and is often distinct even among sister species (4 characters; 16 steps; CI: 0.56).

CONCLUSIONS

We find the internal female genitalia are diverse in Sepsidae and diagnostic for all species. In particular, fast-evolving structures like the ventral receptacle and dorsal sclerite are likely involved in post-copulatory sexual selection. In comparison to behavioral and molecular data, the female structures are evolving 2/3 as fast as the non-constant third positions of the COI barcoding gene. They display less convergent evolution in characters (CI = 0.54) than the third positions or sepsid mating behavior (CICOI = 0.36; CIBEHAV = 0.45).

Natural selection and genital variation: a role for the environment, parasites and sperm ageing?

Male genitalia are more variable between species (and populations) than other organs, and are more morphologically complex in polygamous compared to monogamous species. Therefore, sexual selection has been put forward as the major explanation of genital variation and complexity, in particular cryptic female choice for male copulatory courtship. As cryptic female choice is based on differences between males it is somewhat paradoxical that there is such low within-species variation in male genitalia that they are a prime morphological identification character for animal species. Processes other than sexual selection may also lead to genitalia variation but they have recently become neglected. Here I focus on pleiotropy and natural selection and provide examples how they link genitalia morphology with genital environments. Pleiotropy appears to be important because most studies that specifically tested for pleiotropic effects on genital morphology found them. Natural selection likely favours certain genital morphology over others in various environments, as well as by reducing re-infection with sexually transmitted diseases or reducing the likelihood of fertilisation with aged sperm. Both pleiotropy and natural selection differ locally and between species so may contribute to local variation in genitalia and sometimes variation between monogamous and polygamous species. Furthermore, the multitude of genital environments will lead to a multitude of genital functions via natural selection and pleiotropy, and may also contribute to explaining the complexity of genitalia.

Evolution of the male genitalia: morphological variation of the aedeagi in a natural population of Drosophila mediopunctata

To investigate the size and shape of the aedeagus of Drosophila mediopunctata, we used basic statistics and geometric morphometrics. We estimated the level of phenotypic variation, natural and laboratory heritability as well as the phenotypic correlations between aedeagus and wing measures. The wing was used as an indicator for both body size and shape. Positive significant correlation was obtained for centroid size of aedeagus and wing for field parents and their offspring reared in the laboratory. Many positive significant phenotypic correlations were found among linear measures of both organs. The phenotypic correlations were few for aedeagus and wing shape. Coefficients of variation of the measures were on average larger in the aedeagus than in the wing for offspring reared in laboratory, but not for flies coming from the field. Significant "natural" heritabilities were found for five linear measures of the aedeagus and only one for the wing. Few significant heritabilities were found for aedeagus and wing shape, mostly ones concerning the uniform components. In an exploratory analysis, we found that inversion DS-PC0 is associated with both uniform and nonuniform components of shape, respectively, in the wing and aedeagus. Our results do not support the lock-and-key hypothesis for the male genitalia evolution, but cannot refute the sexual selection and pleiotropy hypotheses.

The genetic basis of traits regulating sperm competition and polyandry: can selection favour the evolution of good- and sexy-sperm?

The good-sperm and sexy-sperm (GS-SS) hypotheses predict that female multiple mating (polyandry) can fuel sexual selection for heritable male traits that promote success in sperm competition. A major prediction generated by these models, therefore, is that polyandry will benefit females indirectly via their sons' enhanced fertilization success. Furthermore, like classic 'good genes' and 'sexy son' models for the evolution of female preferences, GS-SS processes predict a genetic correlation between genes for female mating frequency (analogous to the female preference) and those for traits influencing fertilization success (the sexually selected traits). We examine the premise for these predictions by exploring the genetic basis of traits thought to influence fertilization success and female mating frequency. We also highlight recent debates that stress the possible genetic constraints to evolution of traits influencing fertilization success via GS-SS processes, including sex-linked inheritance, nonadditive effects, interacting parental genotypes, and trade-offs between integrated ejaculate components. Despite these possible constraints, the available data suggest that male traits involved in sperm competition typically exhibit substantial additive genetic variance and rapid evolutionary responses to selection. Nevertheless, the limited data on the genetic variation in female mating frequency implicate strong genetic maternal effects, including X-linkage, which is inconsistent with GS-SS processes. Although the relative paucity of studies on the genetic basis of polyandry does not allow us to draw firm conclusions about the evolutionary origins of this trait, the emerging pattern of sex linkage in genes for polyandry is more consistent with an evolutionary history of antagonistic selection over mating frequency. We advocate further development of GS-SS theory to take account of the complex evolutionary dynamics imposed by sexual conflict over mating frequency.

No evidence for condition-dependent expression of male genitalia in the dung beetle Onthophagus taurus

We examine the condition-dependence of male genitalia in the dung beetle Onthophagus taurus by manipulating the quality of dung provided for larval growth and development. We show that the influence of larval nutrition differed considerably across three different trait classes (sexual, nonsexual and genital). The size of all nonsexual traits varied with dung quality but their allometric slopes remained unchanged. Relative horn length and allometry, but not absolute horn length, showed a high degree of plasticity with differences in dung quality. In contrast, both absolute size and allometry of genitalia were largely unresponsive to changes in dung quality. Male genitalia exhibited intermediate levels of phenotypic variation and lower allometric slopes than both horns and nonsexual traits. Thus, our findings provide little support for good genes hypotheses of genital evolution. We use our findings to discuss a developmental mechanism and selection pressures that may prevent the condition-dependent expression of genitalia.

A CENTENNIAL CELEBRATION FOR QUANTITATIVE GENETICS

Quantitative genetics is at or is fast approaching its centennial. In this perspective I consider five current issues pertinent to the application of quantitative genetics to evolutionary theory. First, I discuss the utility of a quantitative genetic perspective in describing genetic variation at two very different levels of resolution, (1) in natural, free-ranging populations and (2) to describe variation at the level of DNA transcription. Whereas quantitative genetics can serve as a very useful descriptor of genetic variation, its greater usefulness is in predicting evolutionary change, particularly when used in the first instance (wild populations). Second, I review the contributions of Quantitative trait loci (QLT) analysis in determining the number of loci and distribution of their genetic effects, the possible importance of identifying specific genes, and the ability of the multivariate breeder's equation to predict the results of bivariate selection experiments. QLT analyses appear to indicate that genetic effects are skewed, that at least 20 loci are generally involved, with an unknown number of alleles, and that a few loci have major effects. However, epistatic effects are common, which means that such loci might not have population-wide major effects: this question waits upon (QTL) analyses conducted on more than a few inbred lines. Third, I examine the importance of research into the action of specific genes on traits. Although great progress has been made in identifying specific genes contributing to trait variation, the high level of gene interactions underlying quantitative traits makes it unlikely that in the near future we will have mechanistic models for such traits, or that these would have greater predictive power than quantitative genetic models. In the fourth section I present evidence that the results of bivariate selection experiments when selection is antagonistic to the genetic covariance are frequently not well predicted by the multivariate breeder's equation. Bivariate experiments that combine both selection and functional analyses are urgently needed. Finally, I discuss the importance of gaining more insight, both theoretical and empirical, on the evolution of the G and P matrices.

The genetic basis of interspecific differences in genital morphology of closely related carabid beetles

Marked diversification of genital morphology is common in internally fertilizing animals. Although sexual selection may be the primary process controlling genital evolution, factors promoting genital evolution are controversial, and the genetic background of genital morphology is poorly understood. We analyzed the genetic basis of species-specific genital morphologies in carabid beetles of the subgenus Ohomopterus (genus Carabus, Carabidae) using two parapatric species with hybrid zones. Biometric analyses on experimental F(1) and backcross populations revealed that inheritance of genital morphology is polygenic. Applying Lande's modification of the Castle-Wright estimator to population means and variances to estimate the minimum number of genes involved, we found that a relatively small number of loci is responsible for species differences in genital morphology. In addition, joint-scaling tests indicated that the additive genetic effect accounts for most interspecific differences in genital traits, but dominance and epistatic genetic effects also play roles. Overall, the genetic basis of male and female genitalia is fairly simple, enabling these traits to respond quickly to selection pressures and to diverge rapidly. Our results provide insight into the diversification of genital morphology in carabid beetles, and will hopefully stimulate further studies on the genetic basis of genitalia, such as mapping of quantitative trait loci affecting species-specific genital morphology.

Evolutionary trade-off between weapons and testes

It has long been recognized that male mating competition is responsible for the evolution of weaponry for mate acquisition. However, when females mate with more than one male, competition between males can continue after mating in the form of sperm competition. Theory predicts that males should increase their investment in sperm production as sperm competition is increased, but it assumes that males face a trade-off between sperm production and other life-history traits such as mate acquisition. Here, we use a genus of horned beetle, Onthophagus, to examine the trade-off between investment in testes required for fertilizations and investment in weapons used to obtain matings. In a within-species study, we prevented males from developing horns and found that these males grew larger and invested relatively more in testes growth than did males allowed to grow horns. Among species, there was no general relationship between the relative sizes of horns and testes. However, the allometric slope of horn size on body size was negatively associated with the allometric slope of testes size on body size. We suggest that this reflects meaningful evolutionary changes in the developmental mechanisms regulating trait growth, specifically in the degree of nutrition-dependent phenotypic plasticity versus canalization of traits. Finally, we show how this resource allocation trade-off has influenced the evolutionary diversification of weapons, revealing a rich interplay between developmental trade-offs and both pre- and postmating mechanisms of sexual competition.