Sexual selection and the evolution of sperm morphology in sharks

Post‐copulatory sexual selection, and sperm competition in particular, is a powerful selective force shaping the evolution of sperm morphology. Although mounting evidence suggests that post‐copulatory sexual selection influences the evolution of sperm morphology among species, recent evidence also suggests that sperm competition influences variation in sperm morphology at the intraspecific level. However, contradictory empirical results and limited taxonomic scope have led to difficulty in assessing the generality of sperm morphological responses to variation in the strength of sperm competition. Here, we use phylogenetically controlled analyses to explore the effects of sperm competition on sperm morphology and variance in sharks, a basal vertebrate group characterized by wide variation in rates of multiple mating by females, and consequently sperm competition risk. Our analyses reveal that shark species experiencing greater levels of sperm competition produce sperm with longer flagella and that sperm flagellum length is less variable in species under higher sperm competition risk. In contrast, neither the length of the sperm head and midpiece nor variation in sperm head and midpiece length was associated with sperm competition risk. Our findings demonstrate that selection influences both the inter‐ and intraspecific variation in sperm morphology and suggest that the flagellum is an important target of sexual selection in sharks. These findings provide important insight into patterns of selection on the ejaculate in a basal vertebrate lineage.     

Feed‐backs among inbreeding,inbreeding depression in sperm traits,and sperm competition can drive evolution of costly polyandry

Ongoing ambitions are to understand the evolution of costly polyandry and its consequences for species ecology and evolution. Emerging patterns could stem from feed‐back dynamics between the evolving mating system and its genetic environment, defined by interactions among kin including inbreeding. However, such feed‐backs are rarely considered in nonselfing systems. We use a genetically explicit model to demonstrate a mechanism by which inbreeding depression can select for polyandry to mitigate the negative consequences of mating with inbred males, rather than to avoid inbreeding, and to elucidate underlying feed‐backs. Specifically, given inbreeding depression in sperm traits, costly polyandry evolved to ensure female fertility, without requiring explicit inbreeding avoidance. Resulting sperm competition caused evolution of sperm traits and further mitigated the negative effect of inbreeding depression on female fertility. The evolving mating system fed back to decrease population‐wide homozygosity, and hence inbreeding. However, the net overall decrease was small due to compound effects on the variances in sex‐specific reproductive success and paternity skew. Purging of deleterious mutations did not eliminate inbreeding depression in sperm traits or hence selection for polyandry. Overall, our model illustrates that polyandry evolution, both directly and through sperm competition, might facilitate evolutionary rescue for populations experiencing sudden increases in inbreeding.     

Dimorphic ejaculates and sperm release strategies associated with alternative mating behaviors in the squid

Sperm competition is a powerful postcopulatory selective force influencing male adaptations associated with increasing fertilization success, and it is usually related to the evolution of different strategies of ejaculate expenditure between individuals. Ejaculates may also be influenced by additional selective pressures associated with sperm competition, such as timing between insemination and fertilization, female reproductive tract morphology, and fertilization environment. Also, males that adopt alternative mating tactics may face distinct sperm competition pressures, which may lead to the evolution of intraspecific diversity in ejaculates. In loliginid squids, males with alternative reproductive tactics (sneakers and consorts) differ not only in mating behavior, but also transfer spermatophores into two distinct sites within the female. Here, we compared structure and functioning of spermatophores between sneakers and consorts in the squid Doryteuthis plei applying microscopy techniques and in vitro experiments. Sneakers and consorts exhibit differences in spermatophore structure that lead to distinct spermatophoric reactions and spermatangium morphologies. Moreover, in sneakers, sperm release lasts longer and their sperm show an aggregative behavior not detected in consorts. Slow sperm release may be a strategy to guarantee longer sperm provision, given the wide interval between sneaker mating and egg release. For consorts, in turn, intense and quick sperm discharge may be advantageous, as timing between mating and egg‐laying is relatively short. Within the complex squid mating system, factors such as (i) different fertilization sites and (ii) interval between mating and egg release may also influence sperm competition, and ultimately shape the evolution of divergent ejaculates between dimorphic males.     

The evolutionary ecology of testicular function: size isn't everything

Larger testes are considered the quintessential adaptation to sperm competition. However, the strong focus on testis size in evolutionary research risks ignoring other potentially adaptive features of testicular function, many of which will also be shaped by post‐mating sexual selection. Here we advocate a more integrated research programme that simultaneously takes into account the developmental machinery of spermatogenesis and the various selection pressures that act on this machinery and its products. The testis is a complex organ, and so we begin by outlining how we can think about the evolution of testicular function both in terms of the composition and spatial organisation of the testis (‘testicular histology’), as well as in terms of the logical organisation of cell division during spermatogenesis (‘testicular architecture’). We then apply these concepts to ask which aspects of testicular function we can expect to be shaped by post‐mating sexual selection. We first assess the impact of selection on those traits most strongly associated with sperm competition, namely the number and kind of sperm produced. A broad range of studies now support our contention that post‐mating sexual selection affects many aspects of testicular function besides gross testis size, for example, to maximise spermatogenic efficiency or to enable the production of particular sperm morphologies. We then broaden our focus to ask how testicular function is affected by fluctuation in sperm demand. Such fluctuation can occur over an individual's lifetime (for example due to seasonality in reproduction) and may select for particular types of testicular histology and architecture depending on the particular reproductive ecology of the species in question. Fluctuation in sperm demand also occurs over evolutionary time, due to shifts in the mating system, and this may have various consequences for testicular function, for example on rates of proliferation‐induced mutation and for dealing with intragenomic conflict. We end by suggesting additional approaches that could be applied to study testicular function, and conclude that simultaneously considering the machinery, products and scheduling of spermatogenesis will be crucial as we seek to understand more fully the evolution of this most fundamental of male reproductive traits.     

Precopulatory but not postcopulatory male reproductive traits diverge in response to mating system manipulation in Drosophila melanogaster

Competition between males creates potential for pre‐ and postcopulatory sexual selection and conflict. Theory predicts that males facing risk of sperm competition should evolve traits to secure their reproductive success. If those traits are costly to females, the evolution of such traits may also increase conflict between the sexes. Conversely, under the absence of sperm competition, one expectation is for selection on male competitive traits to relax thereby also relaxing sexual conflict. Experimental evolution studies are a powerful tool to test this expectation. Studies in multiple insect species have yielded mixed and partially conflicting results. In this study, we evaluated male competitive traits and male effects on female costs of mating in Drosophila melanogaster after replicate lines evolved for more than 50 generations either under enforced monogamy or sustained polygamy, thus manipulating the extent of intrasexual competition between males. We found that in a setting where males competed directly with a rival male for access to a female and fertilization of her ova polygamous males had superior reproductive success compared to monogamous males. When comparing reproductive success solely in double mating standard sperm competition assays, however, we found no difference in male sperm defense competitiveness between the different selection regimes. Instead, we found monogamous males to be inferior in precopulatory competition, which indicates that in our system, enforced monogamy relaxed selection on traits important in precopulatory rather than postcopulatory competition. We discuss our findings in the context of findings from previous experimental evolution studies in Drosophila ssp. and other invertebrate species.     

SEX‐SPECIFIC INBREEDING DEPRESSION DEPENDS ON THE STRENGTH OF MALE–MALE COMPETITION

Inbreeding depression has become a central theme in evolutionary biology and is considered to be a driving force for the evolution of reproductive morphology, physiology, behavior, and mating systems. Despite the overwhelming body of empirical work on the reproductive consequences of inbreeding, relatively little is known on whether inbreeding depresses male and female fitness to the same extent. However, sex‐specific inbreeding depression has been argued to affect the evolution of selfing rates in simultaneous hermaphrodites and provides a powerful approach to test whether selection is stronger in males than in females, which is predicted to be the consequence of sexual selection. We tested for sex‐specific inbreeding depression in the simultaneously hermaphroditic freshwater snail Physa acuta by comparing the reproductive performance of both sex functions between selfed and outcrossed focal individuals under different levels of male–male competition. We found that inbreeding impaired both male and female reproductive success and that the magnitude of male inbreeding depression exceeded female inbreeding depression when the opportunity for sperm competition was highest. Our study provides the first evidence for sex‐specific inbreeding depression in a hermaphroditic animal and highlights the importance of considering the level of male–male competition when assessing sex differences in inbreeding depression.     

Ejaculate traits in relation to male body size in the eastern mosquitofish Gambusia holbrooki

In the present study, the correlation between sperm number, sperm quality (speed, viability, longevity and length), sperm bundles quality (size and dissolving rate) and male body size has been tested in the eastern mosquitofish Gambusia holbrooki a poecilid species characterized by coercive mating tactics where males do not possess obvious ornaments, and the body size is the key determinant of pre‐copulatory male mating success. The results do not tally with theoretical predictions. Indeed, no correlation between male body size and either sperm or sperm‐bundle traits has been found, evidencing the lack of the theoretically expected trade‐off between the investment in characters involved in mate acquisition and the investment in ejaculate quality. An explanation for the observed pattern comes from the extremely dynamic mating system of G. holbrooki, characterized by variable size‐related male mating success and strong post‐copulatory selective pressure, with all males facing a similar high level of sperm competition. In this situation, a higher investment in growth and maintenance at the expense of ejaculate quality is not expected. These results underscore the necessity to comprehend detailed information on species’ reproductive biology and reproductive environment to understand both the evolution of ejaculate characteristics and possible deviations from theoretical predictions.     

The frequency of multiple paternity predicts variation in testes size among island populations of house mice

Polyandry generates selection on males through sperm competition, which has broad implications for the evolution of ejaculates and male reproductive anatomy. Comparative analyses across species and competitive mating trials within species have suggested that sperm competition can influence the evolution of testes size, sperm production and sperm form and function. Surprisingly, the intraspecific approach of comparing among population variation for investigating the selective potential of sperm competition has rarely been explored. We sampled seven island populations of house mice and determined the frequency of multiple paternity within each population. Applying the frequency of multiple paternity as an index of the risk of sperm competition, we looked for selective responses in male reproductive traits. We found that the risk of sperm competition predicted testes size across the seven island populations of house mice. However, variation in sperm traits was not explained by sperm competition risk. We discuss these findings in relation to sperm competition theory, and other intrinsic and extrinsic factors that might influence ejaculate quality.     

The outcome of sperm competition is affected by behavioural and anatomical reproductive traits in a simultaneously hermaphroditic land snail

Sperm competition is important in species with reproductive strategies that involve multiple mating and prolonged sperm storage such as the simultaneously hermaphroditic land snail Cornu aspersum. Double mating trials in this species have revealed that mating order and courtship behaviour affect paternity success. We investigated the effect of behavioural and anatomical reproductive traits on paternity success from triple mating trials. Triple mating resulted in triple fertilization in 58% of the cases whereas zero paternity was observed in 16% of sperm donors. Third sperm donors achieved higher paternity followed by first and second sperm donors. Snails with a longer epiphallus, the spermatophore forming organ, sired more offspring regardless of their mating order. Genetic compatibility between sperm donor and recipient did not influence paternity success. The results of the present study identified mating order and epiphallus length, as traits affecting the outcome of sperm competition in this species.     

Postmating–prezygotic isolation between two allopatric populations of Drosophila montana: fertilisation success differs under sperm competition

Postmating but prezygotic (PMPZ) interactions are increasingly recognized as a potentially important early‐stage barrier in the evolution of reproductive isolation. A recent study described a potential example between populations of the same species: single matings between Drosophila montana populations resulted in differential fertilisation success because of the inability of sperm from one population (Vancouver) to penetrate the eggs of the other population (Colorado). As the natural mating system of D. montana is polyandrous (females remate rapidly), we set up double matings of all possible crosses between the same populations to test whether competitive effects between ejaculates influence this PMPZ isolation. We measured premating isolation in no‐choice tests, female fecundity, fertility and egg‐to‐adult viability after single and double matings as well as second‐male paternity success (P₂). Surprisingly, we found no PMPZ reproductive isolation between the two populations under a competitive setting, indicating no difficulty of sperm from Vancouver males to fertilize Colorado eggs after double matings. While there were subtle differences in how P₂ changed over time, suggesting that Vancouver males’ sperm are somewhat less competitive in a first‐male role within Colorado females, these effects did not translate into differences in overall P₂. Fertilisation success can thus differ dramatically between competitive and noncompetitive conditions, perhaps because the males that mate second produce higher quality ejaculates in response to sperm competition. We suggest that unlike in more divergent species comparisons, where sperm competition typically increases reproductive isolation, ejaculate tailoring can reduce the potential for PMPZ isolation when recently diverged populations interbreed.     

The Evolution of Large Testes: Sperm Competition or Male Mating Rate?

A positive relationship across species between the extent to which females mate with more than one male and relative testes mass has been demonstrated in a wide range of vertebrate taxa and certain insects. At least two hypotheses, which are not necessarily mutually exclusive, could account for this pattern: (1) the numerical sperm competition hypothesis, which assumes that larger testes enable the male to transfer more sperm to each female, giving the male an advantage in sperm competition and (2) the male mating rate hypothesis, which proposes that larger testes allow the male to produce a greater number of (potentially smaller) ejaculates to engage in frequent copulations with different females. Of these hypotheses, the former has won broad acceptance, while the latter has tended to be dismissed. Here, we argue that the lines of evidence commonly used to support the numerical sperm competition hypothesis in favour of the male mating rate hypothesis are not as clear cut or generally applicable as they are purported to be and that, consequently, the male mating rate hypothesis cannot be excluded with confidence on the basis of the current evidence. Furthermore, some evidence, such as the finding that ejaculate mass and/or sperm number is negatively correlated with testes mass across species in some insects and that larger testes in Drosophila can evolve in response to an increase in the number of females available for mating in the laboratory, provides support for the male mating rate hypothesis. Further work is needed to disentangle the relative effects of these selective pressures on the evolution of testes size.     

The combined effects of pre‐ and post‐copulatory processes are masking sexual conflict over mating rate in Gerris buenoi

In polygynandrous animals, post‐copulatory processes likely interfere with precopulatory sexual selection. In water striders, sexual conflict over mating rate and post‐copulatory processes are well documented, but their combined effect on reproductive success has seldom been investigated. We combine genetic parentage analyses and behavioural observations conducted in a competitive reproductive environment to investigate how pre‐ and post‐copulatory processes influence reproductive success in Gerris buenoi Kirkaldy. Precopulatory struggles had antagonistic effects on male and female reproductive success: efficiently gaining copulations was beneficial for males, whereas efficiently avoiding copulations was profitable for females. Also, high mating rates and an intermediate optimal resistance level of females supported the hypothesis of convenience polyandry. Contrary to formal predictions, high mating rates (i.e. the number of copulations) did not increase reproductive success in males or decrease reproductive success in females. Instead, the reproductive success of both sexes was higher when offspring were produced with several partners and when there were few unnecessary matings. Thus, male and female G. buenoi displayed different interests in reproduction, but post‐copulatory processes were masking the effects of copulatory mating success on reproductive success. Given the high mating rates observed, sperm competition could easily counter the effect of mating rates, perhaps in interaction with cryptic female choice and/or fecundity selection. Our study presents a complex but realistic overview of sexual selection forces at work in a model organism for the study of sexual conflict, confirming that insights are gained from investigating all episodes in the reproduction cycle of polygynandrous animals.     

Evolutionary modeling predicts a decrease in postcopulatory sperm viability as a response to increasing levels of sperm competition

Sperm competition has been found to have a strong influence on the evolution of many male and female reproductive traits. Theoretical models have shown that, with increasing levels of sperm competition, males are predicted to increase ejaculate investment, and there is ample empirical evidence supporting this prediction. However, most theoretical models concern sperm number, and although the predictions are likely to apply to other sperm traits that will affect the sperm competitive ability of males, substantiated predictions are difficult unless the evolution of specific traits is explicitly modeled. Here I present a novel theoretical model aiming at predicting evolutionarily stable sperm viability in relation to female mating frequency in a mating system with internal fertilization. At odds with verbal arguments, this model demonstrates that sperm viability is expected to decrease with increasing female remating rates and thus to decrease with increasing levels of sperm competition. The major reason for this is that, with increasing female remating rates, the prospects of future fertilization success will decrease, which acts to reduce the benefit of long-lived viable sperm. An additional interesting result is that, as the cost of sperm viability increases, the overall energy investment in ejaculates will decrease. These novel results should have a strong impact on future sperm competition studies and will also have implications for our understanding of the evolution of female polyandry.     

TROPICS ACCELERATE THE EVOLUTION OF HYBRID MALE STERILITY IN DROSOPHILA

Understanding the evolutionary mechanisms that facilitate speciation and explain global patterns of species diversity has remained a challenge for decades. The most general pattern of species biodiversity is the latitudinal gradient, whereby species richness increases toward the tropics. Although such a global pattern probably has a multitude of causes, recent attention has focused on the hypothesis that speciation and the evolution of reproductive isolation occur faster in the tropics. Here, I tested this prediction using a dataset on premating and postzygotic isolation between recently diverged Drosophila species. Results showed that while the evolution of premating isolation was not greater between tropical Drosophila relative to nontropical species, postzygotic isolation evolved faster in the tropics. In particular, hybrid male sterility was much greater among tropical Drosophila compared to nontropical species pairs of similar genetic age. Several testable explanations for the novel pattern are discussed, including greater role for sterility‐inducing bacterial endosymbionts in the tropics and more intense sperm–sperm competition or sperm–egg sexual conflict in the tropics. The results imply that processes of speciation in the tropics may evolve at different rates or may even be somewhat different from those at higher latitudes.     

Selection on female remating interval is influenced by male sperm competition strategies and ejaculate characteristics

Female remating rate dictates the level of sperm competition in a population, and extensive research has focused on how sperm competition generates selection on male ejaculate allocation. Yet the way ejaculate allocation strategies in turn generate selection on female remating rates, which ultimately influence levels of sperm competition, has received much less consideration despite increasing evidence that both mating itself and ejaculate traits affect multiple components of female fitness. Here, we develop theory to examine how the effects of mating on female fertility, fecundity and mortality interact to generate selection on female remating rate. When males produce more fertile ejaculates, females are selected to mate less frequently, thus decreasing levels of sperm competition. This could in turn favour decreased male ejaculate allocation, which could subsequently lead to higher female remating. When remating simultaneously increases female fecundity and mortality, females are selected to mate more frequently, thus exacerbating sperm competition and favouring male traits that convey a competitive advantage even when harmful to female survival. While intuitive when considered separately, these predictions demonstrate the potential for complex coevolutionary dynamics between male ejaculate expenditure and female remating rate, and the correlated evolution of multiple male and female reproductive traits affecting mating, fertility and fecundity.     

Differences in sperm morphology in foam‐nesting leptodactyline frogs (Anura,Leptodactylidae)

Sperm morphology is diverse among vertebrates and is influenced by the reproductive strategies adopted by species. In anurans, sperm morphology is associated with reproductive modes and mating systems. Here, we describe the sperm morphology of 11 frog species in the genus Leptodactylus and that of Lithodytes lineatus and discuss the relationship between sperm morphology and species' mating systems. We observed two distinct sperm morphotypes among the leptodactyline species, which differed mostly in head morphology. Type I sperm had triangular head, discrete acrosome vesicle with posterior margin not clearly visible; type II sperm had elongated head, clear acrosomal vesicle with posterior margin clearly visible. These sperm types do not seem to be associated with phylogeny; instead, type II sperm was observed in all polyandrous species analysed and in species with evidences of polyandry. Moreover, sperm of all species presented tail with undulating membrane connected to the axial fibre. We suggest that differences in sperm morphology might be associated with sperm competition to what polyandrous species are subjected. However, natural history observations on polyandrous mating in some species presenting type II sperm and phylogenetic comparative studies are need to elucidate the role of mating systems in the evolution of sperm morphology in leptodactylines.     

Heritable determinants of male fertilization success in the nematode Caenorhabditis elegans

Background  

Sperm competition is a driving force in the evolution of male sperm characteristics in many species. In the nematode Caenorhabditis elegans, larger male sperm evolve under experimentally increased sperm competition and larger male sperm outcompete smaller hermaphrodite sperm for fertilization within the hermaphrodite reproductive tract. To further elucidate the relative importance of sperm-related traits that contribute to differential reproductive success among males, we quantified within- and among-strain variation in sperm traits (size, rate of production, number transferred, competitive ability) for seven male genetic backgrounds known previously to differ with respect to some sperm traits. We also quantified male mating ability in assays for rates of courtship and successful copulation, and then assessed the roles of these pre- and post-mating traits in first- and second-male fertilization success.     

Rates of Evolution of Hominoid Seminal Proteins are Correlated with Function and Expression, Rather than Mating System

In species where females mate with multiple males during a single ovulatory cycle, sperm competition is hypothesized to increase the rate of adaptive evolution of proteins expressed in male reproductive tissues through recurrent selective sweeps (positive selection). The hominoids, comprising apes and humans, are a group of closely related primates with extensive variation in mating behaviors and predicted levels of sperm competition. Since previous studies of individual male reproductive genes have shown evidence of positive selection, we estimated rates of evolution of a comprehensive set of proteins expressed in ejaculated semen. Our results show that these proteins in hominoids do not have elevated rates of nonsynonymous substitutions (Ka) compared with a control dataset of nonreproductive genes. Species with greater sperm competition do not have faster rates of seminal protein evolution. Although at these broad levels our hypotheses were not confirmed, further analyses indicate specific patterns of molecular evolution. Namely, the Ka of seminal genes is more strongly correlated with measures of tissue specificity than nonreproductive genes, suggesting that the former may more readily adapt to tissue-specific functions. Proteins expressed from the seminal vesicles evolve more rapidly than those from other male reproductive tissues. Also, several gene ontology categories show elevated rates of protein evolution, not seen in the control data set. While the generalization that male reproductive genes evolve rapidly in hominoids is an oversimplification, a subset of proteins can be identified that are likely targets for adaptive evolution driven by sexual selection.     

RAPID LOSS OF BEHAVIORAL PLASTICITY AND IMMUNOCOMPETENCE UNDER INTENSE SEXUAL SELECTION

Phenotypic plasticity allows animals to maximize fitness by conditionally expressing the phenotype best adapted to their environment. Although evidence for such adjustment in reproductive tactics is common, little is known about how phenotypic plasticity evolves in response to sexual selection. We examined the effect of sexual selection intensity on phenotypic plasticity in mating behavior using the beetle Callosobruchus maculatus. Male genital spines harm females during mating and females exhibit copulatory kicking, an apparent resistance trait aimed to dislodge mating males. After exposing individuals from male‐ and female‐biased experimental evolution lines to male‐ and female‐biased sociosexual environments, we examined behavioral plasticity in matings with standard partners. While females from female‐biased lines kicked sooner after exposure to male‐biased sociosexual contexts, in male‐biased lines this plasticity was lost. Ejaculate size did not diverge in response to selection history, but males from both treatments exhibited plasticity consistent with sperm competition intensity models, reducing size as the number of competitors increased. Analysis of immunocompetence revealed reduced immunity in both sexes in male‐biased lines, pointing to increased reproductive costs under high sexual selection. These results highlight how male and female reproductive strategies are shaped by interactions between phenotypically plastic and genetic mechanisms of sexual trait expression.