Reproductive senescence: new perspectives in the wild

According to recent empirical studies, reproductive senescence, the decline in reproductive success with increasing age, seems to be nearly ubiquitous in the wild. However, a clear understanding of the evolutionary causes and consequences of reproductive senescence is still lacking and requires new and integrative approaches. After identifying the sequential and complex nature of female reproductive senescence, we show that the relative contributions of physiological decline and alterations in the efficiency of parental care to reproductive senescence remain unknown and need to be assessed in the light of current evolutionary theories of ageing. We demonstrate that, although reproductive senescence is generally studied only from the female viewpoint, age‐specific female reproductive success strongly depends on male–female interactions. Thus, a reduction in male fertilization efficiency with increasing age has detrimental consequences for female fitness. Lastly, we call for investigations of the role of environmental conditions on reproductive senescence, which could provide salient insights into the underlying sex‐specific mechanisms of reproductive success. We suggest that embracing such directions should allow building new bridges between reproductive senescence and the study of sperm competition, parental care, mate choice and environmental conditions.     

The sexually-selected sperm hypothesis: sex-biased inheritance and sexual antagonism

When females are inseminated by more than one male (polyandry) sexual selection continues after insemination in the form of sperm competition and cryptic female choice. The sexually-selected sperm hypothesis proposes that, under the risk of sperm competition, additive variation in male traits determining fertilising efficiency will select for female propensity to be polyandrous in order to increase the probability of producing sons with superior fertilising efficiency. Two factors complicate this prediction: sex-biased transmission of male fertilising efficiency traits and sexual antagonism of sex-limited traits, fostered by sex-biased inheritance. Here, we (i) review the evidence that male traits contributing towards fertilising efficiency are heritable through sex-biased mechanisms, and (ii) explore the evolutionary implications for male and female reproductive strategies caused by both sex-biased transmission and sexual antagonism of fertilising efficiency traits. Many male fertilising efficiency traits are heritable through sex-biased mechanisms and may not necessarily increase female fitness. The predictions of the sexually-selected sperm hypothesis change dramatically under these different mechanisms of inheritance of fertilising efficiency traits, and different fitness pay-offs derived by females from the expression of such traits. Both sex-biased control of fertilising efficiency and sexual antagonism may also be important in explaining the maintenance of the genetic variance and selection potential of fertilising efficiency. We propose that a useful approach to test the sexually-selected sperm hypothesis is to combine studies which identify behavioural and physiological mechanisms explaining variation in reproductive success with artificial selection experiments to infer the underlying evolutionary patterns.     

Maternal inheritance,epigenetics and the evolution of polyandry

Growing evidence indicates that females actively engage in polyandry either to avoid genetic incompatibility or to bias paternity in favor of genetically superior males. Despite empirical support for the intrinsic male quality hypothesis, the maintenance of variation in male fitness remains a conundrum for traditional "good genes" models of sexual selection. Here, we discuss two mechanisms of non-Mendelian inheritance, maternal inheritance of mitochondria and epigenetic regulation of gene expression, which may explain the persistence of variation in male fitness traits important in post-copulatory sexual selection. The inability of males to transmit mitochondria precludes any direct evolutionary response to selection on mitochondrial mutations that reduce or enhance male fitness. Consequently, mitochondrial-based variation in sperm traits is likely to persist, even in the face of intense sperm competition. Indeed, mitochondrial nucleotide substitutions, deletions and insertions are now known to be a primary cause of low sperm count and poor sperm motility in humans. Paradoxically, in the field of sexual selection, female-limited response to selection has been largely overlooked. Similarly, the contribution of epigenetics (e.g., DNA methylation, histone modifications and non-coding RNAs) to heritable variation in male fitness has received little attention from evolutionary theorists. Unlike DNA sequence based variation, epigenetic variation can be strongly influenced by environmental and stochastic effects experienced during the lifetime of an individual. Remarkably, in some cases, acquired epigenetic changes can be stably transmitted to offspring. A recent study indicates that sperm exhibit particularly high levels of epigenetic variation both within and between individuals. We suggest that such epigenetic variation may have important implications for post-copulatory sexual selection and may account for recent findings linking sperm competitive ability to offspring fitness.     

Condition‐dependent mortality exacerbates male (but not female) reproductive senescence and the potential for sexual conflict

Disentangling the relationship between age and reproduction is central to understand life‐history evolution, and recent evidence shows that considering condition‐dependent mortality is a crucial piece of this puzzle. For example, nonrandom mortality of ‘low‐condition’ individuals can lead to an increase in average lifespan. However, selective disappearance of such low‐condition individuals may also affect reproductive senescence at the population level due to trade‐offs between physiological functions related to survival/lifespan and the maintenance of reproductive functions. Here, we address the idea that condition‐dependent extrinsic mortality (i.e. simulated predation) may increase the age‐related decline in male reproductive success and with it the potential for sexual conflict, by comparing reproductive ageing in Drosophila melanogaster male/female cohorts exposed (or not) to condition‐dependent simulated predation across time. Although female reproductive senescence was not affected by predation, male reproductive senescence was considerably higher under predation, due mainly to an accelerated decline in offspring viability of ‘surviving’ males with age. This sex‐specific effect suggests that condition‐dependent extrinsic mortality can exacerbate survival‐reproduction trade‐offs in males, which are typically under stronger condition‐dependent selection than females. Interestingly, condition‐dependent extrinsic mortality did not affect mating success, hinting that accelerated reproductive senescence is due to a decrease in male post‐copulatory fitness components. Our results support the recent proposal that male ageing can be an important source of sexual conflict, further suggesting this effect could be exacerbated under more natural conditions.     

Nontransitivity of paternity in a bird

In many animals reproductive success is determined after insemination by the interaction of male and female processes. While sperm competition is reasonably well understood in some taxa, other processes, such as cryptic female choice and differential early embryo mortality resulting from genetic incompatibilities, are less well understood. The relative importance of these different factors contributing to reproductive success is difficult to assess. Here we control for male-mediated effects (which are often considerable and can mask more subtle processes) through the artificial insemination of known numbers of sperm in the domestic fowl to reveal that male reproductive success is nontransitive across females: the success of a particular male depends on the background against which his sperm compete for fertilization. Two potential processes could account for this effect: cryptic female choice (sperm choice) or differential early embryo mortality. Regardless of the mechanism, nontransitivity of male reproductive success has important evolutionary consequences, including the maintenance of variation in male fitness.     

No intra-locus sexual conflict over reproductive fitness or ageing in field crickets

Differences in the ways in which males and females maximize evolutionary fitness can lead to intra-locus sexual conflict in which genes delivering fitness benefits to one sex are costly when expressed in the other. Trade-offs between current reproductive effort and future reproduction and survival are fundamental to the evolutionary biology of ageing. This leads to the prediction that sex differences in the optimization of age-dependent reproductive effort may generate intra-locus sexual conflict over ageing rates. Here we test for intra-locus sexual conflict over age-dependent reproductive effort and longevity in the black field cricket, Teleogryllus commodus. Using a half-sib breeding design, we show that the most important components of male and female reproductive effort (male calling effort and the number of eggs laid by females) were positively genetically correlated, especially in early adulthood. However, the genetic relationships between longevity and reproductive effort were different for males and females, leading to low genetic covariation between male and female longevity. The apparent absence of intra-locus sexual conflict over ageing suggests that male and female longevity can evolve largely independently of one another.     

Sexual selection,germline mutation rate and sperm competition

Background  

An important component of sexual selection arises because females obtain viability benefits for their offspring from their mate choice. Females choosing extra-pair fertilization generally favor males with exaggerated secondary sexual characters, and extra-pair paternity increases the variance in male reproductive success. Furthermore, females are assumed to benefit from 'good genes' from extra-pair sires. How additive genetic variance in such viability genes is maintained despite strong directional selection remains an evolutionary enigma. We propose that sexual selection is associated with elevated mutation rates, changing the balance between mutation and selection, thereby increasing variance in fitness and hence the benefits to be obtained from good genes sexual selection. Two hypotheses may account for such elevated mutation: (1) Increased sperm production associated with sperm competition may increase mutation rate. (2) Mutator alleles increase mutation rates that are revealed by the expression of condition-dependent secondary sexual characters used by choosy females during their mate choice. M Petrie has independently developed the idea that mutator alleles may account for the maintenance of genetic variation in viability despite strong directional selection.     

Male fertility in natural populations of red deer is determined by sperm velocity and the proportion of normal spermatozoa

Male reproductive success is determined by the ability of males to gain sexual access to females and by their ability to fertilize ova. Among polygynous mammals, males differ markedly in their reproductive success, and a great deal of effort has been made to understand how selective forces have shaped traits that enhance male competitiveness both before and after copulation (i.e., sperm competition). However, the possibility that males also may differ in their fertility has been ignored under the assumption that male infertility is rare in natural populations because selection against it is likely to be strong. In the present study, we examined which semen traits correlate with male fertility in natural populations of Iberian red deer (Cervus elaphus hispanicus). We found no trade-offs between semen traits. Our analyses revealed strong associations between sperm production and sperm swimming velocity, sperm motility and proportion of morphologically normal spermatozoa, and sperm viability and acrosome integrity. These last two variables had the lowest coefficients of variation, suggesting that these traits have stabilized at high values and are unlikely to be related to fitness. In a fertility trial, our results show a large degree of variation in male fertility, and differences in fertility were determined mainly by sperm swimming velocity and by the proportion of morphologically normal sperm. We conclude that male fertility varies substantially in natural populations of Iberian red deer and that, when sperm numbers are equal, it is determined mainly by sperm swimming velocity and sperm morphology.     

THE ECOLOGY OF SEXUAL CONFLICT: BACKGROUND MORTALITY CAN MODULATE THE EFFECTS OF MALE MANIPULATION ON FEMALE FITNESS

Sexual and parental conflicts can arise because males benefit by inducing elevated reproductive effort in their mates. For females, the costs of such manipulation are often manifested later in life, and may therefore covary with female life expectancy. Here, I outline a simple female life‐history model where female life expectancy reflects extrinsic mortality rate, and elevated reproductive effort causes accelerated senescence. Using this model, I show that variation in extrinsic mortality rate can modulate the magnitude and sign of fitness effects that male manipulation has on females. This result has several interesting implications. First, it suggests that the fitness effects of sexual interactions can depend on ecological factors, such as predation, that influence life expectancy. Second, if mortality risk is condition‐dependent but reproductive effort is not fully optimized in relation to individual condition, then sexual conflict intensity may increase with individual condition, selecting for condition‐dependent reproductive strategies. Third, if males vary in manipulativeness, then the fitness effects of mating with a given male phenotype may depend on both female condition and extrinsic mortality rate. Fourth, life span extension in the laboratory can lead to overestimation of sexual and parental conflicts. Life expectancy may therefore be a key factor in sexual coevolution.     

Polyandry and female control: the red flour beetle Tribolium castaneum as a case study

Females of many animal species are polyandrous, and there is evidence that they can control pre- and post-mating events. There has been a growing interest in consequences of polyandry for male and female reproductive success and offspring fitness, and its evolutionary significance. In several taxa, females exhibit mate choice both before and after mating and can influence the paternity of their offspring, enhancing offspring number and quality, but potentially countering male interests. Studying female mating biology and in particular post-copulatory female control mechanisms thus promises to yield insights into sexual selection and the potential of male-female coevolution. Here, we highlight the red flour beetle Tribolium castaneum (Herbst), a storage pest, as a model system to study polyandry, and review studies addressing the effects of polyandry on male sperm competitive ability and female control of post-mating events. These studies show that the outcome of sperm competition in the red flour beetle is influenced by both male and female traits. Furthermore, recent advances suggest that sexual conflict may have shaped reproductive traits in this species.     

Genetic and nutritional effects on male traits and reproductive performance in Tribolium flour beetles

In Tribolium flour beetles and other organisms, individuals migrate between heterogeneous environments where they often encounter markedly different nutritional conditions. Under these circumstances, theory suggests that genotype-by-environment interactions (GEI) may be important in facilitating adaptation to new environments and maintaining genetic variation for male traits subject to directional selection. Here, we used a nested half-sib breeding design with Tribolium castaneum to partition the separate and joint effects of male genotype and nutritional environment on phenotypic variation in a comprehensive suite of life-history traits, reproductive performance measures across three sequential sexual selection episodes, and fitness. When male genotypes were tested across three nutritional environments, considerable phenotypic plasticity was found for male mating and insemination success, longevity and traits related to larval development. Our results also revealed significant additive genetic variation for male mating rate, sperm offence ability (P(2)), longevity and total fitness and for several traits reflecting both larval and adult resource use. In addition, we found evidence supporting GEI for sperm defence ability (P(1)), adult longevity and larval development; thus, no single male genotype outperforms others in every nutritional environment. These results provide insight into the potential roles of phenotypic plasticity and GEI in facilitating Tribolium adaptation to new environments in ecological and evolutionary time.     

EXPERIMENTAL EVOLUTION OF SPERM QUALITY VIA POSTCOPULATORY SEXUAL SELECTION IN HOUSE MICE

Individuals of many species copulate with multiple mates (polygamy). Multiple mating by females (polyandry) promotes sperm competition, which has broad implications for the evolution of the ejaculate. Multigenerational studies of polygamous insects have shown that the removal of sexual selection has profound fitness consequences for females, and can lead to an evolutionary divergence in ejaculate traits. However, the evolutionary implications of polygamous mating across successive generations have not before been demonstrated in a vertebrate. By manipulating the mating system we were able to reinstate postcopulatory sexual selection in a house mouse population that had a long history of enforced monogamy. Following eight generations of selection, we performed sperm quality assays on males from both the polygamous and monogamous selection lines. We applied a principal component analysis to summarize the variation among 12 correlated sperm traits, and found that males evolving under sperm competition had significantly larger scores on the first axis of variation, reflecting greater numbers of epididymal sperm and increased sperm motility, compared to males from lines under relaxed selection. Moreover, we found a correlated response in the size of litters born to females in lines subject to sperm competition. Thus, we present significant evidence that sperm competition has profound fitness consequences for both male and female house mice.     

Bigger testes increase paternity in a simultaneous hermaphrodite,independently of the sperm competition level

Hermaphroditic animals face the fundamental evolutionary optimization problem of allocating their resources to their male vs. female reproductive function (e.g. testes and sperm vs. ovaries and eggs), and this optimal sex allocation can be affected by both pre‐ and post‐copulatory sexual selection. For example, local sperm competition (LSC) – the competition between related sperm for the fertilization of a partner's ova – occurs in small mating groups and can favour a female‐biased sex allocation, because, under LSC, investment into sperm production is predicted to show diminishing fitness returns. Here, we test whether higher testis investment increases an individual's paternity success under sperm competition, and whether the strength of this effect diminishes when LSC is stronger, as predicted by sex allocation theory. We created two subsets of individuals of the simultaneously hermaphroditic flatworm Macrostomum lignano – by sampling worms from either the highest or lowest quartile of the testis investment distribution – and estimated their paternity success in group sizes of either three (strong LSC) or eight individuals (weak LSC). Specifically, using transgenic focal individuals expressing a dominant green‐fluorescent protein marker, we showed that worms with high testis investment sired 22% more offspring relative to those with low investment, corroborating previous findings in M. lignano and other species. However, the strength of this effect was not significantly modulated by the experienced group size, contrasting theoretical expectations of more strongly diminishing fitness returns under strong LSC. We discuss the possible implications for the evolutionary maintenance of hermaphroditism in M. lignano.     

Male sperm storage compromises sperm motility in guppies

Sperm senescence can have important evolutionary implications due to its deleterious effects on sperm quality and offspring performance. Consequently, it has been argued that polyandry (female multiple mating) may facilitate the selection of younger, and therefore competitively superior, sperm when ejaculates from multiple males compete for fertilization. Surprisingly, however, unequivocal evidence that sperm ageing influences traits that underlie sperm competitiveness is lacking. Here, we used a paired experimental design that compares sperm quality between ‘old’ and ‘young’ ejaculates from individual male guppies (Poecilia reticulata). We show that older sperm exhibit significant reductions in sperm velocity compared with younger sperm from the same males. We found no evidence that the brightness of the male''s orange (carotenoid) spots, which are thought to signal resistance to oxidative stress (and thus age-related declines in sperm fitness), signals a male''s ability to withstand the deleterious effects of sperm ageing. Instead, polyandry may be a more effective strategy for females to minimize the likelihood of being fertilized by aged sperm.     

Sexual rest and post‐meiotic sperm ageing in house mice

Fertilization by aged sperm can result in adverse fitness consequences for both males and females. Sperm storage during male sexual rest could provide an environment for post‐meiotic sperm senescence causing a deterioration in the quality of stored sperm, possibly impacting on both sperm performance (e.g. swimming ability) and DNA quality. Here, we compared the proportion of sperm with fragmented DNA, an indicator of structural damage of DNA within the sperm cell, among males that had been sexually rested for approximately 2 months, to that of males that had mated recently. We found no evidence of intra‐epididymal sperm DNA damage or any impairment in sperm performance, and consequently no evidence of post‐meiotic sperm senescence. Our results suggest that male house mice are likely to possess mechanisms that function to ensure that their sperm reserves remain stocked with ‘young’, viable sperm during periods of sexual inactivity. We also discuss the possibility that our experimental design leads to no difference in the age of sperm among males from the two mating treatments. Post‐meiotic sperm senescence is especially relevant under sperm competition. Thus, we sourced mice from populations that differed in their levels of post‐copulatory sexual selection, enabling us to gain insight into how selection for higher sperm production influences the rate of sperm ageing and levels of DNA fragmentation. We found that males from the population that produced the highest number of sperm also had the smallest proportion of DNA‐fragmented sperm and discuss this outcome in relation to selection acting upon males to ensure that they produce ejaculates with high‐quality sperm that are successful in achieving fertilizations under competitive conditions.     

Condition‐dependent ejaculate production affects male mating behavior in the common bedbug Cimex lectularius

Food availability in the environment is often low and variable, constraining organisms in their resource allocation to different life‐history traits. For example, variation in food availability is likely to induce condition‐dependent investment in reproduction. Further, diet has been shown to affect ejaculate size, composition and quality. How these effects translate into male reproductive success or change male mating behavior is still largely unknown. Here, we concentrated on the effect of meal size on ejaculate production, male reproductive success and mating behavior in the common bedbug Cimex lectularius. We analyzed the production of sperm and seminal fluid within three different feeding regimes in six different populations. Males receiving large meals produced significantly more sperm and seminal fluid than males receiving small meals or no meals at all. While such condition‐dependent ejaculate production did not affect the number of offspring produced after a single mating, food‐restricted males could perform significantly fewer matings than fully fed males. Therefore, in a multiple mating context food‐restricted males paid a fitness cost and might have to adjust their mating strategy according to the ejaculate available to them. Our results indicate that meal size has no direct effect on ejaculate quality, but food availability forces a condition‐dependent mating rate on males. Environmental variation translating into variation in male reproductive traits reveals that natural selection can interact with sexual selection and shape reproductive traits. As males can modulate their ejaculate size depending on the mating situation, future studies are needed to elucidate whether environmental variation affecting the amount of ejaculate available might induce different mating strategies.     

SIMULTANEOUS POSITIVE AND NEGATIVE FREQUENCY‐DEPENDENT SELECTION ON SPERM BINDIN,A GAMETE RECOGNITION PROTEIN IN THE SEA URCHIN STRONGYLOCENTROTUS PURPURATUS

Gamete‐recognition proteins often, but not always, evolve rapidly. We explored how variation in sperm bindin influences reproductive success of the sea urchin Strongylocentrotus purpuratus during group spawning in the sea. Despite large variation in male and female abundance and neighbor distances, males with common genotypes had higher reproductive success than males with rare genotypes. However, males with a relatively uncommon proline‐for‐serine substitution were the most successful. Females also showed a fitness consequence of sperm‐bindin genotype, suggesting linkage disequilibrium between the sperm‐bindin locus and the egg receptor locus. Females with common genotypes had higher reproductive success than rare genotypes, but females with relatively uncommon insertions were most successful. Overall, these results suggest that rare male proteins are selected against, as supported by molecular evidence of purifying selection and probably caused by poor matches to the female receptor protein. Within the pool of moderately common to common alleles, however, individuals with less‐common functional variants were favored and probably maintained by negative frequency‐dependent selection. These results support the hypothesis that sperm availability and sexual conflict influence the evolution of gamete recognition systems in broadcast spawners and highlight the benefits of combining fitness measures with molecular signatures for estimation of patterns of selection.     

Introducing mother's curse: low male fertility associated with an imported mtDNA haplotype in a captive colony of brown hares

Mitochondrial DNA mutations create variation in the efficiency of the oxidative phosphorylation pathway and therefore cellular energy production. Mildly deleterious mutations may reduce the performance of sperm cells in particular, due to their high energy requirements and low number of mitochondria, yet have little or no effect on the viability of somatic cells or ova. Mutations will be maintained in the population, despite the fitness cost for males, because mtDNA is passed down the female line. We looked for this so-called mother's curse effect in our captive colony of European brown hares. Significantly reduced male reproductive success was detected for a divergent haplotype that could be traced back to hares imported from a remote population. Median reproductive success for these hares was 0.17 compared to 0.49 for the indigenous haplotypes (Wilcoxon rank-sum, P  = 0.002). No difference was detected for female reproductive success, nor were we able to find a nuclear DNA component to variation in male fertility. Our data are strong evidence for a mother's curse effect persisting despite multiple crossings over seven generations. These data raise important issues relating to the reproductive fitness of small or intermixing populations and have particular implications for the management of populations for conservation.     

Males' evolutionary responses to experimental removal of sexual selection

We evaluated the influence of pre- and post-copulatory sexual selection upon male reproductive traits in a naturally promiscuous species, Drosophila melanogaster. Sexual selection was removed in two replicate populations through enforced monogamous mating with random mate assignment or retained in polyandrous controls. Monogamous mating eliminates all opportunities for mate competition, mate discrimination, sperm competition, cryptic female choice and, hence, sexual conflict. Levels of divergence between lines in sperm production and male fitness traits were quantified after 38-81 generations of selection. Three a priori predictions were tested: (i) male investment in spermatogenesis will be lower in monogamy-line males due to the absence of sperm competition selection, (ii) due to the evolution of increased male benevolence, the fitness of females paired with monogamy-line males will be higher than that of females paired with control-line males, and (iii) monogamy-line males will exhibit decreased competitive reproductive success relative to control-line males. The first two predictions were supported, whereas the third prediction was not. Monogamy males evolved a smaller body size and the size of their testes and the number of sperm within the testes were disproportionately further reduced. In contrast, the fitness of monogamous males (and their mates) was greater when reproducing in a non-competitive context: females mated once with monogamous males produced offspring at a faster rate and produced a greater total number of surviving progeny than did females mated to control males. The results indicate that sexual selection favours the production of increased numbers of sperm in D. melanogaster and that sexual selection favours some male traits conferring a direct cost to the fecundity of females.