Tests of sex allocation theory in simultaneously hermaphroditic animals

Mapping fitness landscapes to interpret sex allocation in hermaphrodites

Sex-allocation theory predicts sex ratios of dioecious organisms, but it has been poor at explaining sex allocation in hermaphrodites in which the assumed trade-off between male and female functions is often obscure. Here, we apply sex-allocation theory to hermaphrodites by mapping components of seasonal reproductive success onto a fitness landscape defined by potentially independent measures of allocation to male and female functions on orthogonal axes. We find that peaks of reproductive success in a perennial hermaphroditic plant reflect the interactive effect of both male and female allocations on self-fertilization and the effects of inbreeding depression. The rugged landscape corresponds well to the complex pattern of sex allocation observed in natural populations in which individuals produce a mix of male and bisexual flowers and express a type of gender diphasy. Our approach may help to interpret common complexities of sex allocation in hermaphroditic plants and animals.

The effects of parasitism on sex allocation of a hermaphroditic acorn barnacle

Sex allocation theory predicts the adaptive allocation of resources to male versus female reproduction in simultaneous hermaphrodites in response to individual characteristics or environmental factors. Because parasites uptake resources from their hosts, their presence could affect the sex allocation of the hosts. We investigated the effects of infestation status and infestation intensity by the rhizocephalan barnacle Boschmaella japonica on reproduction, including sex allocation, of the host intertidal barnacle Chthamalus challengeri. Feeding activity was also examined as a factor related to resource intake. Both male and female reproductive investment decreased with increasing parasite infestation, and the sex allocation of large-infested hosts was more male-biased than that of large uninfested hosts. Moreover, in contrast to the model prediction that male investment does not change under resource limitation, male investment decreased in infested hosts whose resources were taken by parasites. This reduction in male investment could be explained by changes in mating group size, since infested hosts have shorter penises and consequently are able to access fewer mating partners.

Fitness consequences of marine larval dispersal: the role of neighbourhood density, arrangement, and genetic relatedness on survival, growth, reproduction, and paternity in a sessile invertebrate

Dispersal can evolve as an adaptation to escape competition with conspecifics or kin. Locations with a low density of conspecifics, however, may also lead to reduced opportunities for mating, especially in sessile marine invertebrates with proximity-dependent mating success. Since there are few experimental investigations, we performed a series of field experiments using an experimentally tractable species (the bryozoan Bugula neritina) to test the hypothesis that the density, spatial arrangement, and genetic relatedness of neighbours differentially affect survival, growth, reproduction, paternity, and sperm dispersal. We manipulated the density and relatedness of neighbours and found that increased density reduced survival but not growth rate, and that there was no effect of relatedness on survival, growth, or fecundity, in contrast to previous studies. We also manipulated the distances to the nearest neighbour and used genetic markers to assign paternity within known mother-offspring groups to estimate how proximity affects mating success. Distance to the nearest neighbour did not affect the number of settlers produced, the paternity share, or the degree of multiple paternity. Overall, larger than expected sperm dispersal led to high multiple paternity, regardless of the distance to the nearest neighbour. Our results have important implications for understanding selection on dispersal distance: in this system, there are few disadvantages to the limited larval dispersal that does occur and limited advantages for larvae to disperse further than a few 10s of metres.

Life history in Caenorhabditis elegans: from molecular genetics to evolutionary ecology

Life history is defined by traits that reflect key components of fitness, especially those relating to reproduction and survival. Research in life history seeks to unravel the relationships among these traits and understand how life history strategies evolve to maximize fitness. As such, life history research integrates the study of the genetic and developmental mechanisms underlying trait determination with the evolutionary and ecological context of Darwinian fitness. As a leading model organism for molecular and developmental genetics, Caenorhabditis elegans is unmatched in the characterization of life history-related processes, including developmental timing and plasticity, reproductive behaviors, sex determination, stress tolerance, and aging. Building on recent studies of natural populations and ecology, the combination of C. elegans' historical research strengths with new insights into trait variation now positions it as a uniquely valuable model for life history research. In this review, we summarize the contributions of C. elegans and related species to life history and its evolution. We begin by reviewing the key characteristics of C. elegans life history, with an emphasis on its distinctive reproductive strategies and notable life cycle plasticity. Next, we explore intraspecific variation in life history traits and its underlying genetic architecture. Finally, we provide an overview of how C. elegans has guided research on major life history transitions both within the genus Caenorhabditis and across the broader phylum Nematoda. While C. elegans is relatively new to life history research, significant progress has been made by leveraging its distinctive biological traits, establishing it as a highly cross-disciplinary system for life history studies.

Social regulation of reproduction: control or signal?

Traditionally, dominant breeders have been considered to be able to control the reproduction of other individuals in multimember groups that have high variance in reproductive success/reproductive skew (e.g., forced sterility/coercion of conspecifics in eusocial animals; sex-change suppression in sequential hermaphrodites). These actions are typically presented as active impositions by reproductively dominant individuals. However, how can individuals regulate the reproductive physiology of others? Alternatively, all contestants make reproductive decisions, and less successful individuals self-downregulate reproduction in the presence of dominant breeders. Shifting perspective from a top-down manipulation to a broader view, which includes all contenders, and using a multitaxon approach, we propose a unifying framework for the resolution of reproductive skew conflicts based on signalling rather than control, along a continuum of levels of strategic regulation of reproduction.

Sexual conflict, heterochrony and tissue specificity as evolutionary problems of adaptive plasticity in development

Differential gene expression represents a fundamental cause and manifestation of phenotypic plasticity. Adaptive phenotypic plasticity in gene expression as a trait evolves when alleles that mediate gene regulation serve to increase organismal fitness by improving the alignment of variation in gene expression with variation in circumstances. Among the diverse circumstances that a gene encounters are distinct cell types, developmental stages and sexes, as well as an organism's extrinsic ecological environments. Consequently, adaptive phenotypic plasticity provides a common framework to consider diverse evolutionary problems by considering the shared implications of alleles that produce context-dependent gene expression. From this perspective, adaptive plasticity represents an evolutionary resolution to conflicts of interest that arise from any negatively pleiotropic effects of expression of a gene across ontogeny, among tissues, between the sexes, or across extrinsic environments. This view highlights shared properties within the general relation of fitness, trait expression and context that may nonetheless differ substantively in the grain of selection within and among generations to influence the likelihood of adaptive plasticity as an evolutionary response. Research programmes that historically have focused on these separate issues may use the insights from one another by recognizing their shared dependence on context-dependent gene regulatory evolution.

Hermaphroditic origins of anisogamy

Anisogamy-the size dimorphism of gametes-is the defining difference between the male and female sexual strategies. Game-theoretic thinking led to the first convincing explanation for the evolutionary origins of anisogamy in the 1970s. Since then, formal game-theoretic models have continued to refine our understanding of when and why anisogamy should evolve. Such models typically presume that the earliest anisogamous organisms had separate sexes. However, in most taxa, there is no empirical evidence to support this assumption. Here, we present a model of the coevolution of gamete size and sex allocation, which allows for anisogamy to emerge alongside either hermaphroditism or separate sexes. We show that hermaphroditic anisogamy can evolve directly from isogamous ancestors when the average size of spawning groups is small and fertilization is relatively efficient. Sex allocation under hermaphroditism becomes increasingly female-biased as group size decreases and the degree of anisogamy increases. When spawning groups are very small, our model also predicts the existence of complex isogamous organisms in which individuals allocate resources equally to two large gamete types. We discuss common, but potentially unwarranted, assumptions in the literature that could be relaxed in future models. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Macroevolutionary patterns in marine hermaphroditism

Most plants and many animals are hermaphroditic; whether the same forces are responsible for hermaphroditism in both groups is unclear. The well-established drivers of hermaphroditism in plants (e.g., seed dispersal potential, pollination mode) have analogues in animals (e.g., larval dispersal potential, fertilization mode), allowing us to test the generality of the proposed drivers of hermaphroditism across both groups. Here, we test these theories for 1153 species of marine invertebrates, from three phyla. Species with either internal fertilization, restricted offspring dispersal, or small body sizes are more likely to be hermaphroditic than species that are external fertilizers, planktonic developers, or larger. Plants and animals show different biogeographical patterns, however: animals are less likely to be hermaphroditic at higher latitudes-the opposite to the trend in plants. Overall, our results suggest that similar forces, namely, competition among offspring or gametes, shape the evolution of hermaphroditism across plants and three invertebrate phyla.

Precocious Sperm Exchange in the Simultaneously Hermaphroditic Nudibranch, Berghia stephanieae

Sexual systems vary greatly across molluscs. This diversity includes simultaneous hermaphroditism, with both sexes functional at the same time. Most nudibranch molluscs are thought to be simultaneous hermaphrodites, but detailed studies of reproductive development and timing remain rare as most species cannot be cultured in the lab. The aeolid nudibranch, Berghia stephanieae, is one such species that can be cultured through multiple generations on the benchtop. We studied B. stephanieae reproductive timing to establish when animals first exchange sperm and how long sperm can be stored. We isolated age- and size-matched individuals at sequential timepoints to learn how early individuals can exchange sperm. Individuals isolated at 10 weeks post initial feeding (wpf; ∼13 weeks postlaying [wpl]) can produce fertilized eggs. This is 6 weeks before animals first lay egg masses, indicating that sperm exchange occurs well before individuals are capable of laying eggs. Our results indicate that male gonads become functional for animals between 6 mm (∼6 wpf, ∼9 wpl) and 9 mm (∼12 wpf, ∼15 wpl) in length. That is much smaller (and sooner) than the size (and age) of individuals at first laying (12–19 mm; ∼16 wpf, ∼19 wpl), indicating that male and female functions do not develop simultaneously. We also tracked the number of fertilized eggs in each egg mass, which remained steady for the first 10–15 egg masses, followed by a decline to near-to-no fertilization. This dataset provides insights into the precise timing of the onset of functionality of the male and female reproductive systems in B. stephanieae. These data contribute to a broader understanding of reproductive development and the potential for understanding the evolution of diverse sexual systems in molluscs.

Mating and fitness consequences of variation in male allocation in a wind-pollinated plant

In hermaphrodites, the allocation of resources to each sex function can influence fitness through mating success. A prediction that arises from sex allocation theory is that in wind-pollinated plants, male fitness should increase linearly with investment of resources into male function but there have been few empirical tests of this prediction. In a field experiment, we experimentally manipulated allocation to male function in Ambrosia artemisiifolia (common ragweed) and measured mating success in contrasting phenotypes using genetic markers. We investigated the effects of morphological traits and flowering phenology on male siring success, and on the diversity of mates. Our results provide evidence for a linear relation between allocation to male function, mating, and fitness. We find earlier onset of male flowering time increases reproductive success, whereas later flowering increases the probability of mating with diverse individuals. Our study is among the first empirical tests of the prediction of linear male fitness returns in wind-pollinated plants and emphasizes the importance of a large investment into male function by wind-pollinated plants and mating consequences of temporal variation in sex allocation.

Extreme mitochondrial DNA divergence underlies genetic conflict over sex determination

Cytoplasmic male sterility (CMS) is a form of genetic conflict over sex determination that results from differences in modes of inheritance between genomic compartments.^1-3 Indeed, maternally transmitted (usually mitochondrial) genes sometimes enhance their transmission by suppressing the male function in a hermaphroditic organism to the detriment of biparentally inherited nuclear genes. Therefore, these hermaphrodites become functionally female and may coexist with regular hermaphrodites in so-called gynodioecious populations.³ CMS has been known in plants since Darwin's times⁴ but is previously unknown in the animal kingdom.^5-8 We relate the first observation of CMS in animals. It occurs in a freshwater snail population, where some individuals appear unable to sire offspring in controlled crosses and show anatomical, physiological, and behavioral characters consistent with a suppression of the male function. Male sterility is associated with a mitochondrial lineage that underwent a spectacular acceleration of DNA substitution rates, affecting the entire mitochondrial genome-this acceleration concerns both synonymous and non-synonymous substitutions and therefore results from increased mitogenome mutation rates. Consequently, mitochondrial haplotype divergence within the population is exceptionally high, matching that observed between snail taxa that diverged 475 million years ago. This result is reminiscent of similar accelerations in mitogenome evolution observed in plant clades where gynodioecy is frequent,^9,10 both being consistent with arms-race evolution of genome regions implicated in CMS.^11,12 Our study shows that genomic conflicts can trigger independent evolution of similar sex-determination systems in plants and animals and dramatically accelerate molecular evolution.

Evolution: Mitochondrial lodgers can take over in hermaphroditic snails

Mitochondria - the cell's power stations - are inherited uniparentally via eggs, not sperm. In hermaphroditic plants, they sometimes prevent their hosts from making pollen (and sperm), causing cytoplasmic male sterility. New evidence from a hermaphroditic freshwater snail now documents cytoplasmic male sterility in animals.

Evolution of sex allocation plasticity in a hermaphroditic flatworm genus

Sex allocation theory in simultaneous hermaphrodites predicts that optimal sex allocation is influenced by local sperm competition, which occurs when related sperm compete to fertilize a given set of eggs. Different factors, including the mating strategy and the ability to self‐fertilize, are predicted to affect local sperm competition and hence the optimal SA. Moreover, since the local sperm competition experienced by an individual can vary temporally and spatially, this can favour the evolution of sex allocation plasticity. Here, using seven species of the free‐living flatworm genus Macrostomum, we document interspecific variation in sex allocation, but neither their mating strategy nor their ability to self‐fertilize significantly predicted sex allocation among these species. Since we also found interspecific variation in sex allocation plasticity, we further estimated standardized effect sizes for plasticity in response to (i) the presence of mating partners (i.e. in isolation vs. with partners) and (ii) the strength of local sperm competition (i.e. in small vs. large groups). We found that self‐fertilization predicted sex allocation plasticity with respect to the presence of mating partners, with plasticity being lower for self‐fertilizing species. Finally, we showed that interspecific variation in sex allocation is higher than intraspecific variation due to sex allocation plasticity. Our study suggests that both sex allocation and sex allocation plasticity are evolutionarily labile, with self‐fertilization predicting the latter in Macrostomum.

Mitonuclear discordance and patterns of reproductive isolation in a complex of simultaneously hermaphroditic species, the Allolobophora chlorotica case study

Historical events of population fragmentation, expansion and admixture over geological time may result in complex patterns of reproductive isolation and may explain why, for some taxa, the study of mitochondrial (mt) and nuclear (nu) genetic data results in discordant evolutionary patterns. Complex patterns of taxonomic diversity were recently revealed in earthworms for which distribution is largely the result of paleogeographical events. Here, we investigated reproductive isolation patterns in a complex of cryptic species of earthworms in which discordant patterns between mt and nu genetic lineages were previously revealed, the Allolobophora chlorotica aggregate. Using four nu microsatellite markers and a fragment of the cytochrome c oxidase subunit I mt gene, we carried out a parentage analysis to investigate the mating patterns (i) between individuals belonging to two divergent mt lineages that cannot be distinguished with nu markers and (ii) between individuals belonging to lineages that are differentiated both at the mt and nu levels. Amongst the 157 field-collected individuals, 66 adults were used in cross-breeding experiments to form 22 trios based on their assignment to a mt lineage, and 453 obtained juveniles were genotyped. We showed that adults that mated with both their potential mates in the trio produced significantly more juveniles. In crosses between lineages that diverged exclusively at the mt level, a sex-specific pattern of reproduction characteristic to each lineage was observed, suggesting a possible conflict of interest concerning the use of male/female function between mating partners. In crosses between lineages that diverged both at the mt and nu levels, a high production of cocoons was counterbalanced by a low hatching rate, suggesting a post-zygotic reproductive isolation. Different degrees of reproductive isolation, from differential sex allocation to post-zygotic isolation, were thus revealed. Lineages appear to be at different stages in the speciation process, which likely explain the observed opposite patterns of mitonuclear congruence.

Mating behavior and reproductive morphology predict macroevolution of sex allocation in hermaphroditic flatworms

Background

Sex allocation is the distribution of resources to male or female reproduction. In hermaphrodites, this concerns an individual’s resource allocation to, for example, the production of male or female gametes. Macroevolutionary studies across hermaphroditic plants have revealed that the self-pollination rate and the pollination mode are strong predictors of sex allocation. Consequently, we expect similar factors such as the selfing rate and aspects of the reproductive biology, like the mating behaviour and the intensity of postcopulatory sexual selection, to predict sex allocation in hermaphroditic animals. However, comparative work on hermaphroditic animals is limited. Here, we study sex allocation in 120 species of the hermaphroditic free-living flatworm genus Macrostomum. We ask how hypodermic insemination, a convergently evolved mating behaviour where sperm are traumatically injected through the partner’s epidermis, affects the evolution of sex allocation. We also test the commonly-made assumption that investment into male and female reproduction should trade-off. Finally, we ask if morphological indicators of the intensity of postcopulatory sexual selection (female genital complexity, male copulatory organ length, and sperm length) can predict sex allocation.

Results

We find that the repeated evolution of hypodermic insemination predicts a more female-biased sex allocation (i.e., a relative shift towards female allocation). Moreover, transcriptome-based estimates of heterozygosity reveal reduced heterozygosity in hypodermically mating species, indicating that this mating behavior is linked to increased selfing or biparental inbreeding. Therefore, hypodermic insemination could represent a selfing syndrome. Furthermore, across the genus, allocation to male and female gametes is negatively related, and larger species have a more female-biased sex allocation. Finally, increased female genital complexity, longer sperm, and a longer male copulatory organ predict a more male-biased sex allocation.

Conclusions

Selfing syndromes have repeatedly originated in plants. Remarkably, this macroevolutionary pattern is replicated in Macrostomum flatworms and linked to repeated shifts in reproductive behavior. We also find a trade-off between male and female reproduction, a fundamental assumption of most theories of sex allocation. Beyond that, no theory predicts a more female-biased allocation in larger species, suggesting avenues for future work. Finally, morphological indicators of more intense postcopulatory sexual selection appear to predict more intense sperm competition.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01234-1.

Frequent origins of traumatic insemination involve convergent shifts in sperm and genital morphology

Traumatic insemination is a mating behavior during which the (sperm) donor uses a traumatic intromittent organ to inject an ejaculate through the epidermis of the (sperm) recipient, thereby frequently circumventing the female genitalia. Traumatic insemination occurs widely across animals, but the frequency of its evolution, the intermediate stages via which it originates, and the morphological changes that such shifts involve remain poorly understood. Based on observations in 145 species of the free-living flatworm genus Macrostomum, we identify at least nine independent evolutionary origins of traumatic insemination from reciprocal copulation, but no clear indication of reversals. These origins involve convergent shifts in multivariate morphospace of male and female reproductive traits, suggesting that traumatic insemination has a canalizing effect on morphology. We also observed sperm in both the sperm receiving organ and within the body tissue of two species. These species had intermediate trait values indicating that traumatic insemination evolves through initial internal wounding during copulation. Finally, signatures of male-female coevolution of genitalia across the genus indicate that sexual selection and sexual conflict drive the evolution of traumatic insemination, because it allows donors to bypass postcopulatory control mechanisms of recipients.

Environmental effects on the genetic architecture of fitness components in a simultaneous hermaphrodite

Understanding how environmental change affects genetic variances and covariances of reproductive traits is key to formulate firm predictions on evolutionary responses. This is particularly true for sex-specific variance in reproductive success, which has been argued to affect how populations can adapt to environmental change. Our current knowledge on the impact of environmental stress on sex-specific genetic architecture of fitness components is still limited and restricted to separate-sexed organisms. However, hermaphroditism is widespread across animals and may entail interesting peculiarities with respect to genetic constraints imposed on the evolution of male and female reproduction. We explored how food restriction affects the genetic variance-covariance (G) matrix of body size and reproductive success of the simultaneously hermaphroditic freshwater snail Physa acuta. Our results provide strong evidence that the imposed environmental stress elevated the opportunity for selection in both sex functions. However, the G-matrix remained largely stable across the tested food treatments. Importantly, our results provide no support for cross-sex genetic correlations suggesting no strong evolutionary coupling of male and female reproductive traits. We discuss potential implications for the adaptation to changing environments and highlight the need for more quantitative genetic studies on male and female fitness components in simultaneous hermaphrodites.

Evolution of life cycle dimorphism: An example of sacoglossan sea slugs

Many sea slugs of Sacoglossa (Mollusca: Heterobranchia) are sometimes called "solar-powered sea slugs" because they keep chloroplasts obtained from their food algae and receive photosynthetic products (termed kleptoplasty). Some species show life cycle dimorphism, in which a single species has some individuals with a complex life cycle (the mother produces planktotrophic larvae, which later settle in the adult habitat) and others with a simple life cycle (mothers produce benthic offspring by direct development or short-term nonfeeding larvae in which feeding planktonic stages are skipped). Life cycle dimorphism is not common among marine species. In this paper, we ask whether some aspects of the ecology of solar-powered sea slugs have promoted the evolution of life cycle dimorphism in them. We study the population dynamics of the two life-cycle types that differ in summer (one with planktonic life and the other with benthic life), but both have benthic life in other seasons. We obtain the conditions in which two types with different life cycles coexist stably or a single type generating offspring with different life cycles evolves. We conclude that the stable coexistence of two life cycles can evolve if benthic individuals in summer experience strongly density-dependent processes or if the between-year fluctuation of biomass growth in summer is very large. We discuss whether these results match the life cycles of solar-powered sea slugs with life cycle dimorphism.

Reproductive consequences of an extra long-term sperm storage organ

Background

Sperm storage plays a key role in the reproductive success of many sexually-reproducing organisms, and the capacity of long-term sperm storage varies across species. While there are theoretical explanations for why such variation exists, to date there are no controlled empirical tests of the reproductive consequences of additional long-term sperm storage. While Dipterans ancestrally have three long-term sperm organs, known as the spermathecae, Drosophila contain only two.

Results

We identified a candidate gene, which we call spermathreecae (sp3), in which a disruption cause the development of three functional spermathecae rather than the usual two in Drosophila. We used this disruption to test the reproductive consequences of having an additional long-term sperm storage organ. Compared to females with two spermathecae, females with three spermathecae store a greater total number of sperm and can produce offspring a greater length of time. However, they did not produce a greater total number of offspring.

Conclusions

Thus, additional long-term sperm storage in insects may increase female fitness through extending the range of conditions where she produces offspring, or through increasing the quality of offspring via enhanced local sperm competition at fertilization.

Different effects of mating group size as male and as female on sex allocation in a simultaneous hermaphrodite

Sex allocation theory predicts that the optimal sexual resource allocation of simultaneous hermaphrodites is affected by mating group size (MGS). Although the original concept assumes that the MGS does not differ between male and female functions, the MGS in the male function (MGSm; i.e., the number of sperm recipients the focal individual can deliver its sperm to plus one) and that in the female function (MGSf; the number of sperm donors plus one) do not always coincide and may differently affect the optimal sex allocation. Moreover, reproductive costs can be split into "variable" (e.g., sperm and eggs) and "fixed" (e.g., genitalia) costs, but these have been seldom distinguished in empirical studies. We examined the effects of MGSm and MGSf on the fixed and variable reproductive investments in the sessilian barnacle Balanus rostratus. The results showed that MGSm had a positive effect on sex allocation, whereas MGSf had a nearly significant negative effect. Moreover, the "fixed" cost varied with body size and both aspects of MGS. We argue that the two aspects of MGS should be distinguished for organisms with unilateral mating.

Variation in sex allocation plasticity in three closely related flatworm species

Sex allocation (SA) theory for simultaneous hermaphrodites predicts an influence of group size on SA. Since group size can vary within an individual's lifetime, this can favor the evolution of phenotypically plastic SA. In an emerging comparative context, we here report on SA plasticity in three closely related Macrostomum flatworm species, namely Macrostomum janickei, Macrostomum cliftonensis, and Macrostomum mirumnovem. For each species, we experimentally raised worms in three group sizes (isolated, pairs, and octets) and two enclosure sizes (small and large) in all factorial combinations and studied the effects of these factors on different estimates of SA. In addition, we also evaluated whether isolated worms engage in self-fertilization. We found that all species have plastic SA, with M. cliftonensis being more plastic than the other two species, as assessed by comparing standardized effect sizes of (a) the presence/absence of mating partners and (b) the strength of sexual competition. Moreover, we found that sperm production rate-but not sperm morphology-is plastic in M. cliftonensis, and that only M. mirumnovem self-fertilized during our observation period. Our study suggests that both SA and SA plasticity can diverge even between closely related species.

Hermaphrodites and parasitism: size-specific female reproduction drives infection by an ephemeral parasitic castrator

Sex can influence patterns of parasitism because males and females can differ in encounter with, and susceptibility to, parasites. We investigate an isopod parasite (Hemioniscus balani) that consumes ovarian fluid, blocking female function of its barnacle host, a simultaneous hermaphrodite. As a hermaphrodite, sex is fluid, and individuals may allocate energy differentially to male versus female reproduction. We predicted the relationship between barnacle size and female reproductive function influences the distribution of parasites within barnacle populations. We surveyed 12 populations spanning ~400 km of coastline of southern California and found intermediate-sized barnacles where most likely to be actively functioning as females. While it is unclear why larger individuals are less likely to be actively reproducing as females, we suggest this reduced likelihood is driven by increased investment in male reproductive effort at larger sizes. The female function-size relationship was mirrored by the relationship between size and parasitism. We suggest parasitism by Hemioniscus balani imposes a cost to female function, reinforcing the lack of investment in female function by the largest individuals. Within the subset of suitable (=female) hosts, infection probability increased with size. Hence, the distribution of female function, combined with selection for larger hosts, primarily dictated patterns of infection.

The neurogenetics of sexually dimorphic behaviors from a postdevelopmental perspective

Most sexually reproducing animal species are characterized by two morphologically and behaviorally distinct sexes. The genetic, molecular and cellular processes that produce sexual dimorphisms are phylogenetically diverse, though in most cases they are thought to occur early in development. In some species, however, sexual dimorphisms are manifested after development is complete, suggesting the intriguing hypothesis that sex, more generally, might be considered a continuous trait that is influenced by both developmental and postdevelopmental processes. Here, we explore how biological sex is defined at the genetic, neuronal and behavioral levels, its effects on neuronal development and function, and how it might lead to sexually dimorphic behavioral traits in health and disease. We also propose a unifying framework for understanding neuronal and behavioral sexual dimorphisms in the context of both developmental and postdevelopmental, physiological timescales. Together, these two temporally separate processes might drive sex-specific neuronal functions in sexually mature adults, particularly as it pertains to behavior in health and disease.

Reproductive transitions in plants and animals: selfing syndrome, sexual selection and speciation

The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the 'selfing syndrome', in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Genetic control of male production in Daphnia pulex

Daphnia normally reproduce by cyclical parthenogenesis, with offspring sex being determined by environmental cues. However, some females have lost the ability to produce males. Our results demonstrate that this loss of male-producing ability is controlled by a dominant allele at a single locus. We identified the locus by comparing whole-genome sequences of 67 nonmale-producing (NMP) and 100 male-producing (MP) clones from 5 Daphnia pulex populations, revealing 132 NMP-linked SNPs and 59 NMP-linked indels within a single 1.1-Mb nonrecombining region on chromosome I. These markers include 7 nonsynonymous mutations, all of which are located within one unannotated protein-coding gene (gene 8960). Within this single gene, all of the marker-linked NMP haplotypes from different populations form a monophyletic clade, suggesting a single origin of the NMP phenotype, with the NMP haplotype originating by introgression from a sister species, Daphnia pulicaria Methyl farnesoate (MF) is the innate juvenile hormone in daphnids, which induces the production of males and whose inhibition results in female-only production. Gene 8960 is sensitive to treatment by MF in MP clones, but such responsiveness is greatly reduced in NMP clones. Thus, we hypothesize that gene 8960 is located downstream of the MF-signaling pathway in D. pulex, with the NMP phenotype being caused by expression change of gene 8960.

Sex allocation plasticity on a transcriptome scale: Socially sensitive gene expression in a simultaneous hermaphrodite

Phenotypic plasticity can enable organisms to produce optimal phenotypes in multiple environments. A crucial life history trait that is often highly plastic is sex allocation, which in simultaneous hermaphrodites describes the relative investment into the male versus female sex functions. Theory predicts-and morphological evidence supports-that greater investment into the male function is favoured with increasing group size, due to the increasing importance of sperm competition for male reproductive success. Here, we performed a genome-wide gene expression assay to test for such sex allocation plasticity in a model simultaneous hermaphrodite, the free-living flatworm Macrostomum lignano. Based on RNA-Seq data from 16 biological replicates spanning four different group size treatments, we demonstrate that at least 10% of the >75,000 investigated transcripts in M. lignano are differentially expressed according to the social environment, rising to >30% of putative gonad-specific transcripts (spermatogenesis and oogenesis candidates) and tail-specific transcripts (seminal fluid candidates). This transcriptional response closely corresponds to the expected shift away from female and towards male reproductive investment with increasing sperm competition level. Using whole-mount in situ hybridization, we then confirm that many plastic transcripts exhibit the expected organ-specific expression, and RNA interference of selected testis- and ovary-specific candidates establishes that these indeed function in gametogenesis pathways. We conclude that a large proportion of sex-specific transcripts in M. lignano are differentially expressed according to the prevailing ecological conditions and that these are functionally relevant to key reproductive phenotypes. Our study thus begins to bridge organismal and molecular perspectives on sex allocation plasticity.

Genetic and environmental variation in transcriptional expression of seminal fluid proteins

Seminal fluid proteins (SFPs) are crucial mediators of sexual selection and sexual conflict. Recent studies have chiefly focused on environmentally induced plasticity as one source of variation in SFP expression, particularly in response to differing sperm competition levels. However, understanding the evolution of a trait in heterogenous environments requires estimates of both environmental and genetic sources of variation, as well as their interaction. Therefore, we investigated how environment (specifically mating group size, a good predictor of sperm competition intensity), genotype and genotype-by-environment interactions affect seminal fluid expression. To do so, we reared 12 inbred lines of a simultaneously hermaphroditic flatworm Macrostomum lignano in groups of either two or eight worms and measured the expression levels of 58 putative SFP transcripts. We then examined the source of variation in the expression of each transcript individually and for multivariate axes extracted from a principal component analysis. We found that mating group size did not affect expression levels according to the single transcript analyses, nor did it affect the first principal component (presumably representing overall investment in seminal fluid production). However, mating group size did affect the relative expression of different transcripts captured by the second principal component (presumably reflecting variation in seminal fluid composition). Most transcripts were genetically variable in their expression level and several exhibited genotype-by-environment interactions; relative composition also showed high genetic variation. Collectively, our results reveal the tightly integrated nature of the seminal fluid transcriptome and provide new insights into the quantitative genetic basis of seminal fluid investment and composition.

Reproductive differences among species, and between individuals and cohorts, in the leech genus Helobdella (Lophotrochozoa; Annelida; Clitellata; Hirudinida; Glossiphoniidae), with implications for reproductive resource allocation in hermaphrodites

Leeches and oligochaetes comprise a monophyletic group of annelids, the Clitellata, whose reproduction is characterized by simultaneous hermaphroditism. While most clitellate species reproduce by cross-fertilization, self-fertilization has been described within the speciose genus Helobdella. Here we document the reproductive life histories and reproductive capacities for three other Helobdella species. Under laboratory conditions, both H. robusta and H. octatestisaca exhibit uniparental reproduction, apparently reflecting self-fertility, and suggesting that this trait is ancestral for the genus. However, the third species, H. austinensis, seems incapable of reproduction by self-fertilization, so we inferred its reproductive life history by analyzing reproduction in breeding cohorts. Comparing the reproductive parameters for H. robusta reproducing in isolation and in cohorts revealed that reproduction in cohorts is dramatically delayed with respect to that of isolated individuals, and that cohorts of leeches coordinate their cocoon deposition in a manner that is not predicted from the reproductive parameters of individuals reproducing in isolation. Finally, our comparisons of reproductive capacity for individuals versus cohorts for H. robusta, and between different sizes of cohorts for H. austinensis, reveal differences in resource allocation between male and female reproductive roles that are consistent with evolutionary theory.

Sexual selection and inbreeding: Two efficient ways to limit the accumulation of deleterious mutations

Theory and empirical data showed that two processes can boost selection against deleterious mutations, thus facilitating the purging of the mutation load: inbreeding, by exposing recessive deleterious alleles to selection in homozygous form, and sexual selection, by enhancing the relative reproductive success of males with small mutation loads. These processes tend to be mutually exclusive because sexual selection is reduced under mating systems that promote inbreeding, such as self‐fertilization in hermaphrodites. We estimated the relative efficiency of inbreeding and sexual selection at purging the genetic load, using 50 generations of experimental evolution, in a hermaphroditic snail (Physa acuta). To this end, we generated lines that were exposed to various intensities of inbreeding, sexual selection (on the male function), and nonsexual selection (on the female function). We measured how these regimes affected the mutation load, quantified through the survival of outcrossed and selfed juveniles. We found that juvenile survival strongly decreased in outbred lines with reduced male selection, but not when female selection was relaxed, showing that male‐specific sexual selection does purge deleterious mutations. However, in lines exposed to inbreeding, where sexual selection was also relaxed, survival did not decrease, and even increased for self‐fertilized juveniles, showing that purging through inbreeding can compensate for the absence of sexual selection. Our results point to the further question of whether a mixed strategy combining the advantages of both mechanisms of genetic purging could be evolutionary stable.

Experimental evidence for reduced male allocation under selfing in a simultaneously hermaphroditic animal

Self-fertilization is widespread among simultaneously hermaphroditic animals and plants, but is often only facultatively deployed under circumstances that constrain outcrossing. A central prediction of sex allocation (SA) theory is that because exclusive selfing reduces sperm or pollen competition to zero, this should favour extreme economy in resources channelled to the male sex function. We can therefore expect that organisms switching from outcrossing to selfing should reduce their male allocation. However, to date this prediction has received relatively little support in animal taxa, especially compared to plants. Here we show that isolated individuals (under enforced selfing conditions) have a less male-biased SA than do grouped conspecifics (under outcrossing conditions) in the preferentially outcrossing flatworm Macrostomum hystrix This shift arises from a reduced male allocation (testis area) in isolated individuals, although we did not find any evidence for a re-allocation of these resources to the female sex function (i.e. ovary area was unaffected by selfing/outcrossing conditions). Our results provide some of the clearest experimental evidence to date for reduced male allocation under selfing in simultaneously hermaphroditic animals, extending previous findings comparing SA between populations differing in selfing rates to the level of individual plasticity in gametogenesis.

The interaction between sex-specific selection and local adaptation in species without separate sexes

Local adaptation in hermaphrodite species can be based on a variety of fitness components, including survival, as well as both female and male sex-functions within individuals. When selection via female and male fitness components varies spatially (e.g. due to environmental heterogeneity), local adaptation will depend, in part, on variation in selection through each fitness component, and the extent to which genetic trade-offs between sex-functions maintain genetic variation necessary for adaptation. Local adaptation will also depend on the hermaphrodite mating system because self-fertilization alters several key factors influencing selection and the maintenance of genetic variance underlying trade-offs between the sex-functions (sexually antagonistic polymorphism). As a first step to guide intuition regarding sex-specific adaptation in hermaphrodites, we develop a simple theoretical model incorporating the essential features of hermaphrodite mating and adaptation in a spatially heterogeneous environment, and explore the interaction between sex-specific selection, self-fertilization and local adaptation. Our results suggest that opportunities for sex-specific local adaptation in hermaphrodites depend strongly on the extent of self-fertilization and inbreeding depression. Using our model as a conceptual framework, we provide a broad overview of the literature on sex-specific selection and local adaptation in hermaphroditic plants and animals, emphasizing promising future directions in light of our theoretical predictions.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

Asymmetric evolutionary responses to sex-specific selection in a hermaphrodite

Sex allocation theory predicts that simultaneous hermaphrodites evolve to an evolutionary stable resource allocation, whereby any increase in investment to male reproduction leads to a disproportionate cost on female reproduction and vice versa. However, empirical evidence for sexual trade-offs in hermaphroditic animals is still limited. Here, we tested how male and female reproductive traits evolved under conditions of reduced selection on either male or female reproduction for 40 generations in a hermaphroditic snail. This selection favors a reinvestment of resources from the sex function under relaxed selection toward the other function. We found no such evolutionary response. Instead, juvenile survival and male reproductive success significantly decreased in lines where selection on the male function (i.e., sexual selection) was relaxed, while relaxing selection on the female function had no effect. Our results suggest that most polymorphisms under selection in these lines were not sex-antagonistic. Rather, they were deleterious mutations affecting juvenile survival (thus reducing both male and female fitness) with strong pleiotropic effects on male success in a sexual selection context. These mutations accumulated when sexual selection was relaxed, which supports the idea that sexual selection in hermaphrodites contributes to purge the mutation load from the genome as in separate-sex organisms.

Optimal Mating Strategies for Preferentially Outcrossing Simultaneous Hermaphrodites in the Presence of Predators

The optimal timing for initiating reproduction (i.e., the age at first reproduction) is a critical life history trait describing aspects of an individual's resource-allocation strategy. Recent theoretical and empirical work has demonstrated that this trait is also tied to mating system expression when individuals have the opportunity to reproduce via both self-fertilization and cross-fertilization. A strategy of "delayed selfing" has emerged as a "best of both worlds" arrangement where, in the absence of a mate, an individual will delay reproduction (selfing) to "wait" for a mate. Herein, we extend previously developed predictive optimization models for the timing of reproduction to a situation where organisms can allocate their resources to size-dependent and size-independent defensive strategies to counter the threat of predation. By incorporating inducible defenses into a predictive framework for analyzing life history expression and evolution, we can more accurately evaluate the role that allocation strategy plays in altering the optimal waiting time. We compare our model to previous models and empirical results highlighting that incorporation of inducible defenses into the model broadens the parameter space in which a waiting time is expected and often leads to a predicted waiting time that is longer than in the situation without inducible defenses. In particular, a waiting time is predicted to exist regardless of the strength of inbreeding depression in the population.

A targeted in situ hybridization screen identifies putative seminal fluid proteins in a simultaneously hermaphroditic flatworm

BACKGROUND

Along with sperm, in many taxa ejaculates also contain large numbers of seminal fluid proteins (SFPs). SFPs and sperm are transferred to the mating partner, where they are thought to play key roles in mediating post-mating sexual selection. They modulate the partner's behavior and physiology in ways that influence the reproductive success of both partners, thus potentially leading to sexual conflict. Despite the presumed general functional and evolutionary significance of SFPs, their identification and characterization has to date focused on just a few animal groups, predominantly insects and mammals. Moreover, until now seminal fluid profiling has mainly focused on species with separate sexes. Here we report a comprehensive screen for putative SFPs in the simultaneously hermaphroditic flatworm Macrostomum lignano.

RESULTS

Based on existing transcriptomic data, we selected 150 transcripts known to be (a) predominantly expressed in the tail region of the worms, where the seminal fluid-producing prostate gland cells are located, and (b) differentially expressed in social environments differing in sperm competition level, strongly implying that they represent a phenotypically plastic aspect of male reproductive allocation in this species. For these SFP candidates, we then performed whole-mount in situ hybridization (ISH) experiments to characterize tissue-specific expression. In total, we identified 98 transcripts that exhibited prostate-specific expression, 76 of which we found to be expressed exclusively in the prostate gland cells; additional sites of expression for the remaining 22 included the testis or other gland cells. Bioinformatics analyses of the prostate-limited candidates revealed that at least 64 are predicted to be secretory proteins, making these especially strong candidates to be SFPs that are transferred during copulation.

CONCLUSIONS

Our study represents a first comprehensive analysis using a combination of transcriptomic and ISH screen data to identify SFPs based on transcript expression in seminal fluid-producing tissues. We thereby extend the range of taxa for which seminal fluid has been characterized to a flatworm species with a sequenced genome and for which several methods such as antibody staining, transgenesis and RNA interference have been established. Our data provide a basis for testing the functional and evolutionary significance of SFPs.

Twelve fundamental life histories evolving through allocation-dependent fecundity and survival

An organism's life history is closely interlinked with its allocation of energy between growth and reproduction at different life stages. Theoretical models have established that diminishing returns from reproductive investment promote strategies with simultaneous investment into growth and reproduction (indeterminate growth) over strategies with distinct phases of growth and reproduction (determinate growth). We extend this traditional, binary classification by showing that allocation‐dependent fecundity and mortality rates allow for a large diversity of optimal allocation schedules. By analyzing a model of organisms that allocate energy between growth and reproduction, we find twelve types of optimal allocation schedules, differing qualitatively in how reproductive allocation increases with body mass. These twelve optimal allocation schedules include types with different combinations of continuous and discontinuous increase in reproduction allocation, in which phases of continuous increase can be decelerating or accelerating. We furthermore investigate how this variation influences growth curves and the expected maximum life span and body size. Our study thus reveals new links between eco‐physiological constraints and life‐history evolution and underscores how allocation‐dependent fitness components may underlie biological diversity.

The varied ways of being male and female

Our understanding of sexual reproduction is mainly informed by research on gonochorists (i.e., species with separate sexes), including insects, birds, and mammals. But the male and female sexes are not two types of individuals; they actually represent two different reproductive strategies, and in many organisms, these two strategies are distributed among individuals in a population in a variety of ways. For example, sequential hermaphrodites (or sex-changers) exhibit one strategy early in life and later switch to the other, while simultaneous hermaphrodites exhibit both strategies at the same time. There are also many intermediate sexual systems that mix gonochorists and hermaphrodites in the same species and within many organismal groups, shifts occur between these sexual systems. A fascinating collection of six articles in this special issue on Hermaphroditism & Sex Determination impressively documents some important challenges to our understanding of sex determination, and the specification of male and female reproductive function when these need to occur within the same individual rather than in two separate individuals. Moreover, hermaphroditism changes how we need to think about reproductive allocation to sexual functions, how such allocation can be specified, as well as how the sexual system affects sexual conflict and the resulting antagonistic coevolution. Our understanding of sexual reproduction will profit greatly from exploring the varied ways of being male and female. Mol. Reprod. Dev. 84: 94-104, 2017. © 2017 Wiley Periodicals, Inc.

A reconstruction of sexual modes throughout animal evolution

BACKGROUND

Although most extant animals have separate sexes, simultaneous hermaphrodites can be found in lineages throughout the animal kingdom. However, the sexual modes of key ancestral nodes including the last common ancestor (LCA) of all animals remain unclear. Without these data, it is difficult to infer the reproductive-state transitions that occurred early in animal evolution, and thus a broad understanding of the evolution of animal reproduction remains elusive. In this study, we use a composite phylogeny from four previously published studies, two alternative topologies (ctenophores or sponges as sister to the rest of animals), and multiple phylogenetic approaches to conduct the most extensive analysis to date of the evolution of animal sexual modes.

RESULTS

Our analyses clarify the sexual mode of many ancestral animal nodes and allow for sound inferences of modal transitions that have occurred in animal history. Our results also indicate that the transition from separate sexes to hermaphroditism has been more common in animal history than the reverse.

CONCLUSIONS

These results provide the most complete view of the evolution of animal sexual modes to date and provide a framework for future inquiries into the correlation of these transitions with genes, behaviors, and physiology. These results also suggest that mutations promoting hermaphroditism have historically been more likely to invade gonochoristic populations than vice versa.

The evolution of gonad expenditure and gonadosomatic index (GSI) in male and female broadcast-spawning invertebrates

Sedentary broadcast-spawning marine invertebrates, which release both eggs and sperm into the water for fertilization, are of special interest for sexual selection studies. They provide unique insight into the early stages of the evolutionary succession leading to the often-intense operation of both pre- and post-mating sexual selection in mobile gonochorists. Since they are sessile or only weakly mobile, adults can interact only to a limited extent with other adults and with their own fertilized offspring. They are consequently subject mainly to selection on gamete production and gamete success, and so high gonad expenditure is expected in both sexes. We review literature on gonadosomatic index (GSI; the proportion of body tissue devoted to gamete production) of gonochoristic broadcast spawners, which we use as a proxy for gonad expenditure. We show that such taxa most often have a high GSI that is approximately equal in both sexes. When GSI is asymmetric, female GSI usually exceeds male GSI, at least in echinoderms (the majority of species recorded). Intriguingly, though, higher male GSI also occurs in some species and appears more common than female-biased GSI in certain orders of gastropod molluscs. Our limited data also suggest that higher male GSI may be the prevalent pattern in sperm casters (where only males release gametes). We explore how selection might have shaped these patterns using game theoretic models for gonad expenditure that consider possible trade-offs with (i) somatic maintenance or (ii) growth, while also considering sperm competition, sperm limitation, and polyspermy. Our models of the trade-off between somatic tissue (which increases survival) and gonad (which increases reproductive success) predict that GSI should be equal for the two sexes when sperm competition is intense, as is probably common in broadcast spawners due to synchronous spawning in aggregations. Higher female GSI occurs under low sperm competition. Sperm limitation appears unlikely to alter these conclusions qualitatively, but can also act as a force to keep male GSI high, and close to that of females. Polyspermy can act to reduce male GSI. Higher male than female GSI is predicted to be less common (as observed in the data), but can occur when ova/ovaries are sufficiently more resource-intensive to produce than sperm/testes, for which some evidence exists. We also show that sex-specific trade-offs between gonads and growth can generate different life-history strategies for males and females, with males beginning reproduction earlier. This could lead to apparently higher male GSI in empirical studies if immature females are included in calculations of mean GSI. The existence of higher male GSI nonetheless remains somewhat problematic and requires further investigation. When sperm limitation is low, we suggest that the natural logarithm of the male/female GSI ratio may be a suitable index for sperm competition level in broadcast spawners, and that this may also be considered as an index for internally fertilizing taxa.

Organ specific gene expression in the regenerating tail of Macrostomum lignano

Temporal and spatial characterization of gene expression is a prerequisite for the understanding of cell-, tissue-, and organ-differentiation. In a multifaceted approach to investigate gene expression in the tail plate of the free-living marine flatworm Macrostomum lignano, we performed a posterior-region-specific in situ hybridization screen, RNA sequencing (RNA-seq) of regenerating animals, and functional analyses of selected tail-specific genes. The in situ screen revealed transcripts expressed in the antrum, cement glands, adhesive organs, prostate glands, rhabdite glands, and other tissues. Next we used RNA-seq to characterize temporal expression in the regenerating tail plate revealing a time restricted onset of both adhesive organs and copulatory apparatus regeneration. In addition, we identified three novel previously unannotated genes solely expressed in the regenerating stylet. RNA interference showed that these genes are required for the formation of not only the stylet but the whole male copulatory apparatus. RNAi treated animals lacked the stylet, vesicula granulorum, seminal vesicle, false seminal vesicle, and prostate glands, while the other tissues of the tail plate, such as adhesive organs regenerated normally. In summary, our findings provide a large resource of expression data during homeostasis and regeneration of the morphologically complex tail regeneration and pave the way for a better understanding of organogenesis in M. lignano.

Role of parasite transmission in promoting inbreeding: II. Pedigree reconstruction reveals sib-transmission and consequent kin-mating

Even though parasitic flatworms are one of the most species-rich groups of hermaphroditic organisms, we know virtually nothing of their mating systems (selfing or kin-mating rates) in nature. Hence, we lack an understanding of the role of inbreeding in parasite evolution. The natural mating systems of parasitic flatworms have remained elusive due to the inherent difficulty in generating progeny-array data in many parasite systems. New developments in pedigree reconstruction allow direct inference of realized selfing rates in nature by simply using a sample of genotyped individuals. We built upon this advancement by utilizing the closed mating systems, that is, individual hosts, of endoparasites. In particular, we created a novel means to use pedigree reconstruction data to estimate potential kin-mating rates. With data from natural populations of a tapeworm, we demonstrated how our newly developed methods can be used to test for cosibling transmission and inbreeding depression. We then showed how independent estimates of the two mating system components, selfing and kin-mating rates, account for the observed levels of inbreeding in the populations. Thus, our results suggest that these natural parasite populations are in inbreeding equilibrium. Pedigree reconstruction analyses along with the new companion methods we developed will be broadly applicable across a myriad of parasite species. As such, we foresee that a new frontier will emerge wherein the diverse life histories of flatworm parasites could be utilized in comparative evolutionary studies to broadly address ecological factors or life history traits that drive mating systems and hence inbreeding in natural populations.

Sexual selection gradients change over time in a simultaneous hermaphrodite

Sexual selection is generally predicted to act more strongly on males than on females. The Darwin-Bateman paradigm predicts that this should also hold for hermaphrodites. However, measuring this strength of selection is less straightforward when both sexual functions are performed throughout the organism's lifetime. Besides, quantifications of sexual selection are usually done during a short time window, while many animals store sperm and are long-lived. To explore whether the chosen time frame affects estimated measures of sexual selection, we recorded mating success and reproductive success over time, using a simultaneous hermaphrodite. Our results show that male sexual selection gradients are consistently positive. However, an individual's female mating success seems to negatively affect its own male reproductive success, an effect that only becomes visible several weeks into the experiment, highlighting that the time frame is crucial for the quantification and interpretation of sexual selection measures, an insight that applies to any iteroparous mating system.