Delayed selfing as an optimal mating strategy in preferentially outcrossing species: theoretical analysis of the optimal age at first reproduction in relation to mate availability

Waiting for love but not forever: Modeling the evolution of waiting time to selfing in hermaphrodites

Although mixed mating systems involving both selfing and outcrossing are fairly common in hermaphrodites, the mechanisms maintaining mixed mating are still unknown in many cases. In some species, individuals that have not yet found a mating partner delay self-fertilization for some time. This “waiting time” to selfing (WT) can exhibit heritable variation between individuals and is subject to two opposing selection pressures: waiting longer increases the density-dependent probability to encounter a mate within that time and thereby the chance to avoid inbreeding depression (ID) in offspring, but also increases the risk of dying before reproduction. It has long been hypothesized that fluctuations in population density and thus mate availability can lead to stable intermediate WTs, but to our knowledge there are so far no quantitative models that also take into account the joint evolutionary dynamics of ID. We use an individual-based model and a mathematical approximation to explore how delayed selfing evolves in response to density and density fluctuations. We find that at high density, when individuals meet often, WT evolution is dominated by genetic drift; at intermediate densities, strong ID causes WT to increase; and at low densities, ID is purged and WT approaches zero. Positive feedback loops drive the system to either complete selfing or complete outcrossing. Fluctuating density can slow down convergence to these alternative stable states. However, mixed mating, in the sense of either a stable polymorphism in WT, or stable intermediate waiting times, was never observed. Thus, additional factors need to be explored to explain the persistence of delayed selfing.

Is the initiation of selfing linked to a hermaphrodite's female or male reproductive function?

ABSTRACT

There is an ongoing debate about whether simultaneous hermaphrodites capable of selfing should prefer selfing over outcrossing or vice versa. While many theoretical models predict a transmission advantage for alleles that favour selfing, empirical studies often reveal low selfing rates. Despite these considerations, the underlying mechanisms that determine reproductive strategies in simultaneously hermaphroditic animals are poorly understood. In our study on the facultatively selfing free-living flatworm, Macrostomum hystrix, we ask whether the initiation of selfing, as inferred from the differential spatial distribution of received sperm, is linked to an individual's female or male reproductive function. Specifically, the initiation of selfing could (i) be linked to the male function, when an individual is unable to donate sperm to others and hence donates sperm to self, or it could (ii) be linked to the female function, when an individual fails to receive sperm from others-and hence is unable to fertilize its eggs via outcrossing-thus inducing it to self-fertilize. We experimentally created a social environment that allowed focals to outcross via sperm donation, but simultaneously prevented them from receiving sperm-by pairing them with a partner lacking the male copulatory organ-so that fertilization of the focal's eggs was restricted to selfing. Our results suggest that such focals generally do not initiate selfing, while we readily observe selfing in isolated worms. This suggests that in isolated M. hystrix, it is the male function that is linked to the initiation of selfing, likely due to a lack of opportunities for sperm donation.

SIGNIFICANCE STATEMENT

A variety of simultaneously hermaphroditic animals are capable of reproducing via both selfing and outcrossing. While the reproductive choices of such animals can be modelled by the joint action of genetic (e.g. inbreeding depression) and ecological factors (e.g. partner availability), experimental evaluation of theoretical results is often lacking. By manipulating the social environment of focal individuals, we here provide evidence that explores the respective role that the co-occurring male and female sex functions have on the initiation of selfing in a simultaneously hermaphroditic flatworm species. Specifically, our results suggest that the initiation of selfing is linked to the worm's male function. Insights about which function is linked to the initiation of selfing may ultimately help to better understand reproductive decisions in simultaneous hermaphrodites.

How relatedness between mates influences reproductive success: An experimental analysis of self-fertilization and biparental inbreeding in a marine bryozoan

Kin associations increase the potential for inbreeding. The potential for inbreeding does not, however, make inbreeding inevitable. Numerous factors influence whether inbreeding preference, avoidance, or tolerance evolves, and, in hermaphrodites where both self-fertilization and biparental inbreeding are possible, it remains particularly difficult to predict how selection acts on the overall inbreeding strategy, and to distinguish the type of inbreeding when making inferences from genetic markers. Therefore, we undertook an empirical analysis on an understudied type of mating system (spermcast mating in the marine bryozoan, Bugula neritina) that provides numerous opportunities for inbreeding preference, avoidance, and tolerance. We created experimental crosses, containing three generations from two populations to estimate how parental reproductive success varies across parental relatedness, ranging from self, siblings, and nonsiblings from within the same population. We found that the production of viable selfed offspring was extremely rare (only one colony produced three selfed offspring) and biparental inbreeding more common. Paternity analysis using 16 microsatellite markers confirmed outcrossing. The production of juveniles was lower for sib mating compared with nonsib mating. We found little evidence for consistent inbreeding, in terms of nonrandom mating, in adult samples collected from three populations, using multiple population genetic inferences. Our results suggest several testable hypotheses that potentially explain the overall mating and dispersal strategy in this species, including early inbreeding depression, inbreeding avoidance through cryptic mate choice, and differential dispersal distances of sperm and larvae.

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.

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.

Correlation between the timing of autonomous selfing and floral traits: a comparative study from three selfing Gentianopsis species (Gentianaceae)

About 20% of angiosperms employ self-fertilization as their main mating strategy. In this study, we aimed to examine how the selfing timing correlated with floral traits in three Gentianopsis species in which autonomous selfing is achieved through filament elongation. Although the three Gentianopsis species exhibit no significant variation in their capacity for autonomous selfing, flowers of G. grandis last longer, are larger and have a higher corolla biomass, P/O ratios and male biomass allocation than those of G. paludosa, and especially those of G. contorta. Autonomous selfing occurs in the early floral life of G. paludosa and G. contorta and in the later floral life of G. grandis. Seed production mainly results from autonomous selfing in G. paludosa and G. contorta; however, G. grandis could be more described as having a mixed mating system. We suggest that autonomous selfing in later floral life increases the chance of cross-pollination prior to this, while autonomous selfing in early floral life offers a selective advantage to plants by reducing the resource investment in traits that may increase pollinator attraction and visitation.

Timing is everything: Dichogamy and pollen germinability underlie variation in autonomous selfing among populations

PREMISE OF THE STUDY

The evolution of multiple floral traits often underlies the transition from outcrossing to selfing. Such traits can influence the ability to self, and the timing at which selfing occurs, which in turn affects the costs of selfing. Species that display variation in autonomous selfing provide an opportunity to dissect the phenotypic changes that contribute to variability in the mating system.

METHODS

In a common garden, we measured dichogamy and herkogamy in 24 populations of the protandrous mixed-mating herb Campanula americana, and related these to autonomous fruit set (autonomy). We then measured the timing of self-pollen deposition and fruit production in populations with high and low autonomy, and determined whether pollen germinability across floral development contributes to variation in autonomy.

KEY RESULTS

Populations that transitioned more rapidly to female phase displayed elevated autonomous selfing, but herkogamy was unassociated with autonomous selfing. Selfing occurred more rapidly in highly autonomous populations because of greater self-pollen deposition early in female phase. Pollen germinability in low-autonomy populations remained constant across floral development, but in high-autonomy populations it increased after floral anthesis and was highest near the onset of female phase.

CONCLUSIONS

Reduced dichogamy, elevated self-pollen deposition, and higher pollen germination late in male phase contribute to both earlier selfing and greater selfing. These traits vary among populations, likely reflecting past selection on the mating system. While delayed selfing bears fewer fitness costs, the evolution of earlier selfing may be favored if self-pollen availability decreases over floral development.

Exploring the relationship between tychoparthenogenesis and inbreeding depression in the Desert Locust, Schistocerca gregaria

Tychoparthenogenesis, a form of asexual reproduction in which a small proportion of unfertilized eggs can hatch spontaneously, could be an intermediate evolutionary link in the transition from sexual to parthenogenetic reproduction. The lower fitness of tychoparthenogenetic offspring could be due to either developmental constraints or to inbreeding depression in more homozygous individuals. We tested the hypothesis that in populations where inbreeding depression has been purged, tychoparthenogenesis may be less costly. To assess this hypothesis, we compared the impact of inbreeding and parthenogenetic treatments on eight life-history traits (five measuring inbreeding depression and three measuring inbreeding avoidance) in four laboratory populations of the desert locust, Schistocerca gregaria, with contrasted demographic histories. Overall, we found no clear relationship between the population history (illustrated by the levels of genetic diversity or inbreeding) and inbreeding depression, or between inbreeding depression and parthenogenetic capacity. First, there was a general lack of inbreeding depression in every population, except in two populations for two traits. This pattern could not be explained by the purging of inbreeding load in the studied populations. Second, we observed large differences between populations in their capacity to reproduce through tychoparthenogenesis. Only the oldest laboratory population successfully produced parthenogenetic offspring. However, the level of inbreeding depression did not explain the differences in parthenogenetic success between all studied populations. Differences in development constraints may arise driven by random and selective processes between populations.

Self-fertilization, sex allocation and spermatogenesis kinetics in the hypodermically inseminating flatworm Macrostomum pusillum

The free-living flatworm genus Macrostomum is an emerging model system for studying the links between sex allocation, sexual selection and mating system evolution, as well as the underlying developmental and physiological mechanisms responsible for wide intra- and inter-specific variability in reproductive phenotypes. Despite compelling comparative morphological evidence of sexual diversity, detailed experimental work on reproductive behaviour and physiology in Macrostomum has so far been largely limited to just two species, M. lignano and M. hystrix, an obligate and a preferential outcrosser, respectively. In this study, we establish that a third species, M. pusillum, exhibits a combination of reproductive traits strikingly different from both of its congeners. Unlike M. lignano, we demonstrate that M. pusillum does not adjust sex allocation or the speed of spermatogenesis to the prevailing social group size. Macrostomumpusillum's relatively simple sperm morphology likely explains the short spermatogenesis duration we report, and is linked to a hypodermically inseminating mode of fertilization, which we show also means that these worms are capable of self-fertilization. Surprisingly, and unlike M. hystrix, selfing in isolated worms commences after only a short (if any) delay compared with the onset of reproduction in grouped individuals, with little evidence of differential inbreeding depression in 'isolated' progeny. These combined results suggest that, in nature, M. pusillum may be regularly selfing, in contrast to the congeners studied to date. Our findings highlight the rapid and correlated evolution of reproductive traits, and reinforce the utility of the genus Macrostomum for understanding the evolutionary and developmental mechanisms responsible for this diversity.

Exploring the sexual diversity of flatworms: Ecology, evolution, and the molecular biology of reproduction

Flatworms exhibit huge diversity in their reproductive biology, making this group an excellent model system for exploring how differences among species in reproductive ecology are reflected in the physiological and molecular details of how reproduction is achieved. In this review, I consider five key "lifestyle choices" (i.e., alternative evolutionary/developmental outcomes) that collectively encompass much of flatworm sexual diversity, beginning with the decisions: (i) whether to be free-living or parasitic; (ii) whether to reproduce asexually or sexually; and (iii) whether to be gonochoristic (separate-sexed) or hermaphroditic. I then examine two further decisions involving hermaphroditism: (iv) outcrossing versus selfing and (v) the balance of investment into the male versus the female sex function (sex allocation). Collectively, these lifestyle choices set the basic rules for how reproduction occurs, but as I emphasize in the second part of the review, the reproductive biology of flatworms is also greatly impacted by the near-pervasive and powerful pressure of sexual selection, together with the related phenomena of sperm competition and sexual conflict. Exactly how this plays out, however, is strongly affected by the particular combination of reproductive strategies adopted by each species. Mol. Reprod. Dev. 84: 120-131, 2017. © 2016 Wiley Periodicals, Inc.

Hypodermic self-insemination as a reproductive assurance strategy

Self-fertilization occurs in a broad range of hermaphroditic plants and animals, and is often thought to evolve as a reproductive assurance strategy under ecological conditions that disfavour or prevent outcrossing. Nevertheless,selfing ability is far from ubiquitous among hermaphrodites, and may be constrained in taxa where the male and female gametes of the same individual cannot easily meet. Here, we report an extraordinary selfing mechanism in one such species, the free-living flatworm Macrostomum hystrix. To test the hypothesis that adaptations to hypodermic insemination of the mating partner under outcrossing also facilitate selfing, we experimentally manipulated the social environment of these transparent flatworms and then observed the spatial distribution of received sperm in vivo. We find that this distribution differs radically between conditions allowing or preventing outcrossing, implying that isolated individuals use their needle-like stylet (male copulatory organ) to inject own sperm into their anterior body region, including into their own head, from where they then apparently migrate to the site of (self-)fertilization. Conferring the ability to self could thus be an additional consequence of hypodermic insemination, a widespread fertilization mode that is especially prevalent among simultaneously hermaphroditic animals and probably evolves due to sexual conflict over the transfer and subsequent fate of sperm.

High Rates of Self-Fertilization in a Marine Ribbon Worm (Nemertea)

Organisms capable of self-fertilization ("selfing") typically exhibit two evolutionary syndromes: uniting high inbreeding depression with low levels of selfing, or low inbreeding depression with high levels of selfing. We examined the effect of inbreeding on fecundity and time to first reproduction in an apparently self-compatible, simultaneously hermaphroditic marine nemertean worm Prosorhochmus americanus. Adult and juvenile worms were raised in isolation or in pairs. Isolated worms produced significantly more offspring than paired worms (in the adult experiment), and did not exhibit inbreeding avoidance (in the juvenile experiment). The selfing rate of six natural populations was evaluated using 17 species-specific, microsatellite markers, and was consistent with preferential selfing (mean: 0.843, SD: 0.027). Our results showed that P. americanus exhibited an interesting suite of life-history traits, uniting high colonization potential through self-fertilization and high fecundity, with no dispersive larval stage, and with moderate levels of gene flow. We believe that P. americanus is an ideal model system for studies of mating system evolution, inbreeding, and sex allocation.

What happens after inbreeding avoidance? Inbreeding by rejected relatives and the inclusive fitness benefit of inbreeding avoidance

Avoiding inbreeding, and therefore avoiding inbreeding depression in offspring fitness, is widely assumed to be adaptive in systems with biparental reproduction. However, inbreeding can also confer an inclusive fitness benefit stemming from increased relatedness between parents and inbred offspring. Whether or not inbreeding or avoiding inbreeding is adaptive therefore depends on a balance between inbreeding depression and increased parent-offspring relatedness. Existing models of biparental inbreeding predict threshold values of inbreeding depression above which males and females should avoid inbreeding, and predict sexual conflict over inbreeding because these thresholds diverge. However, these models implicitly assume that if a focal individual avoids inbreeding, then both it and its rejected relative will subsequently outbreed. We show that relaxing this assumption of reciprocal outbreeding, and the assumption that focal individuals are themselves outbred, can substantially alter the predicted thresholds for inbreeding avoidance for focal males. Specifically, the magnitude of inbreeding depression below which inbreeding increases a focal male’s inclusive fitness increases with increasing depression in the offspring of a focal female and her alternative mate, and it decreases with increasing relatedness between a focal male and a focal female’s alternative mate, thereby altering the predicted zone of sexual conflict. Furthermore, a focal male’s inclusive fitness gain from avoiding inbreeding is reduced by indirect opportunity costs if his rejected relative breeds with another relative of his. By demonstrating that variation in relatedness and inbreeding can affect intra- and inter-sexual conflict over inbreeding, our models lead to novel predictions for family dynamics. Specifically, parent-offspring conflict over inbreeding might depend on the alternative mates of rejected relatives, and male-male competition over inbreeding might lead to mixed inbreeding strategies. Making testable quantitative predictions regarding inbreeding strategies occurring in nature will therefore require new models that explicitly capture variation in relatedness and inbreeding among interacting population members.

Diet alters delayed selfing, inbreeding depression, and reproductive senescence in a freshwater snail

Reproductive success is a critical fitness attribute that is directly influenced by resource availability. Here, we investigate the effects of diet-based resource availability on three interrelated aspects of reproductive success: a change in mating system based on mate availability, consequent inbreeding depression, and the deterioration of reproductive efficiency with age (senescence). We employed a factorial experimental design using 22 full-sib families of the hermaphroditic freshwater snail Physa acuta to explore these interactions. Individual snails were reared in one of two mate-availability treatments (isolated [selfing] or occasionally paired [outcrossing]) and one of two diet treatments (boiled lettuce or Spirulina, an algae that is rich in protein, vitamins, and minerals). Spirulina-fed snails initiated reproduction at a 13% earlier age and 7% larger size than lettuce-fed snails. Spirulina also resulted in a 30% reduction in the time delay before selfing. Compared to lettuce, a diet of Spirulina increased inbreeding depression by 52% for egg hatching rate and 64% for posthatching juvenile survival. Furthermore, Spirulina led to a 15-fold increase in the rate of reproductive senescence compared with a diet of lettuce. These transgenerational, interactive effects of diet on inbreeding depression and reproductive senescence are discussed in the context of diet-induced phenotypic plasticity.

Purging of inbreeding depression within a population of Oxalis alpina (Oxalidaceae)

PREMISE OF THE STUDY

Variation among individuals in levels of inbreeding depression associated with selfing levels could influence mating system evolution by purging deleterious alleles, but empirical evidence for this association is limited.

METHODS

We investigated the association of family-level inbreeding depression and presumed inbreeding history in a tristylous population of Oxalis alpina (Oxalidaceae).

KEY RESULTS

Mid-styled individuals possessed the greatest degree of self-compatibility (SC) and produced more autogamous capsules than short- or long-styled individuals. Offspring of highly self-compatible mid-styled individuals showed reduced inbreeding depression. Mid-styled plants that produced capsules autogamously exhibited reduced stigma-anther separation compared to mid-styled plants that produced no capsules autogamously. Reduced inbreeding depression was not correlated with stigma-anther separation, suggesting that self-compatibility and autogamy evolve before morphological changes in stigma-anther separation.

CONCLUSIONS

Purging of inbreeding depression occurred in SC mid-styled maternal families. Low inbreeding depression in SC mid-styled plants may lead to retention of the mid-styled morph in populations, despite the occurrence of higher selfing rates in mid-styled relative to short- or long-styled morphs. Variation among individuals in levels of self-fertilization within populations may lead to associations between inbreeding lineages and lower levels of inbreeding depression, influencing the evolution of mating systems.

The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa

BACKGROUND

The cosmopolitan freshwater snail Physa acuta has recently found widespread use as a model organism for the study of mating systems and reproductive allocation. Mitochondrial DNA phylogenies suggest that Physa carolinae, recently described from the American southeast, is a sister species of P. acuta. The divergence of the acuta/carolinae ancestor from the more widespread P. pomilia appears to be somewhat older, and the split between a hypothetical acuta/carolinae/pomilia ancestor and P. gyrina appears older still.

RESULTS

Here we report the results of no-choice mating experiments yielding no evidence of hybridization between gyrina and any of four other populations (pomilia, carolinae, Philadelphia acuta, or Charleston acuta), nor between pomilia and carolinae. Crosses between pomilia and both acuta populations yielded sterile F1 progeny with reduced viability, while crosses between carolinae and both acuta populations yielded sterile F1 hybrids of normal viability. A set of mate-choice tests also revealed significant sexual isolation between gyrina and all four of our other Physa populations, between pomilia and carolinae, and between pomilia and Charleston acuta, but not between pomilia and the acuta population from Philadelphia, nor between carolinae and either acuta population. These observations are consistent with the origin of hybrid sterility prior to hybrid inviability, and a hypothesis that speciation between pomilia and acuta may have been reinforced by selection for prezygotic reproductive isolation in sympatry.

CONCLUSIONS

We propose a two-factor model for the evolution of postzygotic reproductive incompatibility in this set of five Physa populations consistent with the Dobzhansky-Muller model of speciation, and a second two-factor model for the evolution of sexual incompatibility. Under these models, species trees may be said to correspond with gene trees in American populations of the freshwater snail, Physa.

The effects of predation risk on mating system expression in a freshwater snail

Environmental effects on mating system expression are central to understanding mating system evolution in nature. Here, I report the results from a quantitative-genetic experiment aimed at understanding the role of predation risk in the expression and evolution of life-history and mating-system traits in a hermaphroditic freshwater snail (Physa acuta). I reared 30 full-sib families in four environments that factorially contrast predation risk and mate availability and measured age/size at first reproduction, growth rate, a morphological defense, and the early survival of outcrossed/selfed eggs that were laid under predator/no-predator conditions. I evaluated the genetic basis of trade-offs among traits and the stability of the G matrix across environments. Mating reduced growth while predation risk increased growth, but the effects of mating were weaker for predator-induced snails and the effects of predation risk were weaker for snails without mates. Predation risk reduced the amount of time that individuals waited before self-fertilizing and reduced inbreeding depression in the offspring. There was a positive among-family relationship between the amount of time that individuals delayed selfing under predation risk and the magnitude of inbreeding depression. These results highlight several potential roles of enemies in mating-system expression and evolution.

Bateman gradients in hermaphrodites: an extended approach to quantify sexual selection

Sexual selection is often quantified using Bateman gradients, which represent sex-specific regression slopes of reproductive success on mating success and thus describe the expected fitness returns from mating more often. Although the analytical framework for Bateman gradients aimed at covering all sexual systems, empirical studies are biased toward separate-sex organisms, probably because important characteristics of other systems remain incompletely treated. Our synthesis complements the existing Bateman gradient approach with three essential reproductive features of simultaneous hermaphrodites. First, mating in one sex may affect fitness via the opposite sex, for example, through energetic trade-offs. We integrate cross-sex selection effects and show how they help characterizing sexually mutualistic versus antagonistic selection. Second, male and female mating successes may be correlated, complicating the interpretation of Bateman gradients. We show how to quantify the impact of this correlation on sexual selection and propose a principal component analysis on male and female mating success to facilitate interpretation. Third, self-fertilization is accounted for by adding selfed progeny as a separate category of reproductive success to analyses of Bateman gradients. Finally, using a worked example from the snail Biomphalaria glabrata, we illustrate how the extended analytical framework can enhance our understanding of sexual selection in hermaphroditic animals and plants.

Evidence for costs of mating and self-fertilization in a simultaneous hermaphrodite with hypodermic insemination, the Opisthobranch Alderia willowi

Simultaneous hermaphrodites offer the chance to study antagonistic coevolution between the sexes when individuals function in both roles. Traumatic mating by hypodermic insemination has repeatedly evolved in hermaphroditic taxa, but evidence for the fitness costs of such male-advantage traits is lacking. When reared in isolation, specimens of the sea slug Alderia willowi (Opisthobranchia: Sacoglossa) initially laid clutches of unfertilized eggs but 4 days later began self-fertilizing; this is only the third report of selfing in an opisthobranch. Hypodermic insemination may allow selfing in Alderia if penetration of the body wall bypasses internal mechanisms that promote outcrossing. Selfing specimens and slugs reared in pairs had reduced fecundity compared to isolated slugs laying unfertilized clutches, suggesting that hypodermic insemination imposes a cost of mating. Egg production increased for field-caught slugs separated after mating compared to slugs held in pairs, a further indication that accessibility to mates imposes a fitness cost to the female function. Such antagonism can confer a competitive advantage to slugs mating in the male role but diminish reproduction in the female role among hermaphrodites capable of long-term sperm storage. Alderia willowi is also a rare case of poecilogony, with adults producing either planktotrophic or lecithotrophic larvae. Our rearing studies revealed that most slugs switched between expressed development modes at some point; such reproductive flexibility within individuals is unprecedented, even among poecilogonous species.

Are there interactive effects of mate availability and predation risk on life history and defence in a simultaneous hermaphrodite?

Encountering mates and avoiding predators are ubiquitous challenges faced by many organisms and they can affect the expression of many traits including growth, timing of maturity and resource allocation to reproduction. However, these two factors are commonly considered in isolation rather than simultaneously. We examined whether predation risk and mate availability interact to affect morphology and life-history traits (including lifetime fecundity) of a hermaphroditic snail (Physa acuta). We found that mate availability reduced juvenile growth rate and final size. Predator cues from crayfish induced delayed reproduction, but there were no reduced fecundity costs associated with predator induction. Although there were interactive effects on longevity, lifetime fecundity was determined by the number of reproductive days. Therefore, our results indicate a resource-allocation trade-off among growth, longevity and reproduction. Future consideration of this interaction will be important for understanding how resource-allocation plasticity affects the integration of defensive, life-history and mating-system traits.

Prior selfing and the selfing syndrome in animals: an experimental approach in the freshwater snailBiomphalaria pfeifferi

Inbreeding species of hermaphroditic animals practising copulation have been characterized by few copulations, no waiting time (the time that an isolated individual waits for a partner before initiating reproduction compared with paired individuals) and limited inbreeding (self-fertilization) depression. This syndrome, which has never been fully studied before in any species, is analysed here in the highly selfing freshwater snailBiomphalaria pfeifferi. We conducted an experiment under laboratory conditions over two generations (G1and G2) using snails sampled from two populations (100 individuals per population). G1individuals were either isolated or paired once a week (potentially allowing for crosses), and monitored during 29 weeks for growth, fecundity and survival. Very few copulations were observed in paired snails, and there was a positive correlation in copulatory activity (e.g. number of copulations) between the male and female sexual roles. The waiting time was either null or negative, meaning that isolated individuals initiated reproduction before paired ones. G2offspring did not differ in hatching rate and survival (to 28 days) between treatments, but offspring from paired individuals grew faster than those from isolated individuals. On the whole, the self-fertilization depression was extremely low in both populations. Another important result is that paired G1individuals began laying (selfed) eggs several weeks prior to initiating copulation: this is the first characterization of prior selfing (selfing initiated prior to any outcrossing) in a hermaphroditic animal. A significant population effect was observed on most traits studied. Our results are discussed with regard to the maintenance of low outcrossing rates in highly inbreeding species.

The role of natural enemies in the expression and evolution of mixed mating in hermaphroditic plants and animals

Although a large portion of plant and animal species exhibit intermediate levels of outcrossing, the factors that maintain this wealth of variation are not well understood. Natural enemies are one relatively understudied ecological factor that may influence the evolutionary stability of mixed mating. In this paper, we aim for a conceptual unification of the role of enemies in mating system expression and evolution in both hermaphroditic animals and plants. We review current theory and detail the potential effects of enemies on fundamental mating system parameters. In doing so, we identify situations in which consideration of enemies alters expectations about the stability of mixed mating. Generally, we find that inclusion of the enemy dimension may broaden conditions in which mixed mating systems are evolutionarily stable. Finally, we highlight avenues ripe for future theoretical and empirical work that will advance our understanding of enemies in the expression and evolution of mixed mating in their hosts/victims, including examination of feedback cycles between victims and enemies and quantification of mating system-related parameters in victim populations in the presence and absence of enemies.

NO CORRELATION BETWEEN INBREEDING DEPRESSION AND DELAYED SELFING IN THE FRESHWATER SNAIL PHYSA ACUTA

Inbreeding depression, one of the main factors driving mating system evolution, can itself evolve as a function of the mating system (the genetic purging hypothesis). Classical models of coevolution between mating system and inbreeding depression predict negative associations between inbreeding depression and selfing rate, but more recent approaches suggest that negative correlations should usually be too weak or transient to be detected within populations. Empirical results remain unclear and restricted to plants. Here, we evaluate, for the first time, the within-population genetic correlation between inbreeding depression and a trait that controls the amount of self-fertilization (the waiting time) in a self-fertile hermaphroditic animal, the freshwater snail Physa acuta. Using a large quantitative-genetic design (36 grand-families and 348 families), we observe abundant within-population family-level genetic variation for both inbreeding depression (estimated for survival, fecundity, and size) and the degree of behavioral selfing avoidance. However, we detected no correlation between waiting time and inbreeding depression across families. In agreement with recent models, this result shows that mutational variance rather than differential purging accounts for most of the genetic variance in inbreeding depression within a population.

Predominance of outcrossing in Lymnaea stagnalis despite low apparent fitness costs of self-fertilization

We have quantified the natural mating system in eight populations of the simultaneously hermaphroditic aquatic snail Lymnaea stagnalis, and studied the ecological and genetic forces that may be directing mating system evolution in this species. We investigated whether the natural mating system can be explained by the availability of mates, by the differential survival of self- and cross-fertilized snails in nature, and by the effects of mating system on parental fecundity and early survival. The natural mating system of L. stagnalis was found to be predominantly cross-fertilizing. Density of snails in the populations had no relationship with the mating system, suggesting that outcrossing rates are not limited by mate availability at the population densities observed. Contrary to expectations for outcrossing species, we detected no evidence for inbreeding depression in survival in nature with inferential population genetic methods. Further, experimental manipulations of mating system in the laboratory revealed that self-fertilization had no effect on parental fecundity, and only minor effects on offspring survival. Predominance of cross-fertilization despite low apparent fitness costs of self-fertilization is at odds with the paradigm that high self-fertilization depression is necessary for maintenance of cross-fertilization in self-compatible hermaphrodites.

Pollen competition reduces inbreeding depression in Collinsia heterophylla (Plantaginaceae)

We tested two predictions of the hypothesis that competition between self-pollen may mitigate negative genetic effects of inbreeding in plants: (1) intense competition among self-pollen increases offspring fitness; and (2) pollen competition reduces the measured strength of inbreeding depression. We used Collinsia heterophylla (Plantaginaceae), an annual with a mixed mating system, to perform controlled crosses in which we varied both the size of the pollen load and the source of pollen (self vs. outcross). Fitness of selfed offspring was higher in the high pollen-load treatment. Our second prediction was also upheld: inbreeding depression was, on average, lower when large pollen loads were applied (11%) relative to the low pollen-load treatment (28%). The reduction was significant for two fitness components relatively late in the life-cycle: number of surviving seedlings and pollen-tube growth rate in vitro. These findings suggest that intermittent inbreeding, which leads to self-fertilization in plants with genetic loads, may select for traits that enhance pollen competition.

The Cost of Keeping Eggs Fresh: Quantitative Genetic Variation in Females that Mate Late Relative to Sexual Maturation

In many species, females abandon mate choice to ensure that eggs are fertilized before they are lost. But why do females not just maintain oocytes longer if there is a benefit to mate choice? We conducted a quantitative genetic study in the cockroach Nauphoeta cinerea to test whether genetic constraints prevent the evolution of oocyte maintenance or selection against oocyte loss is weak when females mate late relative to sexual maturity. We found standing genetic variation within the population and no evidence for genetic constraints. Levels of genetic variation are of the magnitude found for life-history traits in general, suggesting that this trait has been exposed to selection. We unexpectedly found two categories of females: those that delay reproduction and those that reproduce at a normal time when mating late, which could indicate alternative strategies. However, frequency-dependent selection does not maintain this variation as females that delay always reproduce less well. Given these findings, we suggest that there may be advantages to egg degradation. The evolution of maintenance of fertilizable oocytes over time would then be constrained by the need to maintain the mechanism by which females control the distribution of resources between current and future reproductive events.

Animals mix it up too: the distribution of self-fertilization among hermaphroditic animals

Excluding insects, hermaphroditism occurs in about one-third of animal species, providing numerous opportunities for the evolution of selfing. Here we provide an overview of reproductive traits in hermaphroditic animal species, review the distribution of selfing rates in animals, and test for ecological correlates of selfing. Our dataset (1342 selfing-rate estimates for 142 species) is 97% based on estimates derived from the analysis of population structure (F(IS)-estimates) using genetic markers. The distribution of selfing is slightly U-shaped and differs significantly from the more strongly U-shaped plant distribution with 47% of animal t-estimates being intermediate (falling between 0.2 and 0.8) compared to 42% for plants. The influence of several factors on the distribution of selfing rates was explored (e.g., number of populations studied per species, habitat, coloniality, sessility, or fertilization type), none of which significantly affect the distribution. Our results suggest that genetic forces might contribute to the evolution of self-fertilization to the same extent in animals and plants, although the high proportion of intermediate outcrossing suggests a significant role of ecological factors (e.g., reproductive assurance) in animals. However, we caution that the distribution of selfing rates in animals is affected by various factors that might bias F(IS)-estimates, including phylogenetic underrepresentation of highly selfing and outcrossing species, various genotyping errors (e.g., null alleles) and inbreeding depression. This highlights the necessity of obtaining better estimates of selfing for hermaphroditic animals, such as genotyping progeny arrays, as in plants.

Effects of self-fertilization, environmental stress and exposure to xenobiotics on fitness-related traits of the freshwater snail Lymnaea stagnalis

Genetic and ecological factors may interact in their effects on fitness. Such interactions are thus to be expected between inbreeding and exposure of a population to a toxicant. The magnitude of inbreeding depression is thought to increase in stressful environments. To test this hypothesis, we investigated the combined effects of environmental conditions and inbreeding on fitness in the self-fertile snail Lymnaea stagnalis, using a stress gradient (0-2) applied to a 100 isolated and paired lineages: laboratory control (0), outdoor microcosm control (1) and pesticide exposure under outdoor microcosm conditions (2). Outdoor stress conditions were maintained for 28 days prior to measurements of fitness traits (fecundity, hatching success, and size at hatching) under laboratory conditions, so that delayed environmental effects could be estimated. Under laboratory control conditions, we found significant initial family level heterogeneity for most measured traits, including physiological performances as assessed through energetic biomarkers. Whatever the environmental conditions, inbreeding depression was very low for progeny performances. Negative values of self-fertilization depression (SFD) were obtained. Unexpectedly, SFD showed a negative relationship with the assumed stress intensity, reflecting a higher sensitivity under pairing than under selfing, mostly due to parental fecundity. This suggests that stressful conditions may favour selfing. Stress intensity increased the distribution limits of both depression indices, suggesting that changes in fitness are less predictable in a population under stress. Implications of such findings for environmental risk assessment of pesticides are discussed.

Partial selfing, ecological disturbance and reproductive assurance in an invasive freshwater snail

Although reproductive assurance (RA) might play a central role in the evolution of the selfing rate, this hypothesis has never been seriously investigated in an hermaphroditic animal. We studied the mating system of the freshwater snail Physa acuta in which the availability of mating partners might be highly variable, because this species is an efficient colonizer occupying unstable habitats. A total of 11 populations differing in ecological disturbance regime (water level, openness) and snail densities were monitored over 2 years. The outcrossing rate was estimated in ca 10 families per population using microsatellite markers and the progeny-array approach. Components of fecundity and survival were recorded for each progeny. Predominant outcrossing (t(m)=0.94) was detected, with a few individuals (4%) purely selfing. The outcrossing rate did not explain among-family variation in fitness components. None of the predictions formulated under the RA hypothesis were verified: (i) selfing was related neither to disturbed habitats, nor to temporal density fluctuations, (ii) it was positively related to population density, (iii) it co-occurred with multiple paternity, and (iv) it did not induce delayed reproduction. Explanations for these negative results are discussed in light of other arguments supporting the RA hypothesis in P. acuta, as well as alternative theories explaining the occurrence of partial selfing, as either a genetically fixed or plastic trait.

Optimal reproductive allocation in annuals and an informational constraint on plasticity

In this computational study, we examined optimal reproductive allocation schedules in annual plants whose season lengths vary in predictability. We discuss relationships among season-length predictability, the form of the optimal allocation schedule, the degree of plasticity reflected in the optimal reaction norm, and the competitive consequences of plasticity and bet-hedging. We used an evolutionary algorithm to search the allocation-schedule space for optima, given different distributions of season length. The resulting schedules maximize geometric-mean fecundity under their selecting distributions. We then examined the relative fitness of these schedules in simulated competition among reaction norms optimized for different degrees of season-length predictability. Gradedness of optimal schedules decreases with increasing season-length predictability, and reaction norms comprising highly graded schedules reflect lesser plasticity than norms comprising schedules that are less graded. In simulations, competitively successful genotypes were those that reflected plasticity appropriate to the season-length predictability. Informational constraints in the form of low season-length predictability select for low plasticity and high bet-hedging in allocation. Because an environmental cue must mediate the relationship between environment and fitness, plasticity in reproductive allocation ought to be understood not as a direct response to the selective environment, but rather to cues that are correlated with relevant environmental parameters.

DELAYED SELFING IN RELATION TO THE AVAILABILITY OF A MATING PARTNER IN THE CESTODE SCHISTOCEPHALUS SOLIDUS

A hermaphroditic individual that prefers to outbreed but that has the potential of selfing faces a dilemma: in the absence of a partner, should it wait for one to arrive or should it produce offspring by selfing? Recent theory on this question suggests that the evolutionary solution is to find an optimal delay of reproduction that balances the potential benefit of outcrossing and the cost of delaying the onset of reproduction. Assuming that resources retained from breeding can be reallocated to future reproduction, isolated individuals, compared with individuals with available mates, are predicted to delay their age at first reproduction to wait for future outcrossing. Here, I present empirical support for this idea with experimental data from the hermaphroditic cestode Schistocephalus solidus. I show that individuals breeding alone delay their reproduction and initially produce their eggs at a slow rate relative to cestodes breeding in pairs. This delay is partly compensated for by a later higher egg production, although singly breeding cestodes still pay a cost of overall lower egg production.

Delayed selfing and resource reallocations in relation to mate availability in the freshwater snail Physa acuta

We study the influence of mate availability on the mating behavior of the self-fertile, preferentially outcrossing freshwater snail Physa acuta. Previous optimization theory indicated that mating system interacts with life-history traits to influence the age at first reproduction, providing three testable predictions. First, isolated individuals should reproduce later than individuals with available mates in the expectancy of finding a partner and avoiding the cost of inbreeding. Second, resource reallocation to future fecundity is needed for such reproductive delays to evolve. Third, the reproductive delay can be optimized with respect to life-history traits (e.g., survival, growth) and the mating system (inbreeding depression). Our results largely validate these predictions. First, reproduction is significantly delayed in isolated individuals ("selfers") as compared with individuals frequently exposed to mates ("outcrossers"). Second, delayed reproduction is associated with reallocation to future growth, survival, and fecundity, although fecundity is also affected by the mating system (selfing vs. outcrossing). Third, the reproductive delay found (approximately 2 wk) is consistent with quantitative predictions from optimization models. The delay is largely heritable, which might be partly explained by among-family differences in the amount of inbreeding depression (mating system) but not growth or survival.