Social situation,sperm competition and sex allocation in a simultaneous hermaphrodite parasite,the cestode Schistocephalus solidus

Evolutionary theory predicts an influence of mating group size on sex allocation in simultaneous hermaphrodites. We experimentally manipulated the social situation during reproduction in a simultaneous hermaphrodite parasite, the tapeworm Schistocephalus solidus, by placing worms as singles, pairs or triplets into an in vitro system that replaces the final host. We then determined the reproductive allocation patterns after 24 h (i.e. before the start of egg release) and after 72 h (i.e. around the peak of egg release rate) using stereology. After 24 h, sex allocation strongly depended on worm volume (which is determined in the second intermediate host), but was not significantly affected by the social situation experienced during reproduction. After 72 h, worms in groups had less vesicular sperm (i.e. sperm to be used in future inseminations) than singles. They also stored significantly more received sperm in their seminal receptacles than singles, suggesting that more sperm had been transferred in groups. Moreover, worms in triplets stored significantly more received sperm than worms in pairs, suggesting that they either mated more often and/or transferred more sperm per mating. This suggests a behavioural response to the increased risk of sperm competition in triplets. We further discuss the relative importance of sex allocation decisions at different life‐history stages.     

Trade-off between male and female allocation in the simultaneously hermaphroditic flatworm Macrostomum sp

Sex allocation theory for simultaneous hermaphrodites assumes a direct trade-off between the allocation of resources to the male and female reproductive functions. Empirical support for this basic assumption is scarce, possibly because studies rarely control for variation in individual reproductive resource budgets. Such variation, which can have environmental or genetic sources, can generate a positive relationship between male and female investment and can thus obscure the trade-off. In this study on the hermaphroditic flatworm Macrostomum sp. we tried to control for budget effects by restricting food availability in a standardized way and by using an inbred line. We then manipulated mating group size in a two-way design (two group sizes x two enclosure sizes) in order to induce phenotypic variation in male allocation, and expected to find an opposing correlated response in female allocation. The results suggest that we only managed to control the budget effects under some conditions. Under these the sex allocation trade-off emerged. Under the other conditions we found a strongly positive correlation between male and female allocation. We discuss possible causes for the observed differences.     

Mating frequency and resource allocation to male and female function in the simultaneous hermaphrodite land snail Arianta arbustorum

Sex allocation theory predicts that mating frequency and long‐term sperm storage affect the relative allocation to male and female function in simultaneous hermaphrodites. We examined the effect of mating frequency on male and female reproductive output (number of sperm delivered and eggs deposited) and on the resources allocated to the male and female function (dry mass, nitrogen and carbon contents of spermatophores and eggs) in individuals of the simultaneous hermaphrodite land snail Arianta arbustorum. Similar numbers of sperm were delivered in successive copulations. Consequently, the total number of sperm transferred increased with increasing number of copulations. In contrast, the total number of eggs produced was not influenced by the number of copulations. Energy allocation to gamete production expressed as dry mass, nitrogen or carbon content was highly female‐biased (>95% in all estimates). With increasing number of copulations the relative nitrogen allocation to the male function increased from 1.7% (one copulation) to 4.7% (three copulations), but the overall reproductive allocation remained highly female‐biased. At the individual level, we did not find any trade‐off between male and female reproductive function. In contrast, there was a significant positive correlation between the resources allocated to the male and female function. Snails that delivered many sperm also produced a large number of eggs. This finding contradicts current theory of sex allocation in simultaneous hermaphrodites.     

Determinants of mating and sperm‐transfer success in a simultaneous hermaphrodite

The number of mating partners an individual has within a population is a crucial parameter in sex allocation theory for simultaneous hermaphrodites because it is predicted to be one of the main parameters influencing sex allocation. However, little is known about the factors that determine the number of mates in simultaneous hermaphrodites. Furthermore, in order to understand the benefits obtained by resource allocation into the male function it is important to identify the factors that predict sperm‐transfer success, i.e. the number of sperm a donor manages to store in a mate. In this study we experimentally tested how social group size (i.e. the number of all potential mates within a population) and density affect the number of mates and sperm‐transfer success in the outcrossing hermaphroditic flatworm Macrostomum lignano. In addition, we assessed whether these parameters covary with morphological traits, such as body size, testis size and genital morphology. For this we used a method, which allows tracking sperm of a labelled donor in an unlabelled mate. We found considerable variation in the number of mates and sperm‐transfer success between individuals. The number of mates increased with social group size, and was higher in worms with larger testes, but there was no effect of density. Similarly, sperm‐transfer success was affected by social group size and testis size, but in addition this parameter was influenced by genital morphology. Our study demonstrates for the first time that the social context and the morphology of sperm donors are important predictors of the number of mates and sperm‐transfer success in a simultaneous hermaphrodite. Based on these findings, we hypothesize that sex allocation influences the mating behaviour and outcome of sperm competition.     

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

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

The effect of cryptic female choice on sex allocation in simultaneous hermaphrodites

Sex allocation theory for simultaneous hermaphrodites has focused primarily on the effects of sperm competition, but the role of mate choice has so far been neglected. We present a model to study the coevolution of cryptic female choice and sex allocation in simultaneous hermaphrodites. We show that the mechanism of cryptic female choice has a strong effect on the evolutionary outcome: if individuals remove a fixed proportion of less-preferred sperm, the optimal sex allocation is more female biased (i.e. more biased towards egg production) than without cryptic female choice; conversely, if a fixed amount of sperm is removed, sex allocation is less female-biased than without cryptic female choice, and can easily become male biased (i.e. biased towards sperm production). Under male-biased sex allocation, hermaphroditism can become unstable and the population can split into pure males and hermaphrodites with a female-biased allocation. We discuss the idea that the evolution of sex allocation may depend on the outcome of sexual conflict over the fate of received sperm: the sperm donor may attempt to manipulate or by-pass cryptic female choice and the sperm recipient is expected to resist such manipulation. We conclude that cryptic female choice can have a strong influence on sex allocation in simultaneous hermaphrodites and strongly encourage empirical work on this question.     

Size‐dependent sex allocation in a simultaneous hermaphrodite parasite

Most models of sex allocation distinguish between sequential and simultaneous hermaphrodites, although an intermediate sexual pattern, size‐dependent sex allocation, is widespread in plants. Here we investigated sex allocation in a simultaneous hermaphrodite animal, the tapeworm Schistocephalus solidus, in which adult size is highly variable. Sex allocation was determined using stereological techniques, which allow measuring somatic and reproductive tissues in a common currency, namely volume. We investigated the relationships between individual volume and allocation to different reproductive tissues using an allometric model. One measure of female allocation, yolk gland volume, increased more than proportionally with individual volume. This is in contrast to the measure of male allocation, testis volume, which showed a strong tendency to increase less than proportionally with individual volume. Together these patterns led to sex allocation being strongly related to individual volume, with large individuals being more biased towards female allocation. We discuss these findings in the light of current ideas about size‐dependent sex allocation in, primarily, plants and try to extend them to simultaneous hermaphrodite animals.     

Phenotypically flexible sex allocation in a simultaneous hermaphrodite

Previous studies on sex allocation in simultaneous hermaphrodites have typically focused either on evolutionary or one-time, ontogenetic optimization of sex allocation, ignoring variation within an individual's lifetime. Here, we study whether hermaphrodites also possess facultative sex allocation, that is, a phenotypic flexibility, allowing them to distribute resources to either sex in an opportunistic way during their adult lifetime. We used the simultaneously hermaphroditic free-living flatworm Macrostomum lignano and raised individuals in pairs and groups of eight worms (further called octets) until sexual maturity was reached and sex allocation for the current conditions was expected to be set. Treatment groups were subsequently transferred to the alternative group size, that is, from pairs to octets or from octets to pairs, and compared to two control groups, which were transferred without changing group size. The results show that worms in treatment groups responded as expected by the local mate competition theory for simultaneous hermaphrodites: increasing group size resulted in a shift toward a more male-biased sex allocation and vice versa. These findings reveal that sex allocation in these animals is not fixed during ontogeny, but remains flexible after maturation. We argue that phenotypically flexible sex allocation in hermaphroditic animals may help us to understand the evolution and ecology of hermaphroditism.     

Multiple mating, paternity, and body size in a simultaneous hermaphrodite, Aplysia californica

Sperm displacement and sperm competition prove difficult tomeasure, but are crucial elements in predicting sex allocationstrategies of sperm-storing hermaphrodites. Body size is predictedto affect sex allocation so that within a population, largeanimals invest a greater proportion of resources in female functionthan do small animals. These mating strategies depend on spermdisplacement abilities and lead to similar levels of paternityacross body sizes despite differences in resource level. Thepresent study investigated mating patterns, multiple paternity,and sperm competition in a field population of a simultaneouslyhermaphroditic sea slug, Aplysia californica (California seahare). Animals mating in the female role were larger than themean for the population, indirectly supporting theoretical predictionsfor increased investment in female function with body size.However, contrary to predictions, animals mating in the malerole were not different in size from the population mean orthe animals they inseminated. Individual tagging revealed thatsea slugs are capable of moving across distances that allowfor the sampling of many potential mates, and that they materepeatedly in both sexual roles. Microsatellite paternity analysisdemonstrated that multiple mating in the field leads to multiplepaternity, and last-sperm donors achieve high levels of paternity.There was no effect of body size on paternity. Further paternitystudies are needed to reveal the mechanisms of sperm precedencepatterns in A. californica.     

Sperm competition and sex allocation in simultaneous hermaphrodites

Summary Sex allocation theory is developed for hermaphrodites having frequent copulations and long-term sperm storage. Provided the sperm displacement mechanisms are similar to those known in insects, the ESS allocation to sperm versus eggs satisfies a rather simple rule. There are no data to test this rule, as yet.     

Theoretical influence of female mating status and remating propensity on male sperm allocation patterns

Theoretical models predict that males should allocate more sperm in matings where the immediate risk of sperm competition is high. It has therefore often been argued that males should invest less sperm in matings with virgin females compared with matings with already mated females. However, with relatively polyandrous females, high sperm competition risk will covary with high sperm competition intensity leading to more unpredictable conditions, as high competition intensity should favour smaller ejaculates. With the use of a genetic algorithm, we found that males should allocate more sperm in matings with virgin females when female mating frequency is relatively high, whereas low remating rates will select for higher effort in matings with nonvirgin females. At higher remating rates, first male sperm precedence favours larger ejaculates in matings with virgin females and second male precedence favours the reverse. These results shed some light on several findings that have been difficult to explain adaptively by the hitherto developed theory on sperm allocation.     

Condition dependence of male and female reproductive success: insights from a simultaneous hermaphrodite

Sexually selected traits are predicted to show condition dependence by capturing the genetic quality of its bearer. In separate‐sexed organisms, this will ultimately translate into condition dependence of reproductive success of the sex that experiences sexual selection, which is typically the male. Such condition dependence of reproductive success is predicted to be higher in males than females under conditions promoting intense sexual selection. For simultaneous hermaphrodites, however, sex allocation theory predicts that individuals in poor condition channel relatively more resources into the male sex function at the expense of the female function. Thus, male reproductive success is expected to be less condition dependent than female reproductive success. We subjected individuals of the simultaneously hermaphroditic snail Physa acuta to two feeding treatments to test for condition dependence of male and female reproductive success under varying levels of male–male competition. Condition dependence was found for female, but not for male, reproductive success, meaning that selection on condition is relatively stronger through the female sex function. This effect was consistent over both male–male competition treatments. Decomposition of male and female reproductive performance revealed that individuals in poor condition copulated more in their male role, indicating an increased male allocation to mate acquisition. These findings suggest that sex‐specific condition dependence of reproductive success is at least partially driven by condition‐dependent sex allocation. We discuss the implications of condition‐dependent sex allocation for the evolution of sexually selected traits in simultaneous hermaphrodites.     

Resource-dependent sex-allocation in a simultaneous hermaphrodite

Most sex allocation theory is based on the relationship between the resource investment into male and female reproduction and the consequent fitness returns (often called fitness-gain curves). Here we investigate the effects of resource availability on the sex allocation of a simultaneously hermaphroditic animal, the free-living flatworm Macrostomum lignano. We kept the worms under different resource levels and determined the size of their testes and ovaries over a period of time. At higher resource levels, worms allocated relatively more into the female function, suggesting a saturating male fitness-gain curve for this species. A large part of the observed effect was due to a correlated increase in body size, showing size-dependent sex allocation in M. lignano. However, a significant part of the overall effect was independent of body size, and therefore likely due to the differences in resource availability. Moreover, in accordance with a saturating male fitness-gain curve, the worms developed the male gonads first. As the group size was kept constant, our results contrast with expectations from sex allocation models that deal with local mate competition alone, and with previous experiments that test these models.     

Social group size, potential sperm competition and reproductive investment in a hermaphroditic leech, Helobdella papillornata (Euhirudinea: Glossiphoniidae)

Social group size may affect the potential for sperm competition, and this in turn may favour ontogenetic adjustments in testicular mass according to the likely requirements for sperm and spermatophore production. In a number of comparative analyses of testis mass among vertebrate species that differ in mating system or social organization, increasing potential for sperm competition is associated with larger testis size. Intraspecific phenotypic plasticity should be able to produce the same pattern if social group size is heterogenous and reflects differing degrees of average sperm competition, but this intraspecific effect is less well studied. We tested the effect of social groups on both male and female investment in the simultaneously hermaphroditic leech, Helobdella papillornata. Leeches were placed in groups of one, two, four or eight. Sexual investment at the onset of reproductive maturity was quantified as the total testisac volume for male function and total egg volume for female function. We found that testisac volume (statistically adjusted for body size) showed a significant increase with increasing group size. Total egg volume (also adjusted for body size) was unaffected by group size. Our findings indicate adaptive developmental plasticity in male gonad investment in response to the potential for sperm competition.     

Sex allocation predicts mating rate in a simultaneous hermaphrodite

Sexual selection theory for separate-sexed animals predicts that the sexes differ in the benefit they can obtain from multiple mating. Conventional sex roles assume that the relationship between the number of mates and the fitness of an individual is steeper in males compared with females. Under these conditions, males are expected to be more eager to mate, whereas females are expected to be choosier. Here we hypothesize that the sex allocation, i.e. the reproductive investment devoted to the male versus female function, can be an important predictor of the mating strategy in simultaneous hermaphrodites. We argue that within-species variation in sex allocation can cause differences in the proportional fitness gain derived through each sex function. Individuals should therefore adjust their mating strategy in a way that is more beneficial to the sex function that is relatively more pronounced. To test this, we experimentally manipulated the sex allocation in a simultaneously hermaphroditic flatworm and investigated whether this affects the mating behaviour. The results demonstrate that individuals with a more male-biased sex allocation (i.e. relatively large testes and small ovaries) are more eager to mate compared with individuals with a more female-biased sex allocation (i.e. relatively small testes and large ovaries). We argue that this pattern is comparable to conventional gender roles in separate-sexed organisms.     

Determinants of male ejaculate investment in the cotton bollworm Helicoverpa armigera: mating history,female body size and male age

Many studies demonstrate that ejaculate size may be influenced by male condition, female quality and the risk or intensity of sperm competition. In the present study, the effect of male and female conditions, male mating history and female mating status on ejaculate sperm numbers in the polyandrous moth Helicoverpa armigera is examined. A large variation in ejaculate size is found and, although female body size and male age influence ejaculate size, female age and copula duration do not. Both male and female mating histories have significant effects on ejaculate sperm numbers. Males reduce ejaculate expenditure in successive matings but deliver significantly more apyrene and eupyrene sperm to nonvirgin than to virgin females.     

Size-dependent sex allocation and sexual selection in Aplysia kurodai, a hermaphrodite with nonreciprocal mating

Abstract. For simultaneous hermaphrodites, a male-to-female shift in sex allocation with growth, and weak sexual selection on the male function, is predicted by many theories, although empirical data for both predictions are insufficient for internally fertilizing hermaphrodites with nonreciprocal mating. To address these issues, I studied mating and egg-laying behavior of the sea hare, Aplysia kurodai (Gastropoda: Opisthobranchia) in the laboratory. Both frequency and duration of egg laying increased with body weight, indicating that fecundity increased with weight. On the other hand, frequency and duration of mating as males did not increase with body weight, suggesting that sperm usage was independent of weight. Therefore, sex allocation shifted from male to female functions with growth. The lack of a relationship between body weight and mating activities as males also suggests that there was no "female" choice for large partners. However, the frequency of mating as females increased with body weight, suggesting "male" choice for large partners. This "male" choice is further supported by the presence of size-assortative mating and a longer duration of mating when the female partner was large. In addition, the variance in mating frequency as females was larger than that as males. As a whole, the mating behavior in A. kurodai can be summarized as choosy as males and unchoosy as females, the opposite of the patterns known in most gonochoric and hermaphroditic animals.     

Plant-like mating in an animal: sexual compatibility and allocation trade-offs in a simultaneous hermaphrodite with remote transfer of sperm

The importance of sexual compatibility between mates has only recently been realized in zoological research into sexual selection, yet its study has been central to botanical research for many decades. The reproductive characteristics of remote mating, an absence of precopulatory mate screening, internal fertilization and embryonic brooding are shared between passively pollinated plants and a phylogenetically diverse group of sessile aquatic invertebrates. Here, we further characterize the sexual compatibility system of one such invertebrate, the colonial ascidian Diplosoma listerianum. All 66 reciprocal pairings of 12 genetic individuals were carried out. Fecundities of crosses varied widely and suggested a continuous scale of sexual compatibility. Of the 11 animals from the same population c. 40% of crosses were completely incompatible with a further c. 20% having obvious partial compatibility (reduced fecundity). We are unaware of other studies documenting such high levels of sexual incompatibility in unrelated individuals. RAPD fingerprinting was used to estimate relatedness among the 12 individuals after a known pedigree was successfully reconstructed to validate the technique. In contrast to previous results, no correlation between genetic similarity and sexual compatibility was detected. The blocking of many genotypes of sperm is expected to severely modify realized paternity away from 'fair raffle' expectations and probably reduce levels of intra-brood genetic diversity in this obligatorily promiscuous mating system. One adaptive benefit may be to reduce the bombardment of the female reproductive system by outcrossed sperm with conflicting evolutionary interests, so as to maintain female control of somatic : gametic investment.