Simultaneous Hermaphrodites Research Articles

The Economics of Egg Trading: Mating Rate, Sperm Competition and Positive Frequency-Dependence

Egg trading—the alternating exchange of egg parcels during mating by simultaneous hermaphrodites—is one of the best-documented examples of reciprocity between non-relatives. By offering eggs only to partners who reciprocate, traders increase their reproductive success in the male role, but at a potential cost of delaying or reducing fertilisation of their own eggs. Although several authors have considered the evolutionary stability of egg trading once it has evolved, little attention has been paid to how egg trading can invade a population in the first place. We begin to tackle this problem by formally showing that egg trading is under positive frequency-dependent selection: once the proportion of traders in a population exceeds a certain threshold, egg trading will go to fixation. We show that if mate encounters occur frequently, then the cost of withholding eggs from unreciprocating partners is reduced, making it easier for egg trading to evolve. In contrast, the presence of opportunistic ‘streaking’, where unpaired individuals join mating pairs but contribute only sperm, makes it more difficult for egg trading to invade. This is because streakers weaken the link between the number of eggs an individual can offer and its male-role reproductive success.

First record of the Central Indo-Pacific reef coral Oulastrea crispata in the Mediterranean Sea

A live colony of a non-indigenous zooxanthellate scleractinian coral was found in shallow water at the west coast of Corsica, western Mediterranean. Its diameter of 6 cm suggests that it has already survived for some years. It was identified as Oulastrea crispata, a species native on near-shore coral reefs in the central Indo-Pacific with a high tolerance for low water temperatures at high latitudes. Based on its morphology it can be distinguished from other zooxanthellate colonial scleractinians in the Mediterranean. O. crispata has a reputation of being a successful colonizer because it is able to settle on a wide variety of substrata and because it utilizes various reproductive strategies as simultaneous hermaphrodite and producer of asexually derived planulae. Owing to its original distribution range in temperate and subtropical waters, it is likely that it will be able to meet a suitable temperature regime in the Mediterranean for further range expansion.

Does the cost of a function affect its degree of plasticity? A test on plastic sex allocation in three closely related species of hermaphrodites

Mating opportunities fluctuate in the wild and hermaphrodites have the chance of partitioning reproductive resources between their two sexual functions accordingly, i.e., they have a plastic sex allocation. Plasticity is usually promoted by environmental fluctuations but may be affected by species-specific factors, which may be revealed by comparisons between related species. We tested whether polychaete worms of three related species of simultaneous hermaphrodites, Ophryotrocha diadema, Ophryotrocha adherens and Ophryotrocha gracilis, had plastic male and female allocation. We measured the costs of the female function and investigated whether the costs might affect the magnitude of plasticity in this function. To these aims, we exposed adult worms to three levels of mating opportunities and measured their female and male functions. In our experimental conditions, there was no adjustment in the male function, the cheapest function, whereas the three species differed in how they adjusted their allocation into the female function to mating opportunities. O. diadema and O. adherens worms exhibited highly plastic female allocation, and plasticity was consistent across three measures of female function. In contrast, O. gracilis worms had a fixed female allocation, irrespective of mating opportunities. Additionally, when the sexual functions were relatively costly, their plasticity was greater than when they were relatively cheap. However, the magnitude of the plasticity did not depend solely on species-specific costs of the function, but also on the features of the mating system of each species.

Duration of sperm storage in the simultaneous hermaphrodite Lymnaea stagnalis

Reproductive success is a crucial variable to understand the action of sexual selection, but its quantification is not straightforward because several factors cause nonrandom fertilization success. One of these factors is female sperm storage, which leads to prolonged paternity gain ( paternity longevity). In particular, knowledge about paternity longevity is crucial in simultaneous hermaphrodites, since their ability to self-fertilize adds a further layer of complexity to the quantification of male reproductive success. Here, we investigated the pattern of outcrossing over time after a single mating in a freshwater snail, Lymnaea stagnalis, using a microsatellite marker. We found that offspring are produced from stored sperm for a little over 2 months on average. Furthermore, the pattern of paternity indicates an active role for the female reproductive system in the transport of received sperm to the sperm storage organ. To quantify paternity longevity in a sperm-storing mating system, we propose to use the time when half the offspring are sired by a focal donor (Paternity Longevity 50; PL50), in addition to total storage duration. Intriguingly, we reveal that overall body size is positively correlated with paternity longevity. Although the exact mechanism of sperm storage and its contribution to male reproductive success remain to be revealed, this study provides useful information and new perspectives on sperm storage and sexual selection in this simultaneous hermaphrodite.

Urodasys poculostylis sp. nov., a new stylet-bearing gastrotrich (Macrodasyida) from Capron Shoal, Florida

A new species of marine Gastrotricha, Urodasys poculostylis sp. nov. (Macrodasyida: Macrodasyidae), is described from the sublittoral sediments of Capron Shoal, Florida. The new species is a simultaneous hermaphrodite with a single left testis, a stylet bulb and a frontal organ. The stylet bulb bears a cup-shaped sclerotic stylet up to 20 µm long. Several specimens possessed a large spermatophore within the frontal organ. Urodasys poculostylis sp. nov. can be further distinguished by the presence of three pairs of anterior adhesive tubes distributed in two rows, 14–17 pairs of lateral adhesive tubes with staggered insertions, 5–7 pairs of ventrolateral adhesive tubes, 12–15 pairs of dorsolateral adhesive tubes and two dorsal adhesive tubes.

Reproductive strategies of two invasive sun corals (Tubastraeaspp.) in the southwestern Atlantic

The azooxanthellate sun coralsTubastraea coccineaandT. tagusensisare invasive species which were introduced into Brazilian shallow-water tropical rocky reefs by oil and gas platforms. Both species are simultaneous hermaphrodites and brooders. Histological analysis shows a continuous reproduction with the presence of different stages of development of oocytes, spermatic cysts and larvae occurring together in the same polyps (overlapping). Gametogenesis, embryogenesis, planulation and settlement were observed. Although most of the colonies showed only oocytes, none had only testes. Spermatic cysts were observed in 8.3% ofT. coccineacolonies and in 5.5% of colonies ofT. tagusensis. The maximum diameter of oocyte ofT. tagusensis(1252.5 µm) was greater thanT. coccinea(901.8 µm) and mature spermaries 700 µm in diameter formed the typical arrangement of ‘bouquets’. Although all colonies showed oocytes throughout the year, two reproductive peaks of 3–4 months duration were observed. The larvae ofT. coccineaandT. tagusensishad a competency period of 18 d in aquaria and settled near the parental colony. The investigation of gametogenesis, temporal patterns of reproduction, fecundity and sexual maturation helps to explain the rapid expansion in to new habitats and high population growth in these invasive sun corals.

Reproductive system and spermatozoa ultrastructure support the phylogenetic proximity of Megadasys and Crasiella (Gastrotricha, Macrodasyida)

The reproductive system and the spermatozoon of Megadasys sterreri from Lanzarote (Canary Islands, Spain) were studied at structural and ultrastructural levels. The species is a simultaneous hermaphrodite with cross-fertilization and shows paired gonads, the male anterior and the female posterior, and both gametes mature in a caudo-cephalic direction. Sperm ducts converge on the midline and open into a ventral common pore. Two sexual accessory organs are present in the caudal trunk. A pipe-like frontal organ lies between the ovaries and the caudal organ, and is composed of a long, thin region connected to a large zone containing mature and degenerating spermatozoa. The cigar-like caudal organ is elongate, bulky and is made of an anterior glandulo-muscular region and a posterior muscular one. Spermatozoa are long, filiform cells formed by an acrosome, a nucleus-mitochondrial complex, and a flagellum. The long acrosome is composed of an apical twisted region and a basal straight region. The nucleo-mitochondrial complex is formed by a spring-shaped nucleus surrounding basally the mitochondrion and apically a granular material. The flagellum has a 9x2+2 axoneme, characterized by a dense and prominent central sheath surrounding the central tubules. Megadasys sterreri (Cephalodasyidae) shows the same reproductive layout as Crasiella (Planodasyidae): paired gonads, caudo-cephalic maturation of gametes, sperm ducts converging into a common ventral pore, and two sexual accessory organs. Also the spermatozoa ultrastructure shows two similarities in the two genera: a peculiar prominent central sheath in the axoneme and a similar structure of the basal region of the acrosome. Considering the likely polyphyletic nature of the family Cephalodasyidae, and the sister- taxon relationship of Megadasys and Crasiella that emerged from a recent molecular phylogenetic study, a close relationship between the two taxa appears to be very likely; consequently, we propose to remove Megadasys from the family Cephalodasyidae and affiliate it to the family Planodasyidae.

Demographic costs of sex allocation: hermaphrodites perform better in sparse populations

Ophryotrocha diadema is an outcrossing, simultaneous hermaphroditic polychaete with external fertilization. In isolated pairs, mature worms take turn contributing eggs upon the condition that their partners reciprocate egg donation. In dense populations, these worms do not reciprocate. Instead, they strongly compete for mating in their preferred male role and produce few eggs. This plastic sex allocation may result in an overall different reproductive performance: mean individual reproductive output will be larger in sparse than in dense populations. We tested this hypothesis by measuring the individual reproductive output (paternal and maternal offspring) of worms in sparse and dense replicated populations. In dense populations, mean individual reproductive output was fourfold lower than that in sparse populations. We hypothesise that such dramatic demographic costs are potentially widespread in outcrossing simultaneous hermaphrodites with external fertilization and plastic sex allocation. The reproductive output of hermaphroditic organisms is a function of population density (i.e. the number of conspecifics) and studies on population growth and reproductive performance should take this effect into account.

On the reproduction of the simultaneous hermaphrodite Paroriza prouhoi (Holothuroidea: Synallactidae) in the Porcupine Abyssal Plain, north-east Atlantic

Long-term studies in the abyssal north-east Atlantic (1989–2005) have revealed large-scale changes in the benthic ecosystem and especially in some megafaunal invertebrate taxa over the period 1996–2002, termed the ‘Amperima Event’. Holothurians dominated the megafaunal samples. Temporal patterns in the abundance of holothurians showed a wide spectrum of responses, possibly related to the feeding and reproductive characteristics of the various species. One of the holothurians, the synallactid Paroriza prouhoi, is a simultaneous hermaphrodite, providing a distinct comparison with dioecious reproductive patterns more typical of deep-sea holothurian species. The reproductive biology and abundance/biomass patterns of P. prouhoi were investigated over the period 1989–2005. Paroriza prouhoi produces oocytes with a maximum diameter of ~370 µm. It has asynchronous patterns of gametogenesis both at the individual and population levels. Mean oocyte diameter and the ratio between previtellogenic and vitellogenic oocytes showed no significant differences between the periods prior to and after the Amperima Event. There were no significant differences in abundance or biomass over the time series. We conclude that the energy investment into gamete production by this hermaphrodite did not change over the 16 years examined. It is hypothesized that the feeding characteristics of P. prouhoi lead to a slow, steady and consistent production of gametes despite large-scale changes in organic matter supply to the seabed evident at the time series locality.

A cadaver unearthed: the anatomy of the Japanese living fossil <italic>Stirpulina ramosa</italic> (Bivalvia: Anomalodesmata: Clavagellidae) – the unique specimen in the collections of Emperor Shōwa

The only extant, preserved and complete specimen of Stirpulina ramosa has been discovered in the collections of the late Emperor Shōwa (Hirohito) and is described herein. Hitherto, only shells and the adventitious tubes of this species have been described. This study of the anatomy complements previous ones and adds further information as to how the adventitious tube is formed: that is, by the laying down of an organic, periostracum-like, template that is then biomineralized internally to form the tube and is externally plastered with the clasts that constitute the enclosing burrow wall. Such secretions are produced by pallial lobes that unite on the right side to create the characteristic sutured pleat in this region of the tube. The periostracum enclosing the so-interred animal is then secreted against the internal template of the tube. The shell of S. ramosa and, as a consequence, the musculature, is greatly disfigured as a result of the incorporation of the left valve into the fabric of the adventitious tube. This valve can, however, continue to grow within the tube, especially posteriorly. The right valve remains free inside the tube but grows only a little more anteriorly. The anterior mantle, with a minute pedal gape, is greatly thickened and secretes the watering pot component of the tube. In many anatomical respects, for example simultaneous hermaphroditism, S. ramosa still reflects the basic anomalodesmatan plan and such modifications as there are from the clavagellid form relate principally to the structure, formation and thus functioning of the adventitious tube. This study of S. ramosa has allowed the full spectrum of clavagellid adaptive radiation to be analysed and an evolutionary picture created which suggests that species of Clavagella/Dacosta and Stirpulina are Mesozoic (Late Cretaceous) remnants. Conversely, species of Bryopa and Dianadema are more modern, Late Oligocene and Palaeocene (Cenozoic), respectively, and possibly evolved in association with the emergence of the Indo-West Pacific centre of coral diversity, with a postulated average age of just 30 Myr. © 2013 The Linnean Society of London

Multiple paternity and mate competition in non-selfing, monogamous, egg-trading hermaphrodites

In animals in which the two sexes invest relatively similar amounts of resources in their young, the number of mates is expected to affect male and female reproductive success similarly and gender conflicts on the number of mates may not arise. Correspondingly, in non-selfing, simultaneous hermaphrodites with long-term monogamy, the two partners are expected to alternate repeatedly their sexual roles and invest similarly in their offspring. Therefore, the gender conflict on the number of mating partners should not arise. However, when >2 conspecifics are present, hermaphrodites are known to plastically adjust their behavior and sex allocation and compete for mating repeatedly in the male role. We tested whether this leads to multiple paternities of single egg clutches in experimental replicates of small and large groups of non-selfing, egg-trading, behaviorally monogamous polychaete worms (Ophryotrocha diadema) by using neutral genetic markers to estimate paternity. Multiply fertilized egg cocoons were common in these worms; two or more individuals succeeded in fertilizing the same egg cocoon and mate competition increased with group size. Multiply fertilized egg cocoons had a higher proportion of eggs developing into mature worms than singly fertilized egg cocoons. Possibly singly fertilized cocoons had a lower fertilization rate owing to low sperm counts and aflagellate sperm.

A cadaver unearthed: the anatomy of the Japanese living fossilStirpulina ramosa(Bivalvia: Anomalodesmata: Clavagellidae) - the unique specimen in the collections of Emperor Shōwa

The only extant, preserved and complete specimen of Stirpulina ramosa has been discovered in the collections of the late Emperor Shōwa (Hirohito) and is described herein. Hitherto, only shells and the adventitious tubes of this species have been described. This study of the anatomy complements previous ones and adds further information as to how the adventitious tube is formed: that is, by the laying down of an organic, periostracum-like, template that is then biomineralized internally to form the tube and is externally plastered with the clasts that constitute the enclosing burrow wall. Such secretions are produced by pallial lobes that unite on the right side to create the characteristic sutured pleat in this region of the tube. The periostracum enclosing the so-interred animal is then secreted against the internal template of the tube. The shell of S. ramosa and, as a consequence, the musculature, is greatly disfigured as a result of the incorporation of the left valve into the fabric of the adventitious tube. This valve can, however, continue to grow within the tube, especially posteriorly. The right valve remains free inside the tube but grows only a little more anteriorly. The anterior mantle, with a minute pedal gape, is greatly thickened and secretes the watering pot component of the tube. In many anatomical respects, for example simultaneous hermaphroditism, S. ramosa still reflects the basic anomalodesmatan plan and such modifications as there are from the clavagellid form relate principally to the structure, formation and thus functioning of the adventitious tube. This study of S. ramosa has allowed the full spectrum of clavagellid adaptive radiation to be analysed and an evolutionary picture created which suggests that species of Clavagella/Dacosta and Stirpulina are Mesozoic (Late Cretaceous) remnants. Conversely, species of Bryopa and Dianadema are more modern, Late Oligocene and Palaeocene (Cenozoic), respectively, and possibly evolved in association with the emergence of the Indo-West Pacific centre of coral diversity, with a postulated average age of just 30 Myr. © 2013 The Linnean Society of London

Stage‐ and weather‐dependent dispersal in the brown garden snailCornu aspersum

AbstractDispersal decisions are often condition‐dependent, influenced by the interaction of individual phenotype and environmental conditions. Terrestrial Gastropods are simultaneous hermaphrodites, a reproductive system rarely studied in the context of dispersal. Moreover, the energetic cost of their movement is one of the highest among animals. Despite these features, which make them valuable models to understand the trade‐offs between dispersal and other life‐history traits, their dispersal strategies have been barely explored. We studied the movements of subadults and adults of the brown garden snailCornu aspersumin a semi‐natural 4‐patch network, for 2 months in 2011 (a dry year) and 1 month in 2012 (a wet year). We assessed the effects of life‐history stage (subadult/adult) and weather conditions on dispersal propensity and dispersal speed. Snails were more mobile under humid and warm weather, but nearly all individuals left patches when the relative humidity was close to 100 % in 2012. Because such humidity levels are potentially lethal toC. aspersum, we argue these extreme emigration rates might be an emergency escape response to harmful conditions. Despite a theoretically higher cost of movement, we found that subadults emigrated more, and dispersed faster and further, than adults. Thus, and contrary to what was expected, direct costs of movement do not play the main role in shaping dispersal inC. aspersum. Observed differences between subadults and adults in dispersal behaviour are discussed in the context of intraspecific competition, inbreeding avoidance and relative costs of male and female reproduction.

Do stable environments select against phenotypic plasticity in hermaphroditic sex allocation?

Phenotypic plasticity is the environment-induced change in the phenotype of an organism. Natural selection operates for the ability of individuals to adjust their phenotype to the current environmental conditions when environmental conditions fluctuate. Simultaneous hermaphrodites may exhibit plasticity in sex allocation according to the availability of mates at any particular time. The plasticity in sex allocation has probably evolved under fluctuating mating opportunities, which are usually low but increase when hermaphrodites incur sudden demographic expansion. Here we compare the plasticity in sex allocation in two different populations of the hermaphroditic polychaete worm Ophryotrocha diadema – a laboratory population and a wild one. Worms from the laboratory population were kept under constant crowded conditions for about 200 generations (i.e. they were exposed to high mating opportunities). Worms from the wild population were kept under crowded conditions for 20 generations only. Worms from the laboratory population showed significantly less plasticity in sex allocation than worms from the wild population. Although we cannot rule out the hypotheses that genetic drift or local adaptation played a role in the differences between the two populations, the most likely explanation for our results is that worms of the laboratory population underwent a loss of plasticity in sex allocation because they were kept under constant mating opportunities. In fact, the sensory and regulatory machinery that worms use for exhibiting plastic sex allocation responses is likely to be the same as the machinery that is required for mate searching and sex role synchronization between mating partners. The need to maintain this machinery can explain why worms from the laboratory population diminished their plasticity in sex allocation but did not lose it completely. Therefore our results give some clues as to how plasticity in sex allocation evolves or is constrained.

Williams' Paradox and the Role of Phenotypic Plasticity in Sexual Systems

As George Williams pointed out in 1975, although evolutionary explanations, based on selection acting on individuals, have been developed for the advantages of simultaneous hermaphroditism, sequential hermaphroditism and gonochorism, none of these evolutionary explanations adequately explains the current distribution of these sexual systems within the Metazoa (Williams' Paradox). As Williams further pointed out, the current distribution of sexual systems is explained largely by phylogeny. Since 1975, we have made a great deal of empirical and theoretical progress in understanding sexual systems. However, we still lack a theory that explains the current distribution of sexual systems in animals and we do not understand the evolutionary transitions between hermaphroditism and gonochorism. Empirical data, collected over the past 40 years, demonstrate that gender may have more phenotypic plasticity than was previously realized. We know that not only sequential hermaphrodites, but also simultaneous hermaphrodites have phenotypic plasticity that alters sex allocation in response to social and environmental conditions. A focus on phenotypic plasticity suggests that one sees a continuum in animals between genetically determined gonochorism on the one hand and simultaneous hermaphroditism on the other, with various types of sequential hermaphroditism and environmental sex determination as points along the spectrum. Here I suggest that perhaps the reason we have been unable to resolve Williams' Paradox is because the problem was not correctly framed. First, because, for example, simultaneous hermaphroditism provides reproductive assurance or dioecy ensures outcrossing does not mean that there are no other evolutionary paths that can provide adaptive responses to those selective pressures. Second, perhaps the question we need to ask is: What selective forces favor increased versus reduced phenotypic plasticity in gender expression? It is time to begin to look at the question of sexual system as one of understanding the timing and degree of phenotypic plasticity in gender expression in the life history in terms of selection acting on a continuum, rather than on a set of discrete sexual systems.

Sexual selection and hermaphroditic organisms: Testing theory

Abstract Sexual selection is widespread if not ubiquitous in hermaphroditic organisms. Although many phenomena that have been described as sexual selection in gonochores, (e.g. harem polygamy, multiple mating, elaborate courtship, even secondary sexual characters) can be found in some hermaphrodites, what is more interesting is the ways in which sexual selection in hermaphrodites may differ from dioecious taxa. In hermaphrodites, an individual’s mating success includes its success from both sexual roles. Secondly, in many simultaneously hermaphroditic taxa there is strong evidence of sexual selection and yet the operational sex ratio is 1:1, by definition. Many simultaneous hermaphrodites have elaborate courtship and genital anatomy, suggesting sexual selection plays an important role in reproductive success. Sperm competition and cryptic female choice mean that the number of mates acquired is not necessarily a predictor of reproductive success. Even in simultaneous hermaphrodites with reciprocal mating, variance in reproductive success through the male role and through the female role may differ in a population. Moreover hermaphrodites may choose to emphasize one sexual role over the other. Data suggest that the preferred role varies in hermaphrodites, which creates an opportunity to test fundamental predictions and assumptions of sexual selection theory. Hermaphrodites may vary their emphasis on one sexual role over the other either developmentally or behaviorally in response to environmental or social parameters. How they use this capability in acquiring more or higher quality mates still requires study.

Mating inVeronicella sloanii(Cuvier, 1817) (Veronicellidae)

Abstract: The systellommatophoran slug Veronicella sloanii (Cuvier, 1817), is a simultaneous hermaphrodite. This slug is an agricultural and horticultural pest in Barbados and several islands of the Lesser Antilles. Over the period January-July 2006 and June-August 2010, the mating behavior of this species was determined by ad libitum and focal animal sampling of captive slugs collected from six sites on the island of Barbados, supplemented by observations and length measurements of slugs seen mating in the field. Individuals of Veronicella sloanii mated reciprocally in pairs, but also in a multi-partner ring formation involving three individuals. Two stages in the mating process were identified, courtship and copulation. Courtship was short, less than two minutes in mating events that led directly to copulation (mean 1.87 minutes, range 0.25–2, N = 53). Copulation in contrast was long, lasting on average 1.03 hours (range 0.4–2, N = 40). During mating the penial gland of each partner made contact with th...

Taxonomic and population genetic re-interpretation of two color morphs of the decollate snail, Rumina decollata (Mollusca, Pulmonata) in southern France

The hermaphroditic terrestrial snail Rumina decollata has a mixed breeding system with a high prevalence of self-fertilization. In the Montpellier area (France), the species is represented by a dark and a light color morph. Based on allozyme data, both morphs have been reported as single, homozygous multilocus genotypes (MLG), differing at 13 out of 26 loci, but still showing occasional hybridization. Recent DNA sequence data suggest that each morph is a different phylogenetic species. In order to further evaluate this new taxonomic interpretation, the present contribution explores to what extent populations or color morphs indeed consist of single or few MLG. As such it is shown that both morphs are not single, homozygous MLG, but instead reveal a considerable amount of allelic variation and substantial numbers of heterozygous microsatellite genotypes. This suggests that outcrossing may be more prevalent than previously reported. Nevertheless, both morphs maintain a diagnostic multimarker differentiation in the presence of outcrossing in sympatric conditions, implying that they may be interpreted as species under the biological species concept. Finally, our data challenge the idea that simultaneous hermaphrodites should be either strict selfers or strict outcrossers.

Selfing in a malacostracan crustacean: why a tanaidacean but not decapods

The crustacean class Malacostraca, with over 22,000 species, includes commercially important members, such as crabs, shrimps, and lobsters. A few simultaneous hermaphrodites are known in this group, but self-fertilization was unknown. Here we show, through microscopy and breeding experiments, that the simultaneously hermaphroditic malacostracan Apseudes sp. (order Tanaidacea) can self-fertilize; individuals reared in isolation become hermaphroditic via a male-like phase and produce eggs that develop into fertile adults. Although selfing occurs in crustaceans like the Branchiopoda, in which simultaneous hermaphrodites have the sex ducts united, in decapods the separation of gonadal ducts and gonopores, specialized mating organs, and complex mating behavior appear to have constrained the evolution of selfing. In contrast, in most tanaidaceans, sperm is released externally by a male and reaches the eggs in the female brood pouch, where fertilization occurs. This mode of fertilization permitted Apseudes sp. to achieve selfing without large modifications in morphology or behavior.

SEX ALLOCATION ADJUSTMENT TO MATING GROUP SIZE IN A SIMULTANEOUS HERMAPHRODITE

Sex allocation theory is considered as a touchstone of evolutionary biology, providing some of the best supported examples for Darwinian adaptation. In particular, Hamilton's local mate competition theory has been shown to generate precise predictions for extraordinary sex ratios observed in many separate-sexed organisms. In analogy to local mate competition, Charnov's mating group size model predicts how sex allocation in simultaneous hermaphrodites is affected by the mating group size (i.e., the number of mating partners plus one). Until now, studies have not directly explored the relationship between mating group size and sex allocation, which we here achieve in the simultaneously hermaphroditic flatworm Macrostomum lignano. Using transgenic focal worms with ubiquitous expression of green-fluorescent protein (GFP), we assessed the number of wild-type mating partners carrying GFP+ sperm from these focal worms when raised in different social group sizes. This allowed us to test directly how mating group size was related to the sex allocation of focal worms. We find that the proportion of male investment initially increases with increasing mating group size, but then saturates as predicted by theory. To our knowledge, this is the first direct test of the mating group size model in a simultaneously hermaphroditic animal.