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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.

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 fr...

Summary Cytokine polymorphisms influence the outcomes of parasitic diseases and vary among populations because of their different evolutionary histories and selective pressures imposed by host–pathogen interactions. In this frame, we investigated the frequencies of TNF-α (-308G/A), TGF-β1 (codon 10C/T, codon 25C/G) and IL-10 (-1082A/G) SNPs in 133 individuals from Ouangolodougou, a rural village in Northern Ivory Coast, where malaria and other parasitic diseases are endemic. The SNPs alleles were determined by ARMS-PCR methodology. Allele frequencies of the SNPs investigated were as follows: IL 10 -1082G = 0.741 and -1082A = 0.259; TGF-β1 Codon 10 C = 0.835 and T = 0.165; TGF-β1 Codon 25 G = 0.782 and C = 0.218. For the TNF-α gene, we found high frequencies of the -308A allele (0.305) and heterozygote genotypes (0.594), with a consequent deviation from the Hardy–Weinberg equilibrium. The high heterozygosity at the TNF-α locus suggests a possible selective advantage of the heterozygote genomes, associated with intermediate levels of TNF-α expression, against the infectious agents endemic in Western Africa.

Sex allocation theory predicts that, in hermaphroditic organisms, individuals allocate a fixed amount of resources divided among male and female functions to reproduction and that the proportion devoted to each sex depends on the mating group size. As the mating group size increases, hermaphrodites are predicted to allocate proportionally more resources to the male and less resources to the female function (approaching equal allocation to both sexes) to face increased sperm competition. Up to now little experimental evidence has been provided to support the theory in hermaphroditic animals. Facultative shift between male and female allocation in response to variation in local group size does occur in several taxa but not always in the expected direction and not with similar patterns. In the protandric and then simultaneously hermaphroditic polychaete worm Ophryotrocha diadema reproductive resources are flexibly allocated in the protandrous and the hermaphroditic phase. The cost of male reproduction during adolescence is spread over the whole energy budget of the animal as shown by the shortening of lifespan and the lowering of growth rate in individuals with enhanced male expenditure during the protandrous phase. Moreover, in this species, short term sex allocation adjustments differ from those described in other taxa. Individuals regulate their reproductive output so that where reproductive competitors are present, the number of female gametes is strongly reduced but the number of male gametes (although it changes) is not significantly increased. Resources subtracted from the female function are not directly allocated to sperm production, but to expensive male behaviors that are likely to enhance male reproductive success. These results are discussed in the light of the relevance of sexual selection in large populations of hermaphrodites.

One of the greatest remaining problems in the field of sex allocation is the variation across species of the extent to which individuals adjust their offspring sex ratio or sex allocation according to environmental conditions. Particularly in the field of sex allocation, there is a lack of research on the mechanisms and cues involved. We address this problem in the outcrossing simultaneous hermaphroditic polychaete Ophryotrocha diadema, which shifts sex allocation by suppressing the female function in the presence of reproductive competitors or multiple partners. We document that O. diadema hermaphrodites can evaluate group size by means of a species-specific chemical released by mature individuals in water. The perception of such a water-borne signal is sufficient to trigger the observed shift in sex allocation, i.e., a suppression of the female function, irrespective of the frequency of encounters with conspecifics. Under natural conditions, such a pheromone will favor partners' encounters and the adjustment of individual sex allocation according to the numbers of reproductive competitors and/or potential partners. Copyright 2006.

Sex allocation theory predicts that simultaneous hermaphrodites shift sex allocation facultatively in response to variation in local group size. This study was performed to evaluate the relative investment in each sex function by the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema and to test whether allocation to each sex depends on the number of reproductive competitors. Four experimental groups were set up (in a 2 · 2 factorial design) with small or large group size and with small or large enclosures to control for potential confounding effects of density. We measured the proportion of female and male investment in focal individuals. Results revealed that individuals regulated their reproductive output so that when reproductive competitors were present, the number of female gametes was strongly reduced and the male function increased. In contrast, under monogamy, individuals in small groups produced lower numbers of sperm but had a higher egg output than worms in large groups. Density did not affect sex allocation in our experiment. Our findings provide qualitative support for Local Mate Competition theory, but also show that the pattern of sex allocation specific to this species is more complex than expected by current theory.

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 re...

SUMMARY Hermaphroditism and gonochorism are two contrasting forms of sexuality. Hermaphroditic species are generally seen as species adapted to conditions of low density, stabilized by poor mate search efficiency and high costs of searching. They can adjust allocation of reproductive resources to each sex function in response to current social conditions, making reproduction more efficient, at least in principle. By contrast, gonochorism (separate sexes) is advantageous when mates are frequent, making it ineffective to maintain two sex functions in a single individual. This, however, also rules out the need for a flexible response to mating opportunities as known for hermaphrodites. In the hermaphroditic marine polychaete worm Ophryotrocha diadema we showed earlier that group size is assessed through a chemical cue. In this study we verified the accuracy of the response to gradients of the chemical cue used to assess group size by O. diadema by checking reduction in egg production as the group of partners increases, as expected according to sex allocation theory. Furthermore we compared the effect of such a gradient with a similar gradient in a closely related gonochoric species (O. labronica). Here sex allocation adjustment is not predicted, thus an adaptive change in egg production in response to group-size cues is not expected. In fact, our results show that the group-size effect only occurs in O. diadema and not in O. labronica. Moreover, our study provides evidence of high perceptual abilities of chemical cues in O. diadema, suggesting that perceiving social cues and adjusting sex allocation appropriately are special properties of hermaphrodites.

Because of their double sex functions, hermaphrodites are selected to optimize their investment in the two sex functions. Sex allocation (SA) theory predicts that, in promiscuous mating conditions, simultaneous hermaphrodites should adjust their reproductive investment so as to invest an amount of resources into the male relatively larger than that invested into the female function. In contrast, in monogamy, individuals should invest relatively larger amount of resources into the female function at the expenses of the male function. In the study of SA patterns of simultaneous hermaphrodites little attention has been paid to allocate adjustments costs, which may play an important role in determining variations in SA pattern among species. Indeed, the costs paid for such adjustments may constrain sex investment resulting in suboptimal allocation. We evaluated the costs of SA adjustments on individual fitness in each sexual function in the simultaneous, outcrossing hermaphrodite Ophryotrocha diadema. Following a crossover design, we compared the reproductive success in paternal and maternal offspring of focal hermaphrodites, which were put in replicated monogamous and promiscuous regimes. We document that those hermaphrodites that switched mating regimes and altered their sex investment accordingly did not entail large short-term fitness costs in any sexual function compared to those that were in stable mating regimes. Indeed, individuals changed their sex investment quickly and appropriately to current mating conditions. Hermaphrodites, which had to adjust their SA, did not decrease their maternal or paternal reproductive output with respect to those which did not change their SA. Time needed to shift resources from one to the other sex function is 5 days (the time interval between successive egg layings is of 3 days) indicating that selective pressures for SA adjustments may favour great plasticity and quick adjustments of sex investments in simultaneous hermaphrodites.