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Abstract: 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. [Copyright &y& Elsevier]

Ophryotrocha diademais 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. [ABSTRACT FROM PUBLISHER]

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 wormOphryotrocha 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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]