Sex allocation is a crucial life-history parameter in all sexual organisms. Over the last decades a body of evolutionary theory, sex allocation theory, was developed, which has yielded capital insight into the evolution of optimal sex allocation patterns and adaptive evolution in general. Most empirical work, however, has focused on species with separate sexes. Here I review sex allocation theory for simultaneous hermaphrodites and summarize over 50 empirical studies, which have aimed at evaluating this theory in a diversity of simultaneous hermaphrodites spanning nine animal phyla. These studies have yielded considerable qualitative support for several predictions of sex allocation theory, such as a female-biased sex allocation when the number of mates is limited, and a shift toward a more male-biased sex allocation with increasing numbers of mates. In contrast, many fundamental assumptions, such as the trade-off between male and female allocation, and numerous predictions, such as brooding limiting the returns from female allocation, are still poorly supported. Measuring sex allocation in simultaneously hermaphroditic animals remains experimentally demanding, which renders evaluation of more quantitative predictions a challenging task. I identify the main questions that need to be addressed and point to promising avenues for future research. K E Y W O R D S :Life-history evolution, local mate competition, local resource competition, local sperm competition, sex ratio, sperm competition, trade-offs.Research on sex allocation (SA) investigates how sexual organisms allocate reproductive resources to the male and female function. In "gonochorists" (terms in italic and within quotation marks can be found in the glossary in Appendix 1) SA primarily involves the maternal decision about the sex ratio of her offspring, in "sequential hermaphrodites" it concerns the timing and direction of sex change, and in "simultaneous hermaphrodites" it represents the allocation toward male and female reproductive function (e.g., the production of sperm vs. eggs). SA theory aims to predict the optimal SA that an organism should exhibit under different environmental and social conditions, which makes it a central topic in life-history theory (Charnov 1982;Stearns 1992;De Jong and Klinkhamer 2005). Given its success, SA theory is considered both "a touchstone in the study of adaptation" by W. D. Hamilton (Frank 2002), and the most successful body of evolutionary theory in predicting adaptive evolution (West et al. 2000).A recent book synthesized our understanding of SA patterns in gonochorists (Hardy 2002). It concluded that predictions of SA theory are particularly powerful in species that can easily control SA decisions, a prime example of which are haplodiploid insects. Hermaphrodites should also have an exceptionally flexible SA (Hamilton 1967; Charnov 1982;Michiels 1998; but see West et al. 2005), because in these organisms SA simply represents a decision about how resources are allocated to different organs and behaviors within...
Sex allocation theory for simultaneous hermaphrodites predicts an influence of the mating group size on sex allocation. Mating group size may depend on the size of the group in which an individual lives, or on the density, but studies to date have not distinguished between the two factors. We performed an experiment in which we raised a transparent simultaneous hermaphrodite, the flatworm Macrostomum sp., in different group sizes (pairs, triplets, quartets and octets) and in different enclosure sizes (small and large). This design allows us to differentiate between the effects of group size and density. After worms reached maturity we determined their reproductive allocation patterns from microscopic images taken in vivo. The results suggest that the mating group size is a function of the group size, and not of the density. They support the shift to higher male allocation in larger mating groups predicted by sex allocation theory. To our knowledge, this is the first study that unambiguously shows phenotypically plastic sex allocation in response to mating group size in a simultaneous hermaphrodite.
Sperm are the most diverse of all animal cell types, and much of the diversity in sperm design is thought to reflect adaptations to the highly variable conditions under which sperm function and compete to achieve fertilization. Recent work has shown that these conditions often evolve rapidly as a consequence of multiple mating, suggesting a role for sexual selection and sexual conflict in the evolution of sperm design. However, very little of the striking diversity in sperm design is understood functionally, particularly in internally fertilizing organisms. We use phylogenetic comparative analyses covering 16 species of the hermaphroditic flatworm genus Macrostomum to show that a complex sperm design is associated with reciprocal mating and that this complexity is lost secondarily when hypodermic insemination-sperm injection through the epidermis-evolves. Specifically, the complex sperm design, which includes stiff lateral bristles, is likely a male persistence trait associated with sexual conflicts over the fate of received ejaculates and linked to female resistance traits, namely an intriguing postcopulatory sucking behavior and a thickened epithelium of the sperm-receiving organ. Our results suggest that the interactions between sperm donor, sperm, and sperm recipient can change drastically when hypodermic insemination evolves, involving convergent evolution of a needle-like copulatory organ, a simpler sperm design, and a simpler female genital morphology. Our study documents that a shift in the mating behavior may alter fundamentally the conditions under which sperm compete and thereby lead to a drastic change in sperm design.Platyhelminthes | sexually antagonistic coevolution | simultaneous hermaphrodite | sperm morphology | traumatic insemination P arker's (1-3) far-reaching extension of Darwin's (4) narrow focus on precopulatory mating interactions highlighted that sexual selection continues to operate after mating partners have agreed to mate and that considering postcopulatory sexual selection therefore is crucial to understand the evolution of many reproductive traits (5, 6). This insight has led to extensive research in evolutionary biology that focused on understanding the biology of sperm (7), the most diverse of all animal cell types (8,9). From this research emerged an apparent consensus that the diversity in sperm design-the strikingly variable ways of constructing a sperm-reflects the highly variable physiological and morphological environments in which sperm have to survive, function, and compete for fertilization (5,(10)(11)(12)(13)(14). Moreover, recent studies have documented clearly that these environments can evolve rapidly, probably because of coevolutionary interactions linked to multiple mating and the resulting sexual selection and sexual conflicts (15-21). However, the bewildering diversity in sperm design is poorly understood at the functional level, particularly in internally fertilizing organisms (9,12,22). A recent review on the evolution of sperm morphological diversity concluded...
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