Background: Sex allocation theory is an important branch of life history theory and aims at predicting optimal allocation to male and female reproduction in sexual organisms. In simultaneous hermaphrodites sex allocation theory has classically focussed on allocation to sperm vs. egg production (often measured as testis and ovary size), predicting reallocation away from sperm production to egg production (i.e. a female-biased sex allocation) under social conditions that lead to local sperm competition (i.e. competition between related sperm). Recent theory suggests that this reallocation could also occur to other postcopulatory traits, such as seminal fluids, male genitalia or sperm morphology, and that this can feed back on optimal sex allocation, calling for an extended view of sex allocation. While we have ample evidence that 'gametic sex allocation' shows phenotypic plasticity in different social environments, few examples exist for plasticity in other postcopulatory traits. Still less is known about the evolution of gametic sex allocation and its coevolution with other reproductive traits.

Specific aims: I propose to address these topics in the free-living flatworm genus Macrostomum, a highly suitable model to study sex allocation, sexual selection, and sexual conflict. The proposed research integrates a range of powerful approaches, including phylogenomics, morphometry, electron microscopy, behavioural assays, phenotypic plasticity experiments, comparative analyses, and comparative genomics. Specifically, I propose to:

1)   establish a robust phylogenetic hypothesis of the interrelationships among ~100 Macrostomum species in this highly diverse genus, by generating a phylogenomic dataset using de novo transcriptome sequencing

2)   quantify variation in reproductive morphology and behaviour in these species using in vivo photomicrography and time-lapse video analysis, and advanced imaging approaches for selected species

3)   use the morphological, behavioural, and phylogenomic data to perform comparative analyses of reproductive trait evolution and coevolution, and comparative genomics analyses of genes involved in reproduction

4)   expand the number of Macrostomum species that can be cultured in the laboratory to facilitate more detailed studies on mating behaviour and reproductive trait function

5)   investigate interspecific variation in phenotypic plasticity of reproductive traits in response to changes in social group size in a range of culturable Macrostomum species

6)   study karyotype diversity and genome size variation in a broad range of Macrostomum species to permit an informed selection of candidate species for future genome sequencing, mapping and engineering approaches

Expected value of proposed research: In animals, most studies on sex allocation have focussed on species with separate sexes, while empirical data for hermaphrodites is patchier. Sex allocation may arguably be more complex in simultaneous hermaphrodites, as it can involve allocation to a range of reproductive traits involved in sexual selection. While several comparative studies on reproductive trait evolution and coevolution exist in simultaneous hermaphrodites, none has to date included estimates of gametic sex allocation, which recent theory suggests is crucial. This is particularly important given that sexual selection—and resulting sexual conflicts—can often lead to rapid (co)evolution of male and female reproductive traits and therefore drastically change the context in which gametes function (such as during shifts from reciprocal copulation to hypodermic insemination). The work I propose here will contribute to a conceptual extension of sex allocation theory in hermaphrodites and thereby help to clarify the interplay between sexual selection and sex allocation across the different sexual systems.