The evolution of sex allocation and hypodermic insemination across the flatworm genus Macrostomum

Abstract

Sexual selection in hermaphrodites is thought to result in conflict over mating roles, where individuals will be eager to act as the (sperm) donor but hesitant to act as the (sperm) recipient. Likely due to this conflict, some species engage in elaborate reciprocal mating behaviours such as egg trading or reciprocal copulation, often followed by intense postcopulatory conflict, to control the fate of the received ejaculates (e.g. sperm digestion or mechanical sperm removal). Besides reciprocal mating, another possible solution to the conflict about mating roles is hypodermic insemination, where the male copulatory organ is used to inject ejaculate directly into the tissue of the recipient. Such mating behaviour allows the donor to minimise the recipient’s ability to control the fate of the received ejaculate, which potentially leads to a paternity benefit. Hypodermic insemination appears to be common in hermaphrodites, but it is unclear how frequently it evolves and through what routes it originates. In this thesis, I conducted a large-scale comparative analysis of the flatworm genus Macrostomum, to investigate the origins and the consequences of hypodermic insemination. I have conducted field collections to expand both taxon-sampling and the geographic representation of the genus. By combining detailed morphological documentation and the first phylogenomic analysis of the genus, I documented 89 species that are new to science and present here three taxonomic descriptions that resulted from this work. I, therefore, showed that Macrostomum harbours large undiscovered biodiversity. Analysis of sperm and genital morphology, as well as the location of received sperm, shows that hypodermic insemination has evolved up to 13 times within the genus, thereby almost doubling the number of documented origins of this mating strategy across all hermaphrodites. These origins of hypodermic insemination are associated with consistent changes in the morphology of the male copulatory organ, the female sperm storage organ, and the sperm design. Such consistent correlations imply that these changes are adaptations to hypodermic insemination. I further show that hypodermic insemination likely evolved via initial internal wounding during copulation, leading to internal traumatic insemination and subsequently to the complete loss of copulation. Since hypodermic insemination by-passes several processes that can decrease the level of sperm competition (e.g. cryptic female choice or sperm displacement), its evolution has been proposed to increase the proportion of reproductive resources allocated to sperm production. Contrary to this prediction, my work shows that in Macrostomum, hypodermic insemination is associated with reduced allocation towards sperm production. Most likely, this mating strategy is associated with the ability to self or other factors reducing the intensity of sperm competition. While no data on such a relationship exists in animals, this supports findings in plants, where selfing is associated with reduced investment into pollen production. Finally, I here present evidence that reproduction-related genes evolve at an accelerated rate across the genus, as indicated both by sequence divergence and a decreased probability of identifying homologs with phylogenetic distance. This is the first documented case of such rapid evolution in hermaphrodites and supports a growing body of evidence that sexual selection can drive rapid gene evolution.