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

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.