Evolution of mating behaviour and sex allocation plasticity in simultaneous hermaphrodites

Sexual conflict can arise due to disagreement between the mating partners over the usage of received ejaculate post-mating and can give rise to traits of one sex being costly to the other sex. In simultaneously hermaphroditic organisms, both the male and female sexes are combined in the same individual. This could lead to unique conflict over the sex role taken during mating and its resolution via different mating strategies. These different mating strategies could also influence the sex allocation (SA), defined as the resource allocation towards the male and female reproductive function, and its plasticity, leading to interspecific variation. In my PhD thesis, I examine the evolution of mating behaviour and SA plasticity in simultaneously hermaphroditic flatworms of the genus Macrostomum. This genus contains species exhibiting at least two different mating strategies. One such mating strategy is reciprocal mating, in which both partners mate in both the male and female role simultaneously, with reciprocal transfer of sperm. Another possible strategy is hypodermic insemination, in which species presumably exhibit forced unilateral mating, with sperm being injected via the male copulatory organ into the partner.
In my first chapter, using two reciprocally mating species, M. lignano and its congener M. janickei, I could show that even closely related Macrostomum species can exhibit substantial variation in reproductive traits, including mating behaviour and genital morphology. Interestingly, this variation does not necessarily lead to reproductive isolation, and I showed that while these two species lack premating barriers, there exist some postmating barriers, since in spite of heterospecific matings, very few hybrids were produced. Moreover, using a mate preference assay I demonstrated that the nearly two-fold higher mating rate of M. lignano compared to M. janickei, led to M. lignano engaging predominantly in conspecific matings, while M. janickei ended up mating more often with heterospecific individuals. Thus, mating rate can be an important determinant for shaping heterospecific interactions.
The high mating rates that are often associated with reciprocal mating can lead to intense postcopulatory sexual selection or/and conflict and evolution of female resistance traits. A potential example of a female resistance trait is the postcopulatory ‘suck’ behaviour in Macrostomum, which is hypothesised to be used for removing received ejaculate, during which a worm bends down and places its pharynx on top of its female antrum (sperm receiving organ) and appears to suck. In my second chapter, I provided conclusive evidence that the suck behaviour removes ejaculate out of the antrum in a reciprocally mating species, M. hamatum. I also, examined the evolution of the suck behaviour by documenting behavioural interactions of 64 species, and showed that there is a correlation between the presence, frequency and duration of reciprocal mating and suck, providing support for the hypothesis that the suck behaviour co-evolves with reciprocal mating. Moreover, I showed that the mating strategy of a species can be inferred from a combination of reproductive morphological traits, so called syndromes, in Macrostomum.
Hermaphroditic individuals also face a unique challenge in terms of needing to decide about the allocation of resources towards their male and female function. SA models predict that the amount of resources invested into the male function should vary with local sperm competition (LSC), in which related sperm compete for fertilizing a given set of eggs. In small groups, where there is high LSC, low allocation of resources to the male function is favoured. As the group size increases, LSC decreases and that favours increased allocation towards the male function. Moreover, LSC can vary temporally or spatially during an individual’s lifetime, leading to plasticity in SA. In our experiments, I raised worms in three group sizes (isolated, pairs and octets) and measured their SA, calculated as testis size/(testis size+ovary size). In my third and fourth chapter, in line with the above predictions, I showed plasticity in SA in four Macrostomum species in response to variation in group size, with the species usually exhibiting lower SA in smaller groups. Moreover, theory predicts that any process, such as self-fertilization, which affects the strength of LSC can have an effect on the optimal SA, by changing the shape of the male fitness gain curve. In my fourth chapter, using data for seven Macrostomum species, I showed that there was substantial interspecific variation in SA and SA plasticity. I also, calculated standardized effect sizes for SA plasticity due to the presence of mating partners (i.e. isolated worms vs. worms with partners) and the strength of LSC (i.e. worms in pairs vs. octets) for each species. I showed that while the mating strategy does not have any effect on SA plasticity, there is a significant effect of self-fertilization such that self-fertilizing species had a lower SA plasticity with respect to presence of mating partners. To summarize, my thesis shows that both mating behaviour and SA plasticity can evolve rapidly in the Macrostomum genus.