Mating behavior and reproductive morphology predict macroevolution of sex allocation in hermaphroditic flatworms

Abstract

Background Sex allocation is the distribution of resources to male or female reproduction. In hermaphrodites, this concerns an individual’s resource allocation to, for example, the production of male or female gametes. Macroevolutionary studies across hermaphroditic plants have revealed that the self-pollination rate and the pollination mode are strong predictors of sex allocation. Consequently, we expect similar factors such as the selfing rate and aspects of the reproductive biology, like the mating behaviour and the intensity of postcopulatory sexual selection, to predict sex allocation in hermaphroditic animals. However, comparative work on hermaphroditic animals is limited. Here, we study sex allocation in 120 species of the hermaphroditic free-living flatworm genus Macrostomum. We ask how hypodermic insemination, a convergently evolved mating behaviour where sperm are traumatically injected through the partner’s epidermis, affects the evolution of sex allocation. We also test the commonly-made assumption that investment into male and female reproduction should trade-off. Finally, we ask if morphological indicators of the intensity of postcopulatory sexual selection (female genital complexity, male copulatory organ length, and sperm length) can predict sex allocation. Results We find that the repeated evolution of hypodermic insemination predicts a more female-biased sex allocation (i.e., a relative shift towards female allocation). Moreover, transcriptome-based estimates of heterozygosity reveal reduced heterozygosity in hypodermically mating species, indicating that this mating behavior is linked to increased selfing or biparental inbreeding. Therefore, hypodermic insemination could represent a selfing syndrome. Furthermore, across the genus, allocation to male and female gametes is negatively related, and larger species have a more female-biased sex allocation. Finally, increased female genital complexity, longer sperm, and a longer male copulatory organ predict a more male-biased sex allocation. Conclusions Selfing syndromes have repeatedly originated in plants. Remarkably, this macroevolutionary pattern is replicated in Macrostomum flatworms and linked to repeated shifts in reproductive behavior. We also find a trade-off between male and female reproduction, a fundamental assumption of most theories of sex allocation. Beyond that, no theory predicts a more female-biased allocation in larger species, suggesting avenues for future work. Finally, morphological indicators of more intense postcopulatory sexual selection appear to predict more intense sperm competition.

Evolution of sex allocation plasticity in a hermaphroditic flatworm genus

“…This result suggests that we can probably interpret variation in SA among field-collected worms of different Macrostomum species as being at least partially due to interspecific variation, even if they happen to have been sampled from different group sizes, and may therefore also vary in part due to SA plasticity. Thus, although we cannot currently explain the observed interspecific variation in SA among the species studied here, our study will facilitate research in understanding the evolution of SA patterns across Macrostomum species, by allowing future studies to include SA estimates from field-collected worms (such as in Brand et al, 2022b).…”

Rapid evolution of sex role specialization in a hermaphrodite under sex-limited selection

“…Although this prediction is consistent with our results, changes in sex-specific fitness are unlikely to be mediated by changes in ovary size, since relative ovary area showed a tendency to be negatively related to fitness via the female sex role (figure 3B). Nevertheless, a handful of studies have successfully demonstrated trade-offs between male and female sex function in this and other species (Schärer 2009, Di Bona et al 2015, Picchi and Lorenzi 2019, Brand et al 2022, so perhaps ovary area does not capture the most important aspects of investment in the female sex role. This is in line with results from Janicke et al…”

Evolution and co-evolution of the suck behaviour, a postcopulatory female resistance trait that manipulates received ejaculate

“…Finally, mating frequency (and possibly mating duration) could be positively correlated with allocation towards the male function (e.g., testes). Indeed, studies in Macrostomum have shown interspecific variation in sex allocation towards the male and female functions, such as testes and ovaries (Singh et al, 2020b;Brand et al, 2022a;Singh & Schärer, 2021). This interspecific variation could potentially relate to the mating behaviour, as we can expect species that have a longer mating duration or higher mating frequency to have larger testes, if longer and/or more frequent mating implies that more sperm are transferred (Janicke & Schärer, 2009).…”

“…Species in the genus Macrostomum are small (~0.3 to 3.0 mm body length) aquatic free-living flatworms that are highly transparent, permitting detailed observations of internal structures (for the general morphology see Figure 1A,B). The sperm and eggs are produced in the paired testes and paired ovaries, respectively, with studies documenting inter-and intra-specific variation in both testis and ovary size across the genus (Singh et al, 2020b;Brand et al, 2022a;Singh & Schärer, 2021). The female antrum is located anterior to the male antrum, connected to the outside, respectively, via a female genital opening (also female genital pore or vagina) and the male genital opening (also male genital pore).…”

“…the amount of resources allocated to the male and female function) (van Velzen et al, 2009;Schärer & Pen, 2013). Indeed, studies have documented both inter-and intra-specific variation in sex allocation in Macrostomum (Singh et al, 2020b;Brand et al, 2022a;Singh & Schärer, 2021), with mating behaviour predicting the evolution of a species' sex allocation (Brand et al, 2022a), but not the evolution of its sex allocation plasticity (Singh & Schärer, 2021).…”