Self-fertilization, sex allocation and spermatogenesis kinetics in the hypodermically-inseminating flatworm Macrostomum pusillum

Abstract

The free-living flatworm genus Macrostomum is an emerging model system for studying the links between sex allocation, sexual selection and mating system evolution, as well as the underlying developmental and physiological mechanisms responsible for wide intra-and inter-specific variability in reproductive phenotypes. Despite compelling comparative morphological evidence of sexual diversity, detailed experimental work on reproductive behaviour and physiology in Macrostomum has so far been largely limited to just two species, M. lignano and M. hystrix, an obligate and a preferential outcrosser, respectively. In this study, we establish that a third species, M. pusillum, exhibits a combination of reproductive traits strikingly different from both of its congeners. Unlike M. lignano, we demonstrate that M. pusillum does not adjust sex allocation or the speed of spermatogenesis to the prevailing social group size. Macrostomum pusillum's relatively simple sperm morphology likely explains the short spermatogenesis duration we report, and is linked to a hypodermically inseminating mode of fertilization, which we show also means that these worms are capable of self-fertilization. Surprisingly, and unlike M. hystrix, selfing in isolated worms commences after only a short (if any) delay compared with the onset of reproduction in grouped individuals, with little evidence of differential inbreeding depression in 'isolated' progeny. These combined results suggest that, in nature, M. pusillum may be regularly selfing, in contrast to the congeners studied to date. Our findings highlight the rapid and correlated evolution of reproductive traits, and reinforce the utility of the genus Macrostomum for understanding the evolutionary and developmental mechanisms responsible for this diversity.

Variation in sex allocation plasticity in three closely related flatworm species

“…In contrast, the reciprocal mating syndrome involves reciprocal copulation (e.g., M. lignano , see Schärer, Joss, & Sandner, ), during which both partners reciprocally transfer sperm into the female antrum (sperm receiving organ) of the partner. Both earlier reports of self‐fertilization in Macrostomum are in species that are hypodermically inseminating (e.g., M. hystrix , see Ramm, Vizoso, & Schärer, ; Ramm, Schlatter, Poirier, & Schärer, ; and M. pusillum , Giannakara & Ramm, ), with both species showing a similar needle‐like stylet morphology that potentially facilitates self‐fertilization by allowing self‐injection of sperm. In contrast, the current study documents the first occurrence of self‐fertilization in a reciprocally copulating species, M. mirumnovem , which has a large blunt‐ending stylet (Schärer et al, ), and for which we currently do not understand how selfing is actually achieved.…”

“…Interestingly, an earlier study in M. pusillum-a species that regularly self-fertilizes-has shown that the species does not exhibit any plasticity in SA in response to group size (Giannakara & Ramm, 2017). In contrast, M. hystrix-a facultatively self-fertilizing species-does adjust its SA depending on group size (Winkler & Ramm, 2018).…”

“…Thus, the low allocation into the male function may possibly be due to the fact that self‐fertilization in M. mirumnovem occurs with appreciable frequency. Interestingly, an earlier study in M. pusillum —a species that regularly self‐fertilizes—has shown that the species does not exhibit any plasticity in SA in response to group size (Giannakara & Ramm, ). In contrast, M. hystrix —a facultatively self‐fertilizing species—does adjust its SA depending on group size (Winkler & Ramm, ).…”

“…While SA plasticity in response to mating group size is widespread in hermaphroditic animals (reviewed in Schärer, ), the strength and nature of the plasticity vary both among and within species, with some studies showing plasticity only in the male function (Baeza, ; Hoch & Levinton, ; Janicke et al, ; Schärer & Janicke, ; Schärer & Ladurner, ; Winkler & Ramm, ) or the female function (Lorenzi et al, ; Schleicherová et al, ), while other studies show plasticity in either both functions (Janicke & Schärer, , ) or no plasticity at all (Giannakara & Ramm, ). Although a part of this variation in these estimates of plasticity may reflect biases due to the ease with which male and female allocation can be measured in different study systems, it might also reflect different reproductive modes (with different postcopulatory processes, Schärer & Pen, ), species‐specific costs of plasticity (Schleicherová et al, ), or different evolutionary histories (Schleicherová, Sella, & Lorenzi, ).…”

“…The free-living flatworm genus Macrostomum is an excellent model system for studying variation in SA plasticity, since reproductive organs can be measured accurately, repeatedly, and noninvasively in multiple species in the genus (e.g., Giannakara & Ramm, 2017;Schärer & Ladurner, 2003;Winkler & Ramm, 2018), allowing us to quantify and compare SA across closely related species. Earlier studies on the model organism Macrostomum lignano have shown that it has plastic SA, with SA increasing (i.e., becoming more malebiased) with increasing group size (e.g., Schärer & Ladurner, 2003).…”