No Plastic Responses to Experimental Manipulation of Sperm Competition per se in a Free‐Living Flatworm

Abstract

In the absence of sperm competition evolutionary theory predicts low mating rates and low ejaculate expenditure per mating, and sex allocation theory for simultaneous hermaphrodites predicts a strongly female‐biased sex allocation. In the presence of sperm competition a shift towards a more male‐biased sex allocation and a higher ejaculate expenditure are predicted. The free‐living flatworm Macrostomum lignano has been shown to respond plastically in mating rate, testis size, and sperm transfer to manipulation of the social group size, a proxy of the strength of sperm competition. However, manipulation of social group size may manipulate not only sperm competition, but also other factors, such as food supply and metabolite concentration. In this study we therefore manipulated sperm competition per se by repeatedly exposing individuals to partners that have either mated with rivals or not, while keeping the social group size constant. Our results suggest that M. lignano does not have the ability to detect sperm competition per se, as worms experimentally exposed to the presence or absence of sperm competition did not differ in sex allocation, sperm transfer or mating behavior. A response to our manipulation would have required individual recognition, the ability to detect self‐referencing tags, or tags or traces left by rivals on or in the mating partners. We first discuss the possibility that highly efficient sperm displacement may have decreased the difference between the treatment groups and then propose three alternative cues that may allow M. lignano to respond plastically to the social group size manipulation used in earlier studies: assessment of the mating rate, chemical cues, or tactile cues.

Experimental evidence for reduced male allocation under selfing in a simultaneously hermaphroditic animal

“…The isolated treatment therefore guarantees no sperm competition, whereas the octet treatment likely represents a high sperm competition intensity (worms generally have high numbers of received sperm when in groups, and the mode of fertilization precludes significant displacement). We would argue the paired treatment likely represents a low (rather than no) perceived sperm competition risk, for two reasons: (i) there is the possibility of competition between own and the partner's sperm and (ii) although we do not know the proximate cues involved, evidence from a congener suggests that worms may not recognize individual mating partners, and might instead rely on simpler cues of sperm competition such as mating rate [17].…”

Sex allocation and investment into pre- and post-copulatory traits in simultaneous hermaphrodites: the role of polyandry and local sperm competition

“…On the other hand, mate choice has been shown to involve pre-copulatory assessment of the mating status or novelty of the partner (e.g. [42][43][44], but see [45,46]). It seems probable that this kind of preference is linked to assessing cues that can serve as a proxy for the likelihood of encountering sperm competition, and that they are therefore strongly linked to post-copulatory sexual selection.…”

Variation in sex allocation plasticity in three closely related flatworm species

“…Another possibility could be that in the absence of partners there is little/no sperm expenditure such that the seminal vesicle fills up (Figure 1), which in turn may act as a signal to downregulate sperm production in isolated worms. Furthermore, at least in M. lignano, worms seem to be unable to distinguish between familiar and novel partners (Sandner & Schärer, 2010). This could mean that in pairs, even the single partner may be perceived as representing some sperm competition risk (though sperm competition is absent unless the species self-fertilizes), leading to an increase in allocation into the male function.…”