Biology subjects

Schärer, L.

Publications and source records attributed to Schärer, L..

4 recordsLinked to original sources

bioRxiv2023

Contrasting the form and strength of pre- and postcopulatory sexual selection in a transparent worm with fluorescent sperm

Sexual traits may be selected during multiple consecutive episodes of selection, occurring before, during, or after copulation. The overall strength and shape of selection acting on sexually selected traits may thus be determined by how selection (co-)varies along different episodes. However, it is challenging to measure pre- and postcopulatory phenotypic traits alongside variation in fitness components at each different episode. Here, we used a transgenic line of the transparent flatworm Macrostomum lignano expressing green fluorescent protein (GFP) in all cell types, including sperm cells, enabling in vivo sperm tracking. We exposed GFP(+) focal worms to three groups in which we assessed their mating success, sperm-transfer efficiency, and sperm fertilising efficiency. Moreover, we measured 13 morphological traits on the focal worms to study the fitness landscape in multivariate trait space. We found linear selection on sperm production rate arising from pre- and postcopulatory components, and on copulatory organ shape arising from sperm fertilising efficiency. We further found nonlinear (mostly concave) selection on combinations of copulatory organ and sperm morphology traits arising mostly from sperm-transfer efficiency and sperm fertilising efficiency. Our study shows that contrasting patterns of phenotypic selection are observed by measuring how sexual selection builds-up over consecutive episodes of selection.

evolutionary biology↗

bioRxiv2022

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

Sexual conflicts over the post-mating fate of received ejaculate can favour traits in one sex that are costly to the other. Reciprocally mating hermaphrodites face unique challenges as they mate simultaneously in both the male and female role, potentially leading to receipt of unwanted ejaculate. Reciprocal mating can then give rise to postcopulatory female resistance traits that allow manipulation of received ejaculate. A putative example is the suck behaviour, observed in the flatworm genus Macrostomum. It involves the sperm recipient placing its pharynx over its own female genital opening and appearing to suck, likely removing received ejaculate after mating. The genus also contains hypodermically-inseminating species that presumably exhibit unilateral mating and have not been observed to suck. Here, we examine the evolution of the suck behaviour in Macrostomum, aiming to document the mating behaviour in 64 species. First, we provide videographic evidence that ejaculate is indeed removed during the suck behaviour in a reciprocally mating species, Macrostomum hamatum. Next, we show evolutionary positive correlations between the presence, duration and frequency of reciprocal mating behaviour and the suck behaviour, providing clear evidence that the suck behaviour co-evolves with reciprocal mating behaviour. Finally, we show an association between reproductive behaviour and reproductive morphology, suggesting that reproductive morphology can be used for inferring the behavioural mating strategy of a species. Together our study demonstrates sexual antagonistic coevolution leading to the evolution of a postcopulatory behavioural trait that functions as a female counter-adaptation allowing individuals to gain control over received ejaculate in a hermaphroditic sexual system.

evolutionary biology↗

bioRxiv2020

Evolution of sex allocation plasticity and its predictors in a flatworm genus

Sex allocation (SA) theory in simultaneous hermaphrodites predicts that optimal SA is influenced by local sperm competition (LSC), which occurs when related sperm compete to fertilize a given set of eggs. Different factors, including the mating strategy and the ability to self-fertilize, are predicted to affect LSC and hence the optimal SA. Moreover, since the LSC experienced by an individual can vary temporally and spatially, this can favour the evolution of SA plasticity. Here, using seven species of the free-living flatworm genus Macrostomum, we document sizable interspecific variation in SA, but neither their mating strategy nor their ability to self-fertilize significantly predicted SA among these species. Since we also found considerable interspecific variation in SA plasticity, we further estimated standardized effect sizes for plasticity in response to i) the presence of mating partners (i.e. in isolation vs. with partners) and ii) the strength of LSC (i.e. in small vs. large groups). We found that self-fertilization predicted SA plasticity with respect to the presence of mating partners, with plasticity being lower for self-fertilizing species. Finally, we showed that interspecific variation in SA is higher than intraspecific variation due to SA plasticity. Our study suggests that both SA and SA plasticity are evolutionarily labile, with self-fertilization predicting the latter in Macrostomum.

evolutionary biology↗

bioRxiv2019

Successful mating and hybridisation in two closely related flatworm species despite significant differences in reproductive morphology and behaviour

Speciation is usually a gradual process, in which reproductive barriers between two species accumulate over time. Reproductive traits, like genital morphology and mating behaviour, are some of the fastest diverging characters and can serve as reproductive barriers. The free-living flatworm Macrostomum lignano, an established model for studying sex in hermaphrodites, and its congener M. janickei are closely related, but differ substantially in their male intromittent organ (stylet) morphology. Here, we examine whether these morphological differences are accompanied by differences in behavioural traits, and whether these could represent barriers to successful mating and hybridization between the two species. Our data shows that the two species differ in many aspects of their mating behaviour, with M. janickei having a five-fold longer copulation duration, copulating less frequently, and having a longer and more delayed suck behaviour (a postcopulatory behaviour likely involved in sexual conflict). Interestingly, and despite these significant morphological and behavioural differences, the two species mate readily with each other in heterospecific pairings, often showing behaviours of intermediate duration. Although both species have similar fecundity in conspecific pairings, the heterospecific pairings revealed clear postmating barriers, as only few heterospecific pairings produced F1 hybrids. These hybrids had a stylet morphology that was intermediate between that of the parental species, and they could successfully backcross to both parental species. Finally, in a mate choice experiment we tested if the worms preferentially mated with conspecifics over heterospecifics, since such a preference could represent a premating barrier. Interestingly, the experiment showed that the nearly two-fold higher mating rate of M. lignano caused it to mate more with conspecifics, leading to assortative mating, while M. janickei ended up mating more with heterospecifics. Thus, while the two species can hybridize, the mating rate differences could possibly lead to higher fitness costs for M. janickei compared to M. lignano.

animal behavior and cognition↗