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Reproductive traits are some of the fastest diverging characters and can serve as reproductive barriers. The free-living flatworm  Macrostomum lignano , 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. Despite these differences, the species mate readily with each other in heterospecific pairings. Although both species have similar fecundity in conspecific pairings, the heterospecific pairings revealed clear postmating barriers, as few heterospecific pairings produced F1 hybrids. These hybrids had a stylet morphology that was intermediate between that of the parental species, and they were fertile. Finally, using a mate choice experiment, we show 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 .

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