Abstract
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.
Highlights
Reproductive traits are some of the fastest diverging characters and can serve as reproductive barriers
Do differences in reproductive morphology go along with differences in reproductive behaviour41? And are these differences sufficiently large to function as prezygotic reproductive barriers, leading to reproductive isolation42? Under a scenario of reinforcement in sympatry, we might expect that divergent reproductive traits will serve as effective reproductive barriers
While M. lignano engaged predominantly in conspecific matings, thereby exhibiting assortative mating, M. janickei ended up mating more often with heterospecific individuals, and we suggest that both likely occurred as a result of the higher mating rate of M. lignano compared to M. janickei
Summary
Reproductive traits are some of the fastest diverging characters and can serve as reproductive barriers. Sexual selection can play an important role in evolutionary diversification, reproductive isolation and speciation[20,21], but see 22 This is supported by the fact that reproductive traits, such as mating behaviour and genital morphology, have been shown to diversify faster than other traits[23,24,25,26,27] and can differ markedly even between recently diverged s pecies[25,27,28,29,30], and sometimes even between populations of the same s pecies[26,31,32]. Secondary contact between such previously allopatric species may result in the production of viable and potentially even fertile hybrid offspring
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