Data from: The role of prezygotic isolation mechanisms in the divergence of two parasite species

Background: The formation of reproductive barriers in diverging lineages is a prerequisite to complete speciation according to the biological species concept. In parasites with complex life cycles, speciation may be driven by adaptation to different intermediate hosts, yet diverging lineages can still share the same definitive host where reproduction takes place. In these cases, prezygotic isolation mechanisms should evolve very early and be particularly strong, preventing costly unfavourable matings. In this study, we investigated the importance of prezygotic barriers to reproduction in two cestode species that diverged 20–25mya and show an extraordinary degree of specificity to different intermediate hosts. Both species share the same definitive hosts and hybridize in the laboratory. Yet, natural hybrids have so far not been detected. Methods: We used a combination of different experiments to investigate the role of prezygotic barriers to reproduction in the speciation of these parasites. First, we investigated whether hybridization is possible under natural conditions by exposing lab-reared herring gulls (Larus argentatus, the definitive hosts) to both parasites of either sympatric or allopatric combinations. In a second experiment, we tested whether the parasites prefer conspecifics over parasites from a different species in dichotomous mate choice trials. Results: Our results show that the two species hybridize under natural conditions with parasites originating either from sympatric or allopatric populations producing hybrid offspring. Surprisingly, the mate choice experiment indicated that both parasite species prefer mates of the different species to conspecifics. Conclusions: Neither fundamental constraints against hybridization in a natural host nor assortative mate choice sufficiently explain the persistent segregation of the two tapeworm species in nature. Hence, postzygotic ecological selection against hybrids is presumably the more important driving force limiting gene flow between the two parasite sister species.研究背景：根据生物学物种概念，分化谱系中生殖隔离的形成是完成物种形成的必要前提。对于具有复杂生活史的寄生虫而言，物种形成可能由对不同中间宿主（intermediate host）的适应性演化驱动，但分化后的谱系仍可共享同一终宿主（definitive host）进行繁殖。在此类情形下，合子前隔离（prezygotic isolation）机制应当较早演化形成且强度极高，以避免代价高昂的不适交配。本研究针对两种分化于2000万至2500万年前、对不同中间宿主具有极高特异性的绦虫（cestode）物种，探究了合子前生殖隔离屏障的作用重要性。两种绦虫共享同一终宿主，且可在实验室条件下发生杂交，但迄今尚未在自然环境中发现天然杂交个体。研究方法：本研究结合多项实验，探究合子前生殖隔离屏障在这类寄生虫物种形成过程中的作用。首先，通过将实验室饲养的银鸥（Larus argentatus，即该类寄生虫的终宿主）暴露于同域（sympatric）或异域（allopatric）组合的两种寄生虫中，探究自然条件下杂交是否可行。第二项实验采用二分法配偶选择试验，测试寄生虫是否更倾向于选择同种个体而非异种寄生虫作为配偶。研究结果：结果表明，两种绦虫可在自然条件下发生杂交，无论是同域还是异域种群来源的寄生虫均可产生杂交后代。令人意外的是，配偶选择试验显示，两种寄生虫均更倾向于选择异种个体而非同种个体作为配偶。研究结论：无论是自然宿主内杂交的根本性限制，还是选择性配偶选择，均无法充分解释两种绦虫在自然环境中持续存在的生殖隔离。因此，针对杂交后代的合子后（postzygotic）生态选择，或许是限制这两种寄生姊妹物种（sister species）间基因交流的更关键驱动因素。