Data from: “Darwin’s corollary” and cytoplasmic incompatibility induced by Cardinium may contribute to speciation in Encarsia wasps (Hymenoptera: Aphelinidae)

The potential importance of cytoplasmic incompatibility (CI) – inducing bacterial symbionts in speciation of their arthropod hosts has been debated. Theoretical advances have led to a consensus that a role is plausible when CI is combined with other isolating barriers. However, the insect model systems Nasonia and Drosophila are the only two experimental examples documented. Here we analyzed the components of reproductive isolation between the parasitoid wasp Encarsia suzannae, which is infected by the CI-inducing symbiont Cardinium, and its uninfected sibling species Encarsia gennaro. Laboratory crosses demonstrated that: 1) sexual isolation is incomplete; 2) hybrid offspring production is greatly reduced in the interspecific CI cross; 3) viable hybrids may be produced by curing E. suzannae males of Cardinium with antibiotics; 4) hybrid offspring production in the reciprocal cross is greatly reduced by hybrid inviability due to genetic incompatibilities; 5) hybrid sterility is nearly complete in both directions at the F1 stage. Thus, asymmetrical hybrid incompatibilities and CI act as complementary isolating mechanisms. We propose a new model for contributions of CI symbionts to speciation, with CI reducing gene flow between species in one direction, and in the other, a symbiont sweep resulting in accelerated mtDNA evolution, negative cytonuclear interactions and hybrid incompatibilities.

诱导细胞质不相容（cytoplasmic incompatibility, CI）的细菌共生体在其节肢动物宿主物种形成过程中的潜在重要性，长期以来备受学术争议。理论研究进展已达成共识：当CI与其他生殖隔离屏障结合时，其参与物种形成具备合理性。然而目前仅在两种昆虫模式系统——纳森小蜂（Nasonia）与果蝇（Drosophila）中，记载了相关实验例证。
本研究针对被诱导CI的共生体心杆菌（Cardinium）感染的寄生蜂苏扎纳蚜小蜂（Encarsia suzannae），与其未感染的姊妹种杰纳罗蚜小蜂（Encarsia gennaro）之间的生殖隔离组分展开系统性分析。实验室杂交实验结果显示：1）性隔离并不完全；2）种间CI杂交的子代产出量大幅降低；3）通过抗生素清除苏扎纳蚜小蜂雄性体内的心杆菌后，可获得可育杂种后代；4）正反交的另一组因遗传不相容引发的杂种致死，大幅抑制了子代产生；5）两个杂交方向的子一代（F1）均几乎完全表现出杂种不育。
综上，不对称杂种不相容性与CI共同构成了互补的生殖隔离机制。我们提出全新模型以阐释CI共生体对物种形成的贡献：CI可在一个方向上降低物种种间基因流，而在另一方向上，共生体的快速固定可加速线粒体DNA（mtDNA）进化、引发负向核质互作与杂种不相容性。