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的物种形成过程中所具备的潜在重要性，长期以来存在争议。理论研究的进展已达成共识：当CI与其他生殖隔离屏障协同作用时，其发挥隔离作用具备合理性。但目前仅在昆虫模型系统纳森小蜂（Nasonia）和果蝇（Drosophila）中报道过两例相关实验证据。本研究针对被诱导CI的共生体卡迪纽菌（Cardinium）感染的寄生蜂苏扎纳恩蚜小蜂（Encarsia suzannae）与其未受感染的姊妹种杰纳罗恩蚜小蜂（Encarsia gennaro）之间的生殖隔离组分展开解析，并通过室内杂交实验证实：1）性隔离并不完全；2）种间CI杂交的杂交后代产量大幅下降；3）通过抗生素清除苏扎纳恩蚜小蜂雄性体内的卡迪纽菌，即可获得可存活的杂交后代；4）反交组合的杂交后代产量因遗传不相容引发的杂交致死而显著降低；5）双向杂交的F1代杂交个体几乎完全不育。综上，不对称杂交不相容性与CI共同构成互补的生殖隔离机制。我们提出了全新模型以阐释CI共生体对物种形成的贡献：CI可在一个方向上降低物种间的基因流，而在另一方向上，共生体清扫（symbiont sweep）会加速线粒体DNA（mtDNA）的进化，引发负向核质互作与杂交不相容性。