Data from: Wolbachia do not live by reproductive manipulation alone: infection polymorphism in Drosophila suzukii and D. subpulchrella

Drosophila suzukii recently invaded North America and Europe. Populations in Hawaii, California, New York and Nova Scotia are polymorphic for Wolbachia, typically with <20% infection frequency. The Wolbachia in D. suzukii, denoted wSuz, is closely related to wRi, the variant prevalent in continental populations of D. simulans. wSuz is also nearly identical to Wolbachia found in D. subpulchrella, plausibly D. suzukii's sister species. This suggests vertical Wolbachia transmission through cladogenesis (“cladogenic transmission”). The widespread occurrence of 7-20% infection frequencies indicates a stable polymorphism. wSuz is imperfectly maternally transmitted, with wild infected females producing on average 5-10% uninfected progeny. As expected from its low frequency, wSuz produces no cytoplasmic incompatibility (CI), i.e., no elevated embryo mortality when infected males mate with uninfected females, and no appreciable sex-ratio distortion. The persistence of wSuz despite imperfect maternal transmission suggests positive fitness effects. Assuming a balance between selection and imperfect transmission, we expect a fitness advantage on the order of 20%. Unexpectedly, Wolbachia-infected females produce fewer progeny than do uninfected females. We do not yet understand the maintenance of wSuz in D. suzukii. The absence of detectable CI in D. suzukii and D. subpulchrella makes it unlikely that CI-based mechanisms could be used to control this species without transinfection using novel Wolbachia. Contrary to their reputation as horizontally transmitted reproductive parasites, many Wolbachia infections are acquired through introgression or cladogenesis and many cause no appreciable reproductive manipulation. Such infections, likely to be mutualistic, may be central to understanding the pervasiveness of Wolbachia among arthropods.

斑翅果蝇（Drosophila suzukii）近期入侵北美与欧洲。其在夏威夷、加利福尼亚、纽约以及新斯科舍的种群中，沃尔巴克氏体（Wolbachia）感染呈多态性，感染率通常低于20%。该果蝇体内的沃尔巴克氏体被命名为wSuz，与拟果蝇（Drosophila simulans）大陆种群中流行的wRi变体亲缘关系密切。wSuz还与亚华美果蝇（D. subpulchrella）体内发现的沃尔巴克氏体几乎完全一致，而亚华美果蝇通常被认为是斑翅果蝇的姊妹物种。这表明沃尔巴克氏体可通过成种事件实现垂直传播（即"成种传播"）。7%~20%的感染率在种群中广泛存在，表明该多态性处于稳定状态。wSuz的母系传播并不完全，野外感染的雌蝇平均会产生5%~10%的未感染后代。正如低感染频率所预期的那样，wSuz不会引发细胞质不相容（cytoplasmic incompatibility, CI）：即感染雄蝇与未感染雌蝇交配时，不会导致胚胎死亡率升高，也不会造成显著的性别比例扭曲。尽管母系传播存在缺陷，wSuz仍能在种群中持续存在，这提示其具有正向的适合度效应。假设在选择压力与不完全传播之间存在动态平衡，我们推测wSuz的适合度优势约为20%量级。出乎意料的是，感染沃尔巴克氏体的雌蝇产仔数低于未感染雌蝇。目前我们仍未明确wSuz在斑翅果蝇种群中的维持机制。斑翅果蝇与亚华美果蝇（D. subpulchrella）体内均未检测到细胞质不相容，这意味着若不借助新型沃尔巴克氏体的转感染技术，基于细胞质不相容的防控手段不太可能用于该物种的防治。与沃尔巴克氏体作为水平传播的生殖寄生虫的固有印象相悖，许多沃尔巴克氏体感染是通过基因渐渗（introgression）或成种事件获得的，且多数不会引发显著的生殖操纵。这类大概率属于共生互利的感染，或许是理解沃尔巴克氏体在节肢动物（arthropods）中广泛分布的关键所在。