Data from: Occasional males in parthenogenetic populations of Asobara japonica (Hymenoptera: Braconidae): low Wolbachia titer or incomplete co-adaptation?

Wolbachia are endosymbiotic bacteria known to manipulate the reproduction of their hosts. Some populations of the parasitoid wasp Asobara japonica are infected with Wolbachia and reproduce parthenogenetically, while other populations are not infected and reproduce sexually. Wolbachia infected A. japonica females regularly produce small numbers of male offspring. Because all females in the field are infected and infected females are not capable of sexual reproduction, male production seems to be maladaptive. We investigated why these females nevertheless produce males. We tested three hypotheses: high rearing temperatures could result in higher offspring sex ratios (more males), low Wolbachia titer of the mother could lead to higher offspring sex ratios and/or the Wolbachia infection is of relatively recent origin and not enough time has passed to allow complete co-adaptation between Wolbachia and host. Thirty-three percent of the Wolbachia infected females produced males and 56% of these males were also infected with Wolbachia. Neither offspring sex ratio nor male infection frequency were significantly affected by rearing temperature or Wolbachia concentration of the mother. The mitochondrial DNA sequence of one of the uninfected populations was identical to that of two of the infected populations. Therefore, the initial Wolbachia infection of A. japonica must have occurred recently. Mitochondrial sequence variation among infected populations suggests that the spread of Wolbachia through the host populations involved horizontal transmission. We conclude that the occasional male production by Wolbachia infected females is most likely a maladaptive side-effect of incomplete co-evolution between symbiont and host in this relatively young infection.

沃尔巴克氏体（Wolbachia）是一类已知可操控宿主生殖的内共生细菌。部分寄生蜂（parasitoid wasp）日本索氏茧蜂（Asobara japonica）的种群被沃尔巴克氏体感染后，以孤雌生殖（parthenogenesis）的方式繁殖；而未感染的种群则进行有性生殖。感染沃尔巴克氏体的雌蜂通常会产生少量雄性后代。由于野外所有雌蜂均已被感染，且感染雌蜂无法进行有性生殖，雄性后代的产生看似属于适应不良的性状。我们针对这一现象展开研究，检验了三项假说：其一，较高的饲养温度可提升子代性比，即产生更多雄性后代；其二，母体的沃尔巴克氏体滴度（Wolbachia titer）较低会提升子代性比；其三，沃尔巴克氏体感染的起源相对较新，尚未有足够时间完成共生体与宿主间的完全协同适应（co-adaptation）。实验结果表明，33%的感染沃尔巴克氏体的雌蜂产生了雄性后代，其中56%的雄性个体也携带沃尔巴克氏体感染。饲养温度与母体沃尔巴克氏体滴度均未对子代性比或雄性感染率产生显著影响。其中一个未感染种群的线粒体DNA序列（mitochondrial DNA sequence）与两个感染种群的完全一致，据此可推断日本索氏茧蜂的初始沃尔巴克氏体感染必然发生在较近的时期。感染种群间的线粒体序列变异表明，沃尔巴克氏体在宿主种群中的传播涉及水平传播（horizontal transmission）。我们最终得出结论：感染沃尔巴克氏体的雌蜂偶尔产生雄性后代，极有可能是该较新感染事件中，共生体与宿主间未完成协同进化（co-evolution）所带来的适应不良副作用。