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）是一类已知可调控宿主生殖的内共生细菌。部分寄生蜂日本异索茧蜂（Asobara japonica）种群感染有沃尔巴克氏体，行孤雌生殖；而未感染的种群则进行有性生殖。感染沃尔巴克氏体的雌性日本异索茧蜂常会产生少量雄性后代。由于野外所有雌性个体均被感染，且受感染雌性无法进行有性生殖，雄性产出似乎属于适应不良现象。我们针对此类雌性为何仍会产生雄性展开了研究，检验了三项假说：一是较高的饲养温度可提升后代性比（产生更多雄性）；二是母体内沃尔巴克氏体滴度较低会提高后代性比；三是沃尔巴克氏体感染起源相对较新，尚未有足够时间完成沃尔巴克氏体与宿主的完全协同适应。结果显示，33%的感染沃尔巴克氏体的雌性个体产生了雄性后代，其中56%的雄性个体也被沃尔巴克氏体感染。饲养温度与母体沃尔巴克氏体滴度均未对后代性比或雄性感染频率产生显著影响。某一未感染种群的线粒体DNA序列与两个感染种群完全一致，由此可见，日本异索茧蜂的初始沃尔巴克氏体感染必然发生于晚近时期。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事件。我们最终得出结论：受感染雌性偶尔产生雄性后代，极有可能是这种相对年轻的共生体-宿主协同进化不完全所带来的适应不良副效应。