The effects of outbreeding on a parasitoid wasp fixed for infection with a parthenogenesis-inducing Wolbachia symbiont

Trichogramma wasps can be rendered asexual by infection with the maternally inherited symbiont Wolbachia. Previous studies indicate the Wolbachia strains infecting Trichogramma wasps are host-specific, inferred by failed horizontal transfer of Wolbachia to novel Trichogramma hosts. Additionally, Trichogramma can become dependent upon their Wolbachia infection for the production of female offspring, leaving them irreversibly asexual, further linking host and symbiont. We hypothesized Wolbachia strains infecting irreversibly asexual, resistant to horizontal transfer Trichogramma would show adaptation to a particular host genetic background. To test this, we mated Wolbachia-dependent females with males from a Wolbachia-naïve population to create heterozygous wasps. We measured sex ratios and fecundity, a proxy for Wolbachia fitness, produced by heterozygous wasps, and by their recombinant offspring. We find a heterozygote advantage, resulting in higher fitness for Wolbachia, as wasps will produce more offspring without any reduction in the proportion of females. While recombinant wasps did not differ in total fecundity after ten days, recombinants produced fewer offspring early on, leading to an increased female-biased sex ratio for the whole brood. Despite the previously identified barriers to horizontal transfer of Wolbachia to and from Trichogramma pretiosum, there were no apparent barriers for Wolbachia to induce parthenogenesis in these non-native backgrounds. This is likely due to the route of infection being introgression rather than horizontal transfer, and possibly the co-evolution of Wolbachia with the mitochondria rather than the nuclear genome. These results help to elucidate the mechanisms by which Wolbachia adapt to hosts and the evolution of host-symbiont phenotypes.

赤眼蜂（Trichogramma wasps）感染母系遗传的共生体沃尔巴克氏体（Wolbachia）后可转为无性生殖模式。既往研究表明，侵染赤眼蜂的沃尔巴克氏体菌株具有宿主特异性，该结论由沃尔巴克氏体无法水平转移（horizontal transfer）至新型赤眼蜂宿主的实验结果推导而来。此外，赤眼蜂可依赖其携带的沃尔巴克氏体产生雌性后代，进而陷入不可逆的无性生殖状态，进一步强化了宿主与共生体之间的紧密关联。我们提出假说：侵染不可逆无性生殖、且难以发生水平转移的赤眼蜂的沃尔巴克氏体菌株，会对特定宿主遗传背景产生适应性。为验证该假说，我们将依赖沃尔巴克氏体的雌性赤眼蜂与未接触沃尔巴克氏体种群的雄性赤眼蜂进行交配，以构建杂合子蜂群。我们分别测定了杂合子蜂群及其重组后代的性比与繁殖力（fecundity）——繁殖力可作为沃尔巴克氏体适合度的替代指标。研究结果显示，杂合子优势可提升沃尔巴克氏体的适合度：赤眼蜂可产生更多后代，且雌性后代比例未出现任何下降。尽管重组蜂群在10天后的总繁殖力与对照组无显著差异，但早期繁殖的后代数量更少，进而导致整个子代群体的雌性偏向性比例升高。尽管此前已明确沃尔巴克氏体向耶氏赤眼蜂（Trichogramma pretiosum）发生水平转移存在障碍，但在这些非原生遗传背景下，沃尔巴克氏体诱导孤雌生殖（parthenogenesis）并未出现明显障碍。这一现象可能是由于感染途径为渐渗作用（introgression）而非水平转移，也可能与沃尔巴克氏体与线粒体（mitochondria）而非核基因组（nuclear genome）的协同进化有关。本研究结果有助于阐明沃尔巴克氏体对宿主的适应性机制，以及宿主-共生体表型的演化过程。