Department of Entomology, University of California Riverside, Riverside, CA, USA.
Heredity (Edinb). 2017 Dec;119(6):411-417. doi: 10.1038/hdy.2017.53. Epub 2017 Sep 13.
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 10 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.
沃尔巴克氏体可通过感染母体遗传共生体沃尔巴克氏体而被转化为无性生殖。先前的研究表明，感染赤眼蜂的沃尔巴克氏体菌株具有宿主特异性，这是通过沃尔巴克氏体向新的赤眼蜂宿主的水平转移失败推断出来的。此外，赤眼蜂可能依赖于其沃尔巴克氏体感染来产生雌性后代，从而使其不可逆地成为无性生殖，进一步将宿主和共生体联系在一起。我们假设感染不可逆无性生殖、抗水平转移的赤眼蜂沃尔巴克氏体菌株会表现出对特定宿主遗传背景的适应性。为了验证这一点，我们让依赖沃尔巴克氏体的雌性与来自沃尔巴克氏体未感染种群的雄性交配，以产生杂合蜂。我们测量了杂合蜂及其重组后代的性别比例和繁殖力，这是沃尔巴克氏体适应性的一个指标。我们发现杂合蜂具有杂种优势，从而提高了沃尔巴克氏体的适应性，因为这些蜂产生的后代更多，而雌性的比例没有任何降低。虽然重组蜂在 10 天后的总繁殖力没有差异，但重组蜂在早期的繁殖力较低，导致整个蜂群的雌性偏多性别比例增加。尽管之前已经确定了沃尔巴克氏体向和从桃蚜赤眼蜂转移的障碍，但沃尔巴克氏体在这些非本地背景下诱导孤雌生殖似乎没有明显的障碍。这可能是由于感染途径是渐渗而不是水平转移，并且可能是沃尔巴克氏体与线粒体而不是核基因组的共同进化。这些结果有助于阐明沃尔巴克氏体适应宿主和宿主-共生体表型进化的机制。
