Department of Biology, University of Pennsylvania, Philadelphia, PA, USA.
Department of Biology, Centre for Social Evolution, University of Copenhagen, Copenhagen, Denmark.
J Evol Biol. 2017 Feb;30(2):225-234. doi: 10.1111/jeb.13012. Epub 2016 Nov 25.
Social insect sex and caste ratios are well-studied targets of evolutionary conflicts, but the heritable factors affecting these traits remain unknown. To elucidate these factors, we carried out a short-term artificial selection study on female caste ratio in the ant Monomorium pharaonis. Across three generations of bidirectional selection, we observed no response for caste ratio, but sex ratios rapidly became more female-biased in the two replicate high selection lines and less female-biased in the two replicate low selection lines. We hypothesized that this rapid divergence for sex ratio was caused by changes in the frequency of infection by the heritable bacterial endosymbiont Wolbachia, because the initial breeding stock varied for Wolbachia infection, and Wolbachia is known to cause female-biased sex ratios in other insects. Consistent with this hypothesis, the proportions of Wolbachia-infected colonies in the selection lines changed rapidly, mirroring the sex ratio changes. Moreover, the estimated effect of Wolbachia on sex ratio (~13% female bias) was similar in colonies before and during artificial selection, indicating that this Wolbachia effect is likely independent of the effects of artificial selection on other heritable factors. Our study provides evidence for the first case of endosymbiont sex ratio manipulation in a social insect.
社会性昆虫的性别和职别比例是进化冲突的研究热点，但影响这些特征的可遗传因素仍不清楚。为了阐明这些因素，我们对法老小家蚁的雌性职别比例进行了短期人工选择研究。在三代的双向选择中，我们没有观察到职别比例的反应，但在两个重复的高选择系中，性别比例迅速变得更加雌性偏斜，而在两个重复的低选择系中则变得不那么雌性偏斜。我们假设，这种快速的性别比分化是由可遗传细菌共生体沃尔巴克氏体感染频率的变化引起的，因为初始繁殖种群的沃尔巴克氏体感染情况不同，而沃尔巴克氏体已知会导致其他昆虫的雌性偏斜性别比。与这一假设一致的是，选择系中沃尔巴克氏体感染群体的比例迅速变化，与性别比的变化相吻合。此外，沃尔巴克氏体对性别比的估计影响（约 13%的雌性偏斜）在人工选择前后的群体中相似，表明这种沃尔巴克氏体效应可能独立于人工选择对其他可遗传因素的影响。我们的研究为社会性昆虫中首例共生体性别比操纵提供了证据。
