Data from: Artificial selection on ant female caste ratio uncovers a link between female-biased sex ratios and infection by Wolbachia endosymbionts

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.

社会性昆虫的性比与品级比是进化冲突中被广泛研究的对象，但影响这些性状的可遗传因子仍未明确。为阐明这些因子，我们针对法老蚁（Monomorium pharaonis）的雌性品级比开展了短期人工选择实验。经过三代双向选择实验，我们未观察到品级比出现响应，但在两个重复的高选择品系中，性比快速向雌性偏倚方向演化，而两个重复的低选择品系的性比雌性偏倚程度则有所降低。我们推测，这种性比的快速分化是由可遗传细菌内共生体沃尔巴克氏体（Wolbachia）的感染频率变化所导致的：其一，初始繁殖种群的沃尔巴克氏体感染情况存在差异；其二，已知沃尔巴克氏体可在其他昆虫中引发雌性偏倚的性比。与该假说一致，选择品系中被沃尔巴克氏体感染的蚁群比例快速发生了变化，与性比的变化趋势完全吻合。此外，在人工选择前及选择过程中的蚁群中，沃尔巴克氏体对性比的预估影响（约13%的雌性偏倚）基本一致，这表明该沃尔巴克氏体的效应大概率独立于人工选择对其他可遗传因子的影响。本研究首次为社会性昆虫中存在内共生体操控性比的现象提供了实证依据。