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.

社会性昆虫的性别比例与品级比例是进化冲突研究中被广泛探究的靶标性状，但调控这些性状的可遗传因子仍未明确。为阐明此类因子，我们针对法老蚁（Monomorium pharaonis）的雌性品级比例开展了短期人工选择实验。历经三代双向选择后，我们未观察到品级比例出现响应，但在两个重复的高选择品系中，性别比例迅速向雌性偏倚方向演化，而在两个重复的低选择品系中，性别比例的雌性偏倚程度则显著降低。我们推测，这种性别比例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：因为初始繁育种群的沃尔巴克氏体感染状态存在差异，且已知沃尔巴克氏体可在其他昆虫中引发雌性偏倚的性别比例。该假说与实验结果一致：选择品系中感染沃尔巴克氏体的蚁群比例快速变化，且这一变化与性别比例的变化高度吻合。此外，在人工选择开始前及选择过程中的蚁群中，沃尔巴克氏体对性别比例的估计效应（约13%的雌性偏倚）保持一致，这表明沃尔巴克氏体的该效应大概率不受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调控性别比例的现象提供了实验证据。