Data from: Ants exhibit asymmetric hybridization in a mosaic hybrid zone

Research on hybridization between species provides unparalleled insights into the pre- and post-zygotic isolating mechanisms that drive speciation. In social organisms, colony-level incompatibilities may provide additional reproductive barriers not present in solitary species, and hybrid zones offer an opportunity to identify these barriers. Here, we use genotyping-by-sequencing to sequence hundreds of markers in a hybrid zone between two socially polymorphic ant species, Formica selysi and Formica cinerea. We characterize the zone, determine the frequency of hybrid workers, infer whether hybrid queens or males are produced, and investigate whether hybridization is influenced by colony social organization. We also compare cuticular hydrocarbon profiles and aggression levels between the two species. The hybrid zone exhibits a mosaic structure. The asymmetric distribution of hybrids skewed toward F. cinerea suggests a pattern of unidirectional nuclear gene flow from F. selysi into F. cinerea. The occurrence of backcrossed individuals indicates that hybrid queens and/or males are fertile, and the presence of the F. cinerea mitochondrial haplotype in 97% of hybrids shows that successful F1 hybrids will generally have F. cinerea mothers and F. selysi fathers. We found no evidence that social organization contributes to speciation, since hybrids occur in both single-queen and multiple-queen colonies. Strongly differentiated cuticular hydrocarbon profiles and heightened interspecific aggression further reveal that species recognition cues are both present and perceived. The discovery of fertile hybrids and asymmetrical gene flow is unusual in ants, and this hybrid zone will therefore provide an ideal system with which to investigate speciation in social insects.

物种间杂交研究可为解析驱动物种形成（speciation）的合子前隔离机制（pre-zygotic isolating mechanisms）与合子后隔离机制（post-zygotic isolating mechanisms）提供无与伦比的研究视角。在社会性生物中，群体水平的不相容性可能构成独居物种所不具备的额外生殖隔离屏障，而杂交带则为识别这类隔离机制提供了绝佳契机。本研究采用测序基因分型（genotyping-by-sequencing）技术，对两种社会多态性蚂蚁——撒切切林蚁（Formica selysi）与灰林蚁（Formica cinerea）——之间的杂交带内数百个遗传标记进行测序。我们对该杂交带开展了系统表征，明确了杂交工蚁的出现频率，推断杂交蚁后与雄性个体是否可产生，并探究了群体社会组织是否会对杂交过程产生影响。此外，我们还比较了两个物种的表皮碳氢化合物图谱（cuticular hydrocarbon profiles）与攻击行为水平。该杂交带呈现镶嵌结构。杂交个体的不对称分布偏向灰林蚁，这表明存在从撒切切林蚁向灰林蚁的单向核基因流模式。回交个体的存在说明杂交蚁后和/或雄性个体具备可育性；而97%的杂交个体携带灰林蚁的线粒体单倍型（mitochondrial haplotype），则表明成功产生的F1杂交个体通常以灰林蚁为母本、撒切切林蚁为父本。我们未发现社会组织对物种形成存在贡献的证据，因为杂交个体既存在于单蚁后群体，也存在于多蚁后群体中。显著分化的表皮碳氢化合物图谱与增强的种间攻击（interspecific aggression）进一步证实，物种识别信号既存在且可被感知。在蚂蚁类群中，可育杂交个体与不对称基因流的发现较为罕见，因此该杂交带将成为研究社会性昆虫物种形成的理想实验体系。