Species recognition limits mating between hybridizing ant species

Identifying mechanisms limiting hybridization is a central goal of speciation research. Here, we studied pre-mating and post-mating barriers to hybridization between two ant species, Formica selysi and Formica cinerea. These species hybridize in the Rhône valley in Switzerland, where they form a mosaic hybrid zone, with limited introgression from F. selysi into F. cinerea. There was no sign of temporal isolation between the two species in the production of queens and males. With choice experiments, we showed that queens and males strongly prefer to mate with conspecifics. Yet we did not detect post-mating barriers caused by genetic incompatibilities. Specifically, hybrids of all sexes and castes were found in the field and F1 hybrid workers did not show reduced viability compared to non-hybrid workers. To gain insights into the cues involved in species recognition, we analyzed the cuticular hydrocarbons of queens, males and workers and staged dyadic encounters between workers. Cuticular hydrocarbon profiles differed markedly between species, but were similar in F. cinerea and hybrids. Accordingly, workers also discriminated species, but they did not discriminate F. cinerea and hybrids. We discuss how the CHC-based recognition system of ants may facilitate the establishment of pre-mating barriers to hybridization, independent of hybridization costs.

解析限制杂交的机制，是物种形成研究的核心目标之一。本研究针对蚁属两个物种——Formica selysi与Formica cinerea——之间的杂交交配前隔离障碍（pre-mating barriers）与交配后隔离障碍（post-mating barriers）展开探究。这两个物种在瑞士罗讷河谷存在自然杂交现象，并形成镶嵌式杂交带，且仅存在有限的基因渐渗（introgression）：仅由Formica selysi向Formica cinerea方向发生基因交流。在雌蚁与雄蚁的繁育节律上，两物种未表现出时间隔离（temporal isolation）的迹象。通过选择实验，我们发现雌蚁与雄蚁均显著倾向于与同种个体（conspecifics）交配。然而，本研究未检测到由遗传不相容性引发的交配后隔离障碍。具体而言，野外环境中可获取所有性别与品级的杂交后代，且F1杂交工蚁的生存能力并未较非杂交工蚁出现下降。为解析物种识别所涉及的信号线索，我们对雌蚁、雄蚁及工蚁的表皮碳氢化合物（cuticular hydrocarbons, CHC）进行了分析，并设置了工蚁间的二元相遇实验。不同物种间的表皮碳氢化合物谱型存在显著差异，但Formica cinerea与杂交个体的谱型较为相似。相应地，工蚁同样能够区分不同物种，却无法区分Formica cinerea与杂交个体。本研究最后探讨了基于表皮碳氢化合物的蚂蚁识别系统，如何在不依赖杂交代价的前提下，助力杂交交配前隔离障碍的形成。