Data from: Early and dynamic colonization of Central America drives speciation in Neotropical army ants

The emergence of the Isthmus of Panama is one of the most important events in recent geological history, yet its timing and role in fundamental evolutionary processes remain controversial. While the formation of the isthmus was complete around three million years ago (Ma), recent studies have suggested prior intercontinental biotic exchange. In particular, the possibility of early intermittent land bridges facilitating colonization constitutes a potential mechanism for speciation and colonization before full closure of the isthmus. To test this hypothesis, we employed genomic methods to study the biogeography of the army ant genus Eciton, a group of keystone arthropod predators in Neotropical rainforests. Army ant colonies are unable to disperse across water and are therefore ideally suited to study the biogeographic impact of land bridge formation. Using a reduced representation genome sequencing approach, we show that all strictly Central American lineages of Eciton diverged from their respective South American sister lineage between 4 – 7 Ma, significantly prior to the complete closure of the isthmus. Furthermore, three of the lineage pairs form extensive and coincident secondary contact zones in Costa Rica and Nicaragua, with no evidence of gene flow. Such a discrete and repeated biogeographic pattern indicates at least two waves of army ant dispersal into Central America that were separated by significant genetic divergence times. Thus, by integrating phylogenomic, population genomic, and geographic evidence, we show that early colonization of Central America across the emerging Isthmus of Panama drove parallel speciation in Eciton army ants.

巴拿马地峡（Isthmus of Panama）的形成是近代地质历史中最为重要的事件之一，但其形成时间以及其在基础演化进程中的作用仍存在争议。尽管该陆桥的最终形成距今约300万年前（Ma），但近期研究却提出了跨大陆生物交流更早发生的观点。具体而言，存在早期间歇性陆桥促进物种扩散的可能性，这一机制或可解释陆桥完全闭合之前的物种形成与扩散过程。为验证这一假说，我们采用基因组学方法研究了行军蚁属（Eciton）的生物地理学特征——该类群是新热带区雨林中的关键节肢动物捕食者。行军蚁群落无法跨水域扩散，因此非常适合用于研究陆桥形成对生物地理分布的影响。通过简化基因组测序（reduced representation genome sequencing）技术，我们发现所有严格分布于中美洲的行军蚁属演化支，其与对应的南美姊妹演化支的分化时间介于400万至700万年前（Ma），显著早于巴拿马地峡的完全闭合时期。此外，其中三对演化支在哥斯达黎加与尼加拉瓜境内形成了大范围且重合的次生接触带，且未检测到基因流（gene flow）的迹象。这种离散且重复出现的生物地理模式表明，行军蚁至少通过两波扩散进入中美洲，且两次扩散之间存在显著的遗传分化时间间隔。综上，我们通过整合系统基因组学（phylogenomic）、群体基因组学（population genomic）以及地理学证据，证实了通过新兴巴拿马地峡早期向中美洲的殖民过程，驱动了行军蚁属的平行物种形成。