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 3 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 and 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 Panamá drove parallel speciation in Eciton army ants.

Understanding microbial partnerships with the medicinally and economically important crop Cannabis has the potential to affect agricultural practice by improving plant fitness and production yield. Furthermore, Cannabis presents an interesting model to explore plant-microbiome interactions as it produces numerous secondary metabolic compounds. Here we present the first description of the endorhiza-, rhizosphere-, and bulk soil-associated microbiome of five distinct Cannabis cultivars. Bacterial communities of the endorhiza showed significant cultivar-specificity. When controlling cultivar and soil type the microbial community structure was significantly different between plant cultivars, soil types, and between the endorhiza, rhizosphere and soil. The influence of soil type, plant cultivar and sample type differentiation on the microbial community structure provides support for a previously published two-tier selection model, whereby community composition across sample types is determined mainly by soil type, while community structure within endorhiza samples is determined mainly by host cultivar.

1. Sodium is often a limiting nutrient for terrestrial animals, and may be especially sought by herbivores. Leafcutter ants are dominant herbivores in the Neotropics, and leafcutter foraging may be affected by nutritional demands of the colony and/or the demands of their symbiotic fungal mutualists. We hypothesized that leafcutter colonies are sodium limited, and that leafcutter ants will therefore forage specifically for sodium.2. Previous studies demonstrated that leafcutter Atta cephalotes Linnaeus workers preferentially cut and remove paper baits treated with NaCl relative to water control baits. Atta cephalotes colonies in this study were presented with baits offering NaCl, Na 2 SO 4 , and KCl to test whether leafcutters forage specifically for sodium. Sucrose and water were used as positive and negative controls, respectively.3. Atta foragers removed significantly more of the baits treated with NaCl and Na 2 SO 4 than the KCl treatment, which did not differ from water. The NaCl and Na 2 SO 4 treatments were collected at similar rates. We conclude A. cephalotes forage specifically for sodium rather than for anions (chloride) or solutes in general. This study supports the hypothesis that leafcutter ants are limited by, and preferentially forage for, sodium.