10: A phylogenomic approach to understanding the symbiosis between Acropyga ants and xenococcine mealybugs

A major question in evolutionary biology is how mutualistic symbioses evolve and remain stable over long periods of time. Classic theory predicts that mutualisms will tend to become parasitic over time, but fossils and molecular dating indicate that some mutualisms have been stable for tens of millions of years. Herder ants, those that tend other insects as 'livestock', have great potential to serve as models for studying the evolution of animal-to-animal symbioses. This study investigates the obligate associations between Acropyga ants and xenococcid mealybugs. Acropyga ants practice a unique form of herding where unmated alate Acropyga queens vertically transmit mealybugs by carrying a pregnant adult female with them on their mating flights (called trophophoresy). This single mealybug serves as a seed individual for the next generation of symbionts in the new colony. This mode of symbiont transmission has persisted for at least 20 million years. What is lending stability to these mutualisms? Current research predicts that, in this type of agricultural mutualism, a hosts' control over the dispersal and reproduction of symbionts is essential to the stability of the relationship. Trophophoresy potentially gives Acropyga ants control over both the dispersal and reproduction of xenococcid populations. This study seeks to address the former of these two aspects (dispersal), exploring the history of associations between Acropyga and xenococcids as shaped by trophophoresy. This study could become a new model in the study of ant agriculture and is the first one to address any group of herder ants from a phylogenomics perspective.