Ben Rubin at Tambopata National Reserve, Peru, photographing a Pseudomyrmex ant going into its nest entrance.
Ben's first forays into biology included work on a wheat breeding farm and several bird behavior projects. However, his nascent interests in animal behavior soon pushed his focus in the direction of social insects and he began his undergraduate thesis project on the colony structure of an east African acacia-ant, Crematogaster mimosae. Acacia-ants nest in and feed on the hollow thorns and extra-floral nectar produced by their host acacia plants. In exchange, the ants aggressively protect their plants from herbivores including large mammals such as elephants and giraffes. Without their ant partners, the acacias are quickly killed by herbivory and disease.
As a graduate student at the University of Chicago, Ben still studies acacia-ants but instead of Crematogaster his current study organisms are members of the genus Pseudomyrmex and live in the neotropics. Despite being distantly related, these two groups of ants have evolved almost identical mutualistic behaviors with similar plants. Unfortunately for Ben and anyone that has ever helped him in the field, Pseudomyrmex species have a painful sting, unlike the Crematogasters that he used to study, and few field days end before Ben has been covered in welts. On the other hand, Pseudomyrmex is a neotropical genus and during his graduate career, Ben has traveled to the Florida Keys, Arizona, Costa Rica, and the Peruvian Amazon to collect ants for his project.
Most species of Pseudomyrmex are non-mutualists, nest non-specifically in dead twigs, and are very timid. However, within Pseudomyrmex, plant-ant mutualism has evolved convergently at least three times leading to three groups of species nesting in three different types of plants: acacia, Triplaris, and Tachigali. Triplaris and Tachigali trees do not produce thorns or nectar like acacias so the ants nest instead in their hollow trunks and petioles and feed on the honeydew from cohabitating scale insects. Regardless of plant partner, the plant-ant behavior is nearly identical.
The convergence of mutualism in Pseudomyrmex makes it an ideal group for exploring the evolution of such an amazing behavior. There are two main avenues for Ben's research. The first is comparative genomics of the three mutualistic species groups with their non-mutualistic sister clades. Ben is sequencing the genomes of representative species from each of these clades and comparing the mutualists to non-mutualists to find the determinants of mutualistic behavior. Consistent differences between mutualists and non-mutualists will show the areas of the genome associated with these differences in behavior. Specifically, genes associated with aggression, olfaction and digestion are likely to be subject to particular selection pressures in mutualists.
The other half of Ben's research focuses on the bacterial endosymbionts of Pseudomyrmex plant-ants. Bacteria are thought to enrich the protein-poor diets of many species of herbivorous ants and plant-ant mutualists are likely candidates to harbor such symbionts as their diets are almost exclusively nectar based. In a collaboration with the Earth Microbiome Project, an initiative with the lofty goal of characterizing the global bacterial community, Ben is leveraging recent technological advances in DNA sequencing to compare the bacterial diversity in plant-ant mutualists and non-mutualists. Using the same phylogenetic framework as in the comparative genomics half of his research he will determine if particular bacterial lineages are present in all plant-ants and are potentially necessary for evolving such a specialized behavior.
Ben is generally very excited about applying next-generation sequencing technology to non-model organisms to study questions that were previously beyond the scope of any individual research program. Ideally, his research will utilize such techniques to help us understand the evolution of plant-ant mutualism and behavior more generally.
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