Disease defence through generations: leaf-cutter ants and their symbiotic bacteria

Microbial ecology of animals is taking on significance in the modern dialogue for the biology of species. Similar to a nuclear genome, the entire bacterial assemblage maintains an ancestral signal of the host's evolution leading to cophylogeny between the host and the microbes they harbour (Brucker & Bordenstein 2012b). The stability of such associations is of great interest as they provide a means for species to acquire new traits and genetic diversity that their own genomes lack (McFall-Ngai et al. 2013). The role of gut microbiota, for example, in host health and nutrition is widely recognized and a shared characteristic among animals. The role of bacteria colonizing the outside surfaces of animals is less well understood, but rather than random colonization, these microbes on skin, cuticles, scales and feathers in many cases provide benefits to the host. The symbiosis of leaf-cutter ants, their fungus gardens and their microbiota is a fascinating and complex system. Whether culture-independent bacterial diversity on the cuticle of leaf-cutter ants is high or highly constrained by subcuticular gland secretions is one prominent question. In this issue of Molecular Ecology, Andersen et al. (2013) show that leaf-cutting ants, Acromyrmex echinatior, maintain a dominant and colony-specific bacterium called Pseudonocardia on their cuticles (the laterocervical plates in particular). This bacterium is involved in protecting the ants and their fungal gardens from disease. Other fungus-gardening attine species as well as soil and vegetation can harbour Pseudonocardia. However, it was previously unknown how stable the bacterial strain-ant colony association was through the lifetime of the colony.