Remaining flexible in old alliances: Functional plasticity in constrained mutualisms

Central to any beneficial interaction is the capacity of partners to detect and respond to significant changes in the other. Recent studies of microbial mutualists show their close integration with host development, immune responses, and acclimation to a dynamic external environment. While the significance of microbial players is broadly appreciated, we are just beginning to understand the genetic, ecological, and physiological mechanisms that generate variation in symbiont functions, broadly termed "symbiont plasticity" here. Some possible mechanisms include shifts in symbiont community composition, genetic changes via DNA acquisition, gene expression fluctuations, and variation in symbiont densities. In this review, we examine mechanisms for plasticity in the exceptionally stable mutualisms between insects and bacterial endosymbionts. Despite the severe ecological and genomic constraints imposed by their specialized lifestyle, these bacteria retain the capacity to modulate functions depending on the particular requirements of the host. Focusing on the mutualism between Blochmannia and ants, we discuss the roles of gene expression fluctuations and shifts in bacterial densities in generating symbiont plasticity. This symbiont variation is best understood by considering ant colony as the host superorganism. In this eusocial host, the bacteria meet the needs of the colony and not necessarily the individual ants that house them.