Microbial systems offer a rich and powerful way to understand the evolution of social behaviors. These social behaviors allow microbes to exert dramatic effects on their environment, often constructing a niche that is favorable to them. Selfish and mutually beneficially behaviors can be understood through an individual fitness benefit, while altruism and spite have provided an evolutionary puzzle. Our lab is working to solve that puzzle. Spite has long been considered a theoretical possibility, but not a social behavior of any consequence in nature. After all, how can a behavior that is harmful to both the actor and the recipient persist? The answer lies in who is the recipient of the spiteful act and who might indirectly benefit. In other words, spite might more easily be understood as “indirect altruism,” whereby the actor’s kin benefit by the harm directed to unrelated individuals.
Microbial Social Behavior
Antagonistic interactions among bacteria mediated via bacteriocins have been proposed as the premier example of spite in nature. Bacteriocins are bacteriocidal toxins, noted for their narrow killing spectrum. Bacteriocin production is costly, and often, the producer cell lyses to release the toxin. As a dead producer can receive no direct benefit from its action, bacteriocin production fits the definition of spite. Moreover, individuals closely related to the producer cell are often immune to the bacteriocin’s effect and thus may benefit by the destruction of competitors.
Xenorhabdus bacteria can inhibit each other’s growth by via the bacteriocin, xenorhabdicin. Xenorhabdicin is phage derived and similar to the R-type pyocins of Pseudomonas. Our work on natural isolates of Xenorhabdus was the first to show that intraspecific bacteriocin-based antagonisms can occur over a spatially relevant scale and thus support the existence of spite in nature. Moreover, by examining the genetic relatedness and spatial distance between interacting isolates, our studies support recent theoretical predictions suggesting that population structure is key to the evolution of spite and that spite benefits kin and can lower virulence. We are currently examining the genetic architecture underlying the phenotypic diversity in bacteriocin-based interactions of our natural isolates. We are also examining the regulation of bacteriocin production.