Immune response of the ant Camponotus floridanus against pathogens and its obligate mutualistic endosymbiont

Numerous insect species harbor mutualistic endosymbionts that play a role in nutrient cycling or confer other fitness benefits to their hosts. Insect hosts face the problem of having to maintain such mutualistic bacteria while staging an immune response towards pathogens upon infection. In addition, hosts may regulate the number of endosymbionts present in their tissues via the innate immune system. Camponotus floridanus ants harbor the obligate endosymbiont Blochmannia floridanus in specialized midgut cells and ovaries. We identified genes transcriptionally induced in response to septic injury by suppression subtractive hybridization (SSH). Among these were genes involved in pathogen recognition (e.g. GNBP), signal transduction (e.g. MAPK-kinase), antimicrobial activity (e.g. defensin and hymenoptaecin), or general stress response (e.g. heat shock protein). A quantitative analysis of immune-gene expression revealed different expression kinetics of individual factors and also characteristic expression profiles after injection of gram-negative and gram-positive bacteria. Likewise, B. floridanus injected into the hemocoel elicited a comparable immune response of its host C. floridanus. Thus, the host immune system may contribute to controlling the endosymbiont population.

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► We identified 35 infection-inducible genes in the ant Camponotus floridanus.
► Expression kinetics following septic injury was documented for 9 genes.
► Genes were also upregulated when the endosymbiont was injected into the hemocoel.
► The endosymbiont Blochmannia may thus still induce an immune response in its ant host.
► We conclude that the immune system plays a role in regulation of the endosymbiont.