Induced and Constitutive Immune Defenses in Flies

Organisms use induced and constitutive defenses to protect themselves against natural enemies. Induced responses are activated upon an attack, whereas constitutive defenses are always active. Induced and constitutive defenses are well-studied in plant systems; however, the relative usage and regulation of these defenses in most organisms are still unclear. In addition, theoretical research on the evolution of induced and constitutive defenses suffers from the lack of realistic models of immune defenses. To address these gaps in knowledge, I identified induced and constitutive defenses and regulatory mechanisms controlling the two defenses in flies. I have also devised realistic mathematical models to identify factors that shape the evolution of the induced and constitutive defenses in different environments. To those ends, I have analyzed RNA-seq data from house fly (Musca domestica) and Lucilia sericata infection experiments and found immune genes that are induced upon infection and those that are expressed regardless of the infection status (i.e., constitutive). Next, I have identified transcription factors that regulate induced and constitutive immune genes. To gain insight into the evolution of induced and constitutive defenses, I devised detailed mathematical models of induced and constitutive defenses and compared their performance under realistic simulations of host-pathogen interactions. I have shown that the extent to which an organism relies on induced or constitutive defenses is dependent upon its life history. I found evidence for the involvement of pleiotropic transcription factors in the regulation of constitutive defenses. Also, I showed that different organisms use different regulatory mechanisms to activate induced and constitutive defenses. Finally, my theoretical work predicts that environmental uncertainty favors the evolution of induction over constitutive defenses.