THE ROLE OF THE MICROBIOME IN TRANS-GENERATIONAL IMMUNE PRIMING OF T. CASTANEUM

Abstract

Trans-generational immune priming is a phenomenon recently documented in insects in which the offspring of previously infected mothers inherit a survival advantage over larvae from uninfected mothers. This form of immunological “memory,” analogous to adaptive immunity in vertebrates, has provided the scientific community with an important mechanistic puzzle: in the absence of heritable antibody-conferred resistance, how is this survival advantage transmitted from parent to offspring? Despite a proliferating scientific literature that has revealed much about the immunogenetic manifestations of immune priming, the mechanistic basis by which immune advantage is imparted trans-generationally in insects remains poorly understood. I approached this question through a framework that identified two potential sources of inheritance: maternally provisioned immune factors (i.e., immune factors supplied within the egg) and socially inherited microbial flora (i.e., gut-colonizing bacteria found on the egg). In this thesis, I used Tribolium castaneum, the red flour beetle, and Bacillus thuringiensis (Bt), an entomopathogenic bacterium, to perform a cross-fostering experiment in which I controlled and manipulated the bacterial exposures of larvae from primed and unprimed mothers. This allowed me to parse out the influences of maternal inheritance and microbial inheritance on survival and immune gene expression as they relate to the primed larval phenotype. From my survival analysis, I found that maternal egg provisioning plays a more predominant role in producing the primed phenotype, but that the microbial inheritance may still have a modulatory influence that varies depending on maternal priming status. Although my analyses of gene expression patterns across maternal and microbial exposures revealed few conclusive trends, my combined investigations into survival and gene expression have provided testable conceptual explanations for observed patterns, and pave the way for continued investigation of the role of the microbiome in trans-generational immune priming.