Symbiont-induced odorant binding proteins mediate insect host hematopoiesis.

Abstract

Symbiotic bacteria assist in maintaining homeostasis of the animal immune system. However, the molecular mechanisms that underlie symbiont-mediated host immunity are largely unknown. Tsetse flies (<i>Glossina</i> spp.) house maternally transmitted symbionts that regulate the development and function of their host's immune system. Herein we demonstrate that the obligate mutualist, <i>Wigglesworthia</i>, up-regulates expression of <i>odorant binding protein six</i> in the gut of intrauterine tsetse larvae. This process is necessary and sufficient to induce systemic expression of the hematopoietic RUNX transcription factor <i>lozenge</i> and the subsequent production of crystal cells, which actuate the melanotic immune response in adult tsetse. Larval <i>Drosophila's</i> indigenous microbiota, which is acquired from the environment, regulates an orthologous hematopoietic pathway in their host. These findings provide insight into the molecular mechanisms that underlie enteric symbiont-stimulated systemic immune system development, and indicate that these processes are evolutionarily conserved despite the divergent nature of host-symbiont interactions in these model systems.