Finalist EPFL doctorate Award 2015 – Jeremy Herren

Spiroplasma poulsonii is a maternally transmitted bacterial endosymbiont that is naturally associated with Drosophila melanogaster. To persist in its host, Spiroplasma poulsonii must evade host immunity, acquire metabolites to sustain bacterial growth and achieve high fidelity vertical transmission while minimizing detrimental effects on host fitness. These requirements provide the central framework for interactions between host and endosymbiont. We establish that the availability of lipids contained in hemolymph-localized transport lipoproteins mediates the rate of Spiroplasma growth.

Diaclyglyceride is used by Spiroplasma for the production of cardiolipin, a bacterial-specific lipid that cannot be obtained directly from the host. Host hemolymph lipids are required for Spiroplasma growth, which is demonstrated by the finding that RNAi-mediated knockdown of ApoLpp expression inhibits Spiroplasma growth. In addition, we analyzed the mechanism that underlies Spiroplasma’s vertical transmission, and provide evidence that these bacteria use the yolk uptake machinery to colonize the germ line. We observed that Spiroplasma was endocytosed into oocytes within yolk granules during the vitellogenic stages of oogenesis. Mutations that disrupt yolk uptake by oocytes inhibit vertical Spiroplasma transmission and lead to an accumulation of these bacteria outside the oocyte. Impairment of yolk secretion by the fat body results in Spiroplasma not reaching the oocyte and a severe reduction of vertical transmission.