Mapping possible evolutionary routes to endosymbiosis by globally profiling loss-of-function mutations that limit bacterial proliferation within aphids
Many organisms have beneficial and mutually dependent relationships with vertically inherited microbial endosymbionts that are thought to be descended from pathogens. According to this hypothesis, these microbes evolved to become less harmful to their hosts after they established by invading host cells or tissues. Serratia symbiotica CWBI-2.3T (CWBI) is a recently cultured relative of aphid endosymbionts that primarily colonizes the aphid gut. When it escapes the gut or is injected into aphids, CWBI can colonize developing embryos. However, it grows rapidly to a high density, reduces aphid survival, and stunts or kills aphid offspring. This study used transposon insertion sequencing (Tn-seq) to understand what gene loss events in this "protosymbiont" might evolve to stabilize its association with aphids as an endosymbiont.