Host Defense via Symbiosis in Drosophila

Host-associated microbes have often been studied as pathogens and the causes of disease, but symbiotic microbes that benefit their hosts are now known to be ubiquitous. In particular, insects possess a diversity of bacteria that can defend against natural enemies—Anopheles mosquitoes, for example, were recently shown to host a gut bacterium that confers refractoriness to malaria parasites [1]. In Drosophila, a key model of infection and immunity, fascinating examples of defense are accumulating, and two lineages of bacteria that infect the genus are now known to be defensive: Wolbachia and Spiroplasma (Figure 1). Both are vertically transmitted, both are facultative in Drosophila in that they are not strictly required by the host, and both infect Drosophila melanogaster. Here, we summarize what is known of Drosophila as an intriguing and emerging model of defensive symbiosis. Drosophila is an incredibly diverse genus with thousands of species, many of which are infected by Wolbachia and Spiroplasma [2]. As maternally transmitted symbionts, Wolbachia and Spiroplasma came to attention in Drosophila through their ability to manipulate host reproduction to favor their own transmission. Wolbachia are notorious for doing this by inducing cytoplasmic incompatibility (CI), whereby matings between Wolbachia-infected males and uninfected females result in the production of few to no offspring [3], providing selective pressure to maintain and rapidly spread Wolbachia in host populations. Though Spiroplasma are not known to induce CI, both Spiroplasma and Wolbachia can selfishly distort host sex ratios through male-killing in Drosophila, selectively killing the male offspring of infected females [3,4]. Many strains of Wolbachia and Spiroplasma, though, do not have such manipulative tendencies, and it has largely been a mystery how they are maintained in host populations. The discovery that they can defend against enemies has gone a long way in explaining their persistence, and has begun to shift our perception of many facultative inherited symbionts from that of manipulative parasites toward helpful mutualists.