Cell Division by Longitudinal Scission in the Insect Endosymbiont Spiroplasma poulsonii

Spiroplasma bacteria are highly motile bacteria with no cell wall and a helical morphology. This clade includes many vertically transmitted insect endosymbionts, including Spiroplasma poulsonii, a natural endosymbiont of Drosophila melanogaster. S. poulsonii bacteria are mainly found in the hemolymph of infected female flies and exhibit efficient vertical transmission from mother to offspring. As is the case for many facultative endosymbionts, S. poulsonii can manipulate the reproduction of its host; in particular, S. poulsonii induces male killing in Drosophila melanogaster. Here, we analyze the morphology of S. poulsonii obtained from the hemolymph of infected Drosophila. This endosymbiont was not only found as long helical filaments, as previously described, but was also found in a Y-shaped form. The use of electron microscopy, immunogold staining of the FtsZ protein, and antibiotic treatment unambiguously linked the Y shape of S. poulsonii to cell division. Observation of the Y shape in another Spiroplasma, S. citri, and anecdotic observations from the literature suggest that cell division by longitudinal scission might be prevalent in the Spiroplasma clade. Our study is the first to report the Y-shape mode of cell division in an endosymbiotic bacterium and adds Spiroplasma to the so far limited group of bacteria known to utilize this cell division mode.