Natural variation in population densities and vertical transmission rates of a Spiroplasma endosymbiont in Drosophila hydei

A bacterium belonging to the genus Spiroplasma, an endosymbiont of the fly Drosophila hydei, is vertically transmitted through host egg cytoplasm. To infer vertical transmission rates of Spiroplasma in natural populations of D. hydei, the infection status of Spiroplasma was examined for offspring produced by Spiroplasma-positive females that were collected in two geographical populations. In both populations, nearly half of the broods consisted of only infected offspring. Infection frequencies of the rest of the broods ranged from 0.364 to 0.975. Quantitative PCR demonstrated that the Spiroplasma titers in the whole body of wild-caught females were highly variable (1.81?×?10⁶–5.60?×?10⁸ cells per insect). Contrary to our expectations, however, the Spiroplasma titers did not account for the variation in infection frequencies among offspring (i.e., vertical transmission rates). These results suggest that the spatial distribution of Spiroplasma, particularly in somatic tissues and germ tissues, is highly variable among host individuals, which may be caused by environmental stochasticity or some unknown effects.