Effect of Iron on the Defensive Mutualism of Drosophila Flies and Spiroplasma Bacteria
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Maternally-transmitted associations between endosymbiotic bacteria and insects are pervasive in nature, and vary from commensalism to mutualism to parasitism. The association between the fruit fly Drosophila melanogaster and the maternally-inherited bacterium Spiroplasma involves reproductive parasitism and an apparent defensive mutualism. When attacked by the parasitoid wasp, Leptopilina boulardi, Spiroplasma-infected flies have a higher survival rate than their Spiroplasma-free counterparts. The mechanisms by which Spiroplasma prevents successful development of wasps are not understood, except that wasps exhibit slower growth when developing within a Spiroplasma-infected fly. One possible mechanism may involve competition between Spiroplasma and the wasp for a limiting resource. In this study, we investigated the role of iron in the Spiroplasma-mediated defense against the parasitoid wasp Leptopilina boulardi. Iron levels in the fly host were manipulated by rearing flies on diets that differed in the amount of iron available. Growth of the wasp larvae was monitored by measuring body length at 0, 72, and 144 hours post-oviposition. In the absence of Spiroplasma, iron levels had little effect on wasp growth. In the presence of Spiroplasma however, iron levels had a significant effect on wasp growth. The growth rate of wasps was lowest in the high-iron treatment and highest in the low-iron treatment. Indeed, in the latter treatment, the growth rate of the wasp resembled that of wasps reared in Spiroplasma-free hosts. These results imply that while competition for iron does not seem to be the mechanism of host defense, iron plays a role in the Spiroplasma-mediated defense against L. boulardi.
SubjectSpiroplasma, Drosophila, Parasitoid, Wasp, Mutualism, Defensive Mutualism, Symbioses, Evolution, Fly, Leptopilina
Winter, Caitlyn (2013). Effect of Iron on the Defensive Mutualism of Drosophila Flies and Spiroplasma Bacteria. Honors and Undergraduate Research. Available electronically from