The effect of iron on the defensive mutualism of Spiroplasma bacteria and Drosophila flies

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 144 hours post-oviposition.  Fly survival and wasp success rates were also measured.  Both in the presence and absence of Spiroplasma, iron level did not have a significant effect on wasp growth.  Additionally, iron level had no significant effects on larva to pupa or pupa to fly survival rates, or on wasp success rates.   These results imply that competition for iron is not the mechanism of host defense in the Spiroplasma conferred protection against parasitoid wasp attack in Drosophila.