Male killing Spiroplasma protects Drosophila melanogaster against two parasitoid wasps

Abstract

Maternally transmitted associations between endosymbiotic bacteria and insects are diverse and widespread in nature. Owing to imperfect vertical transmission, many heritable microbes have evolved compensational mechanisms to enhance their persistence in host lineages, such as manipulating host reproduction and conferring fitness benefits to host. Symbiont-mediated defense against natural enemies of hosts is increasingly recognized as an important mechanism by which endosymbionts enhance host fitness. Members of the genus Spiroplasma associated with distantly related Drosophila hosts are known to engage in either reproductive parasitism (i.e., male killing) or defense against natural enemies (the parasitic wasp Leptopilina heterotoma and a nematode). A male-killing strain of Spiroplasma (strain Melanogaster Sex Ratio Organism (MSRO)) co-occurs with Wolbachia (strain wMel) in certain wild populations of the model organism Drosophila melanogaster. We examined the effects of Spiroplasma MSRO and Wolbachia wMel on Drosophila survival against parasitism by two common wasps, Leptopilina heterotoma and Leptopilina boulardi, that differ in their host ranges and host evasion strategies. The results indicate that Spiroplasma MSRO prevents successful development of both wasps, and confers a small, albeit significant, increase in larva-to-adult survival of flies subjected to wasp attacks. We modeled the conditions under which defense can contribute to Spiroplasma persistence. Wolbachia also confers a weak, but significant, survival advantage to flies attacked by L. heterotoma. The host protective effects exhibited by Spiroplasma and Wolbachia are additive and may provide the conditions for such cotransmitted symbionts to become mutualists. Occurrence of Spiroplasma-mediated protection against distinct parasitoids in divergent Drosophila hosts suggests a general protection mechanism.

Figure 1

Fly larva-to-adult survival, larva-to-pupa survival, pupal mortality and wasp success in the four endosymbiont infection treatments (S=Spiroplasma; W=Wolbachia) and in the three wasp treatments. (a) No wasp control. (b) Lh treatment. (c) Lb treatment. P-values shown for each effect: Spiroplasma infection state; Wolbachia infection state; their interaction; and fly strain (isoline). For isoline, only significant P-values are shown (see Supplementary Table S1). Bars: white, proportion of fly larvae that survived to adulthood; gray, proportion of fly larvae that survived to pupation; black, proportion of total pupae that failed (neither fly nor wasp emerged); dotted, exposed fly larvae that gave rise to eclosing wasps. Error bars: s.e.

Figure 2

Comparison of wasp oviposition frequency among fly larvae from the four endosymbiont treatments (S=Spiroplasma; W=Wolbachia). Lh=flies subjected to L. heterotoma; Lb=flies subjected to L. boulardi. Error bars: s.e.

Figure 3

Comparison of wasp growth rate within fly larvae from the four endosymbiont infection treatments (S=Spiroplasma; W=Wolbachia). (a) Lh egg/larvae body length (mean±s.e.) through 72 h after wasp attack. (b) Lb egg/larvae body length through 144 h after wasp attack.

Figure 4

Relationship of wasp parasitism rate (P) to Spiroplasma prevalence (I) in the fly population for (a) Lh and (b) Lb, according to the larva-to-adult survival advantage conferred by Spiroplasma MSRO, as estimated directly from our experiments (solid line), and an adjusted fitness advantage accounting for reduced longevity and fecundity of adult flies surviving a wasp attack (dashed line; see text for details). Gray areas indicate the range of prevalences (1–17.7%) reported for Spiroplasma MSRO in natural populations of D. melanogaster.