Evidence for metabolic provisioning by a common invertebrate endosymbiont, Wolbachia pipientis, during periods of nutritional stress

Abstract

Wolbachia are ubiquitous inherited endosymbionts of invertebrates that invade host populations by modifying host reproductive systems. However, some strains lack the ability to impose reproductive modification and yet are still capable of successfully invading host populations. To explain this paradox, theory predicts that such strains should provide a fitness benefit, but to date none has been detected. Recently completed genome sequences of different Wolbachia strains show that these bacteria may have the genetic machinery to influence iron utilization of hosts. Here we show that Wolbachia infection can confer a positive fecundity benefit for Drosophila melanogaster reared on iron-restricted or -overloaded diets. Furthermore, iron levels measured from field-collected flies indicated that nutritional conditions in the field were overall comparable to those of flies reared in the laboratory on restricted diets. These data suggest that Wolbachia may play a previously unrecognized role as nutritional mutualists in insects.

Figure 1. Total iron content of adult D. melanogaster females as determined by inductive coupled plasma mass-spectrometry.

Field collected flies are represented by white bars and lab reared flies by black bars. Flies reared on cornmeal fly diet contained approximately half the amount of iron as flies reared on high iron food, and approximately twice the amount of flies reared on low iron diets. All observations of lab-reared flies are statistically different from each other (t test; p<0.0001; groups a, c and d). The total iron content for three of the four field caught fly populations were significantly lower than those reared on normal food, where no Wolbachia fecundity advantage was observed (t test; p<0.0001; group b). The total iron content of flies collected from the St Lucia field site (t test; p>0.9999) was not significantly different from flies reared under low iron conditions where Wolbachia-associated fecundity benefits were observed. Standard error bars are indicated. Analysis was performed on pools of 10 adult female flies. A total of 10 pools were examined for each of the four defined diets; 4 pools of flies were examined for field-collected flies.

Figure 2. Mean fecundity measures of female D. melanogaster reared on low iron food (tea).

The total number of eggs laid by a single female was counted over a three-day period and the average calculated. Standard error bars are indicated; replicate numbers are noted within the columns. Uninfected females are denoted by an open bar, Wolbachia-infected females by a filled bar. Female flies reared on cornmeal fly diet are described as “Control.” Mean fecundities that are significantly different are denoted by * (p<0.05; ANOVA) or ** (p<0.001; Mann-Whitney U Test).

Figure 3. Mean fecundity measures of female D. melanogaster reared on low iron food (BPS).

The total number of eggs laid by a single female was counted over a three-day period and the average calculated. Standard error bars are indicated; replicate numbers are noted within the columns. Uninfected females are denoted by an open bar, Wolbachia-infected females by a filled bar. Female flies reared on cornmeal fly diet are described as “Control.” Mean fecundities that are significantly different are denoted by * (p<0.05; ANOVA).

Figure 4. Mean fecundity measures of female D. melanogaster reared on high iron food (FeCl₃).

The total number of eggs laid by a single female was counted over a three-day period and the average calculated. Standard error bars are indicated; replicate numbers are noted within the columns. Uninfected females are denoted by an open bar, Wolbachia-infected females by a filled bar. Female flies reared on cornmeal fly diet are described as “Control.” Mean fecundities that are significantly different are denoted by *(p<0.05) or ** (p<0.001; Mann-Whitney U Test).