Give us the tools and we will do the job: symbiotic bacteria affect olive fly fitness in a diet-dependent fashion

Abstract

Olive flies (Bactrocera oleae) are intimately associated with bacteria throughout their life cycle, and both larvae and adults are morphologically adapted for housing bacteria in the digestive tract. We tested the hypothesis that these bacteria contribute to the adult fly's fitness in a diet-dependent fashion. We predicted that when dietary protein is superabundant, bacterial contribution will be minimal. Conversely, in the absence of protein, or when only non-essential amino acids are present (as in the fly's natural diet), we predicted that bacterial contribution to fitness will be significant. Accordingly, we manipulated diet and the presence of bacteria in female olive flies, and monitored fecundity--an indirect measure of fitness. Bacteria did not affect fecundity when females were fed a nutritionally poor diet of sucrose, or a protein-rich, nutritionally complete diet. However, when females were fed a diet containing non-essential amino acids as the sole source of amino nitrogen, egg production was significantly enhanced in the presence of bacteria. These results suggest that bacteria were able to compensate for the skewed amino acid composition of the diet and may be indispensable for wild adult olive flies that subsist mainly on nitrogen-poor resources such as honeydew.

Figure 1.

Effect of the antibiotic treatment on the bacterial population housed within the oesophageal diverticulum as estimated by total bacteria counts. Incorporating the antibiotic piperacillin into all three diets (S, sugar; NE, sugar and non-essential amino acids; F, full diet) decreased the number of bacteria in the gut. Antibiotic-treated females: shaded box. Non-treated females: unshaded box. *p < 0.05; ^†p = 0.065.

Figure 2.

Mean number of eggs laid by females as affected by the presence (shaded box) or absence (unshaded box) of antibiotics in the diet. Fecundity significantly depended on the presence of bacteria only when females were offered NE amino acids as the sole source of nitrogen in their diet (diet NE). *p < 0.001.

Figure 3.

Oviposition pattern of females fed on sugar and non-essential amino acids (diet NE) throughout the 20 day treatment period. Antibiotic-treated females: shaded box. Non-treated females: unshaded box. *p < 0.0005.