Influence of the symbiont Wolbachia on life history traits of the cabbage root fly (Delia radicum).

Wolbachia is an endocellular bacteria infecting arthropods and nematodes and is only transmitted vertically by females via the cytoplasm of the egg. It is often a manipulator of host reproduction, causing cytoplasmic incompatibility, thelytokous parthenogenesis, feminization or male killing, which all increase the proportion of infected females in the population. However, Wolbachia can modify life history traits of the host without causing the above phenotypes and each species illustrates the variability of relationships between this remarkably versatile symbiont and its many hosts. We have measured maternal transmission and the impact of a natural Wolbachia infection in the cabbage root fly Delia radicum, a major agricultural pest. We used a population that is polymorphic for the infection to ensure similar genetic and microbiome backgrounds between groups. Maternal transmission of the infection was 100% in our sample. We found no evidence of cytoplasmic incompatibility, thelytokous parthenogenesis, feminization nor male killing. Wolbachia infection significantly reduced hatch rate in infected eggs (by 10%) but improved larvo-nymphal viability sufficiently so that infected eggs nevertheless yielded as many adults as uninfected ones, albeit with a 1.5% longer total development time. Starved and infected ovipositing females suffered significantly reduced viability (20% higher mortality during a 3-day oviposition period) than uninfected females, but mortality was not higher in starved virgin females nor in starved males, suggesting that the energetic cost of the infection is only revealed in extreme conditions. Wolbachia had no effect on egg hatch time or offspring size. The apparently 100% vertical transmission and the significant but mutually compensating effects found suggest that infection might be nearly benign in this host and might only drift slowly, which would explain why the infection rate has been stable in our laboratory (approximately 50% individuals infected) for at least 30 generations.