Wolbachia infection and incompatibility dynamics in experimental selectionlines

High and low levels of Wolbachia-induced cytoplasmic incompatibility (CI) w ere selected for in the parasitic wasp Nasonia vitripennis, in the single-i nfected strain Ti277. After nine generations of selection, males from lines selected for high incompatibility level (PII lines) were significantly mor e incompatible with uninfected females (AsymC) than the maternal strain. Th e reverse response, a full compatibility with AsymC, was observed in eight out of 12 lines selected for low incompatibility (LO lines), correlated wit h loss of Wolbachia infection. Bacterial density estimates in the eggs of s ome HI lines increased significantly. The procedure for line maintenance re sulted in introgression of AsymC nuclear genome into the Ti277 background. Significant changes of CI level and bacterial density due to the introgress ion were also observed in the control lines, possibly reflecting an effect of host genotype on bacterial density and CI. After selection had been rela xed for six generations, bacterial density in the five high-infected HI lin es declined back to a level comparable to the other lines. The data are con sistent with the 'bacterial dosage' model, but with an upper threshold of b acterial infection above which there is no correlation between infection le vel and CI level. We further investigate the maternal transmission of bacte rial density by a mother-daughter regression on bacterial density. The patt ern observed is consistent with a density dependent regulation of bacterial numbers around an 'equilibrium' density, independent of any effects of CI. The equilibrium value is likely to be determined by both bacterial strain and host genotype.