Cytoplasmic incompatibility (Cl) in Drosophila simulans is related to infection of the germ line by a rickettsial endosymbiont (genus Wolbachia). Wolbachia were transferred by microinjection of egg cytoplasm into uninfected eggs of both D. simulans and D. melanogaster to generate infected populations. Transinfected strains of D. melanogaster with lower densities of Wolbachia than the naturally infected D. simulans strain did not express high levels of Cl. However, transinfected D. melanogaster egg cytoplasm, transferred back into D. simulans, generated infected populations that expressed Cl at levels near those of the naturally infected strain. A transinfected D. melanogaster line selected for increased levels of Cl expression also displayed increased symbiont densities. These data suggest that a threshold level of infection is required for normal expression of Cl and that host factors help determine the density of the symbiont in the host.
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