WOLBACHIA SUPERINFECTIONS AND THE EXPRESSION OF CYTOPLASMIC INCOMPATIBILITY

Strains of Drosophila simulans from Riverside, California (DSR) and Ha waii (DSH) harbour distinct strains of the cytoplasmic incompatibility microorganism Wolbachia, resulting in the expression of bidirectional incompatibility when crossed. D. simulans lines carrying both of thes e (superinfected) Wolbachia strains were generated by the transfer of infected DSH cytoplasm into DSR embryos by microinjection. The superin fected flies were unidirectionally incompatible with both DSR and DSH individuals. As a result of this pattern, the superinfected state was observed to replace single infections in laboratory populations. The a bility of the superinfection to spread was modulated by the production of singly infected offspring from superinfected mothers: strain segre gation was observed under crowded larval rearing conditions. An invers e correlation between the penetrance of the cytoplasmic incompatabilit y phenotype and the degree of larval crowding was also observed. The f indings have implications for the evolution of bidirectionally incompa tible strains, and lead to the prediction that superinfections should be relatively common in field populations. Evidence for a natural supe rinfection in the mosquito Aedes albopictus is discussed. The results also have applied significance for the generation of insect lines capa ble of driving desirable genes into populations already infected with Wolbachia, thus allowing repeated opportunities for population replace ment.