POPULATION-DYNAMICS OF THE WOLBACHIA INFECTION CAUSING CYTOPLASMIC INCOMPATIBILITY IN DROSOPHILA-MELANOGASTER

Field populations of Drosophila melanogaster are often infected with W olbachia, a vertically transmitted microorganism. Under laboratory con ditions the infection causes partial incompatibility in crosses betwee n infected males and uninfected females. Here we examine factors influ encing the distribution of the infection in natural populations. We sh ow that the level of incompatibility under field conditions was much w eaker than in the laboratory. The infection was not transmitted with c omplete fidelity under field conditions, while field males did not tra nsmit the infection to uninfected females and Wolbachia did not influe nce sperm competition. There was no association between field fitness as measured by fluctuating asymmetry and the infection status of adult s. Infected field females were smaller than uninfecteds in some collec tions fr-om a subtropical location, but. not in other collections from the same location. Laboratory cage studies showed that the infection did not change in frequency when populations were maintained at a low larval density but it decreased in frequency at a high larval density. Monitoring of infection frequencies in natural populations indicated stable frequencies in some populations but marked fluctuations in othe rs. Simple models suggest that the infection probably provides a fitne ss benefit for the host ill order to persist in populations. The exact nature of this benefit remains elusive.