WIDESPREAD OCCURRENCE OF THE PROTEOBACTERIA WOLBACHIA AND PARTIAL CYTOPLASMIC INCOMPATIBILITY IN DROSOPHILA-MELANOGASTER

Most populations of Drosophila melanogaster over the world are polymor phic for infection by the maternally inherited Proteobacteria Wolbachi a pipientis (average rate of infection 34 %) responsible in the host s pecies for partial cytoplasmic incompatibility (the percentage of unha tched eggs varies from negligible up to 77 %, with an average of 46,7 %). Partial 16S rDNA sequences of bacteria of flies from four populati ons were identical and indistinguishable from the D. simulans symbiont type R. In crosses to an infected reference strain, all infected indi viduals seemed to belong to the same incompatibility type. In addition , in D. melanogaster, cytoplasm infection is irrespective of mtDNA hap lotypes, but not in D. simulans where this bacterial lineage is only p resent in a terminal branch in the mtDNA haplotype tree. It can be inf erred that the cytoplasm of D. melanogaster was infected probably once , independently of D. simulans, early in the evolution of the species, and was subsequently lost several times. The level of incompatibility is related to the level of infection of the sperm cysts observed in y oung males, but infected females, whatever the strength of incompatibi lity determined by the males of their strains, are compatible with the most heavily infected males. The percentage of infected cytoplasms ha s not changed during the last decades and the maintenance of infection polymorphism is problematic.