RICKETTSIAL RELATIVE ASSOCIATED WITH MALE KILLING IN THE LADYBIRD BEETLE (ADALIA-BIPUNCTATA)

A cytoplasmically inherited microorganism associated with male killing in the two-spot ladybird beetle, Adalia bipunctata, is shown to be cl osely related to bacteria in the genus Rickettsia. Sequencing of a PCR -amplified product of the 16S genes coding for rRNA (16S rDNA) shows t he organism associated with male killing in ladybirds to share a commo n ancestry with the Rickettsias relative to other genera (eg, Anaplasm a, Ehrlichia, and Cowdria). The rickettsial 16S rDNA product is found in four strains of ladybird beetle showing male embryo lethality and i s absent from two uninfected strains and an antibiotic-cured strain. I n addition, a revertant strain that had naturally lost the male-killin g trait failed to amplify the rickettsial 16S rDNA product. Use of PCR primers for a 17-kDa protein antigen which is found only in rickettsi as also resulted in an amplified product from infected strains. Uninfe cted, cured, and revertant strains and insect species infected with re lated bacteria (cytoplasmic-incompatibility bacteria from Nasonia wasp s) failed to amplify the product. Discovery of a close relative of ric kettsias associated with sex ratio distortion in insects has implicati ons for the evolution and population dynamics of this bacterial genus.