The Type IV Secretion System (T4SS) is an efficient pathway with which bacteria can mediate the transfer of DNA and/or proteins to eukaryotic cells. In Wolbachia pipientis, a maternally inherited obligate endosymbiont of arthropods and nematodes, two operons of vir genes, virB3-B6 and virB8-D4, encoding a T4SS were previously identified and characterized at two separate genomic loci. Using the largest data set of Wolbachia strains Studied so far, we show that vir gene sequence and organization are strictly conserved among 37 Wolbachia strains inducing various phenotypes Such as cytoplasmic incompatibility, feminization, or oogenesis in their arthropod hosts. In sharp contrast, extensive variation of genomic sequences flanking the virB8-D4 operon Suggested its distinct location among Wolbachia genomes. Long term conservation of the T4SS may imply maintenance of a functional effector translocation system in Wolbachia, thereby Suggesting the importance for the T4SS in Wolbachia biology and Survival inside host cells. (c) 2009 Elsevier Inc. All rights reserved.