Horizontal transfer of genes and genomes in an endosymbiotic system: Investigation of the movement of intracellular bacteria and related gene transfer

The genus Wolbachia is comprised of intracellular gram negative bacteria capable of infecting a wide range of insect hosts. In arthropods, Wolbachia are reproductive parasites that can cause cytoplasmic incompatibility, parthenogenesis, feminization or male killing, each of which enhances the vertical transmission of the endosymbiont in a host population. Horizontal transmission of Wolbachia between different species can be shown by incongruence of phylogenies of Wolbachia and their hosts along with Wolbachia's widespread distribution. Wolbachia is a model for understanding the horizontal transfer of genes and genomes and the role of mobile genetic elements, including bacteriophages in host-symbiont interactions. My study provides the groundwork for understanding multitrophic interactions among insects, bacteria, and viruses in Wolbachia-host systems.;This study examines horizontal transfer of Wolbachia and associated WO bacteriophages in two host-parasitoid systems based on agronomically important pests Plutella xylostella and Ceutorhynchus obstrictus. Phylogenetic analysis shows that Wolbachia and WO have been horizontally transferred between P. xylostella and the parasitoid Diadegma insulare but not between C. obstrictus and its parasitoid T. lucidus. Results from this study suggest that horizontal transfer of Wolbachia requires longer-term host/parasitoid associations.;Comparative genomic methods were used to identify the core genome of the replicating bacteriophage WO. This study shows that all replicating WO bacteriophages include modules for DNA packaging and head assembly and tail morphogenesis. Remnant prophages lack the tail morphogenesis module and many genes associated with DNA packaging. One bacteriophage gene implicated in the establishment and maintenance of Wolbachia symbiosis is a DNA adenine methyltransferase (MTase) containing a ParB-like nuclease domain. Adenine methylation of DNA in bacteria has numerous functions including gene regulation and host-pathogen interactions. This study reveals that this gene is widespread in prokaryotes and has undergone horizontal transfer between distantly related bacterial species, but not between Wolbachia strains. Examining the genomic position of this gene in each prokaryotic genome revealed a conserved association with bacteriophage DNA packaging genes.