The insect endosymbiont Sodalis glossinidius utilizes a type III secretionsystem for cell invasion

Sodalis glossinidius is a maternally transmitted secondary endosymbiont res iding intracellularly in tissues of the tsetse flies, Glossina spp. In this study, we have used Tn5 mutagenesis and a negative selection procedure to derive a S, glossinidius mutant that is incapable of invading insect cells in vitro and is aposymbiotic when microinjected into tsetse, This mutant st rain harbors Tn5 integrated into a chromosomal gene sharing high sequence i dentity with a type III secretion system invasion gene (invC) previously id entified in Salmonella enterica. With the use of degenerate PCR, we have am plified a further six Sodalis inv/spa genes sharing high sequence identity with type III secretion system genes encoded by Salmonella pathogenicity is land 1. Phylogenetic: reconstructions based on the inv/spa genes of Sodalis and other members of the family Enterobacteriaceae have consistently ident ified a well-supported clade containing Sodalis and the enteric pathogens S higella and Salmonella, These results suggest that Sodalis may have evolved from an ancestor with a parasitic intracellular lifestyle, possibly a latt er-day entomopathogen. These observations lend credence to a hypothesis sug gesting that vertically transmitted mutualistic endosymbionts evolve from h orizontally transmitted parasites through a parasitism-mutualism continuum.