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

Sodalis glossinidius is a maternally transmitted secondary endosymbiont residing intracellularly in tissues of the tsetse flies,Glossina spp. In this study, we have used Tn5 mutagenesis and a negative selection procedure to derive aS. glossinidius mutant that is incapable of invading insect cellsin vitro and is aposymbiotic when microinjected into tsetse. This mutant strain harbors Tn5 integrated into a chromosomal gene sharing high sequence identity with a type III secretion system invasion gene (invC ) previously identified inSalmonella enterica . With the use of degenerate PCR, we have amplified a further sixSodalis inv /spa genes sharing high sequence identity with type III secretion system genes encoded bySalmonella pathogenicity island 1. Phylogenetic reconstructions based on theinv /spa genes ofSodalis and other members of the familyEnterobacteriaceae have consistently identified a well-supported clade containingSodalis and the enteric pathogensShigella andSalmonella . These results suggest thatSodalis may have evolved from an ancestor with a parasitic intracellular lifestyle, possibly a latter-day entomopathogen. These observations lend credence to a hypothesis suggesting that vertically transmitted mutualistic endosymbionts evolve from horizontally transmitted parasites through a parasitism–mutualism continuum.