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

摘要

Sodalis glossinidius is a maternally transmitted secondary endo- symbiont 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 a S. glossinidius mutant that is incapable of invading insect cells in vitro and is aposymbiotic when microinjected into tsetse. This mutant strain harbors Tn5 integrated into a chromosomal gene sharing high sequence iden- tity with a type Ⅲ secretion system invasion gene (invO previously identified in Salmonella enterica. With the use of degenerate PCR, we have amplified a further six Sodalis inv/spa genes sharing high sequence identity with type Ⅲ secretion system genes encoded by Salmonella pathogenicity island 1. Phylogenetic reconstructions based on the inv/spa genes of Sodalis and other members of the family Enterobacterisceae have consistently identified a well- supported clade containing Sodalis and the enteric pathogens Shigella and Salmonella. These results suggest that Sodalis may have evolved from an ancestor with a parasitic intracellular life- style, possibly a latter-day entomopathogen. These observations lend credence to a hypothesis suggesting that vertically transmit- ted mutualistic endosymbionts evolve from horizontally transmit- ted parasites through a parasitism--mutualism continuum.