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 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 identity with a type III secretion system invasion gene (invC) 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 Enterobacteriaceae 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 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.
Dale
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Sir Alexander Robertson Center for Tropical Veterinary Medicine, Royal Dick School of Veterinary Studies, University of Edinburgh, Easter Bush, Roslin, Midlothian EH25 9RG, Scotland, United Kingdom. cdale@ed.ac.uk
Young
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SA
Haydon
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Welburn
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D000330:Aedes; D000818:Animals; D000942:Antigens, Bacterial; D001426:Bacterial Proteins / Q000235:genetics / Q000502:physiology; D001483:Base Sequence; D002460:Cell Line; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D004755:Enterobacteriaceae / Q000145:classification / Q000235:genetics* / Q000378:metabolism / Q000472:pathogenicity*; D005260:Female; D005798:Genes, Bacterial; D017403:In Situ Hybridization / Q000379:methods; D008562:Membrane Glycoproteins; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D010802:Phylogeny; D016133:Polymerase Chain Reaction / Q000379:methods; D006180:Proton-Translocating ATPases / Q000235:genetics / Q000502:physiology; D013559:Symbiosis; D014370:Tsetse Flies / Q000382:microbiology*
D000942:Antigens, Bacterial; D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D008562:Membrane Glycoproteins; C055764:S-layer proteins; C101105:SpaQ protein, Salmonella; C088597:invB protein, Salmonella; C088598:invC protein, Salmonella typhimurium; C075518:invA protein, Bacteria; D006180:Proton-Translocating ATPases