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Proc Natl Acad Sci U S A. 2002 Sep 17;99(19):12397-402. Epub 2002 Sep 4.

Type III secretion systems and the evolution of mutualistic endosymbiosis.

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  • 1Department of Ecology and Evolutionary Biology, Center for Insect Science, and Department of Biochemistry and Molecular Biophysics, University of Arizona, Tucson, AZ 85721.


The view that parasites can develop cooperative symbiotic relationships with their hosts is both appealing and widely held; however, there is no molecular genetic evidence of such a transition. Here we demonstrate that a mutualistic bacterial endosymbiont of grain weevils maintains and expresses inv/spa genes encoding a type III secretion system homologous to that used for invasion by bacterial pathogens. Phylogenetic analyses indicate that inv/spa genes were present in presymbiotic ancestors of the weevil endosymbionts, occurring at least 50 million years ago. The function of inv/spa genes in maintaining symbiosis is demonstrated by the up-regulation of their expression under both in vivo and in vitro conditions that coincide with host cell invasion.

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