Degenerative evolution and functional diversification of type-III secretion systems in the insect endosymbiont Sodalis glossinidius

Sodalis glossinidius, a maternally transmitted endosymbiont of tsetse flies, maintains two phylogenetically distinct type-III secretion systems encoded by chromosomal symbiosis regions designated SSR-1 and SSR-2. Although both symbiosis regions are closely related to extant pathogenicity islands with similar gene inventories, SSR-2 has undergone novel degenerative adaptations in the transition to mutualism. Notably, SSR-2 lacks homologs of genes found in SSR-1 that encode secreted effector proteins known to facilitate the host cell cytoskeletal rearrangements necessary for bacterial entry and uptake into eukaryotic cells. Also, as a result of relaxed selection, SSR-2 has undergone inactivation of genes encoding components of the type-III secretion system needle substructure. In the current study, we used quantitative PCR to determine the expression profiles of ysaV (SSR-1) and invA (SSR-2) transcripts when S. glossinidius infects an insect cell line, and we used an invasion assay to characterize the phenotype of an S. glossinidius mutant that lacks the ability to produce an OrgA protein that is required for function of the SSR-2 secretome. Whereas SSR-1 is required for bacterial invasion of host cells and ysaV is expressed when bacteria contact host cells, SSR-2 is required for bacterial proliferation after entry, and invA is only expressed in the intracellular stage of infection. These results demonstrate that degenerative genetic adaptations in SSR-2 have promoted functional diversification of the Sodalis SSR-2 type-III secretion system.