Systematic Investigation into Endoriftia Flagellar System Suggests Possibility of System Co-Option for Additional Functionality

Abstract

The overarching goal of this study is to contribute to the development of the Candidatus Endoriftia persephone-Riftia pachyptila symbiotic association as a biological model system with a more specific goal being to systematically investigate the symbiont's flagellar system. While long thought to possess the flagellum for motility during its free-living life stage, no systematic investigation into the system and functions were discovered(Klose et al., 2015; Millikan et al., 1999). This is a significant gap in efforts to understand this association, particularly since endosymbionts frequently co-opt/re-functionalize their flagellar system for roles in symbiosis association mechanisms, often via additional functionalities distinct from motility itself (Aschtgen et al., 2019; Chaban et al., 2015). Furthermore, in ongoing energy metabolism experiments, flagellar genes were observed to be actively expressed in the Riftia endosymbionts(J. H. Mitchell et al., 2019). Assuming this is not about motility given the microbes' habitat at the time of transcript collection, such active expression seems to indicate the Ca. Endoriftia persephone flagellar system may have additional functionality during endosymbiosis. To elucidate this further, this study systematically investigated the system itself and expression across various environmental treatments, finding it complete with no obvious global pattern between treatments. However, one comparison did show clear patterns of flagellar gene up-regulation and subsequent analysis of these expressed components suggested F-T3SS mediated export of non-flagellar effector proteins as the additional function. This was further supported by the prediction of large amounts of candidate type III secreted effectors in Ca. Endoriftia persephone while also demonstrating a lack of the NF-T3SS.