Using RNA interference to investigate cell-cell interactions in a nascent phototrophic endosymbiosis

Abstract

The origins of eukaryotic cellular complexity arose from an early endosymbiosis between two or more ancestral cells - a mutual association whereby two became one to form a single integrated system. The relationship between the ciliate, Paramecium bursaria, and its green algal endosymbiont represents a nascent facultative endosymbiosis, meaning both host and endosymbiont are capable of surviving independently. As such, P. bursaria represents a valuable model system for investigating the cell-cell interactions of emergent endosymbioses. Intimately associated host-pathogen symbioses are subject to sRNA mediated interactions that act to determine the fate of the relationship, yet the existence of similar mechanisms within endosymbiotic systems is unclear. RNA interference (RNAi) is a form of post-transcriptional gene silencing dependent upon conserved sRNA processing pathways. In this thesis, I confirm the presence of an active siRNA-based RNAi pathway in the endosymbiotic system P. bursaria, validating host processing of exogenously derived double-stranded RNA (dsRNA) and substantiating its use as a tool for high-throughput reverse genetic screening. I additionally demonstrate host processing of single-stranded RNA (ssRNA), and identify an RNAi-mediated cost to host fitness for mass elimination of the endosymbiont. In doing so, I postulate a potential new factor in the maintenance of this endosymbiotic cell-cell interaction – RNAi mediated RNA-RNA ‘collisions’. RNAi ‘collisions’ between endosymbiont and host mRNA sharing sufficient sequence identity may, under certain conditions such as endosymbiont digestion, impose an RNAi-mediated physiological cost upon the host through knock-down of host gene expression. A cost to host fitness for elimination of the endosymbiont would transiently shift the dynamics of the interaction in favour of retention, punishing the host for termination of the association compared to co-operating sister cells, and promoting stability of the endosymbiotic cell-cell interaction.