P. bursaria RNAi machinery processes algal endosymbiont derived RNA. A cost to host growth is incurred upon elimination of endosymbiotic algae. This cost is partly derived from RNAi mediated RNA-RNA interactions or collisions between endosymbiont and host mRNA sharing a high level of sequence identity. These RNA-RNA interactions act to punish the host for breakdown of the endosymbiosis, a factor that may drive stability.