Immune-like glycan-sensing and horizontally-acquired glycan-processing orchestrate host control in a microbial endosymbiosis

Endosymbiosis was a key factor in the evolution of eukaryotic cellular complexity. Yet the mechanisms that allow host regulation of intracellular symbionts, a pre-requisite for stable endosymbiosis and subsequent organelle evolution, are largely unknown. Here, we describe an immune-like glycan-sensing/processing network, partly assembled through horizontal gene-transfers (HGTs), that enables Paramecium bursaria to control its green algal endosymbionts. Using phylogenetics, RNA-interference (RNAi), and metabolite exposure experiments, we show that P. bursaria regulates endosymbiont destruction using glycan-sensing/processing - a system that includes a eukaryotic-wide chitin-binding chitinase-like protein (CLP) localized to the host phago-lysosome. RNAi of CLP alters expression of eight host glycan-processing genes, including two prokaryote-derived HGTs, during endosymbiont destruction. Furthermore, glycan-sensing/processing dynamically regulates endosymbiont number in P. bursaria, plasticity crucial to maximize host fitness across ecological conditions. CLP is homologous to a human phagocyte-associated innate immune factor, revealing how immune functions can be alternatively adapted and expanded, partly through HGT, enabling endosymbiotic control.